Possible dose reduction by dose-rate measurements using mobile phones/tablets combined with tabulated imaging procedure/radiation doses
Author(s):
Christoph Hoeschen;
William W. Orrison M.D.;
Rolf Dieter Klein;
Mathias M. A. Reichl;
Peter Cartwright
Show Abstract
The purpose of our study is to demonstrate the effectiveness of using smartphones for measuring ionizing radiation and
to combine the findings with radiation exposure from medical imaging procedures for recording individual annual/lifetime
radiation dose and provide improved radiation protection.
We developed an application for smartphones which can use the properties of the video camera chip in such smartphones
together with highly sufficient statistical evaluation of the signals to detect ionizing radiation. We could show that this
application can be used in a large range of dose rates from natural backgrounds to high dose rate pulsed radiation like in
fluoroscopic radiation or CT investigations.
We could also show that these kinds of systems might help to provide better radiation protection by advising medical
staff to use radiation protection material in best dose saving ways.
Development of matched virtual and physical breast phantoms based on patient data
Author(s):
Nooshin Kiarashi;
Gregory M. Sturgeon;
Loren W. Nolte;
Joseph Y. Lo;
James T. Dobbins III;
William P. Segars;
Ehsan Samei
Show Abstract
Physical phantoms are essential for the development, optimization, and clinical evaluation of x-ray systems. These
phantoms are used for various tests such as quality assurance testing, system characterization, reconstruction evaluation,
and dosimetry. They should ideally be capable of serving as ground truth for purposes such as virtual clinical trials.
Currently, there is no anthropomorphic 3D physical phantom commercially available. We present our development of a
new suite of physical breast phantoms based on real patient data. The phantoms were generated from the NURBS-based
extended cardiac-torso (XCAT) breast phantoms, which were segmented from patient dedicated breast computed
tomography data. High-resolution multi-material 3D printing technology was used to fabricate the physical models.
Glandular tissue and skin were presented by the most radiographically dense photopolymer available to the printer,
mimicking a 75% glandular tissue. Adipose tissue was presented by the least radiographically dense photopolymer,
mimicking a 35% glandular tissue. The glandular equivalency was measured by comparing x-ray images of samples of
the photopolymers available to the printer with those of breast tissue-equivalent materials. The mammographic
projections and tomosynthesis reconstructed images of fabricated models showed great improvement over available
phantoms, presenting a more realistic breast background.
Design considerations for a new high resolution Micro-Angiographic Fluoroscope based on a CMOS sensor (MAF-CMOS)
Author(s):
Brendan Loughran;
S. N. Swetadri Vasan;
Vivek Singh;
Ciprian N. Ionita;
Amit Jain;
Daniel R. Bednarek;
Albert H. Titus;
Stephen Rudin
Show Abstract
The detectors that are used for endovascular image-guided interventions (EIGI), particularly for neurovascular
interventions, do not provide clinicians with adequate visualization to ensure the best possible treatment outcomes.
Developing an improved x-ray imaging detector requires the determination of estimated clinical x-ray entrance
exposures to the detector. The range of exposures to the detector in clinical studies was found for the three modes of
operation: fluoroscopic mode, high frame-rate digital angiographic mode (HD fluoroscopic mode), and DSA mode.
Using these estimated detector exposure ranges and available CMOS detector technical specifications, design
requirements were developed to pursue a quantum limited, high resolution, dynamic x-ray detector based on a CMOS
sensor with 50 μm pixel size. For the proposed MAF-CMOS, the estimated charge collected within the full exposure
range was found to be within the estimated full well capacity of the pixels. Expected instrumentation noise for the
proposed detector was estimated to be 50-1,300 electrons. Adding a gain stage such as a light image intensifier would
minimize the effect of the estimated instrumentation noise on total image noise but may not be necessary to ensure
quantum limited detector operation at low exposure levels. A recursive temporal filter may decrease the effective total
noise by 2 to 3 times, allowing for the improved signal to noise ratios at the lowest estimated exposures despite
consequent loss in temporal resolution. This work can serve as a guide for further development of dynamic x-ray
imaging prototypes or improvements for existing dynamic x-ray imaging systems.
Image performances of multi-resolution technology for dynamic detector
Author(s):
Takaaki Ito;
Fumito Nariyuki;
Yoshihiro Okada
Show Abstract
The multi-resolution technology was developed for dynamic flat panel detectors for X-ray imaging. This multi-resolution
technology allows us to switch between the 1 x 1 mode (150 μm square) and the 2 x 2 mode (300 μm square)
instantaneously, using an external control. We developed a 17" x 17" dynamic detector using this multi-resolution
technology. This novel dynamic detector has a high sensitivity and a high-speed readout and it can reduce the radiation
exposure dose and deliver a smooth image. The key feature of our multi-resolution technology is capable of reading 4
pixel signals simultaneously. The sensitivity and the readout speed in the 2 x 2 mode were 4 times higher than those in
the 1 x 1 mode. The multi-resolution technology was implemented using a unique thin film transistor structure; that is,
one pixel has two switches, each of which are turned on/off depending on the readout mode. As a result, the dynamic
detector with a large active area of 17" x 17" realized a high detective quantum efficiency value of 40% under the low
radiation of RQA5 20 nGy and a high-speed readout of 30 frames/sec. This multi-resolution technology made it possible
to reduce the radiation exposure dose in a variety of applications.
Design of a 3D field emission electron source with improved emission current density and cathode life
Author(s):
Yiming Xu;
Dongsong Li;
Jian Zhang
Show Abstract
Field emission electron sources have been increasingly investigated and practiced as cold electron sources for many Xray
generation mechanisms especially in certain medical imaging applications, such as computed tomography (CT). In a
field emission electron source, the emission current and cathode life are the two key performance parameters of interests.
Conventional field emission electron source in the form of a 2-dimensional single surface cathode design is often
undergoing the bottleneck of limited emission current. Higher current can be obtained by increasing the strength of
driving electric field. However this is at the expense of a reduced cathode life. In this paper we present a novel field
emission electron source design based on a 3-dimensional semi-enclosed cavity structure, by utilizing the cavity’s
multiple inner faces as electron emission surfaces. The cavity has one bottom inner face open for electron ejection, the
area of which was kept the same as a conventional single surface cathode to maintain the same initial electron beam
cross section. The extended emission area of the new cathode design due to its 3-dimensional structure provides a higher
electron emission current while maintaining the same beam cross section, resulting in an improved effective emission
current density. We have demonstrated a significantly increased electron emission current from this 3-dimensional
cathode under a constant electric field, without over-driving the cathode field emitters. Alternatively this new cathode
design can also achieve the same total emission current with a lower electric field compared with a conventional 2-
dimensional cathode, which will significantly extend its life.
A novel sensor for high throughput preclinical radiotracer imaging
Author(s):
Haris Kudrolli;
Harish B. Bhandari;
Katherine L. Byrne;
Simon R. Cherry;
Gregory S. Mitchell;
Hamid Sabet;
Vivek Nagarkar
Show Abstract
Preclinical imaging is a cornerstone of translational research, as all therapeutic drugs need to be
tested for efficacy and toxicity on animals prior to human trials. Optical imaging techniques, such as
bioluminescence and multispectral fluorescence imaging, currently dominate preclinical functional
imaging despite their depth dependent limitations on quantitation and sensitivity. Translating drugs
developed with these techniques to clinical models can therefore be difficult. Hence, clinically relevant
nuclear imaging techniques, such as SPECT and PET, are therefore becoming increasingly used in
preclinical imaging. Dedicated preclinical SPECT and PET systems are now available, but for many
preclinical research groups this requires a significant investment in new equipment.
Performance analysis of several generations of flat-panel x-ray imagers based on polycrystalline silicon TFTs
Author(s):
Larry E. Antonuk;
Youcef El-Mohri;
Qihua Zhao;
Martin Koniczek;
Albert Liang;
Hao Jiang;
John McDonald;
Robert A. Street;
Jeng-Ping Lu
Show Abstract
Active matrix flat-panel imagers (AMFPIs) have become ubiquitous in medical imaging environments. AMFPIs are
based on two-dimensional pixelated arrays coupled to various x-ray converter materials that provide either indirect or
direct detection of the incident x-ray radiation. However, the capabilities of this technology are severely constrained by
the underlying solid-state properties of the amorphous silicon semiconductor material employed in the thin-film
transistors present in each array pixel. The considerably higher electron and hole mobilities of polycrystalline silicon, a
semiconductor material that (like amorphous silicon) is well suited to fabrication of transistors for large area electronics,
provide the potential to overcome these constraints by increasing the overall gain of the system relative to the electronic
additive noise. To explore this potential, a series of prototype arrays based on increasingly complex pixel designs
employing polycrystalline silicon transistors is under development by our collaboration. The designs include several
generations of active pixel arrays that incorporate sophisticated pixel-level amplifier circuits with the goal of improving
imaging performance. In this paper, an initial analysis of the noise and DQE performance of selected prototype pixel
circuit designs will be presented. The results are based on a combination of Monte Carlo -based circuit simulations and
cascaded systems analysis, supplemented with information obtained from measurements performed on poly-Si
transistors. The paper concludes with a brief discussion of the potential for, and challenges associated with, the creation
of single photon counting arrays based on poly-Si TFTs.
Monte-Carlo simulations of a coded-aperture x-ray scatter imaging system for molecular imaging
Author(s):
Anuj J. Kapadia;
Manu N. Lakshmanan;
Kalyani Krishnamurthy;
Pooyan Sahbaee;
Amarpreet Chawla;
Scott Wolter;
Ken Maccabe;
David Brady;
Ehsan Samei
Show Abstract
In this work, we demonstrate the ability to determine the material composition of a sample by measuring coherent scatter
diffraction patterns generated using a coded-aperture x-ray scatter imaging (CAXSI) system. Most materials are known
to exhibit unique diffraction patterns through coherent scattering of low-energy x-rays. However, clinical x-ray imagers
typically discard scatter radiation as noise that degrades image quality. Through the addition of a coded aperture, the
system can be sensitized to coherent scattered photons that carry information about the identity and location of the
scattering material. In this work, we demonstrate this process using a Monte-Carlo simulation of a CAXSI system. A
simulation of a CAXSI system was developed in GEANT4 with modified physics libraries to model coherent scatter
diffraction patterns in materials. Simulated images were generated from 10 materials including plastics, hydrocarbons,
and biological tissue. The materials were irradiated using collimated pencil- and fan-beams with energies of 160 kVp.
The diffraction patterns were imaged using a simulated 2D detector and mathematically deconstructed using an
analytical projection model that accounted for the known x-ray source spectrum. The deconstructed diffraction patterns
were then matched with a library of known coherent scatter form-factors of different materials to determine the identity
of the scatterer at different locations in the object. The results showed good agreement between the measured and known
scatter patterns from the materials, demonstrating the ability to image and identify materials at different 3D locations
within an object using a projection-based CAXSI system.
Effect of denoising on the quality of reconstructed images in digital breast tomosynthesis
Author(s):
Marcelo A. C. Vieira;
Predrag R. Bakic;
Andrew D. A. Maidment
Show Abstract
Individual projection images in Digital Breast Tomosynthesis (DBT) must be acquired with low levels of radiation,
which significantly increases image noise. This work investigates the influence of a denoising algorithm and the
Anscombe transformation on the reduction of quantum noise in DBT images. The Anscombe transformation is a
variance-stabilizing transformation that converts the signal-dependent quantum noise to an approximately signalindependent
Gaussian additive noise. Thus, this transformation allows for the use of conventional denoising algorithms,
designed for additive Gaussian noise, on the reduction of quantum noise, by working on the image in the Anscombe
domain. In this work, denoising was performed by an adaptive Wiener filter, previously developed for 2D
mammography, which was applied to a set of synthetic DBT images generated using a 3D anthropomorphic software
breast phantom. Ideal images without noise were also generated in order to provide a ground-truth reference. Denoising
was applied separately to DBT projections and to the reconstructed slices. The relative improvement in image quality
was assessed using objective image quality metrics, such as peak signal-to-noise ratio (PSNR) and mean structural
similarity index (SSIM). Results suggest that denoising works better for tomosynthesis when using the Anscombe
transformation and when denoising was applied to each projection image before reconstruction; in this case, an average
increase of 9.1 dB in PSNR and 58.3% in SSIM measurements was observed. No significant improvement was observed
by using the Anscombe transformation when denoising was applied to reconstructed images, suggesting that the
reconstruction algorithm modifies the noise properties of the DBT images.
Comparative studies on exposure conditions and reconstruction algorithms in limited angle tomography
Author(s):
Kwang Eun Jang;
Jiyoung Choi;
Jongha Lee;
Younghun Sung;
Jae Hak Lee;
SeongDeok Lee
Show Abstract
Digital breast tomosynthesis (DBT) has been investigated as a promising alternative to conventional X-ray
mammography for breast cancer screening. By reconstructing 3D volumetric images from multiple 2D projections
measured over a limited angular range, it can offer depth-directional information and improve both sensitivity
and specificity of cancer detection in dense breasts. The diagnostic performance of DBT can be affected by
a number of imaging parameters. The angular range of scan orbit is one of the most crucial factors, since
it determines the depth-directional resolution. Recently, we proposed the wide angle tomosynthesis based on
voltage modulations of X-ray source. By using X-rays with large penetration power on exterior positions, it
can acquire high-SNR projections over a wide angular range. In this paper, we present comparative studies on
exposure conditions in DBT, including narrow and wide angle scan using an invariant tube voltage of X-ray
source, and wide angle scan with the voltage modulation technique. In addition, we compared the conventional
reconstruction methods with recently proposed IDIR algorithms. In preliminary studies, the wide-angle scheme
with proposed IDIR algorithm showed superior performances in detecting abnormal lesions over conventional
approaches.
Stationary chest tomosynthesis using a CNT x-ray source array
Author(s):
Jing Shan;
Pavel Chtcheprov;
Andrew W. Tucker;
Yueh Z. Lee;
Xiaohui Wang;
David Foos;
Michael D. Heath;
Jianping Lu;
Otto Zhou
Show Abstract
Chest tomosynthesis is an imaging modality that provides 3D sectional information of a patients thoracic cavity
using limited angle x-ray projections. Studies show that tomosynthesis can improve the detection of subtle
lung nodules comparing to conventional radiography at a lower radiation dose than CT. In the conventional
design, the projection images are collected by mechanically moving a single x-ray source to different viewing
angles. We investigated the feasibility of stationary chest tomosynthesis using the distributed CNT x-ray source
array technology, which can generate a scanning x-ray beam without any mechanical motion. A proof-of-concept
system was constructed using a short linear source array and a at panel detector. The performance of the source
including the flux was evaluated in the context of chest imaging. The bench-top system was characterized and
images of a chest phantom were acquired and reconstructed. The preliminary results demonstrate the feasibility
of stationary chest tomosynthesis using the CNT x-ray source array technology.
Proposing a new velocity profile for continuous x-ray tube motion in digital breast tomosynthesis
Author(s):
Raymond J. Acciavatti;
Predrag R. Bakic;
Andrew D. A. Maidment
Show Abstract
In digital breast tomosynthesis (DBT), a 3D image of the breast is generated from x-ray projections at various angles.
There are two mechanisms for acquiring projection images in DBT, step-and-shoot motion and continuous tube motion.
The benefit of continuous tube motion is shorter scan time and hence less patient motion; the trade-off is focal spot
blurring. To minimize focal spot blurring in a system with continuous tube motion, this study proposes a new velocity
profile for the x-ray tube during the scan. Unlike existing systems for which the x-ray tube has constant angular velocity,
we investigate a smoothly-varying tube velocity that approaches zero during each projection and is larger between
projections. With this unique design, the filtered backprojection reconstruction of a sinusoidal test object was calculated,
and modulation was determined at various frequencies. It is shown that the newly proposed tube velocity yields
increased modulation in the reconstruction relative to a conventional system with continuous tube motion. The
modulation in the re-designed system differs minimally from an analogous step-and-shoot system operated with the same
scan time. This improvement in image quality was validated with reconstructions of microcalcifications in computer
breast phantoms. It is known that continuous tube motion reduces the contrast of microcalcifications relative to stepand-shoot systems; we show that the newly proposed tube motion increases the contrast of microcalcifications compared
to conventional continuous tube motion. In conclusion, this work proposes a strategy for optimizing the velocity of tube motion in DBT.
Optimization of clinical protocols for contrast enhanced breast imaging
Author(s):
Yue-Houng Hu;
David A. Scaduto;
Wei Zhao
Show Abstract
Contrast enhanced (CE) breast imaging has been proposed as a method to increase the sensitivity and specificity of
breast cancer detection. Because malignant lesions often exhibit angiogenesis, the uptake of radio-opaque contrast agents (e.g. iodine) results in increased attenuation compared to the background tissue. Both planar CE digital mammography (CE-DM) and digital breast tomosynthesis (CE-DBT) have been proposed, using temporal or dual energy (DE) subtraction to remove tissue backgrounds. In the current study, we apply a cascaded linear systems model approach to analyze CE techniques with DE subtraction for designing a diagnostic imaging study, including the effects of contrast dynamics. We apply the model for both CE-DM and CE-DBT to calculate the ideal observer signal-to-noise ratio (SNR)
for the detection of I contrast objects of different sizes and concentrations. The calculation of this figure-of-merit (FOM) was be used to optimize CE clinical imaging protocols.
Demonstration of a scatter correction technique in digital breast tomosynthesis
Author(s):
Christy R. Inscoe;
Andrew W. Tucker;
Otto Z. Zhou;
Jianping Lu
Show Abstract
We have recently developed a method of using a distributed x-ray source array to obtain images with scatter correction
for tomographic reconstruction of an object. The method consists of obtaining x-ray images of the object with and
without the primary beam sampling apparatus. In this study, we report the results of applying the scatter correction
method for breast tomosynthesis imaging using the carbon nanotube x-ray based stationary Digital Breast Tomosynthesis
(s-DBT) system developed at UNC. The unique design of s-DBT system makes it possible to estimate the image of the
scatter profile of the object with very low dose, and without significant increase in acquisition time. An anthropomorphic
breast phantom was used for quantitative analysis of the change in contrast and scatter-to-primary ratio. Our results
suggest that the scatter correction method is effective and can be used for enhanced contrast.
Study of image quality in digital breast tomosynthesis by subpixel reconstruction
Author(s):
Yao Lu;
Heang-Ping Chan;
Jun Wei;
Lubomir Hadjiiski;
Ravi Samala
Show Abstract
In digital breast tomosynthesis (DBT), the reconstructed image quality of small objects such as microcalcifications are
limited by the detector resolution and the reconstruction voxel dimensions used, in addition to the inherent in-plane and
inter-plane blurring due to limited-angle image acquisition. We are investigating the effects of subpixel reconstruction
on the image quality of microcalcifications. In this study, subpixel projection images were generated by polynomial
interpolation of the gray level values of the original projection images to reduce the pixel pitch by a factor of 2 to 4. The
voxel dimensions in the reconstruction volume were varied. The in-plane voxel dimensions were chosen to match the
pitch in the subpixel projection images while the voxel dimension in the depth-direction was varied systematically to
study the relative effects. DBT scans of human subjects with microcalcifications were acquired with a GE prototype
DBT system at 0.1 mm x 0.1 mm pixel pitch. In addition, computer modeling of the same system was used to generate
simulated DBT projections of small spheres as a surrogate for microcalcifications. DBT volumes corresponding to the
subpixel projections were reconstructed with SART. The FWHMs of the line profiles of the simulated
microcalcifications on their in-focus DBT slices and FWHMs of the inter-plane artifact spread function (ASF) in the
depth-direction were used for comparison of reconstruction quality. The results indicated that subpixel reconstruction
increased the object contrast and reduced the inter-plane blurring to a certain extent. Further work is underway to study
the dependence of the trade-off between image quality and reconstruction voxel dimensions on the image acquisition
parameters (total scan angle, angular increment) of the DBT system and the properties of the objects of interest.
Quantitative analysis of an enlarged area solid state x-ray image intensifier (SSXII) detector based on electron multiplying charge coupled device (EMCCD) technology
Author(s):
Setlur Nagesh Swetadri Vasan;
P. Sharma;
V. Singh;
A. Jain;
Ciprian N. Ionita;
A. H. Titus;
A. N. Cartwright;
D. R. Bednarek;
S. Rudin
Show Abstract
Present day treatment for neurovascular pathological conditions involves the use of devices with
very small features such as stents, coils, and balloons; hence, these interventional procedures demand high resolution xray
imaging under fluoroscopic conditions to provide the capability to guide the deployment of these fine endovascular
devices. To address this issue, a high resolution x-ray detector based on EMCCD technology is being developed. The
EMCCD field-of-view is enlarged using a fiber-optic taper so that the detector features an effective pixel size of 37 μm
giving it a Nyquist frequency of 13.5 lp/mm, which is significantly higher than that of the state of the art Flat Panel
Detectors (FPD). Quantitative analysis of the detector, including gain calibration, instrumentation noise equivalent
exposure (INEE) and modulation transfer function (MTF) determination, are presented in this work. The gain of the
detector is a function of the detector temperature; with the detector cooled to 50 C, the highest relative gain that could be
achieved was calculated to be 116 times. At this gain setting, the lowest INEE was measured to be 0.6 μR/frame. The
MTF, measured using the edge method, was over 2% up to 7 cycles/ mm. To evaluate the performance of the detector
under clinical conditions, an aneurysm model was placed over an anthropomorphic head phantom and a coil was guided
into the aneurysm under fluoroscopic guidance using the detector. Image sequences from the procedure are presented
demonstrating the high resolution of this SSXII.
Intrinsic and total system performance evaluation for a newly developed Solid State X-ray Image Intensifier (SSXII) detector
Author(s):
V. Singh;
S. N. Swetadri Vasan;
A. Jain;
P. Sharma;
D. R. Bednarek;
S. Rudin
Show Abstract
The new Solid State X-ray Image Intensifier (SSXII) is a high-resolution, high-sensitivity, real-time region-ofinterest
(ROI) x-ray imaging detector. Evaluations were made of both standard linear systems metrics (MTF, DQE)
and total system performance with generalized linear systems metrics (GMTF, GDQE) including scatter and
geometric un-sharpness for simulated clinical conditions.
The SSXII is based on a 1k x 1k EMCCD sensor coupled to a 300 μm thick CsI(Tl) phosphor through a 2.88:1 fiber
optic taper resulting in a 37 μm effective pixel size and an effective 3.7 cm x 3.7 cm square field-of-view (FOV).
Standard methods were used to calculate MTF, NNPS and DQE. Generalized metrics were calculated and compared
for three different magnifications (1.03, 1.11 and 1.2) and three different focal spots (0.3 mm, 0.5 mm and 0.8 mm)
for a scatter fraction of 0.28.
For an RQA5 spectrum, at 5 cycles/mm the MTF was found to be 0.06 and DQE was 0.04, while the DQE(0) was
0.60. Focal spot un-sharpness and scatter significantly degrades the GMTF and GDQE performance of the detector.
A low frequency drop is caused by scatter and is almost independent of focal spot size and magnification. The
degradation for middle range frequencies is caused by geometric un-sharpness and increases with focal spot size and
magnification. This degradation was least in the case of the small focal spot and almost independent of
magnification.
In spite of this degradation, the high resolution SSXII with a small FOV may have a significant impact on ROI
image-guided neuro-interventions since it demonstrates far better performance than standard current detectors.
Uncertainty of Monte Carlo variance estimates: application to the simulation of x-ray imaging detectors
Author(s):
Aldo Badano;
Frank W. Samuelson
Show Abstract
Knowledge of the uncertainty associated with Monte Carlo estimates is useful for determining when to stop a
simulation run when statistical
uctuations fall below a desired tolerance level, and for designing and analyzing
variance reduction techniques. In this work, we discuss how to analytically calculate the uncertainty of Monte
Carlo variance estimates from higher order moments of the distribution of events. In addition, we show how
these expressions can be incorporated in the study of x-ray imaging detectors to manage runtime and precision
of the simulation. Our analysis can be used to design variance reduction techniques for Monte Carlo simulations
when, as in many cases in imaging, the variance and not the mean, is the quantity of interest.
Two methods for simulation of dense tissue distribution in software breast phantoms
Author(s):
Joseph H. Chui;
Rongping Zeng;
David D. Pokrajac;
Subok Park;
Kyle J. Myers;
Andrew D. A. Maidment;
Predrag R. Bakic
Show Abstract
Software breast phantoms have been developed for use in evaluation of novel breast imaging systems. Software
phantoms are flexible allowing the simulation of wide variations in breast anatomy, and provide ground truth for the
simulated tissue structures. Different levels of phantom realism are required depending on the intended application.
Realistic simulation of dense (fibroglandular) tissue is of particular importance; the properties of dense tissue – breast
percent density and the spatial distribution – have been related to the risk of breast cancer. In this work, we have
compared two methods for simulation of dense tissue distribution in a software breast phantom previously developed at
the University of Pennsylvania. The methods compared are: (1) the previously used Gaussian distribution centered at
the phantom nipple point, and (2) the proposed combination of two Beta functions, one modeling the dense tissue
distribution along the chest wall-to-nipple direction, and the other modeling the radial distribution in each coronal
section of the phantom. Dense tissue distributions obtained using these methods have been compared with distributions
reported in the literature estimated from the analysis of breast CT images. Qualitatively, the two methods produced
rather similar dense tissue distributions. The simulation based upon the use of Beta functions provides more control
over the simulated distributions through the selection of the various Beta function parameters. Both methods showed
good agreement to the clinical data, suggesting both provide a high level of realism.
Simulation of anatomical texture in voxelized XCAT phantoms
Author(s):
Jason Bond;
D. Frush;
Ehsan Samei;
W. P. Segars
Show Abstract
While great advances are made toward making highly realistic, surface models of the human
anatomy, very little has been done to fill these bounded surfaces with models of anatomical
texture. We propose a method whereby realistic anatomically-based computed tomography (CT)
texture can be incorporated into voxelized versions of the 4D extended cardiac-torso (XCAT)
phantom. Our source of texture comes from patient CT scans from the Duke CT imaging
database. These image-sets were de-noised using anisotropic diffusion. Two organs were
selected from which texture was obtained, liver and lungs. From each organ, multiple regions of
interest (ROIs) were taken and tiled side-by-side to create a larger image. Textures for the liver
and lungs were extrapolated using ImageQuilting, based on the tiled images. Next, a NURBSbased
XCAT phantom was voxelized at the same resolution as the textures. The texture was then
placed in the voxelized phantoms. Finally, CT simulations of the phantoms with and without the
textures were compared against each other, using the power spectral density. This work shows
that there is a way whereby the XCAT phantoms can be textured to give more realistic
appearance in CT simulations. It is anticipated that this method would find great use in making
projections of the XCAT phantom look more realistic and allow for the phantoms to not only be
utilized in dosimetrical evaluations, but in image quality studies as well.
Multi-energy performance of a research prototype CT scanner with small-pixel counting detector
Author(s):
S. Kappler;
A. Henning;
B. Krauss;
F. Schoeck;
K. Stierstorfer;
T. Weidinger;
T. Flohr
Show Abstract
We have investigated the multi-energy performance of our most recent prototype CT scanner with CdTe-based
counting detector. With its small pixel pitch of 225 μm this device is prepared for the high X-ray fluxes occurring
in clinical CT. Each of these pixels is equipped with two adjustable counters. The ASIC architecture of the
detector allows configuration of the counter thresholds in chess patterns, enabling data acquisition in up to four
energy bins. We have studied the material separation capability of counting CT with respect to potential clinical
applications. Therefore we have analyzed contrast and noise properties in material decomposed CT images using
up to four base materials. We have studied contrast agents containing iodine, gadolinium, or gold, and the
body-like materials calcium, fat, and water. We describe the mathematical framework used in this work and
demonstrate the general multi-energy capability of counting CT with simulations and experimental data from
our prototype scanner. To prove the clinical relevance of our studies we compare the results to those obtained
with well-established dual-kVp techniques recorded at same patient dose and with identical image sharpness.
Measurements of a dual-energy fast photon counting CdTe detector with integrated charge sharing correction
Author(s):
Christer Ullberg;
Mattias Urech;
Niclas Weber;
Anders Engman;
Anna Redz;
Fredrik Henckel
Show Abstract
Photon counting detectors offer some unique features for x-ray imaging. If designed correctly, photon counting detectors
have no readout noise and no dark counts. This is an important feature in for example low dose CT imaging where the
total dose is distributed over a large number of projections from different angles. In addition to this it is also possible to
incorporate pulse height discrimination of each photon event, thus enabling the recording of images from multiple
energy intervals in a single exposure.
We demonstrate the performance of a newly developed dual-energy fast photon counting detector with 100μm pixel size
that can be read out up to 1000fps. It consists of a three side buttable ASIC that is bump bonded to a CdTe converter.
The very high conversion efficiency of CdTe makes the detector suitable for a wide range of applications requiring high
spatial resolution at low doses. The efficiency of the detector is maintained all the way out to the edge of the chip which
opens up the possibility to build larger detectors still fulfilling medical requirements.
The novel detector incorporates a charge sharing correction feature and the effect of this function is demonstrated using
the DQE measurements for different spectra as well as with spectrum reconstruction from Cd109 and Am241 radioactive
sources. We show that this charge sharing correction feature affects the properties of NPS and MTF, and the energy
resolution is greatly enhanced.
Measurements are also compared to a simulation model for the detector system.
Threshold optimization for efficient contrast imaging with quantum counting CT detectors
Author(s):
T. Weidinger;
T. M. Buzug;
T. Flohr;
S. Kappler;
F. Schöck;
K. Stierstorfer
Show Abstract
Photon counting detectors are expected to bring along various clinical benefits in CT imaging. Among the benefits of these detectors is their intrinsic spectral sensitivity that allows to resolve the incident X-ray spectrum. Their capability for multi-energy imaging enables material segmentation, but it is also possible to use the spectral information to create fused gray-scale CT images with improved imaging properties.
We have developed and investigated an optimization method that maximizes the image contrast-to-noise ratio, making use of the spectral information in data recorded with a counting detector with up to six energy thresholds.
The resulting merged gray-scale CT images exhibit significantly improved CNR2 for a number of clinically
established, potentially novel and hypothetical contrast agents in the thin absorber approximation.
In this work we motivate and describe the optimization method, provide the deduced optimal sets of threshold energies and mixing weights, and summarize the maximally achievable gain in CNR2 for each contrast agent under study.
Modeling photon-counting detectors for x-ray CT: spectral response
and pulse-pileup effects and evaluation using real data
Author(s):
J. Cammin;
J. Xu;
William C. Barber;
J. S. Iwanczyk;
N. E. Hartsough;
K. Taguchi
Show Abstract
Spectral computed tomography (CT) with photon-counting detectors (PCDs) has the potential to substantially advance
diagnostic CT imaging by reducing image noise and dose to the patient, by improving contrast and tissue specificity, and
by enabling molecular and functional imaging. The current PCD technology, however, is limited by two main factors:
imperfect energy measurement (spectral response effects, SRE) and limited count rate (pulse-pileup effects, PPE, due to
detector dead-times). The overall goal of our research is to develop compensation algorithms for these sources of data
distortions, to demonstrate that PCDs are suitable for clinical CT, and to identify key clinical applications for spectral
CT with PCDs. We have already developed an iterative compensation scheme that includes a forward projection model
of the imaging chain and that can compensate for either SRE or PPE distortions separately by maximizing a penalized
log-likelihood function. In this paper we describe the evaluation of a combined, cascaded SRE and PPE model for PCDs
and compare the models to data acquired with an experimental table-top PCD-CT system. The separation into count-rate
independent effects (SRE) and count-rate dependent effects (PPE) allows cascading the forward model. First, the SRE
model is evaluated using low count rates. Then the PPE model is cascaded and the combined SRE+PPE model is
evaluated. Several different attenuators were used, including K-edge materials and the models and data were compared for various count-rate conditions.
Cascaded-systems analyses of photon-counting x-ray detectors
Author(s):
Jesse Tanguay;
Seungman Yun;
Ho Kyung Kim;
Ian A. Cunningham
Show Abstract
Single-photon counting (SPC) x-ray imaging has the potential to improve image quality and enable new advanced energy-dependent methods. Recently, cascaded systems analysis (CSA) has been extended to the description of the detective quantum efficiency (DQE) of SPC detectors. In this article we apply the new CSA approach to the description of the DQE of hypothetical direct-conversion selenium (Sc) and cadmium zinc telluride (CdZnTc) detectors including the effects of poly-energetic x-ray spectra, stochastic conversion of x-ray energy to electron hole (c-h) pairs, depth-dependent collection of e-h pairs using the Hecht relation, additive electronic noise, and thresholding. Comparisons arc made to an energy-integrating model. For this simple model, with the exception of thick (1- 10 mm) Sc-bascd convertors, we found that the SPC DQE was 5-20 %greater than that of the energy integrating model. This trend was tnw even when additive noise was included in the SPC model and excluded from the energy-integrating model. However, the DQE of SPC detectors with poor collection efficiency (such as thick (<1 mm) Sc detectors) and high levels of additive noise can be degraded by 40-90 % for all energies and x-ray spectra considered. vVhile photon-counting approaches arc not yet ready for routine diagnostic imaging, the available DQE is equal to or higher than that of conventional energy-integrating detectors under a wide range of x-ray energies and convertor thickness. However, like energy-integrating detectors, the DQE of SPC detectors will be degraded by the combination of poor collection efficiency and high levels of additive noise.
Achieving sub-pixel resolution using CZT-based photon counting detectors for dedicated breast CT
Author(s):
Andrey Makeev;
John McGrath;
Martin Clajus;
Scott Snyder;
Stephen J. Glick
Show Abstract
Semiconductor based photon-counting detectors for x-ray CT have a number of advantages over energy integrat ing detectors, including reduced electronic and Swank noise, increased dynamic range, capability of spectral CT for material decomposition, and improved SNR characteristics through energy weighting. Quite a few clinical applications could benefit from high-resolution spectral CT. For example, in breast CT the visualization of mi crocalcifications and assessment of tumor microvasculature after contrast enhancement require spatial resolution on the order of 100 μm or better. A straightforward approach to increasing spatial resolution by decreasing the detector pixel size, leads to two major problems: 1) fabricating circuitry with small pixels becomes very costly, and 2) inter-pixel charge spreading can obviate any improvement in spatial resolution. In this study, we have used computer simulations to investigate position estimation algorithms that utilize charge sharing to achieve sub-pixel position resolution. To study these algorithms, we model a simple detector geometry with a pixellated
5 x 5 anode array, and use conditional probability functions modeling electron-hole charge transport in CZT. We used COMSOL Multiphysics software to map the distribution of charge pulses in the detector. Performance of two x-ray interaction position estimation algorithms were evaluated: 1) method of maximum likelihood, and
2) a fast, practical algorithm that can be realistically implemented in a readout ASIC, providing identification
of the quadrant of the pixel in which interaction occurred. Both methods exhibit good sub-pixel resolution performance, however their actual efficiency is limited by electronic noise.
Dual energy iodine contrast imaging with mammography and tomosynthesis
Author(s):
Baorui Ren;
Chris Ruth;
Yiheng Zhang;
Andrew Smith;
Don Kennedy;
Bernadette O'Keefe;
Ian Shaw;
Cornell Williams;
Zhen Ye;
Elena Ingal;
Brad Polischuk;
Zhenxue Jing
Show Abstract
We have developed dual energy (DE) iodine contrast imaging functions with a commercial mammography and
tomosynthesis system. Our system uses a tungsten target x-ray tube and selenium direct conversion detector.
Conventional low energy (LE) images were acquired with existing Rh, Ag and Al filters at the screening doses while the
high energy images (HE) were acquired with new Cu filters at half of the screening doses. In DE 2D mode, a pair of LE
and HE images was taken with one second delay time between and with anti-scatter grid. In DE 3D mode, 22 views of
alternating LE and HE were taken over 15 degrees angle in seven seconds without grid while tube was scanned
continuously. We used log-subtraction algorithm to obtain clean DE images with the subtraction factor K derived
empirically. In 3D mode, the subtraction was applied to each pair of LE and HE slices after reconstruction. The x-ray
technique optimization was done with simulation and phantom study. We performed both phantom and patient studies to
demonstrate the advantage of iodine contrast imaging. Among several new things in our work, a selenium detector
optimized for DE imaging was tested and a large dose advantage was demonstrated; 2D and 3D DE images of a breast
under same compression were acquired with a unique DE combo mode of the system, allowing direct image quality
comparison between 2D and 3D modes. Our study showed that new DE system achieved good image quality. DE
imaging is be a promising modality to detect breast cancer.
An application of pre-computed backprojection based penalized-likelihood (PPL) image reconstruction on stationary digital breast tomosynthesis
Author(s):
Shiyu Xu;
Jianping Lu;
Otto Zhou;
Ying Chen
Show Abstract
Stationary Digital Breast Tomosynthesis (s-DBT) is a carbon nanotube based breast imaging device with fast image acquisition and decent resolution. In this paper, we investigate several representative reconstruction methods with the recently improved s-DBT system and also introduce a two-step reconstruction strategy with Pre-computed Backprojection based penalized-likelihood (PPL). This strategy reconstructs three dimensional (3-D) images with a desired resolution properties by choosing the corresponding smoothing parameter, which is evaluated in advance by studying simulated data. Our experiments show that the current s-DBT system has been greatly improved with respect to the performance of image reconstructions. PPL method exhibits controllable pixel precision, high image contrast and low noise on reconstructed images. Therefore, the enhanced Contrast Noise Ratio (CNR) from PPL method benefits both micro-calcifications and mass of the breast-equivalent phantom.
Efficient synthesis of virtual projections from a tomosynthesis data set using a 2D image processing method
Author(s):
Frank Dennerlein;
Anna Jerebko;
Andreas Fieselmann;
Thomas Mertelmeier
Show Abstract
A new algorithm is suggested to compute one or several virtual projection images directly from cone-beam data
acquired in a tomosynthesis geometry. One main feature of this algorithm is that it does not involve the explicit
reconstruction of a 3D volume, and a subsequent forward-projection operation, but rather operates using solely
2D image processing steps. The required 2D processing is furthermore based on the use of pre-computed entities,
so that a significant speed-up in the computations can be obtained. The presented algorithm can be applied
to a variety of CT geometries, and is here investigated for a mammography application, to simulate virtual
mammograms from a set of low-dose tomosynthesis projection images. A first evaluation from real measured
data is given.
Towards visual-search model observers for mass detection in breast tomosynthesis
Author(s):
Beverly A. Lau;
Mini Das;
Howard C. Gifford
Show Abstract
We are investigating human-observer models that perform clinically realistic detection and localization tasks as a
means of making reliable assessments of digital breast tomosynthesis images. The channelized non-prewhitening
(CNPW) observer uses the background known exactly task for localization and detection. Visual-search observer
models attempt to replicate the search patterns of trained radiologists. The visual-search observer described in
this paper utilizes a two-phase approach, with an initial holistic search followed by directed analysis and decision
making. Gradient template matching is used for the holistic search, and the CNPW observer is used for analysis
and decision making. Spherical masses were embedded into anthropomorphic breast phantoms, and simulated
projections were made using ray-tracing and a serial cascade model. A localization ROC study was performed
on these images using the visual-search model observer and the CNPW observer. Observer performance from
the two computer observers was compared to human observer performance. The visual-search observer was able
to produce area under the LROC curve values similar to those from human observers; however, more research is
needed to increase the robustness of the algorithm.
Simulation of 3D DLA masses in digital breast tomosynthesis
Author(s):
Alaleh Rashidnasab;
Premkumar Elangovan;
Oliver Diaz;
Alistair Mackenzie;
Kenneth Young;
David Dance;
Kevin Wells
Show Abstract
Digital breast tomosynthesis (DBT) is suggested to have superior performance compared to 2D mammography in terms
of cancer visibility, especially in the case of dense breasts. However, the overall performance of tomosynthesis for
screening applications, and the manner in which tomosynthesis should be optimally used for screening remains unclear.
This motivates the development of software tools that can insert user-defined synthetic pathology of realistic appearance
into clinical tomosynthesis images for subsequent use in virtual clinical trials. We present a method for inserting lesions
grown using Diffusion Limited Aggregation, previously validated in 2D mammograms, into clinical DBT images. A
preliminary pilot study was used to validate the realism of the masses, wherein three readers each viewed 19 cases and
rated the realism of the inserted masses. Each case included a simulated mass inserted in the tomosynthesis projections and the counterpart digital 2D mammogram. These results show that masses can be successfully embedded in the tomosynthesis projections and can produce visually authentic DBT images containing synthetic pathology. These results will be used to further optimize the appearance of these masses in DBT for an upcoming validation.
Reconstruction method incorporating the object-position dependence of visibility loss in dark-field imaging
Author(s):
Udo van Stevendaal;
Zhentian Wang;
Thomas Köhler;
Gerhard Martens;
Marco Stampanoni;
Ewald Roessl
Show Abstract
Dark-field imaging has the potential to overcome limitations in computed tomography (CT) investigating relatively
weakly absorbing material. However, an object-position dependence of the visibility loss in dark-field
imaging is observed. This effect might be negligible for small objects, but, for acquisition geometries using fanangle
apertures and field of views as those in human CT scanners, the object-position dependence of visibility
loss has to be taken into consideration if the scattering structure within the object is in the range of the grating
periods, i.e. micrometer. This work examines the effect of object-position dependent visibility loss in dark-field
imaging experimentally, investigates its consequences and presents an algorithm which solves the corresponding
reconstruction problem.
Grating-based dark-field breast imaging
Author(s):
Jens Rieger;
Florian Bayer;
Jürgen Durst;
Karl Gödel;
Wilhelm Haas;
Florian Horn;
Thilo Michel;
Georg Pelzer;
André Ritter;
Thomas Weber;
Andrea Zang;
Gisela Anton
Show Abstract
Grating-based X-ray phase-contrast imaging (XPCI) is a promising modality to increase soft-tissue contrast in medical imaging and especially in the case of mammography. Several groups worldwide have performed investigations on grating-based Talbot-Lau X-ray imaging of breast tissue, but in most cases focussed on the soft tissue contrast enhancement of the differential phase image.
In this contribution, we present promising measurements with a Talbot-Lau interferometer of several mastectomy breast tissue samples especially focussed on the sensitivity of the dark-field signal of microcalcifications and with a comparable dose value to conventional mammography. We can present a contrast improvement for calcifications in surrounding breast tissue for the dark-field image by a factor of 10 related to the attenuation
image.
How to determine detection performance of a DPC-CT
system from a conventional cone beam CT system?
Author(s):
Ke Li;
John Garrett;
Nicholas Bevins;
Joseph Zambelli;
Guang-Hong Chen
Show Abstract
Although the relative ease of implementation and compact nature of grating-based differential phase contrast
CT (DPC-CT) has sparked tremendous enthusiasm for potential medical applications, the pros and cons of this
imaging method remains to be addressed before an actual clinical system can be constructed. To address these
unknowns, either numerical simulations or direct hardware implementations can be used. However, both approaches
have their limitations. It is highly desirable to develop a research method to enable imaging performance
prediction for a future DPC-CT system from the performance of an available absorption CT (ACT) system. In
this paper, a theoretical framework was developed to accurately predict the noise properties and detection performance
of DPC-CT from that of conventional ACT. The framework was derived based on a fundamental noise
relationship between DPC-CT and ACT and was experimentally validated. An example has been given in the
paper on how the framework can be utilized to predict model observer detectability index of a DPC breast CT
constructed based on an existing absorption breast CT. This framework is expected to become a valuable tool in
addressing the following questions: (i) With a fixed radiation dose in a particular clinical application, how well
can a specific detection/discrimination imaging task can be performed provided that an existing ACT scanner is
modified into a DPC-CT by inserting a grating interferometer, which is characterized by a few design parameters
(e.g., pitches and duty cycles of the gratings, relative distance between the gratings, etc.) into the ACT system?
(ii) If a DPC-CT system can outperform an ACT for certain detection/discrimination tasks under the constraint
of identical radiation dose to the image object, how would one optimize design parameters of the gratings in
order to maximize its potential clinical benefits?
Edge illumination and coded-aperture X-ray phase-contrast imaging: increased sensitivity at synchrotrons and lab-based translations into medicine, biology and materials science
Author(s):
M. Endrizzi;
P. C. Diemoz;
M. B. Szafraniec;
C. K. Hagen;
T. P. Millard;
C. E. Zapata;
P. R. T. Munro;
K. Ignatyev;
M. Marenzana;
R. D. Speller;
A. Olivo
Show Abstract
The edge illumination principle was first proposed at Elettra (Italy) in the late nineties, as an alternative method
for achieving high phase sensitivity with a very simple and flexible set-up, and has since been under continuous
development in the radiation physics group at UCL. Edge illumination allows overcoming most of the limitations
of other phase-contrast techniques, enabling their translation into a laboratory environment. It is relatively
insensitive to mechanical and thermal instabilities and it can be adapted to the divergent and polychromatic
beams provided by X-ray tubes. This method has been demonstrated to work efficiently with source sizes up
to 100m, compatible with state-of-the-art mammography sources. Two full prototypes have been built and
are operational at UCL. Recent activity focused on applications such as breast and cartilage imaging, homeland
security and detection of defects in composite materials. New methods such as phase retrieval, tomosynthesis
and computed tomography algorithms are currently being theoretically and experimentally investigated. These
results strongly indicate the technique as an extremely powerful and versatile tool for X-ray imaging in a wide
range of applications.
Compact hard X-ray grating interferometry for table top phase contrast micro CT
Author(s):
T. Thüring;
S. Hämmerle;
S. Weiss;
J. Nüesch;
J. Meiser;
J. Mohr;
C. David;
M. Stampanoni
Show Abstract
Today’s commercial X-ray micro computed tomography (CT) specimen systems are based on microfocus sources,
2D pixel array cameras and short source-to-detector distances (i.e. cone-beam configurations). High resolution
is achieved by means of geometric magnification. The further development of such devices to acquire phase and
scattering contrast images can dramatically enhance their range of applications. Due to the compact geometries,
which imply a highly diverging beam, the gratings must be curved to maintain highest imaging performance
over a large field of view. We report about the implementation of extremely compact Talbot and Talbot-
Lau type grating interferometers which are compatible to the geometry of typical micro CT systems. For the
analytical description of the imaging system, formulas are presented describing the dependency of the sensitivity
on geometric parameters, camera and source parameters. Further, the imaging pipeline consisting of the data
acquisition protocol, radiographic phase retrieval and tomographic image reconstruction is illustrated. The
reported methods open the way for an immediate integration of phase and scattering contrast imaging on table
top X-ray micro CT scanners.
High energy x-ray phase-contrast imaging using glancing angle grating interferometers
Author(s):
D. Stutman;
J. W. Stayman;
M. Finkenthal;
J. H. Siewerdsen
Show Abstract
The Talbot-Lau grating interferometer enables refraction based imaging with conventional X-ray tubes, offering the
promise of a new medical imaging modality. The fringe contrast of the normal incidence interferometer is however
insufficient at the >40 keV photon energies needed to penetrate thick body parts, because the thin absorption gratings used in the interferometer become transparent. To solve this problem we developed a new interferometer design using gratings at glancing incidence. For instance, using 120 μm thick Au gratings at 10° incidence we increased several fold the interferometer contrast for a spectrum with ~58 keV mean energy. Tests of DPC-CT at 60-80kVp using glancing angle interferometers and medically relevant samples indicate high potential for clinical applications. A practical design for a slot-scan DPC-CT system for the knee is proposed, using glancing angle gratings tiled on a single substrate.
Type II beam hardening artifacts in phase contrast imaging
Author(s):
Nicholas Bevins;
Ke Li;
Joseph Zambelli;
Guang-Hong Chen
Show Abstract
The effects of beam hardening have previously been extended from absorption imaging to phase contrast imaging,
showing a similar, albeit reduced, effect in the phase images. The effect of beam hardening on the interferometer
performance, however, has not been demonstrated. In this work, the visibility reduction on a differential phase
contrast imaging system due to spectral changes as a result of beam hardening is demonstrated. The implication
of this reduction is an artificial increase in noise for the phase contrast image through highly-attenuating regions
of the object. In addition, false signal will be recorded in the dark-field image, which normally shows only
highly-scattering objects and interfaces. The results show that with added beam filtration, the effect is reduced,
just as with more traditional beam hardening artifacts. However, the effect also means that one must also take
into account the desired imaging task when determining the system’s design energy.
Model observer and human observer performance studies in
differential phase contrast CT
Author(s):
Ke Li;
Nicholas Bevins;
Joseph Zambelli;
Guang-Hong Chen
Show Abstract
This study investigates the possibility of using the model observer detectability to represent human detection
performance in differential phase contrast CT (DPC-CT). Five model observers were investigated, including
the prewhitening (PW) observer, the prewhitening observer with eye filter and internal noise (PWEi), the nonprewhitening
(NPW) observer, the non-prewhitening observer with eye filter (NPWE), and the channelized
Hotelling observer (CHO). Human 2AFC experiments involving four physicist observers were also performed to
provide reference to the model observer results. The contrast thresholds required for 92% correct decision rate
in 2AFC tests were evaluated for each observer. While all five model observers show good correlations with
human observers, the CHO generated the best quantitative agreement with human observer results. Compared
with the time-consuming human observer method, the model observer method is of much higher efficiency for
the evaluation and optimization of DPC-CT.
Intensity Modulated CT implemented with a dynamic bowtie filter
Author(s):
Timothy P. Szczykutowicz;
Charles Mistretta
Show Abstract
Current advances in CT dose reduction and image quality improvement mechanisms rely on moving towards a more patient / image specific approach. For example, statistical reconstruction algorithms tailor reconstruction weights to patient attenuation values and CT protocols make use of different kVp and mAs settings for different body regions and sizes. In this paper, for the first time to our knowledge, experimental results are presented in which a dynamic bowtie filter was used to tailor the imaging dose during a CT acquisition. The device used to implement a dynamic bowtie is referred to as a digitial beam attenuator (DBA). A non-DBA (non-modulated) CT scan was also performed for comparison. At half of the imaging dose, the noise uniformity of the DBA CT images was 37% better than the non-DBA scan. The use of a dynamic bowtie filter may also prove useful in photon counting CT (PCCT) where high fluence rates and large differences in fluence across projections and from view to view make implementing PCCT difficult. Results are presented in this paper showing dynamic range requirements of 7.9 and 13.8 for DBA and non-DBA scans respectively. The results also show that when using the DBA, the dose delivered can be reduced by 1.9 times for the small phantom used in this study. The differences in dynamic range and dose are somewhat small compared to what would be seen clinically due to the small size of the phantom used in the study. Using a more clinically relevant phantom, dynamic range differences of 22 times and dose reductions on the order of 4 times can be observed in which the effects of the DBA will be more dramatic.
Noise reduction in material decomposition for low-dose dual-energy cone-beam CT
Author(s):
W. Zbijewski;
G. Gang;
A. S. Wang;
J. W. Stayman;
K. Taguchi;
J. A. Carrino;
J. H. Siewerdsen
Show Abstract
Purpose: Dual-energy cone-beam CT (DE-CBCT) is an emerging technology with potential application in diagnostic
imaging and image-guided interventions. This paper reports DE-CBCT feasibility and investigates decomposition
algorithms for maximizing low-dose performance for reconstruction-based DE decomposition. A framework of binary
decision theory is used to examine the accuracy of DE decompositions obtained from analytical reconstructions of
differentially filtered low-energy (LE) and high-energy (HE) data and from penalized likelihood (PL) reconstructions
with differential regularization using quadratic and total variation penalties.
Methods: Accurate DE-CBCT decomposition benefits from consideration of all system noise components. Filtered
backprojection (FBP) reconstruction-based decomposition was investigated with differential filtering of LE and HE data.
Penalized likelihood reconstruction-based decomposition with differential regularization was hypothesized to further
improve low-dose performance, especially when coupled with regularization through a total variation edge preserving
penalty that encourages piecewise smooth images. Performance of decomposition was assessed in terms of a binary
hypothesis framework of sensitivity, specificity, and accuracy. Studies involved experiments on a DE-CBCT testbench,
phantoms of variable material type and concentration, and cadavers (knee arthrography).
Results: Studies support the overall feasibility of accurate, low-dose DE-CBCT at concentration down to 5 mg/ml
(iodine), dose ~3-6 mGy, and accuracy of material classification ~90%. Reconstruction-based decomposition with
quadratic PL performed comparably to FBP. PL with a total variation penalty provided edge preservation and piecewise
smooth images that aided DE classification and achieved improved performance over FBP and quadratic PL, reaching
accuracy of ~0.98 for 2 mg/mL iodine at 3.2 mGy, compared to approx. 0.9 for FBP and quadratic PL.
Conclusions: Accurate material decomposition with DE-CBCT is feasible at low dose and benefits from a rigorous
assessment of noise mechanisms among various reconstruction-based techniques. The work points to the potential for
non-linear iterative reconstruction methods for high-quality decomposition at low material concentration and dose.
A novel temporal recovery technique to enable cone beam CT perfusion imaging using an interventional C-arm system
Author(s):
Jie Tang;
Min Xu;
Kai Niu;
Kevin Royalty;
Kari Pulfer;
Charles Strother;
Guang-Hong Chen
Show Abstract
In the current workflow of ischemic stroke management, it is highly desirable to obtain perfusion information with the
C-arm CBCT system in the interventional room. Due to hardware limitations, the data acquisition speed of the current Carm
CBCT systems is relatively slow and only 7 time frames are available for a 45 s perfusion study. In this study, a
novel temporal recovery method was proposed to recover contrast enhancement curves in C-arm CBCT perfusion
studies. The proposed temporal recovery problem is a constrained optimization problem. Two numerical methods were
used to solve the proposed problem. The feasibility of proposed temporal recovery method was validated with numerical
experiments. Both solvers can achieve a satisfactory solution for the temporal recovery problem, while the result of the
Bregman algorithm is more accurate than that from the CG. In vivo animal studies were used to demonstrated the
improvement of the proposed method in C-arm CBCT perfusion. A stoked canine model was scanned with both C-arm
CBCT and diagnostic CT. Perfusion defects can be clearly indentified from the cerebral blood flow (CBF) map of
diagnostic CT perfusion. Without the temporal recovery technique, these defects can hardly be identified from the CBCT CBF map. After applying the proposed temporal recovery method, the CBCT CBF map well correlates with the CBF
map from diagnostic CT.
Reconstruction from truncated projections in cone-beam CT using an efficient 1D filtering
Author(s):
Yan Xia;
Andreas Maier;
Hannes G. Hofmann;
Frank Dennerlein;
Kerstin Mueller;
Joachim Hornegger
Show Abstract
In X-ray imaging, a reduction of the field of view (FOV) is proportional to a reduction in radiation dose. The resulting
truncation, however, is incompatible with conventional tomographic reconstruction algorithms. This problem has been
studied extensively. Very recently, a novel method for region of interest (ROI) reconstruction from truncated projections
with neither the use of prior knowledge nor explicit extrapolation has been published, named Approximated Truncation
Robust Algorithm for Computed Tomography (ATRACT). It is based on a decomposition of the standard ramp filter into a
2D Laplace filtering (local operation) and a 2D Radon-based filtering step (non-local operation).
The 2D Radon-based filtering that involves many interpolations complicates the filtering procedure in ATRACT, which
essentially limits its practicality. In this paper, an optimization for this shortcoming is presented. That is to apply ATRACT
in one dimension, which implies that we decompose the standard ramp filter into the 1D Laplace filter and a 1D convolutionbased
filter. The convolution kernel was determined numerically by computing the 1D impulse response of the standard
ramp filtering coupled with the second order anti-derivative operation. The proposed algorithm was evaluated by using a
reconstruction benchmark test, a real phantom and a clinical data set in terms of spatial resolution, computational efficiency
as well as robustness of correction quality.
The evaluation outcomes were encouraging. The proposed algorithm showed improvement in computational performance
with respect to the 2D ATRACT algorithm and furthermore maintained reconstructions of high accuracy in presence
of data truncation.
Development and spatial resolution characterization of a dedicated pulsed x-ray, cone-beam breast CT system
Author(s):
Peymon Gazi;
Kai Yang;
George Burkett;
John Boone
Show Abstract
Dedicated breast CT (bCT) technology may be useful for patients with high risk of developing breast cancer. Previous
studies have shown that bCT outperforms mammography in the visualization of mass lesions, however mammography is
superior in identifying microcalcifications. The Breast Tomography Project at UC Davis has led to development of three
dedicated breast CT scanners that produce high resolution, fully tomographic images, overcoming tissue superposition
effects found in mammography while maintaining an equivalent radiation dose. Over 600 patients have been imaged in
an ongoing clinical trial. The first patient scan was performed on the latest bCT scanner developed at UC Davis, called
Cambria, on April 12, 2012. The main differences between Cambria and the previous scanners are in using a pulsed xray
source (generator and tube) instead of continuous x-ray sources, and also in using the non-binning mode of the flatpanel
fluoroscopic detector. The spatial resolution characteristics of the new scanner were investigated and the results
show significant improvement in the overall MTF properties. Based on these results, it was concluded that using the
pulsed x-ray tube, we were able to restore the MTF degradation caused by motion blurring effect that exists in previous
generations of bCT. Moreover, MTF analysis shows that using the detector in the native acquisition mode (1 x 1) results
in superior spatial resolution which will likely bring considerable improvement to the delineation of microcalcifications.
Development of a phantom-based methodology for the assessment of quantification performance in CT
Author(s):
Baiyu Chen;
Ehsan Samei
Show Abstract
The quantification of lung nodule volume from CT images provides valuable information for cancer diagnosis and
staging. However, the usefulness of quantification depends on its precision. Direct assessment of the volume
quantification precision involves multiple steps and can become intractable for a multiplicity of protocols. To assess
quantification precision efficiently, we developed a prediction model, named the estimability index (e’). e’ provides a prediction of precision based on the characteristics of image noise and resolution, the nodule being quantified, and the segmentation software. It was further calibrated against empirical precision for 45 protocols of various reconstruction algorithms, slice thickness, and dose level. Results showed a strong correlation established between e’ and the empirical precision across all 45 protocols, demonstrating e’ as an effective surrogate of quantification precision. This study provides a useful framework for the optimization of CT protocols in terms of quantification
precision. It also enables fast assessment of protocol compliance in terms of precision for biomarker quantification.
Soft-tissue imaging in low-dose, C-arm cone-beam CT using statistical image reconstruction
Author(s):
Adam S. Wang;
Sebastian Schafer;
J. Webster Stayman;
Yoshi Otake;
Marc S. Sussman;
A. Jay Khanna;
Gary L. Gallia;
Jeffrey H. Siewerdsen
Show Abstract
C-arm cone-beam CT is an emerging tool for intraoperative imaging, but currently exhibits modest soft-tissue imaging capability. This work adapts a spectrum of statistical iterative reconstruction approaches to C-arm CBCT and investigates performance in imaging of low-contrast tasks pertinent to soft-tissue surgical guidance. Experiments involved a mobile C-arm and phantoms and cadavers presenting soft-tissue structures imaged using 3D FBP and penalized likelihood reconstruction. Statistical reconstruction - especially non-quadratic PL - boosted soft-tissue image quality through reduction of noise and artifacts, therefore presenting promise for interventional imaging. Further investigation of task-specific performance may overcome conventional tradeoffs in noise, resolution, and dose.
Modeling and control of nonstationary noise characteristics in filtered-backprojection and penalized likelihood image reconstruction
Author(s):
G. J. Gang;
J. W. Stayman;
W. Zbijewski;
J. H. Siewerdsen
Show Abstract
Purpose: Nonstationarity of CT noise presents a major challenge to the assessment of image quality. This work presents
models for imaging performance in both filtered backprojection (FBP) and penalized likelihood (PL) reconstruction that
describe not only the dependence on the imaging chain but also the dependence on the object as well as the nonstationary
characteristics of the signal and noise. The work furthermore demonstrates the ability to impart control over the imaging
process by adjusting reconstruction parameters to exploit nonstationarity in a manner advantageous to a particular
imaging task.
Methods: A cascaded systems analysis model was used to model the local noise-power spectrum (NPS) and modulation
transfer function (MTF) for FBP reconstruction, with locality achieved by separate calculation of fluence and system
gain for each view as a function of detector location. The covariance and impulse response function for PL
reconstruction (quadratic penalty) were computed using the implicit function theorem and Taylor expansion.
Detectability index was calculated under the assumption of local stationarity to show the variation in task-dependent
image quality throughout the image for simple and complex, heterogeneous objects. Control of noise magnitude and
correlation was achieved by applying a spatially varying roughness penalty in PL reconstruction in a manner that
improved overall detectability.
Results: The models provide a foundation for task-based imaging performance assessment in FBP and PL image
reconstruction. For both FBP and PL, noise is anisotropic and varies in a manner dependent on the path length of each
view traversing the object. The anisotropy in turn affects task performance, where detectability is enhanced or
diminished depending on the frequency content of the task relative to that of the NPS. Spatial variation of the roughness
penalty can be exploited to control noise magnitude and correlation (and hence detectability).
Conclusions: Nonstationarity of image noise is a significant effect that can be modeled in both FBP and PL image
reconstruction. Prevalent spatial-frequency-dependent metrics of spatial resolution and noise can be analyzed under
assumptions of local stationarity, providing a means to analyze imaging performance as a function of location throughout
the image. Knowledgeable selection of a spatially-varying roughness penalty in PL can potentially improve local noise
and spatial resolution in a manner tuned to a particular imaging task.
Keywords: cascaded systems analysis, nonstationarity, filtered backprojection, penalized-likelihood reconstruction,
noise-power spectrum, covariance matrix, imaging task, detectability index
Preliminary investigation of the frequency response and distortion
properties of nonlinear image processing algorithms
Author(s):
Jered R. Wells;
James T. Dobbins III
Show Abstract
Assessment of the resolution properties of nonlinear imaging systems is a useful but challenging task. While the
modulation transfer function (MTF) fully describes contrast resolution as a function of spatial frequency for linear
systems, an equivalent metric does not exist for systems with significant nonlinearity. Therefore, this preliminary
investigation attempts to classify and quantify the amount of scaling and distortion imposed on a given image signal as
the result of a nonlinear process (nonlinear image processing algorithm).
As a proof-of-concept, a median filter is assessed in terms of its principle frequency response (PFR) and distortion
response (DR) functions. These metrics are derived in frequency space using a sinusoidal basis function, and it is shown
that, for a narrow-band sinusoidal input signal, the scaling and distortion properties of the nonlinear filter are described
exactly by PFR and DR, respectively. The use of matched sinusoidal basis and input functions accurately reveals the
frequency response to long linear structures of different scale. However, when more complex (multi-band) input signals
are considered, PFR and DR fail to adequately characterize the frequency response due to nonlinear interaction effects
between different frequency components during processing.
Overall, the results reveal the context-dependent nature of nonlinear image processing algorithm performance, and they
emphasize the importance of the basis function choice in algorithm assessment. In the future, more complex forms of
nonlinear systems analysis may be necessary to fully characterize the frequency response properties of nonlinear
algorithms in a context-dependent manner.
Scatter correction with kernel perturbation
Author(s):
Josh Star-Lack;
Mingshan Sun
Show Abstract
X-ray scatter degrades image contrast, uniformity and CT number accuracy in cone-beam computed tomography
(CBCT). Correction methods based on the scatter kernel superposition (SKS) technique are efficient and suitable for
many clinical applications but still produce residual errors due to limitations in the scatter kernel models. To reduce
these errors, we propose to generate a first-pass reconstruction using a set of default SKS parameters followed by limited
Monte Carlo simulations that are then used to perturb and refine key kernel parameters in order to obtain an improved
second-pass correction. To test the approach, we used the fast adaptive scatter kernel model (fASKS) employing
asymmetric kernels for the first-pass scatter correction and then used GEANT4 to simulate scatter-to-primary ratios in
selected projections allowing for refined scatter estimates. The results show that a minimal number of projections require
simulation in order to adequately perturb scatter kernel parameters for all projections. Compared to the default
asymmetric kernels, the refined kernels reduced CT number errors from 24 HU to 15 HU in a large pelvis phantom
resulting in a more uniform and accurate image.
Splitting-based statistical X-ray CT image reconstruction with blind gain correction
Author(s):
Hung Nien;
Jeffrey A. Fessler
Show Abstract
Variational methods are useful for solving ill-posed inverse imaging problems by minimizing a cost function
with a data fidelity term and a regularization term. For statistical X-ray computed tomography (CT) image
reconstruction, penalized weighted least-squares (PWLS) criteria with edge-preserving regularization can improve
quality of the reconstructed image compared to traditional filtered back-projection (FBP) reconstruction.
Nevertheless, the huge dynamic range of the statistical weights used in PWLS image reconstruction leads to a
highly shift-variant local impulse response, making effective preconditioning difficult. To overcome this problem,
iterative algorithms based on variable splitting were proposed recently. However, existing splitting-based iterative
algorithms do not consider the (unknown) gain fluctuations that can occur between views. This paper
proposes a new variational formulation for splitting-based iterative algorithms where the unknown gain parameter
vector and the image are estimated jointly with just simple changes to the original algorithms. Simulations
show that the proposed algorithm greatly reduces the shading artifacts caused by gain fluctuations yet with
almost unchanged computational complexity per iteration.
Tradeoff between noise properties and local impulse response in statistical prior image constrained compressed sensing
Author(s):
Pascal Thériault Lauzier;
Yinsheng Li;
Guang-Hong Chen
Show Abstract
The increase in the use of CT scanning in the clinical setting is raising concerns from the medical community. In order to reduce the dose of ionizing radiation imparted to patients during CT scans, statistical image recon struction was proposed. This family of algorithm aims at improving image noise characteristics by modeling the stochastic x-ray detection process in the reconstruction algorithm. It was shown however that statistical recon struction may lead to an anisotropic spatial resolution. In this abstract, we study this tradeoff in the context of a statistical formulation of the dose reduction using prior image constrained compressed sensing framework (DR-PICCS). Two numerically-simulated phantoms and a dataset acquired in vivo were used for this evaluation. It is demonstrated that the inclusion of a statistical model in DR-PICCS may whiten the NPS and uniformize the noise spatial distribution in the image. However, the images may suffer from an anisotropic spatial resolution while the images reconstructed using DR-PICCS without statistical model have more isotropic spatial resolution. Due to the flexibility offered in PICCS, a specially-designed prior image processing method has been used in statistical DR-PICCS to palliate for the anisotropy in spatial resolution.
Overcoming nonlinear partial volume effects in known-component reconstruction of Cochlear implants
Author(s):
J. W. Stayman;
H. Dang;
Y. Otake;
Wojciech Zbijewski;
J. Noble;
B. Dawant;
R. Labadie;
J. P. Carey;
J. H. Siewerdsen
Show Abstract
Nonlinear partial volume (NLPV) effects can be significant for objects with large attenuation differences and fine detail
structures near the spatial resolution limits of a tomographic system. This is particularly true for small metal devices like
cochlear implants. While traditional model-based approaches might alleviate these artifacts through very fine sampling
of the image volume and subsampling of rays to each detector element, such solutions can be extremely burdensome in
terms of memory and computational requirements. The work presented in this paper leverages the model-based approach
called “known-component reconstruction” (KCR) where prior knowledge of a surgical device is integrated into the estimation.
In KCR, the parameterization of the object separates the volume into an unknown background anatomy and a
known component with unknown registration. Thus, one can model projections of an implant at very high spatial resolution
while limiting the spatial resolution of the anatomy - in effect, modeling NLPV effects where they are most significant.
We present modifications of the KCR approach that can be used to largely eliminate NLPV artifacts, and demonstrate
the efficacy of the modified technique (with improved image quality and accurate implant position estimates) for
the cochlear implant imaging scenario.
Impact of norm selections on the performance of prior image constrained compressed sensing (PICCS)
Author(s):
Yinsheng Li;
Jie Tang;
Guang-Hong Chen
Show Abstract
Advances have been made in recent years in computed tomography (CT) as a result of the development and
implementation of new iterative reconstruction methods. Prior Image Constrained Compressed Sensing (PICCS)
is one such iterative reconstruction method which iteratively minimizes an objective function to approach a target
image. To date, published studies have employed the L1 norm in the minimization of the objective function. In
this study, we investigate the use of Lp norms with p > 1 and investigate how image quality depends on the selection of the Lp norm used in the minimization of the objective function.
Comparative evaluation of linear interpolation models for
iterative reconstruction in X-ray CT
Author(s):
K. Schmitt;
H. Schöndube;
K. Stierstorfer;
J. Hornegger;
F. Noo
Show Abstract
The forward projection operator is a key component of every iterative reconstruction method in X-ray computed
tomography (CT). Besides the choices being made in the definition of the objective function and associated
constraints, the forward projection model affects both bias and noise properties of the reconstruction.
In this work, we compare three important forward projection models that rely on linear interpolation: the
Joseph method, the distance-driven method, and the image representation using B-splines of order n = 1. The
comparison focuses on bias and noise in the image as a function of the resolution. X-ray CT data that are
simulated in fan-beam geometry with two different magnification factors are used.
Dose reduction in CT with correlated-polarity noise reduction: comparable image quality at half the dose with projection space processing
Author(s):
James T. Dobbins III;
Jered R. Wells;
W. P. Segars
Show Abstract
Correlated-polarity noise reduction (CPNR) is a novel noise reduction technique that uses a statistical approach to reduce
noise while maintaining excellent resolution and a “normal” noise appearance. It is applicable to any type of medical
imaging, and we introduced it at SPIE 2011 for reducing dose three-fold in radiography while maintaining excellent
image quality. In this current work, we demonstrate for the first time its use in reducing the noise in CT images as a
means of reducing the dose in CT. Simulated chest CT images were generated using the XCAT phantom and Poisson
noise was added to simulate a conventional full-dose CT image and a half-dose CT image. CPNR was applied to the
half-dose images in projection image space, and then the images were reconstructed using filtered backprojection with a
Feldkamp methodology. The resulting CPNR processed half-dose images showed essentially equivalent relative
standard deviation in the central heart region to the full-dose images, and about 0.7 times that in half-dose images that
were not processed with CPNR. This noise reduction was consistent with a two-fold reduction in dose that is possible
with CPNR in CT. The CPNR images demonstrated virtually identical sharpness of vessels and no apparent artifacts.
We conclude that CPNR shows strong promise as a new noise reduction method for dose reduction in CT. CPNR could
also be used in combination with model-based iterative reconstruction techniques for yet further dose reduction.
A moving blocker system for cone-beam computed tomography scatter
correction
Author(s):
Luo Ouyang;
Kwang Song;
Timothy Solberg;
Jing Wang
Show Abstract
Scatter contamination in cone-beam computed tomography (CBCT) degrades the image quality by introducing shading
artifacts. In our previous study, a moving-blocker-based approach was proposed to simultaneously estimate scatter and
reconstruct the complete volume within field of view (FOV) from a single CBCT scan. Promising results were obtained
from simulation studies. In this work, we implemented the moving blocker system on a LINAC on-board kV CBCT
imaging system. A physical attenuator (i.e., "blocker") consisting equal spaced lead strips was mounted on a linear
actuator. A step motor connected to the actuator drove the blocker to move back and forth along gantry rotation axis
during CBCT acquisition. Scatter signal was estimated from the blocked region of imaging panel, and interpolated into
the un-blocked region. A sparseness prior based statistical iterative reconstruction algorithm was used to reconstruct
CBCT images from un-blocked projections after the scatter signal was subtracted. Experimental studies were performed
on both a Catphan phantom and an anthropomorphic pelvis phantom to evaluate performance of the moving blocker
system. The scatter-induced shading artifacts were substantially reduced in the images acquired with the moving blocker
system. CT number error reduced in selected regions of interest of the Catphan phantom from 318 to 17. It also
decreased in those of the pelvis phantom from 239 to 10. We demonstrated for the first time that the moving blocker
system could successfully estimate the scatter signal in projection data, reduce the imaging dose and obtain complete
volumetric information within the FOV using a single scan.
Optimized control of a dynamic, prepatient attenuator
Author(s):
Scott S. Hsieh;
Norbert J. Pelc
Show Abstract
Dynamic attenuators are beam shaping filters that can customize the x-ray illumination field to the clinical task and for
each view. These dynamic attenuators replace traditional attenuators (or “bowtie filters”) and decrease radiation dose,
dynamic range, and scatter when compared to their static counterparts. We propose a one-dimensional dynamic
attenuator that comprises multiple wedges with axially-dependent triangular cross-sections, and which are translated in
the axial direction. These wedges together produce a time-varying, piecewise-linear attenuation function. We investigate
different control methods for this attenuator and estimate the ability of the dynamic attenuator to reduce dose while
maintaining the peak variance of the scan. With knowledge of the patient anatomy, the dynamic attenuator can be
controlled by solving a convex optimization problem. This knowledge could be determined from a low dose pre-scan.
Absent this information, various heuristics can be used. We simulate the dynamic attenuator on datasets of the thorax,
abdomen, and a targeted scan of an abdominal aortic aneurysm. The dose and scatter-to-primary ratio (SPR) are
estimated using Monte Carlo simulations, and the noise is calculated analytically. Compared to a system using the
standard bowtie with typical mA modulation, dose reductions of 50% are observed. Compared to an optimized, patientspecific mA modulation, the typical dose reduction is 30%. If the dynamic attenuator is controlled with a heuristic, typical dose reductions are also 30%. The gains are larger in the targeted scan. The SPR is also reduced by 20% in the abdomen. We conclude that the dynamic attenuator has significant potential to reduce dose without increasing the peak variance of the scan.
Technical feasibility of CT perfusion using a C-Arm CBCT system
Author(s):
Min Xu;
Jie Tang;
Kevin Royalty;
Guang-Hong Chen
Show Abstract
It is highly desirable to obtain perfusion information with the C-arm CBCT system in the interventional room. However,
due to hardware limitations, it is still elusive to achieve cone-beam CT perfusion measurements. In this study, we
performed a systematic study to investigate what the main limiting factors are that need to be addressed for future C-arm
cone beam CT perfusion imaging. To do so, we performed systematic numerical simulation studies using a diagnostic
CT perfusion data set. Specifically, a forward projection was performed to simulate cone-beam CT perfusion experiment
with C-arm CBCT geometry and temporal behavior. Different x-ray delays after contrast injection have been simulated
with this method. The view angle undersampling artifacts, shading artifacts from dynamic objects, and the importance of
arterial input function (AIF) for perfusion study were investigated in this study with different x-ray delay times. From the
simulation results, it was found that the view angle undersampling artifacts do not have much impact on perfusion maps.
The shading artifacts from dynamic object were shown to have a negligible effect on the NRMSE in perfusion maps. The
accuracy of AIF is an important but not a dominating factor for perfusion studies. C-arm CBCT cannot accurately
recover the slowly changing contrast in brain tissues due to the low temporal resolution. Therefore, to enable cone beam
C-arm CT perfusion measurement, it is critical to improve the temporal behavior of CBCT by either employing new
hardware upgrades or introducing new software methods.
An online motion- and misalignment-correction method for medical flat-detector CT
Author(s):
Julia Wicklein;
Yiannis Kyriakou;
Willi A. Kalender;
Holger Kunze
Show Abstract
Misalignment-Correction in C-arm-based flat-detector CT (FD-CT) is a frequently discussed problem. To avoid artifacts
caused by geometrical instabilities, numerous methods for misalignment correction were investigated. Most of them
make use of a foregoing calibration routine, based on scanning a specific phantom. The aim of this study is to develop
and evaluate an online image-content-based calibration technique without using any kind of marker or calibration
phantom. The introduced method is based on a gradient descent method, minimizing an entropy criterion which is used
to optimize the underlying geometry parameters of the acquisition system. It is formed as multistep approach, including a
global, local and projection wise optimization. This enables the elimination of general system misalignments, as well as a
reduction of streak artifacts and the adjustment of patient motion artifacts. Phantom and patient measurements with the
C-arm FD-CT system Artis Zeego (Siemens AG, Healthcare Sector, Forchheim, Germany) were used to validate the
algorithm for realistic applications. It reduced most of the actual misalignment and increased image quality drastically.
Phantom-studies, starting from the standard system geometry without a foregoing calibration showed very good results.
Online-calibration is possible with our approach and therefore, the limitation to predefined scan-protocols is obsolete.
The evaluation of patient datasets brought out the same conclusions and provides the implication of simultaneous patient
motion compensation.
Can motion compensated reconstruction improve 'best phase' reconstruction in Cardiac CT?
Author(s):
H. Bruder;
C. Rohkohl;
T. Allmendinger;
H. Schöndube;
R. Raupach;
K. Stierstorfer;
T. Flohr
Show Abstract
Based on a phantom study with a realistic coronary vessel phantom, we investigated if motion compensated cardiac CT
reconstruction can improve best phase image quality with respect to motion artifacts and patency of coronary vessel
lumen. Basically, tracking based methods (with and without improvement of temporal resolution) deriving the motion
fields by a registration-like procedure are compared to optimization based methods optimizing objective functions while
minimizing artifact levels (e.g. Motion Artifact Metric Optimization (MAM) Reconstruction).
Using the MAM technique, the motion field is iteratively calculated with a steepest descent update equation minimizing
a motion artifact metric.
We evaluated patency of the vessel lumen, the normalized cross correlation (NCC) of the respective reconstruction data
with the ground truth data and a best phase improvement index correlating the motion compensated reconstruction data
to the non-compensated FDK-based reconstruction data. It will be shown that the MAM technique is superior to the
tracking methods. The latter proved to be more or less susceptible to template matching and, or erroneous template size.
The value of MAM is also demonstrated evaluating clinical data. In particular it is beneficial for patients with high heart
rates as well as for dose optimized scan protocols because it does not need over-radiation.
Positron emission tomography coincidence detection with photon polarization correlation
Author(s):
Aimee L. McNamara;
Kinwah Wu;
David Boardman;
Mark I. Reinhard;
Zdenka Kuncic
Show Abstract
Two-photon annihilation quanta are emitted in a pure quantum state and when detected in coincidence, the
photon pairs possess orthogonal polarizations. We propose that this polarization correlation can be exploited in
Positron Emission Tomography (PET), which relies crucially on accurate coincidence detection of photon pairs.
In this proof of concept study, we investigate how photon polarization information can be exploited in PET
imaging by developing a method to discern true coincidences using the polarization correlation of annihilation
pairs. We demonstrate that the unique identification of true photon pairs with their polarization correlation can
dramatically enhance overall PET image quality, especially for high emission rates, when conventional, energy-
based coincidence detection methods become increasingly unreliable. Our results suggest that polarization-based
coincidence detection offers new prospects for in vivo molecular imaging with next-generation PET systems.
Liver imaging: image quality evaluation and comparison between single and dual energy protocols
Author(s):
Yuan Yao;
Alec J. Megibow;
Norbert J. Pelc
Show Abstract
Purpose: Some qualitative studies report a preference for blended dual-energy (DE) CT images over single energy (SE)
images for liver CT imaging at the same dose. This is counter to theoretical expectations for simple tasks. We
hypothesized that perhaps the broad spectrum of DE might be beneficial for a combination of tasks. We compare the
CNR of SE and blended DE images for single and composite tasks, in part to see if they explain the preference.
Methods: We simulated pre- and post-contrast SE abdominal CT imaging at various kVp but at constant average dose.
Next, 80kVp and 140kVp scans with different dose allocations, dose matched to the SE images, were simulated. DE
images were blended linearly with optimized blending ratios. The CNRs of liver against other soft tissues were used as a
composite image quality metric for evaluation and comparison between the SE and DE protocols. In addition, the
combination of the CNR of many tissue pairs pre- and post-contrast.
Results: The CNR of pre-contrast single kVp imaging mostly increases with increasing tube voltage while 90kVp or
lower energy yields higher CNR for post-contrast images, depending on the differential iodine concentration of each
tissue. Similar trends are seen in the DE blended CNR curves. Results from the composite multi-CNR metric
demonstrate that the SE protocol has better performance.
Conclusions: Our study showed that an optimized SE protocol produces higher CNR, even for a range of tasks. This
suggests that the reason for the radiologist preference must be something other than a fundamental advantage of DE.
Joint reconstruction of activity and attenuation map using LM SPECT emission data
Author(s):
Abhinav K. Jha;
Eric Clarkson;
Matthew A. Kupinski;
Harrison H. Barrett
Show Abstract
Attenuation and scatter correction in single photon emission computed tomography (SPECT) imaging often
requires a computed tomography (CT) scan to compute the attenuation map of the patient. This results in
increased radiation dose for the patient, and also has other disadvantages such as increased costs and hardware
complexity. Attenuation in SPECT is a direct consequence of Compton scattering, and therefore, if the scattered
photon data can give information about the attenuation map, then the CT scan may not be required. In this
paper, we investigate the possibility of joint reconstruction of the activity and attenuation map using list-
mode (LM) SPECT emission data, including the scattered-photon data. We propose a path-based formalism
to process scattered-photon data. Following this, we derive analytic expressions to compute the Cram´er-Rao
bound (CRB) of the activity and attenuation map estimates, using which, we can explore the fundamental limit
of information-retrieval capacity from LM SPECT emission data. We then suggest a maximum-likelihood (ML)
scheme that uses the LM emission data to jointly reconstruct the activity and attenuation map. We also propose
an expectation-maximization (EM) algorithm to compute the ML solution.
Experimental study of optimal energy weighting in energy-resolved CT using a CZT detector
Author(s):
Franco Rupcich;
Taly Gilat-Schmidt
Show Abstract
Recent advances in energy-resolved CT can potentially improve contrast-to-noise ratio (CNR), which could subsequently reduce dose in conventional and dedicated breast CT. Two methods have been proposed for optimal energy weighting: weighting the energy- bin data prior to log normalization (projection- based weighting) and weighting the energy-bin data after log normalization (image-based weighting). Previous studies suggested that optimal projection-based and image-based energy weighting provide similar CNR improvements for energy resolved CT compared to photon-counting or conventional energy-integrating CT. This study experimentally investigated the improvement in CNR of projection-based and image-based weighted images relative to photon counting for six different energy-bin combinations using a bench top system with a CZT detector. The results showed CNR values ranged between 0.85 and 1.01 for the projection- based weighted images and between 0.91 and 1.43 for the image- based weighted images, relative to the CNR for the photon-counting image. The range of CNR values demonstrates the effects of energy-bin selection on CNR for a particular energy weighting scheme. The non-ideal spectral response of the CZT detector caused spectral tailing, which appears to generally reduce the CNR for the projection-based weighted images. Image-based weighting increased CNR in five of the six bin combinations despite the non-ideal spectral effects.
Objective assessment of penalized maximum likelihood reconstruction with sparsity-promoting penalty for Myocardial
perfusion SPECT imaging
Author(s):
Joyeeta Mitra Mukherjee;
Joyoni Dey;
Michael A. King;
Souleymane Konate
Show Abstract
Novel methods of reconstructing the tracer distribution in myocardial perfusion images are being considered for lowcount
and sparse sampling scenarios. Few examples of low count scenarios are when the amount of radioisotope
administered or the acquisition time is lowered, in gated studies where individual gates are reconstructed. Examples of
sparse angular sampling scenarios are patient motion correction in traditional SPECT where few angles are acquired at
any given pose and in multi-pinhole SPECT where the geometry is sparse and truncated by design. The reconstruction
method is based on the assumption that the tracer distribution is sparse in the transform domain, which is enforced by a
sparsity-promoting penalty on the transform coefficients. In this work we investigated the curvelet transform as the
sparse basis for myocardial perfusion SPECT. The objective is to determine if myocardial perfusion images can be
efficiently represented in this transform domain, which can then be exploited in a penalized maximum likelihood (PML)
reconstruction scheme for improving defect detection in low-count/ sparse sampling scenarios. The performance of this
algorithm is compared to standard OSEM with 3D Gaussian post-filtering using bias-variance plots and numerical
observer studies. The Channelized Non-prewhitening Observer (CNPW) was used for defect detection task in a “signalknown-
statistically” LROC study. Preliminary investigations indicate better bias-variance characteristics and superior
CNPW performance with the proposed curvelet basis. However, further assessment using more defect locations and
human observer evaluation is needed for clinical significance.
Spectra optimization for dual-energy contrast-enhanced breast CT
Author(s):
Pablo Milioni de Carvalho;
Ann-Katherine Carton;
Sylvie Saab-Puong;
Răzvan Iordache;
Serge Muller
Show Abstract
This work investigates a dual-energy subtraction technique for cone-beam breast CT combined with an iodinated
contrast agent. Simulations were performed to obtain optimally enhanced iodine-equivalent and morphological images.
The optimal x-ray beam energies and average glandular dose allocation between the LE and HE images were identified.
Cylindrical phantoms were simulated with 10, 14 and 18 cm diameters and composed of 50% fibroglandular breast tissue
equivalent material. They contained spherical lesion inserts composed of 0, 25, 75 and 100% fibroglandular equivalent
tissues, homogeneous mixtures of 50% fibroglandular equivalent tissue and 0.5, 1.0, 2.5 and 5.0 mg/cm3 iodine, as well
as pure calcium hydroxyapatite, emulating calcifications. An acquisition technique with 600 projection images is
proposed. Only primary x-ray photons were simulated and a perfect energy-integrating detector was considered. LE and
HE beams ranging from 20 keV to 80 keV were investigated. The LE and HE projections were reconstructed using a
filtered backprojection algorithm. The LE volume provided the morphological image while the iodine-equivalent volume
was obtained by recombining the LE and HE volumes. Contrast-to-noise ratio (CNR) between the spherical inserts and
background breast tissue normalized to the square root of the total AGD (CNRD) was used as figure-of-merit for lesion
detectability. Based on maximizing CNRD, a 30keV/34keV LE/HE pair and a ~50/50% LE/HE AGD allocation were
found to provide the best possible performance for iodine and morphological imaging for an average size breast.
Measurement of breast-tissue x-ray attenuation by spectral mammography: first results on cyst fluid
Author(s):
Erik Fredenberg;
David R. Dance;
Paula Willsher;
Miriam von Tiedemann;
Kenneth C. Young;
Matthew G. Wallis
Show Abstract
Knowledge of x-ray attenuation is essential for developing and evaluating x-ray imaging technologies. For instance,
techniques to better characterize cysts at mammography screening would be highly desirable to reduce recalls, but
the development is hampered by the lack of attenuation data for cysts. We have developed a method to measure xray
attenuation of tissue samples using a prototype photon-counting spectral mammography unit. Spectral (energyresolved)
images were acquired and the image signal was mapped to two known reference materials, which were
used to derive the x-ray attenuation as a function of energy. We have measured the attenuation of 25 samples of
breast cyst fluid. Spectral measurements of water samples showed consistent results compared to published
attenuation values.
Model observer detectability as a substitute for contrast detail analysis in routine digital mammography quality control
Author(s):
Elena Salvagnini;
Kim Lemmens;
Hilde Bosmans;
Lara Struelens;
Nicholas W. Marshall
Show Abstract
This work investigated the substitution of CDMAM contrast detail (c-d) analysis with detectability (d’) from a nonprewhitening
eye filter model observer (NPWE) for routine quality control (QC). Routinely acquired QC data for 53
systems were analyzed: 13 computed radiography (CR) and 40 integrated detector (DR) systems. For a given system,
threshold gold thickness from the c-d analysis (T) was calculated from 16 images and compared against d’ calculated for
0.1 and 0.25 mm diameter discs. The d’ data were calculated from the routine 50 mm PMMA AEC image and the
measured pre-sampling detector modulation transfer function (MTF). Threshold gold thickness T and d’ were plotted as
function of MGD and compared. The Fuji CR and the Agfa CR systems had the highest T values compared to the other
systems. The Hologic systems were found to have a low value of T, compared to the other systems, for both disc
diameters. The NPWE results reflected the performance seen for T data for the majority of the systems with the
exception of the Fuji CR and Konica CR systems. The Hologic systems gave unexpectedly low d’ results or
unexpectedly low T values. The correspondence between the two quality indices was examined with the Pearson
correlation statistical test. This test was not applicable to the GE Essential systems because all systems are grouped
together at the same working point so the result of r is about 0. For all other groups of systems the test gave good results
(larger than -0.65).
Intensity standardization in breast MR images improves tissue quantification
Author(s):
Shandong Wu;
Jayaram K. Udupa;
Aikaterini Marinaki;
Susan P. Weinstein;
Despina Kontos
Show Abstract
Computerized algorithms are increasingly being developed for quantifying breast MRI features for facilitating lesion
detection and breast tissue segmentation in various clinical applications. One of the current impediments is the intensity
non-standardness of the breast tissue in the acquired MR images across different cases, scanners, and/or patients. This
degrades the performance of quantitative image processing. In this work, we investigate the usefulness of post-hoc
intensity standardization of breast MR images by using a landmark-based nonlinear intensity mapping algorithm. The
standardization algorithm is applied after correction of the images for background bias field non-uniformity. We then
quantitatively compare the percentage coefficient of variation (%CV) of image intensity in the fibroglandular (e.g.,
dense) tissue region before and after standardization to evaluate the standardization procedure. In our experiments, we
use 9 representative 3D bilateral breast MRI scans/cases constituting 18 breasts (a total of 504 tomographic breast MRI
slices), in which we observe a significant decrease of the %CV in the standardized images, indicating that
standardization significantly reduces the intensity variation for the fibroglandular tissue across these cases. Furthermore,
we demonstrate for two segmentation methods that the standardization process leads to improved segmentation of the
fibroglandular tissue. Our work suggests that intensity standardization following bias field correction may serve as an
effective preprocessing step to support improved quantitative breast MR image processing and analysis, particularly for
breast density quantification.
Conventional mammographic image generation in dual-energy digital mammography
Author(s):
Xi Chen;
Robert M. Nishikawa;
Xuanqin Mou
Show Abstract
Dual-energy digital mammography (DEDM) can generate tissue-subtracted calcification image for improving the
detectability of breast calcifications. However, the masses, if present, are missing in the tissue-subtracted calcification
image. This paper proposes an algorithm to generate conventional mammographic image by DEDM images based on a
multi-scale decomposition and reconstruction architecture with Gaussian filters. Firstly, calibration coefficients are
measured at different kVp to correct the original LE and HE. Secondly, the LE and HE images are decomposed into
multi-scale components. Thirdly, the components at different scale of the two images are weighted based on a similarity
measure to generate new components. Finally, the conventional mammographic image is reconstructed by these new
components using noise suppression technique. The proposed method was validated by different breast phantoms on two
commercially available full-field digital mammography systems. Results show that the method is effective and the
reconstructed image has similar grayscale, contrast and noise level to the corresponding conventional mammogram.
Therefore, both the calcification image and conventional mammogram-like image can be generated; the patient will not
need more exposure to get the conventional mammogram in DEDM.
Testing realism of software breast phantoms: texture analysis of synthetic mammograms
Author(s):
Predrag R. Bakic;
Brad Keller;
Yuanjie Zheng;
Yan Wang;
James C. Gee;
Despina Kontos;
Andrew D. A. Maidment
Show Abstract
Software breast phantoms have been developed for pre-clinical validation of breast imaging systems. Realism is of great
importance for the acceptance and the range of applications of breast phantoms. In this paper we have assessed the
phantom realism based upon the analysis of mammographic texture properties. Texture analysis is of interest since it
reflects the spatial tissue distribution, which is known to correlate with breast cancer risk. We compared texture
properties of synthetic mammograms generated using software breast phantoms with clinical data. A total of 133
phantom images were synthesized using software phantoms developed at the University of Pennsylvania. The phantoms
were designed using two different anatomy simulation methods: an octree-based recursive partitioning method and a
region growing method. The synthetic images were generated assuming a clinically used acquisition geometry and
mono-energetic x-ray beam with no scatter. The clinical data included 60 anonymized mammograms selected
retrospectively from screening cases at the University of Pennsylvania. The same postprocessing was applied to clinical
and phantom images. The texture analysis was performed using fully automated software which extracts a battery of
features from analyzed images. The histograms of texture properties extracted from phantom images were compared
with those from clinical mammograms, separately for the two anatomy simulation methods. The histogram agreement
was quantified using symmetrized Kulback-Leibler divergence. We observed good agreement for most of the analyzed
25 features. In more than a half of the features, the octree-based simulation method yielded better agreement with
clinical data as compared with the region growing method.
Estimation of patient dose with standard and low-dose MDCT fluoroscopy protocols for lung biopsy
Author(s):
F. Zanca;
A. Jacobs;
W. De Wever;
W. Crijns;
R. Oyen;
H. Bosmans
Show Abstract
Purpose: This study aimed at assessing patient dose with standard and low-dose MDCT fluoroscopy protocols
for lung biopsy.
Materials and Methods: The low-dose protocol used lower tube potential (80 kV) respect to the standard
protocol (120 kV); all other scanning parameters were left unchanged. Data from sixty-nine (69) CT fluoroscopy
(CTF) lung interventions were prospectively collected and included in the study. Nineteen (19) procedures where
performed using the standard protocol (120 kV), while the remaining 50 where performed with the low-dose
protocol ( 80 kV). Effective patient dose was calculated using the dose-length product information, while peak
entrance skin dose was measured with EBT2 gafchromic films. Lesion size, total fluoroscopy time, success rate
and complication rate were also recorded. The Mann-Whitney U test was used to assess statistical significant
difference in terms of lesion size and fluoroscopy time, between the two study groups. Fisher’s test was used to
assess significant difference in terms of success and conclusiveness.
Results: The median effective patient dose was 5.4 mSv (minimum 2.4 mSv, maximum 18.8 mSv; 19
procedures) for the standard protocol and 1.1 mSv (minimum 0.4 mSv, maximum 4.5 mSv; 50 procedures) for
the low-dose protocol (p<0.01). The median peak entrance skin dose was 268 mGy (95-899 mGy) and 141 mGy
(38-410 mGy) for the standard and low-dose protocol respectively (p<0.01). There was no significant difference
(p=0.95) in mean lesion size between the two groups: standard protocol 2.7 cm (min 0.9 cm, max 10.7 cm); lowdose
protocol 2.6 cm (min 1.0 cm, max 7.9 cm). Average fluoroscopy time was 1.4 min (range 0.6-5.0 min) and
1.3 min (range 0.4-4.5 min), respectively for the standard and low-dose protocol (p-value=0.36). Biopsy
performed using the low-dose protocol was technically successful in 98% of the cases and complication rate was
18%, compared to 100% and 10% for the standard protocol. No statistical significant difference was found
between the two groups ( p< 0.05).
Conclusions: High patient entrance skin dose (up to 899 mGy) and high effective patient dose (up to 18.8 mSv)
can occur for standard CTF lung biopsy protocol. Simple means, like lowering the kV, allow reducing patient
dose significantly, with skin doses now far below the 2 Gy level of deterministic effects. Occupational doses,
occasionally of concern in high work load regimes, are expected to follow the same trend.
Radiation dose reduction in dual-energy CT using Prior Image Constrained Compressed Sensing: image quality evaluation in virtual monochromatic imaging
Author(s):
Stephen Brunner;
John Garrett;
Yinsheng Li;
Jie Tang;
Kari Pulfer;
Jiang Hsieh;
Howard Rowley;
Guang-Hong Chen
Show Abstract
Dual-energy CT has the potential to overcome many of the limitations of routine single-energy CT scanning, such a,.., the potential to provide quantitative imaging via electron density, effective atomic munber, and virtual monochromatic imaging and the potential to completely eliminate beam-hardening artifacts via projection space decomposition. While the potential clinical benefit is strong, a possible barrier to more frequent clinical use of dual-energy CT scanning is radiation dose for high quality images. While image quality in dual-energy CT depends on a munber of factors, including dose partitioning, the choice of kV pair, and the amount of pre filtration used, a munber of strategies have been employed to improve image quality in dual-energy CT. Four main methods are: (1) increa,..,e the radiation dose, (2) increase the slice thickness, (3) perform voxel averaging, or (4) use noise reduction algorithms. While these methods offer options for improving image quality, ideally, it is desirable not to have to increase radiation dose or sacrifice spatial resolution (in the x-y plane or in the z-direction). Therefore, it is the purpose of this work to investigate the application of Prior Image Constrained Compressed Sensing (PICCS) in dual-energy CT to reduce radiation dose without sacrificing image quality. In particular, we investigate the use of PICCS in dual-energy CT to generate material density images at half the radiation dose of a commonly used gemstone spectral imaging (GSI) protocol. lVIaterial density images are generated using half the radiation dose, and virtual monochromatic images are generated as a linear combination of half-dose material density images. In this abstract, qualitative and quantitative evaluation are provided to assess the performance of PICCS relative to FBP images at the full dose level and at the half dose level.
Dependence of radiation dose on area and volumetric mammographic breast density estimation
Author(s):
H. Jing;
B. Keller;
Jae Young Choi;
R. Crescenzi;
E. Conant;
A. Maidment;
D. Kontos
Show Abstract
Mammographic breast density is a strong risk factor for breast cancer. Studies on imaging dose in mammography
have primarily focused on imaging quality and diagnostic accuracy, while little work has been done on
understanding its effect on the estimation of breast density. Studies on the effect of dose on mammographic density
estimation can be useful in dose reduction for the purpose of density estimation and monitoring. In this study, we
investigate the dependence of percent area (PD%) and volumetric (VD%) breast density estimation on imaging dose
using an anthropomorphic breast phantom (Rachel, Gammex). A set of digital mammograms were obtained with a
GE Senographe 2000D FFDM system, using 220 unique combinations of different imaging physics, namely
target/filter, kVp and mAs. Specifically, 8 different mAs settings were defined as corresponding to 10%, 20%, 40%,
70%, 100%, 150%, 200% and 300% of 1.8 mGy reference average glandular dose (AGD) for standard phototimed
exposure. Breast density was estimated using fully-automated FDA-cleared software (Quantra v.2.0, Hologic Inc.).
The obtained estimates were analyzed to study the effect of the imaging dose, using ANOVA and linear regression.
Results show that there is a statistically significant dependence of density estimation on x-ray imaging dose
(p-value=0.014 and <0.001 for PD% and VD%, respectively), while the actual variation of the estimation across the
different levels of dose is relatively low (standard deviation of 2.87% and 0.66% for PD% and VD% respectively),
the differences could be significant when breast density measures are used for risk estimation.
A real-time radiation dose monitoring system for patients
and staff during interventional fluoroscopy using a
GPU-accelerated Monte Carlo simulator and an automatic
3D localization system based on a depth camera
Author(s):
Andreu Badal;
Fahad Zafar;
Han Dong;
Aldo Badano
Show Abstract
Radiation monitoring systems able to accurately track the radiation dose received by the patient and the medical staff during interventional fluoroscopy can be used to minimize the likelihood and severity of radiation-induced skin injuries and estimate the accumulated organ doses. We describe a method to monitor doses in real time using automatic sensors in the imaging room and a CPU-accelerated computer simulator. The Monte Carlo simulation code MC-GPU is used to estimate patient and staff doses due to primary and scattered radiation, along with the associated statistical uncertainties. The geometrical configuration of the irradiation is automatically determined and updated using data from a depth camera that tracks the location and posture of each person in the imaging room. A virtual x-ray source graphical interface is used to manually trigger the simulations. The implemented computational framework separates the simulation of the x-ray transport through the patient and the operator bodies into two coupled, sequential simulations. The initial simulation uses the patient anatomy and a c-arm source model with a collimated cone beam emitted from a point focal spot. During this simulation a large phase space file with the energy, position and direction of x rays scattered in the direction of the operator is created. The phase space file is then used as the input radiation source for the following simulation with the operator anatomy model. Particle recycling is employed as a variance reduction technique to maximize the information obtained from the limited number of particles scattered towards the operator. For a typical image acquisition, a patient skin dose map can be displayed at the operator's monitor within 10 seconds with a peak skin dose error below 1%. This work demonstrates that a dose monitoring system based on accurate Monte Carlo simulations can be used to estimate in real-time the average and peak organ doses for both the patient and the staff in interventional fluoroscopy, and provide timely information regarding possible overdoses while the imaging procedure is being performed.
Projection-based dose metric: accuracy testing and applications for CT design
Author(s):
Xiaoyu Tian;
Zhye Yin;
Bruno De Man;
Ehsan Samei
Show Abstract
The purpose of this study was to develop and validate a projection-based dose metric that
enables computationally efficient dose estimation. The two physical quantities determining dose, absorbed
energy and mass, were estimated in projection space. The absorbed energy was estimated using the
difference between the imparted energy and detected energy. The mass was estimated using the area under
the attenuation profile. A series of phantom simulations were conducted to test the metric’s applicability for
multi-material phantoms, different kVp settings, and bowtie filters. Projection-based dose estimates were
benchmarked against results from the Monte Carlo (MC) simulation. The projection-based dose metric
shows a strong linear correlation with MC dose estimates (R2 > 0.96). The prediction errors for projection-based dose metric are below 14%. This study demonstrates a computationally efficient and relatively
accurate dose estimation method based on the projection data. It further suggests the possibility to achieve
real-time and patient-specific dose optimization when applied prior to a CT scan.
Organ dose in chest CT: effect of modulation scheme on estimation accuracy
Author(s):
Xiang Li;
William P. Segars;
Ehsan Samei
Show Abstract
The purpose of this study was to evaluate how different implementations of the tube
current modulation (TCM) technology affect organ dose conversion factors in chest CT
and how organ dose can be accurately estimated for various modulation schemes.
Computational phantom of a normal-weight female patient was used. A method was
developed to generate tube current (mA) modulation profiles based on the attenuation of
the phantom, taking into account the geometry of the CT system as well as the x-ray
energy spectrum and bowtie filtration in a CT scan. The mA for a given projection angle
was calculated as a power-law function of the attenuation along this projection. The
exponent of this function, termed modulation control strength, was varied from 0 to 1 to
emulate the effects of different TCM schemes. Organ dose was estimated for a chest scan
for each modulation scheme and was subsequently normalized by volume-weighted CT
dose index (CTDIvol) to obtain conversion factors. The results showed that the conversion
factors are second-order polynomial functions of the modulation control strength. The
conversion factors established for a fixed-mA scan may be used to estimate organ dose in
a TCM scan. For organs on the periphery of the scan coverage, the best accuracy is
achieved when using CTDIvol computed from the average mA of the entire scan. For
organs inside the scan coverage, the best accuracy is achieved when using CTDIvol
computed from the volume-averaged mA values of all the axial slices containing the
organ.
Task based assessment of a motion compensation algorithm via simulation of a moving stenotic vessel
Author(s):
Brian E. Nett;
Jed D. Pack;
Darin Okerlund
Show Abstract
An analysis of a task based simulation study of coronary artery imaging via computed tomography (CT). Evaluation of standard filtered backprojection (FBP) reconstruction and motion compensated reconstruction of a moving cylindrical vessel that contains a hyper-intense lesion. Multiple conditions are simulated including: varying rest times of the vessel and varying motion orientations. A reference image with no motion was used for all comparisons. The images were segmented and quantitative metrics for accurate segmentation were compared. The motion compensated images have consistent error metrics with respect to the static case for all rest times. The FBP reconstructions were visually inferior for shorter rest times and had significantly inferior metrics. This is the first demonstration of equivalent performance for a given task when the rest times are reduced well below the temporal aperture of the acquisition, using either advanced algorithms or different data acquisition such as multi-source geometries.
Grid artifact reduction based on homomorphic filtering in digital radiography imaging
Author(s):
Dong Sik Kim;
Sanggyun Lee;
Jung Kee Yoon
Show Abstract
In digital radiography imaging, the x-ray images are formed based on a multiplicative model, in which the
projected image of an object is multiplied by the antiscatter grid shadow. Hence, the formed image is amplitude-
modulated by the grid shadow and the resultant modulated terms appear as the grid artifacts. Since the
bandwidths of the modulated terms are as wide as that of the projected image, we should employ relatively
wide-bandwidth band-stop filters (BSFs) to reduce the grid artifacts. When we apply such BSFs, the object to
be recovered is prone to distortion due to the wide filter bandwidth. In this paper, to reduce the signal bandwidth
of the grid shadow images in reduction of the grid artifacts, applying BSFs based on the homomorphic operation
is proposed by employing the logarithmic function. By taking the logarithm of the formed image, we can separate
the multiplicative grid component from both projected image and the exposure of x-rays. Hence, by employing
a relatively narrow and fixed-bandwidth BSFs, we can efficiently alleviate the grid artifacts independently of
the strength of the grid artifacts comparing to the conventional linear approaches. For real x-ray images, the
superior performance of the proposed approach is compared in this paper.
Atlas-based linear volume-of-interest (ABL-VOI) image correction
Author(s):
A. K. Maier;
Z. Jiang;
J. Jordan;
C. Riess;
H. G. Hofmann;
J. Hornegger
Show Abstract
Volume-of-interest imaging offers the ability to image small volumes at a fraction of the dose of a full scan.
Reconstruction methods that do not involve prior knowledge are able to recover almost artifact-free images. Although
the images appear correct, they often suffer from the problem that low-frequency information that would be included in a
full scan is missing. This can often be observed as a scaling error of the reconstructed object densities. As this error is
dependent on the object and the truncation in the respective scan, only algorithms that have the correct information about
the extent of the object are able to reconstruct the density values correctly.
In this paper, we investigate a method to recover the lost low-frequency information. We assume that the correct scaling
can be modeled by a linear transformation of the object densities. In order to determine the correct scaling, we employ an
atlas of correctly scaled volumes. From the atlas and the given reconstruction volume, we extract patch-based features
that are matched against each other. Doing so, we get correspondences between the atlas images and the reconstruction
VOI that allow the estimation of the linear transform.
We investigated several scenarios for the method: In closed condition, we assumed that a prior scan of the patient was
already available. In the open condition test, we excluded the respective patient’s data from the matching process. The
original offset between the full view and the truncated data was 133 HU on average in the six data sets. The average
noise in the reconstructions was 140 HU. In the closed condition, we were able to estimate this scaling up to 9 HU and in
open condition, we still could estimate the offset up to 23 HU.
Design and analysis of a calibration-method for stereo-optical
motion tracking in MRI using a virtual calibration phantom
Author(s):
Martin Hoßbach;
Johannes Gregori;
Stefan Wesarg;
Matthias Günther
Show Abstract
Motion tracking for head motion compensation in MRI has been a research topic for several years. However,
literature is not giving much attention to the calibration of such setups. We present a method to calibrate the
coordinate systems of a stereo-optical camera setup mounted to the MRI head coil. Though using a simple setup
and visible instead of infrared light for tracking, it is possible to achieve a sub-millimeter tracking precision.
Blue water-filled spheres are positioned throughout the whole MRI imaging volume and detected in images
of the tracking cameras as well as MRI scans. In order to register the coordinate systems of both camera system
and MRI scanner, a heuristic-enhanced brute-force approach is used to match detected spheres in the different
images. Then, a rigid transformation is calculated and applied to the cameras' external parameters to align the
coordinate systems.
The precision of our setup was evaluated using leave-one-out cross validation both for the camera calibration
and the scanner coordinate system registration. We found that the cameras' locations and orientations are
correct within 0:03mm and 0:03°, using a number of 45 spheres. Evaluation of the MRI coordinate system
registration showed an average reprojection error of 1:1 mm.
Influence of a feature point jitter of 0:5 px is 0:03mm for a point close to the cameras and 0:3mm for a point
close to the back of the patient's head. Tracked poses are correct within 0:17mm and 0:001.°
Truncation correction for VOI C-arm CT using scattered radiation
Author(s):
Bastian Bier;
Andreas Maier;
Hannes G. Hofmann;
Chris Schwemmer;
Yan Xia;
Tobias Struffert;
Joachim Hornegger
Show Abstract
In C-arm computed tomography, patient dose reduction by volume-of-interest (VOI) imaging is of increasing
interest for many clinical applications. A remaining limitation of VOI imaging is the truncation artifact when
reconstructing a 3D volume. It can either be cupping towards the boundaries of the field-of-view (FOV) or an
incorrect offset in the Hounsfield values of the reconstructed voxels.
In this paper, we present a new method for correction of truncation artifacts in a collimated scan. When
axial or lateral collimation are applied, scattered radiation still reaches the detector and is recorded outside of
the FOV. If the full area of the detector is read out we can use this scattered signal to estimate the truncated
part of the object. We apply three processing steps: detection of the collimator edge, adjustment of the area
outside the FOV, and interpolation of the collimator edge.
Compared to heuristic truncation correction methods we were able to reconstruct high contrast structures
like bones outside of the FOV. Inside the FOV we achieved similar reconstruction results as with water cylinder
truncation correction. These preliminary results indicate that scattered radiation outside the FOV can be used
to improve image quality and further research in this direction seems beneficial.
A papillary muscle guided motion estimation method for gated cardiac imaging
Author(s):
Jizhe Wang;
George S. K. Fung;
Tao Feng;
Benjamin M. W. Tsui
Show Abstract
This research aims to develop a new feature guided motion estimation method for the left ventricular wall in gated
cardiac imaging. The guiding feature is the “footprint” of one of the papillary muscles, which is the attachment of the
papillary muscle on the endocardium. Myocardial perfusion (MP) PET images simulated from the 4-D XCAT phantom,
which features papillary muscles, realistic cardiac motion with known motion vector field (MVF), were employed in the
study. The 4-D MVF of the heart model of the XCAT phantom was used as a reference. For each MP PET image, the 3-
D “footprint” surface of one of the papillary muscles was extracted and its centroid was calculated. The motion of the
centroid of the “footprint” throughout a cardiac cycle was tracked and analyzed in 4-D. This motion was extrapolated to
throughout the entire heart to build a papillary muscle guided initial estimation of the 4-D cardiac MVF. A previous
motion estimation algorithm was applied to the simulated gated myocardial PET images to estimate the MVF. Three
different initial MVF estimates were used in the estimation, including zero initial (0-initial), the papillary muscle guided
initial (P-initial), and the true MVF from phantom (T-initial). Qualitative and quantitative comparison between the
estimated MVFs and the true MVF showed the P-initial provided more accurate motion estimation in longitudinal
motion than the 0-initial with over 70% improvement and comparable accuracy with that of the T-initial. We concluded
that when the footprint can be tracked accurately, this feature guided approach will significantly improve the accuracy
and robustness of traditional optical flow based motion estimation method.
Noise reduction of low-dose computed tomography using the multi-resolution total variation minimization algorithm
Author(s):
Cheng-Ting Shih;
Shu-Jun Chang;
Yan-lin Liu;
Jay Wu
Show Abstract
Computed tomography (CT) has become a popular tool in radiologic diagnosis due to the ability of obtaining highresolution
anatomical images. However, radiation doses to patients are substantial and can increase the risk of cancer
incidence. Although lowering the tube current is a direct way to reduce absorbed doses, insufficient photon numbers can
cause severe quantum mottle and subsequently degrade the diagnostic value of CT images. In this study, we proposed an
algorithm for noise reduction of low-dose computed tomography (LDCT) based on the multiresolution total variation
minimization (MRTV) method. The discrete wavelet transform was used to decompose the CT image into high- and lowfrequency
wavelet coefficients. The total variation minimization with suitable tuning parameters was then applied to
reduce the variance among the wavelet coefficients. The noise-reduced image was reconstructed by the inverse wavelet
transform. The results of the Shepp-Logan phantom added with Gaussian white noise showed that the noise was
eliminated effectively and the SNR in the three compartments was increased from 2.04, 20.69 and 0.09 to 19.45, 187.77
and 0.27, respectively. In the CT image of the water phantom acquired with 50-mAs tube currents, the MRTV improved
the smoothness of the water compartment. The average SNR was increased from 0.14 to 0.98, which is even better than
the CT image acquired by 200 mAs. In the clinical head CT image with a tube current of 9.12 mAs, the MRTV
successfully removed the severe noise in the parenchyma, and SNR was increased from 0.982 to 3.452 in average. In
addition, the details of the septal structure of the sinus cavity were maintained. We conclude that the MRTV approach
can effectively reduce the image noise caused by the tube current insufficiency, and thereby could improve the
diagnostic value of LDCT images.
Monte Carlo modeling of field angle-dependent spectra for x-ray imaging systems
Author(s):
Edward B. Gindele
Show Abstract
The photon spectrum for X-ray capture systems is a function of the emission field angle. Spectrum variability is the
most pronounced for cone-beam computed tomography (CBCT) systems with wide field angles operating close to the
anode angle limit. Filtration devices also contribute to the change in the photon spectrum with an emission field angle,
especially for variable-thickness filters, e.g., bow-tie filters. The change in the photon spectrum is primarily due to the
distance traversed through the anode and filtration materials with emission field angles. Although Monte Carlo X-ray
simulations can include the materials and geometries for these source assembly elements, the computational
requirements are considered prohibitive. As a consequence, most X-ray Monte Carlo simulation implementations ignore
emission field angle spectral effects. Our approach uses a probabilistic rejection scheme to model the emission field
angle spectral effects within the context of a Monte Carlo simulation tool. A bounding spectrum is constructed that
supersedes all possible spectrums, i.e., for all emission field angles. Photons are generated with the bounding spectrum
and rejected or accepted based on the probability of transmission through the cascade of anode and filtration materials
relative to a pre-calculated maximum probability of transmission. The resultant photon spectrum properly models the
intensity and spectral shape of the emitted photons as a function of the emission field angle. The modeling accuracy
improvement over the constant spectrum approximation was calculated for a CBCT system for anode voltages ranging
from 50 Kvp to 110 Kvp. The maximum improvement in predicted primary and scatter signals was approximately 5%
for a system configuration employing a simple filtration and 25% for a CBCT system employing a bow-tie filter with
less than a 10% additional computation cost.
Fast iterative beam hardening correction based on frequency splitting in computed tomography
Author(s):
Qiao Yang;
Matthias Elter;
Ingo Schasiepen;
Nicole Maass;
Joachim Hornegger
Show Abstract
In computed tomography (CT), the nonlinear characteristics of beam hardening are due to the polychromaticity of X-rays, which severely degrade the CT image quality and diagnostic accuracy. The correction of beam hardening has been an active area since the early years of CT, and various techniques have been developed. State of-the-art works on multi-material beam hardening correction (BHC) are mainly based on segmenting datasets into different materials, and correcting the non-linearity iteratively. Those techniques are limited in correction effectiveness due to inaccurate segmentation. Furthermore, most of them are computationally intensive. In this study, we introduce a fast BHC scheme based on frequency splitting with the fact that beam hardening artifacts mainly contain in the low frequency components and take more iterations to be corrected in comparison with high frequency components. After low-pass filtering and correcting artifacts at down-sampled projections, an artifact reduced high resolution reconstruction will be obtained by incorporating the original edge information from the high frequency components. Evaluations in terms of correction accuracy and computational efficiency are performed using simulated and real CT datasets. In comparison to the BHC algorithm without frequency splitting, the proposed accelerated algorithm yields comparable results in correcting cupping and streak artifacts with tremendously reduced computational effort. We conclude that the presented framework can achieve a significant speedup while still obtaining excellent artifact reduction. This is a significant practical advantage for clinical as well as industrial CT.
Removing intra plane blurring in dental panoramas
Author(s):
Christian Hofmann;
Michael Knaup;
Marc Kachelrieß
Show Abstract
Dental imaging often requires to gather information from a curved plane that covers the upper and lower jaw. To acquire these data one may either use a CT or a DVT scan followed by extracting the desired curved plane from the volumetric CT data, or one may decide to work at much lower dose levels and acquire a panoramic radiograph, which is also known as an orthopantomogram, or short as panorama.
The panorama is acquired by moving the x-ray source and detector arrangement such that the x-ray cone intersects the curved plane in a preferably perpendicular way. Due to the small size of specialized panorama x-ray detectors, that often run in the so-called time-delayed integration (TDI) mode, the cone is collimated such that it is only a few millimeters wide in the fan direction. In this situation the fan angle is much smaller than the cone angle.
Assuming an imaging system based on flat detectors the panoramic imaging corresponds to digital x-ray tomosynthesis taken in a curved plane. Of course, panoramic imaging suffers from significant blurring between adjacent planes, as it is inherent to all tomosynthesis techniques. To reduce this intra plane blurring we propose an approach that uses a form filter to simultaneously acquire the data for a typical orthopantomogram and low-dose data with a significantly increased fan-angle, sufficient to perform a low-dose FDK reconstruction. The bow-tie takes care to reduce dose in that extended region to about 1% compared to the dose in the central region. The total dose remains constant. These data will be combined resulting in an orthopantomogram with improved image quality due to reduced intra plane blurring as one is able to subtract the unwanted background structures arising from off focus objects as for example the cervical vertebrae. We conducted simulations to validate our approach. For that we use a volumetric CT data set of a patient's head from which we generated rawdata.
The simulation results show that with our approach panoramic imaging with a flat detector offers the possibility to improve image quality without additional costs in patient dose.
Cascaded-systems analyses of the DQE of double-Z x-ray detectors including photoelectric, coherent and incoherent interactions
Author(s):
Seungman Yun;
Jesse Tanguay;
Ho Kyung Kim;
Ian A. Cunningham
Show Abstract
Image quality in diagnostic x-ray detectors is limited by statistical properties governing how, and where, x-ray
energy is deposited in a detector. This in turn depends on the physics of underlying x-ray interactions, and
the development of theoretical models of x-ray interaction physics is therefore a critical step in optimal detector
design and assessment. While cascaded-systems analyses are often used to describe image signal and noise in many systems, it has always been assumed there is only a single element (single Z) with which all x rays interact
even though most commonly used and promising candidates are compound materials. In addition, coherent and
incoherent scattering and their effects on image quality are usually ignored but may be important in some situa-
tion such as in low-Z atoms with high x-ray energies. We present a theoretical model of energy deposition within
a double-Z x-ray detector material that addresses the nature of energy absorption following photoelectric and
incoherent interactions and the effects of coherent scatter prior to energy deposition by photoelectric interactions.
A cascaded systems approach is used to describe the transfer of signal and noise in terms of the modulation
transfer function (MTF), Wiener noise power spectrum (NPS), and detective quantum efficiency (DQE). The
model is validated by comparing Monte Carlo simulation results with CsI and PbI2 double-Z materials. Excellent
agreement is obtained for each metric over the entire diagnostic energy range up to 10 cycles/mm. It is shown
that in all cases tested, a combination of two single-Z models weighted by the atomic density of each atom type
gives equivalent results to the more comprehensive double-Z model within a few percent. This result suggests
the simpler model is adequate and may be preferred for the optimal design of conventional radiography detectors and the estimation of x-ray imaging performance of novel photoconductor materials.
Hybrid EID algorithm for PCD/EID-CT systems
Author(s):
Katsuyuki Taguchi;
George S. K. Fung;
Qiulin Tang;
Jochen Cammin
Show Abstract
One of the major obstacles toward photon counting detector (PCD)-based clinical x-ray CT systems is the large count
rates, because when operated under too intense x-rays, pulse pileup effects (PPEs) due to coincident photons distort the
spectrum recorded by PCDs. In this paper we discuss a strategy using a hybrid detector, which consists of PCDs for the
central part of the detector [which corresponds to a central small field-of-view (FOV) of the object] and energy
integrating detectors (EIDs) for the peripheral part, to achieve the following three goals: 1) to minimize the PPEs; 2) to
produce accurate spectral images for the small FOV; and 3) to provide conventional CT images for the entire FOV. The
third goal requires a solution to exterior problem, because the central part of EID data is missing. The spectral data
obtained by PCDs carry richer information than the intensity data obtained by EIDs; however, performing a simple
weighted summation of counts from multi-energy windows of PCD would not produce realistic EID data, as the
spectrum recorded by PCD could be skewed by spectral response effects (SREs) and PPEs. We propose a unique
approach for the hybrid PCD/EID-CT system in this paper.
Cardiac deformation indices derived from motion estimated x-ray computed tomography
Author(s):
Liwei Jiang;
Qiulin Tang;
Katsuyuki Taguchi
Show Abstract
Cardiac deformation indices such as strain, strain rate, and time to maximal strain are valuable in early detection of heart
disease and hold tremendous prognostic value in assessing patients recovering from heart failure. These indices have
been previously measured in modalities such as Doppler echocardiography and magnetic resonance imaging. However,
cardiac deformation has not been well characterized in X-ray computed tomography (CT), a modality whose balance of
high acquisition speed and good temporal and spatial resolution makes it highly desirable in the clinical setting. The
current work, an extension of our group’s previously published cardiac motion estimation algorithm, calculates
deformation indices such as strain, strain rate, maximum strain, and time to maximal strain from four-dimensional
motion vector field data. Along each dimension, calculations are made between every adjacent pair of grid points, thus
striving for simplicity while yielding an increase in spatial resolution over approaches that divide the heart into a number
of segments. Results are visualized as semi-transparent color maps superimposed on CT image slices in the short-axis
view and the two long-axis views. Results agree with the expected behavior of myocardial contraction. Animal studies
are underway to better assess the physiologic accuracy of the calculated deformation indices as well as to compare the
results with those from other imaging modalities. The present work and its further refinements may yield rich yet easily
accessible information for clinicians in early diagnosis and follow-up monitoring.
Metal artifact reduction based on beam hardening correction and statistical iterative reconstruction for X-ray computed tomography
Author(s):
Yanbo Zhang;
Xuanqin Mou
Show Abstract
Metal artifact is a main cause to degrade CT image quality, but there is still no standard solution to this issue. The cause of introduction of metal artifacts is due to several physical effects, in which beam hardening and noise are two major factors. Accordingly, in this paper these two factors are alleviated by using beam hardening correction based on polynomial fitting and statistical iterative reconstruction based on Poisson log-likelihood approach. Unlike other metal artifact reduction (MAR) methods by using iterative image reconstruction from polychromatic projection dataset, the proposed method in this work does not require a priori knowledge about the X- ray spectrum and attenuations of the materials to be reconstructed. A conventional linear interpolation MAR algorithm and two MAR methods based on beam hardening correction are performed for comparison. Simulation results illustrate that the proposed method can suppress metal artifacts greatly and restore low contrast tissues well.
A model-based volume restoration approach for Monte Carlo scatter correction in image reconstruction of cone beam CT with limited field of view
Author(s):
Guozhi Zhang;
Reinhilde Jacobs;
Hilde Bosmans
Show Abstract
Scatter remains a major cause of image artifacts in cone beam CT (CBCT). To correct the scatter for improved image reconstruction, the Monte Carlo simulation technique has been useful in estimating the scatter distribution. This technique, however, requires a 3D computational phantom of the patient, which is typically obtained from the reconstructed 3D image and is not fully available for CBCT scans with limited field of view. This study proposes a novel approach to restore the volume of the patient in such cases by use of a standard patient model and image registration techniques. As demonstrated for the 6 x 6 em oral CBCT scan, a full-field image of the model could be registered to the truncated patient image and was then successfully imported to Monte Carlo simulation for scatter estimation. Scatter correction was achieved by subtracting the Monte Carlo scatter data from the raw data on a projection-by-projection basis. Compared to the original data, the reconstructed image with scatter correction showed reduced streak artifacts and an improved contrast resolution of up to 15% between the soft and bony tissue. This approach exploits the low-frequency characteristic of the scatter distribution. The procedure could use more suitable models, and it could in a next step be further automated. It would then be an interesting candidate to improve image quality in practice.
A method to characterize the radiation output from a cone beam
O-arm using a device for dose and dose profile scanning measurement
Author(s):
Lars Herrnsdorf;
Marcus Söderberg
Show Abstract
The O-arm system is a mobile intraoperative imaging system that is comprised of fluoroscopy and cone beam CT. The
configuration of the O-arm system with absence of patient table and a broad beam width (165 mm in isocenter) brings
new practical and physical requirements on how to perform dose measurements. The purpose of this study was to
describe a method that overcomes this and makes it possible to characterize the radiation output from the O-arm system.
A holder with a clamp and a flexible ball joint that can orientate the radiation detector support and the Mover that can be
adjusted to hold the dose detector in a horizontal position was used. Evaluation of the dose response for three different
dose detectors of different active length (0.3, 23.1 and 100 mm) was made for three different beam qualities.
Furthermore the dose profile free in air to control the possible heel effect and width of the x-ray field during rotation was
measured and the dose rate waveform was analyzed. The FWHM of the dose profile was 162 mm. The dose response of
the three detectors is reported. The average dose response was lower for the detector with longer active length due to the
influence of the dose profile shape. From dynamic measurement total exposure time, pulse width, and the number of
pulses were verified. In conclusion, an external horizontal hanging holder with mover option helps to assist to make dose
measurement easier and enables characterize the radiation output from the O-arm system.
Volume of interest CT implemented with a dynamic bowtie filter
Author(s):
Timothy P. Szczykutowicz;
Charles Mistretta
Show Abstract
Often in clinical CT imaging situations, a clinician only requires a small volume of interest (VOI) within aCT slice to be imaged. Unfortunately, due to some fundamental constraints in CT image reconstruction, the entire slice must be irradiated even when a small VOI is all that is clinically required. This produces excess dose for the patient, excess scatter which degrades image quality and increases clinician dose, and may effect patient care because clinicians may limit their use of CT in the interventional setting due to the previous two factors. This paper shows experimental results in which a digital beam attenuator (DBA) is used to modulate the patient dose as a function of view and gantry angle to enhance the SNR in a small VOI relative to the surrounding areas. It was found that the noise was lower by 50% inside the VOI for DBA enabled VOL This promising result was obtained while providing an incident fluence reduction of 13 times on average for all regions not corresponding to the VOI relative to a non-VOI scan.
Radiation dose reduction and CNR enhancement in C-arm cone beam CT
Author(s):
Kai Niu;
Jie Tang;
Kevin Royalty;
Orhan Ozkan;
Charles Strother;
Beverly Aagaard-Kienitz;
Kari A. Pulfer;
Guang-Hong Chen
Show Abstract
In this work we applied dose reduction using the prior image constrained compress sensing (DR-PICCS) method
on a C-arm cone beam CT system. DR-PICCS uses a smoothed image as the prior image. After applying DRPICCS,
the final image will have noise variance inherited from the prior image and spatial resolution from the
projection data. In order to investigate the dose reduction of DR-PICCS, three different dose levels were used in
C-arm scans of animal subjects using a Siemens Zeego C-arm system under an IACUC protocol. Image volumes
were reconstructed using the standard FBP and DR-PICCS algorithms(total of 160 images). These images were
randomly mixed and presented to three experienced interventional radiologists(each having more than twenty
years reading experience) to review and score using a five-point scale. After statistical significance testing, the
results show that DR-PICCS can achieve more than 60% dose reduction while keeping the same image quality.
And if we compare FBP and DR-PICCS at the same dose level the results show that DR-PICCS will generate
higher quality images.
Motion detection in cone-beam computed tomography incorporating a geometric calibration approach
Author(s):
Rizza D. Pua;
Boyeol Yoo;
Chang Hwan Kim;
Seungryong Cho
Show Abstract
This work proposed a motion detection method for cone-beam computed tomography (CBCT) that utilizes a calibration
phantom of known geometry as the motion detector and an established geometric calibration protocol to provide the
motion information. An initial numerical study regarding the consequences of motion and its correction was conducted
with a Shepp-Logan and an XCAT phantom. Motion artifacts were induced by acquiring the projections in a simple
saddle trajectory scan. Since the scanning trajectory is set, the magnitude of motion for each projection view is already
known, the correction of motion can then be efficiently implemented. Motion correction was done prior to the
backprojection process of the filtered backprojection (FBP) image reconstruction algorithm. Results showed that motion
correction improved the image quality of the reconstructed images. For a known or unknown scanning trajectory, the
geometric calibration method can define the geometric information of a scanning system. In the current work,
projections of a calibration phantom of known geometry were acquired from a saddle trajectory scan, and geometric
parameters for selected projection views were successfully computed from the projection matrix provided by the
geometric calibration method. Further studies will involve an experimental investigation wherein a calibration phantom
is attached to a randomly moving object and scanned in a circular trajectory. Utilizing the parameters extracted from the
geometric calibration, an accurate description of the object motion can be used and adapted for motion correction.
Infinite impulse response filtering for cone beam tomography
Author(s):
Karl Barth;
Frank Dennerlein;
Thomas Brunner;
Andreas Fieselmann;
Rainer Graumann
Show Abstract
In computed tomography (CT) or conebeam tomography (CBT), filtered backprojection (FBP) has been known as an
efficient technique for reconstructing 3D-volumes from acquired projection data. Plain backprojection only would
result in systematically blurred objects. To compensate for this blurring, convolution-based filters have been derived that
are non-local and sampled with 2048 coefficients or more, dependent on the projection data size. This filtering operation
can be classified as finite impulse response (FIR) filtering. In terms of image quality, ideally-derived kernels sometimes
amplify too much the high frequency noise (e.g. due to X-ray quantum noise) from the input projections. In practice,
regularized filters are often preferred, damping higher frequencies while preserving the sharpness and signal dynamics
needed for the reconstructed 3D-objects. From discrete systems theory, another filter type with infinite impulse response
(IIR) has been known. Because such a filter recursively uses backward components, it requires very few coefficients
while the long-range filter effect is preserved. In the presented work, IIR filters have systematically been designed and
tested. They have been adjusted for the correction of a blurring system transfer function as well as for high-frequency
noise suppression. Image quality has carefully been inspected by reconstruction of phantom data and clinical cases. It has
been found that the filtering step in CBT/FBP can be realized as a recursive filter only, i.e. in self-contained IIRnotation,
including adaptions like e.g. apodisation. The number of filtering operations is significantly reduced hereby. So
with IIR filtering an efficient alternative for FBP filtering is available.
ML reconstruction of cone-beam projections acquired by a flat-panel rotational X-ray device
Author(s):
Tim Pfeiffer;
Robert Frysch;
Sebastian Gugel;
Georg Rose
Show Abstract
The steadily growing computational power of modern hardware allows use of more sophisticated reconstruction
methods. We present an implementation of the maximum likelihood (ML) method, a previously studied method,
for the case of a flat-panel rotational X-ray device. Contrary to the related principle of algebraic reconstruction
(ART), the ML method takes into consideration the physical properties of X-radiation, especially the corpuscular
character and the associated Poisson distribution of the measured number of photons. The basic principle is the
maximization of the joint probability of all measured projections with respect to the attenuation coefficients of
all voxels of the object. The application of the ML optimization procedure finally generates an iterative scheme
for the update of the attenuation coefficients. For this, in each step an accurate estimation of the forward
projections (FP) is mandatory. We use an approximate calculation of the footprints of single voxels based on
separable trapezoids. The resulting enormous computational effort is handled by an efficient implementation
on GPGPU (General-purpose computing on graphics processing units). As a first look, using data from 133
projections of a sheep head acquired by means of a flat-panel rotational angiography system, we compare the
reconstruction by the ML-based method with the gold standard - the Feldkamp filtered back projection (FBP)
procedure. The results reveal a clearly reduced amount of streak artifacts as well as less blurring in the statistical
reconstruction method.
A new approach for prospectively gated cardiac rotational angiography
Author(s):
Stijn De Buck;
Dieter Dauwe;
Jean-Yves Wielandts;
Piet Claus;
Stefan Janssens;
Hein Heidbuchel;
Dieter Nuyens
Show Abstract
Cardiac rotational angiography (RA) is well suited for 3-D cardiac imaging during catheter based interventions
but remained limited to static images or was characterized by high dose patient radiation dose. We present a
new prospective imaging technique that is capable of imaging the dynamics of the cardiac cavities in a single
C-arm run during the intervention with a relatively low dose.
By combining slow atrial pacing to obtain a stable heart rhythm and a single C-arm rotation with imaging
at a regular imaging interval, a prospective 4DRA is established. Pacing interval and imaging framerate can be
adapted such that a single cardiac phase is imaged multiple times and a motion free state is imaged from different
equiangular positions. A practical implementation of this technique was realized in which the cardiac cavities
are imaged while pacing at 105 bpm (574 msec) and imaging at approximately 15 fps. A number of animal
experiments were conducted in which the technique was applied and MR imaging was performed subsequently.
Quantitative comparison was made by manual contouring of the left ventricle in the RA and MR images of both
end-systolic and end-diastolic phases.
Reconstructed images of the individual cardiac phases showed all four chambers and important vessels in
spite of substantial image noise. 4DRA and MR absolute surface distance errors amounted to 2:8 ± 0:7 mm,
which is acceptable. Further, no systematic difference could be identified. Finally, it is expected that the effective
dose of a clinical protocol with 381 images will be lower than the current retrospective gated RA protocols.
Simulation study of cone beam CT for visualizing cell clusters in breast biopsies
Author(s):
C. Laamanen;
R. J. LeClair
Show Abstract
The feasibility of cone beam CT (CBCT) for differentiating normal epithelium from invasive carcinoma is investigated
via a simulation study. The phantom consisted of a 5 mm long 5mm diameter cylinder of a 50:50
mixture of fibrous and fatty tissue. Normal epithelium and invasive carcinoma were each modeled as epithelium
and connective tissue compartments with respective cross-sectional dimensions of 158 by 161 μm and 131 by
161 μm . For normal epithelium, 125 cells were placed in the compartments with a higher concentration in the
basal layer. For the invasive carcinoma, 314 cells were spread out sporadically. Cells were modeled as 5.67 μm
diameter spheres. The attenuation coefficients used in the simulation were those of fat for epithelium, 80:20
mixture of fibrous and fat for the connective tissue and water for cells. A point source and 50 μm detector
pixels were assumed. Scatter from the phantom is negligible and was neglected. Three hundred projections were
acquired at magnification 10 in vacuum using a 26 kV spectrum. The preliminary study suggest a potential
application of CBCT for visualizing cell clusters. The contrast was slightly improved by using a 5 to 10 keV
uniform spectrum.
Single-scan energy-selective imaging on cone-beam CT: a preliminary study
Author(s):
Xue Dong;
Tianye Niu;
Lei Zhu
Show Abstract
On an onboard cone-beam CT (CBCT) system, the poly-energetic beam generated by current commercial x-ray
tubes hardens as it penetrates the object. The beam-hardening effect results in CT image artifacts of up to 100 HU,
especially around dense objects (e.g. bones). We propose to insert a half-blocked beam filter between x-ray source
and object, such that any ray passing through the object is filtered once from one of the opposite directions in a
single full scan. The conventional data processing of dual energy imaging is then applied to obtain material
decomposition as well as CT images with no beam-hardening artifacts. In this preliminary study, we demonstrate
the method feasibility using computer simulations. The beam filter thickness is optimized to balance the CT
number accuracy and the noise level on the reconstructed images via both analytical calculation and measurement
of image noise. A calibration phantom is designed to obtain the decomposition function for energy selective
imaging by projection measurements without the knowledge of x-ray spectrum and detector response. Using the
optimized beam blocker and the calibration phantom, we simulate the data acquisition using scan settings of a
commercial CBCT system. Our proposed approach significantly suppresses beam-hardening artifacts and reduces
the image error from 65 HU to 9 HU in the selected regions of interest on a head phantom. The method also
successfully decomposes bone and soft tissue, with 95% accuracy.
An integrated x-ray/optical tomography system for pre-clinical radiation research
Author(s):
S. Eslami;
Y. Yang;
J. Wong;
M. S. Patterson;
I. Iordachita
Show Abstract
The current Small Animal Radiation Research Platform (SARRP) is poor for localizing small soft tissue targets for
irradiation or tumor models growing in a soft tissue environment. Therefore, an imaging method complementary to x-ray CT is required to localize the soft tissue target’s Center of Mass (CoM) to within 1 mm. In this paper, we report the
development of an integrated x-ray/bioluminescence imaging/tomography (BLI/BLT) system to provide a pre-clinical,
high resolution irradiation system. This system can be used to study radiation effects in small animals under the conebeam computed tomography (CBCT) imaging guidance by adding the bioluminescence imaging (BLI) system as a
standalone system which can also be docked onto the SARRP. The proposed system integrates two robotic rotating
stages and an x-ray source rated at maximum 130 kVp and having a small variable focal spot. A high performance and
low noise CCD camera mounted in a light-tight housing along with an optical filter assembly is used for multiwavelength BL imaging and tomography. A three-mirror arrangement is implemented to eliminate the need of rotating the CCD camera for acquiring multiple views. The mirror system is attached to a motorized stage to capture images in angles between 0-90o (for the standalone system). Camera and CBCT calibration are accomplished.
Image reconstruction of arc cone-beam CT with reprojection: a preliminary study
Author(s):
Shih-Chung B. Lo;
Matthew T. Freedman
Show Abstract
A reprojection reconstruction is proposed for limited angle cone-beam CT. This approach can be used as an
improved method for any non-reprojection method such as total vibration (TV) minimization based and/or
projection onto convex sets (POCS) methods in cone-beam CT reconstruction. Since the reprojection and
backprojection are mutually inversed operations, the reprojection as line integral can be implemented with
backprojector currently used in most CT systems. In this study, we focused on the effect of reprojection
reconstruction with a small arc cone-beam CT scan. A human head phantom and a fiber glass plate were
used for the study with 15 projections spanning 45° of the scan. The results showed that the ringing artifacts
and edge unsharpness are greatly improved by the reprojection reconstruction method. We also found that an
arc scan covering larger projection areas of the object would result in a much greater reconstruction result
than a scan covering smaller projection areas with and without reprojection reconstruction method.
Evaluation of adaptation strengths of CARE Dose 4D in pediatric CT
Author(s):
Marcus Söderberg;
Sonny La
Show Abstract
The motivation of this study is the general lack of knowledge regarding the efficiency and the appropriate use of the
adaptation strengths of Siemens automatic exposure control system CARE Dose 4D. The purpose was to evaluate the
effect on radiation absorbed dose using different adaptation strengths of CARE Dose 4D in three routine pediatric CT
protocols. A pediatric anthropomorphic whole body phantom was used to simulate a 4 year old patient. CT scans were
performed with a Siemens SOMATOM Definition Flash using three different pediatric protocols: neck, thorax, and
abdomen. The characteristic of the tube current modulation was similar for all adaptation strengths. The difference is the
extent of decrease in tube current. The degree of dose reduction using CARE Dose 4D and CARE kV compared using a
fix effective mAs was 34-57%, 51-88%, and 56-91% for neck, thorax, and abdomen protocol, respectively. Accordingly,
there is a large difference in radiation dose dependent on the adaptation strength: a factor of 1.5, 4.5, and 4.6 for neck,
thorax, and abdomen protocol, respectively. The adaptation strengths can be used to obtain user-specified modifications
of image quality or radiation dose to the patient. Radiologists and medical physicists need to be aware of the large
differences between the adaptation strengths, and such differences are useful when attempting strategies to optimize CT
radiation dose.
Alternative noise map estimation methods for CT images
Author(s):
Daxin Shi
Show Abstract
In this work, we propose three alternative methods to estimate noise map for CT images. Our methods are
generalizations to the existing even-and-odd views approach proposed by Hsieh and Thibault [1]. Method one in this
work estimates noise map from images reconstructed from three sets of independent views. Method two deals with
images reconstructed by using two sets of correlated views. Our method three generates the noise map from two images
reconstructed from two sets of independent views while the number of views in each set is unequal. Physical phantom
data were employed to validate our proposed noise map estimation methods. In comparison to the existing method, our
alternative methods yield reasonably accurate noise map estimation.
FPGA-based forward and back-projection operators for tomographic reconstruction
Author(s):
Kyungchan Jin;
Sangyup Song
Show Abstract
Artifacts in computerized tomography (CT), such as metal streak effects, can also be reduced using the iterative
reconstruction approach. The main issue in iterative method is the computation cost by efficient implementation of the
forward and back-projection operations, which are the dominant cost in all iterative reconstruction algorithms. We
designed a field programmable gate array (FPGA)-based operators for iterative forward and back-projection method to
solve the artifact model. The projection method in CT reconstruction is to retrieve the volumetric image based on
observed projection image and makes reconstruction errors minimize. The FPGA-based operators contain only the
iterative reconstruction operations from tomographic projections, and the filtering of detector data and the geometry
correction between detector and object are done by host CPU processor. For FPGA design, we used Impulse C package,
C-to-FPGA tool including the use of streaming and pipelining for high performance. We evaluated the FPGA-based
projection on Shepp-Logan phantom data with metal streak artifact. Simulation results show that the FPGA-based
operators can reduce the computation time of iterative reconstruction, while still providing accuracy comparable to CPU
or GPU-based reconstruction.
Modelling and simulation of a respiratory motion monitor using a continuous wave Doppler radar in near field
Author(s):
Florian Pfanner;
Thomas Allmendinger;
Thomas Flohr;
Marc Kachelrieß
Show Abstract
To avoid motion artifacts in medical imaging or to minimize exposure of healthy tissues in radiation therapy
medical devices are often synchronized with the patient’s respiration. Today’s respiratory motion monitors require
additional effort in preparing the patient, such as mounting of a motion belt or the placement of an optical reflector
on the patient breast, and they are not able to measure internal organ motion without implanting markers. An
interesting alternative to assess the person’s respiratory motion is a continuous wave Doppler radar. By placing
the antennas close to the body, the radar waves propagate into the body and are reflected on boundaries between
body tissues, for example between muscle and adipose tissue or on the outline of organs.
To evaluate the radar system, a macroscopic simulation model is created to study the radar measurement
process of human beings. To check the theoretical considerations of the model, measurements performed by a
robot are used. Simulation of human respiratory motion is done by using computed tomography (CT) datasets,
reconstructed at different respiratory phases.
Feasibility study on multiple fan-beam data acquisition for low-dose helical CT
Author(s):
Taewon Lee;
Miran Park;
Yunjeong Lee;
Insoo Kim;
Bumsoo Han;
Seungryong Cho
Show Abstract
In computed tomography (CT) imaging, radiation dose delivered to the patient is one of the major concerns. Many CT
developers and researchers have been making efforts to reduce radiation dose. Sparse-view CT takes projections at
sparser view-angles and provides a viable option to reducing radiation dose. However, a fast power switching of an x-ray
tube, which is needed for the sparse-view sampling, can be challenging in many CT systems. We have recently proposed
a novel alternative approach to sparse-view circular CT that can be readily incorporated in the existing CT systems.
Instead of switching the x-ray tube power, we proposed to use a multi-slit collimator placed between the x-ray source
and the patient to partially block the x-ray beam thereby reducing the radiation. In this study, we performed a simulation
study based on numerically acquired projection data to demonstrate a feasibility of using a multi-slit collimator in a
helical CT. The XCAT phantom was used and a numerical collimator has been made to apply on the projection data.
Numerical multi-slit collimator was designed to have equal size of slit-openings and radio-opaque rectangular areas, and
the length dimension of the slits is perpendicular to the rotation axis. For image reconstruction, we used a total-variation
minimization (TV) algorithm which has shown its out-performance in many sparse-view CT applications. We
demonstrated that the proposed multiple fan-beam helical CT can provide a useful low-dose scanning option.
Statistical CT noise reduction with multi-scale decomposition and penalized weighted least square for incomplete projection data
Author(s):
Shaojie Tang;
Xiangyang Tang
Show Abstract
Tremendous efforts have been devoted to decreasing x-ray radiation dose in diagnostic CT while maintaining
the image quality. The statistical noise reduction with iterative algorithm in the projection domain has been one of the
major research subjects in CT technologies. Previously, we have proposed a statistical noise reduction with multi-scale
decomposition and penalized weighted least square (PWLS) in the projection domain, in which the Markov Random
Field (MRF) penalty function is incorporated. In this work, by taking the variation or irregularity of sampling interval
along each dimension of the projection domain, we extend our previous method to deal with the situations of incomplete
projection data, covering sparse view sampling, latitudinal data truncation and photon starvation. Using the computersimulated
projection data of a performance phantom and the FORBILD thorax phantom, we evaluate and verify the
performance of the proposed method.
Bregman regularized statistical image reconstruction method and application to prior image constrained
compressed sensing (PICCS)
Author(s):
Yinsheng Li;
Pascal Theriault Lauzier;
Jie Tang;
Guang-Hong Chen
Show Abstract
Recently, the Statistical Image Reconstruction (SIR) and compressed sensing (CS) framework has shown promise
in the x-ray computed tomography (CT) community. In this paper, we propose to establish an equivalence
between the unconstrained optimization problem and a constrained optimization with explicit data consistency
term. The immediate consequence of the equivalence is to enable one to use the well-developed optimization
method to solve the constrained optimization problem to refine the solution of the corresponding unconstrained
optimization problem. As an application of this equivalence, the method was used to develop a convergent and
numerically efficient implementation for the prior image constrained compressed sensing (PICCS).
A new padding scheme for local tomograpy in tomographic microscopy
Author(s):
Yongsheng Pan;
Francesco De Carlo
Show Abstract
Tomographic microscopy using synchrotron radiation provides high-resolution structure details on the scale of
microns. The field of view (FOV) of the microscopy system, however, is usually limited by the detector size.
For example, a typical CCD camera used for data acquisition is of size 2048 by 2048. In many cases this CCD
camera is not large enough to provide complete information required for accurate reconstruction, and the local
tomography problem hereby arises. On the other hand, the huge dataset generated by tomographic microscopy
asks for a highly efficient solution with no a priori information necessary. A new padding scheme is therefore
proposed for the local tomography issue. It first pads the projection data using the boundary value inside the
FOV, which is specified by the detector size, followed by a zero-value padding to 1.5 times the FOV length.
The boundary-value padding removes the energy deposition and cupping artifact in reconstruction results from
local tomography, while the zero-value padding reduces the drift of the intensity values caused by fully boundary
padding. The combination of two padding schemes keeps advantages of fully zero-value padding and fully
boundary-value padding, while avoiding their disadvantages. Quantitative analysis using synthetic data shows
that the proposed method outperforms fully zero-value padding and fully boundary-value padding in terms of
accuracy and ease for post processing. Experimental results for real data are also provided to demonstrate the
effectiveness of the proposed method.
Influence of metal segmentation on the quality of metal artifact reduction methods
Author(s):
Maik Stille;
Bärbel Kratz;
Jan Müller;
Nicole Maass;
Ingo Schasiepen;
Matthias Elter;
Imke Weyers;
Thorsten M. Buzug
Show Abstract
In computed tomography, star shape artifacts are introduced by metal objects, which are inside a patient's
body. The quality of the reconstructed image can be enhanced by applying a metal artifact reduction method. Unfortunately, a method that removes all such artifacts in order to make the images valuable for medical diagnosis remains to be found. In this study, the influence of metal segmentation is investigated. A thresholding technique, which is the state of the art in the field, is compared with a manual segmentation. Results indicate that a more accurate segmentation can lead to a preservation of important anatomical details, which are of high value for medical diagnosis.
TV-Stokes strategy for sparse-view CT image reconstruction
Author(s):
Yan Liu;
Lin Chen;
Hao Zhang;
Ke Wang;
Jianhua Ma;
Zhengrong Liang
Show Abstract
This paper introduces a new strategy to reconstruct computed tomography (CT) images from sparse-view projection data
based on total variation stokes (TVS) strategy. Previous works have shown that CT images can be reconstructed from
sparse-view data by solving a constrained TV problem. Considering the incompressible property of the voxels along the
tangent direction of isophote lines, a tangent vector is consolidated in this newly-proposed algorithm for normal vector
estimation. Then, a minimization problem based on this estimated normal vector is addressed and resolved in
computation. The to-be-estimated image is obtained by executing this two-step framework iteratively with projection
data fidelity constraints. By introducing this normal vector estimation, the edge information of the image is well
preserved and the artifacts are efficiently inhibited. In addition, the new proposed algorithm can mitigate the staircase
effects which are usually observed from the results of the conventional constrained TV method. In this study, the TVS
method was evaluated by patients’ brain raw data which was acquired from Siemens SOMATOM Sensation 16-slice CT
scanner. The results suggest that the proposed TVS strategy can accurately reconstruct the brain images and produce
comparable results relative to the TV-projection onto convex sets (TV-POCS) method and its general case: adaptiveweighted
TV-POCS (AwTV-POCS) method from 232,116 projection views. In addition, an improvement was observed
when using only 77 views for TVS method compared to the AwTV/TV-POCS methods. In the quantitative evaluation,
the TVS method showed adequate noise-resolution property and highest universal quality index value.
A comparison study of sinogram- and image-domain penalized re-weighted least-squares approaches to noise reduction for low-dose cone-beam CT
Author(s):
Hao Zhang;
Yan Liu;
Hao Han;
Jing Wang;
Jianhua Ma;
Lihong Li;
Zhengrong Liang
Show Abstract
Reducing X-ray exposure to the patients is one of the major research efforts in the computed tomography (CT) field, and
one of the common strategies to achieve it is to lower the mAs setting (by lowering the X-ray tube current and/or
shortening the exposure time) in currently available CT scanners. However, the image quality from low mAs acquisition
is severely degraded due to excessive quantum noise, if no adequate noise control is applied during image
reconstruction. Different from filter-based algorithms, statistical reconstruction algorithms model the statistical property
of the noise using a cost function and minimize the cost function for an optimal solution in statistical sense. The
algorithms have shown to be feasible and effective in both sinogram and image domain. In our previous researches, we
proposed penalized reweighted least-squares (PRWLS) approaches to sinogram noise reduction and image
reconstruction for low-dose CT imaging, which are in this statistical category. This work is a continuation of the
research along this direction and aims to compare the reconstruction quality of two different PRWLS implementations
for low-dose cone-beam CT reconstruction: (1) PRWLS sinogram restoration followed by analytical Feldkamp-Davis-
Kress reconstruction, (2) fully iterative PRWLS image reconstruction. Inspired by our recent study on the variance of
low-mAs projection data in presence of electric noise background, a more accurate weight was adopted in the weighted
least-squares term. An anisotropic quadratic form penalty was utilized in both PRWLS implementations to preserve
edges during noise reduction. Experiments using the CatPhan® 600 phantom and anthropomorphic head phantom were
carried to study the relevant performance of these two implementations on image reconstruction. The results revealed
that the implementation (2) can outperform implementation (1) in terms of noise-resolution tradeoff measurement and
analysis of the reconstructed small objects due to its matched image edge-preserved penalty in the image domain.
However, those gains are offset by the cost of increased computational time. Thus, further examination of real patient
data is necessary to show the clinical significance of the iterative PRWLS image reconstruction over the PRWLS
sinogram restoration.
Background filtering for accuracy improvement in computed tomography with iterative region-of-interest reconstruction
Author(s):
Keisuke Yamakawa;
Shinichi Kojima
Show Abstract
Two methods for preventing the deterioration of the accuracy of iterative region-of-interest (ROI) reconstruction are
proposed. Both methods apply filters; the first one applies them to the whole region the outside the region of interest
(outside ROI) without distinguishing objects (“method 1”, hereafter); and the second one applies them to only the air and
patient-table regions while masking other objects outside the ROI (“method 2”). The effectiveness of both methods was
evaluated in terms of simulated CT values by using two different phantoms. Method 1 reduced the artifact intensity level
by 86% (at most) compared with that obtained with the conventional method. In the case of an object with high
attenuation coefficient, method 2 decreases the level more than method 1. In other words, method 2 improves
reconstruction accuracy without causing deterioration by the filters. By selecting either method 1 or 2 in accordance with
the attenuation coefficient in regions of objects to be imaged, it is possible to reduce the error level compared with the
conventional method.
Co-registered image quality comparison in hybrid iterative reconstruction techniques: SAFIRE and SafeCT
Author(s):
Seungwan Lee;
Aran Shima;
Sarabjeet Singh;
Mannudeep K. Kalra;
Hee-Joung Kim;
Synho Do
Show Abstract
Iterative reconstruction techniques (IRTs) are used to reduce the radiation dose significantly and suppress the noise in
computed tomography (CT) imaging. However, the image from IRTs is unacceptable to the human visual system due to
the presence of outlier and non-Gaussian noise structure. The conventional noise measurements, such as mean and
standard deviation, are limited to provide the information of noise characteristics of an image comprehensively when the
undesirable noise structure happens on the image. In this study, the images reconstructed by using Weighted filtered
back-projection (WFBP) and image-based IRTs (SAFIRE and SafeCT) were compared in terms of conventional noise
statistics, high-order noise statistics, modulation transfer function (MTF), and slice sensitivity profile (SSP) with
different levels of radiation dose. The results showed that the noise characteristics, which were considered with
conventional and high-order noise statistics, and spatial resolution characteristics were different for the IRTs,
reconstruction parameters, and levels of radiation dose. This study can contribute to the optimization of image quality
from IRT with the reduction of radiation dose and development of IRT which overcomes the issue caused by undesirable
noise structures.
Iterative CT reconstruction using continuous model
Author(s):
Yongsheng Pan;
Daxin Shi;
Alexander A. Zamyatin
Show Abstract
A typical iterative CT reconstruction using SART involves ray-driven forward projection and voxel-driven back-
ward projection. Bilinear interpolation is usually applied on image data for forward projection, and linear
interpolation is usually applied on projection data for backward projection, when both data are represented
using discrete samples in 2D fan-beam geometry. The applied interpolations, however, may affect the spatial
resolution, bias and noise properties of the reconstruction. A basis function (such as blob and spline) is therefore
applied to formulate a continuous model for the image data to reduce bias. In this paper we propose to apply
the blob representation on the projection data and explore its effectiveness. In this way we use continuous model
for the data of projection difference during backward projection, and we avoid the linear interpolation in this
process. Experimental results show that the proposed scheme is able to provide higher spatial resolution than
linear interpolation, while introducing more local variations in the reconstruction. However, the introduced local
variations may be reduced with the combination of total variation (TV) minimization. The proposed scheme is
therefore able to provide improved spatial resolution while keeping low local variations in reconstructions.
Image reconstruction from limited-angle range projections
Author(s):
Nan Du;
Yusheng Feng;
Artyom M. Grigoryan
Show Abstract
This paper describes a new approach for reconstructing images from a finite number of projections. The rayintegrals
of the image f(x, y) are transformed uniquely into the ray-sums of the discrete image fn,m on the
Cartesian lattice. This transformation allows for calculating the tensor representation of the discrete image, when
the image is considered as the sum of direction images, or splitting-signals carrying the spectral information of
the image at frequency-points of different subsets that cover the Cartesian lattice. These subsets are intersected
and this property of redundancy is used to reduce the angular range of projections. The proposed approach is
presented for parallel projections and the continuous model. Preliminary results show very good results of image reconstruction when the angular range scanned is 27° and down to 10°.
Impact of noise level and edge sharpness of a prior image on the performance of Prior Image Constrained Compressed
Sensing (PICCS)
Author(s):
Yinghua Tao;
Jie Tang;
Michael A. Speidel;
Guang-Hong Chen
Show Abstract
Dose reduction using prior image constrained compressed sensing (DR-PICCS) is a method of CT reconstruction which utilizes prior image information in a compressed sensing framework to significantly reduce the noise in images acquired at low dose. The purpose of this study was to investigate the impact of edge sharpness and noise level in the prior image data on the resulting DR-PICCS images. Projection data from a 100 rnA CT myocardial perfusion scan of a swine was combined with numerically simulated projections of vessels of varying geometry (diameter = 4, 3, 2 mm) and contrast levels (600, 400, 200 HU enhancement). Bilateral and mean filters were applied to generate prior images, which were then used with the DR-PICCS algorithm. Vessel diameter, effective blurring kernel, and vessel intensity were compared among prior images as well as among the corresponding PICCS images. Although the filters produced prior images with significantly different spatial resolution characteristics at similar noise levels, these differences were mitigated in DR-PICCS images and the DR-PICCS had improved fidelity in comparison to the priors.
Evaluation of reconstructed images from sparse data on the micro-CT system
Author(s):
Dae-Hong Kim;
Hee-Joung Kim;
Pil-Hyun Jeon
Show Abstract
The quantitative estimation was performed to verify the effect the low dose imaging with fewer projection data using
micro-CT with total variation (TV) minimization method. To assess the image contrast and noise, sparse data were
obtained from the projection data of the phantom and the mouse. Filtered-backprojection (FBP) images of the projection
data of the phantom and the mouse were used as a reference image reconstructed with 400 projection data. Universal
quality index (UQI) was used as an image similarity metric for comparison between TV minimization algorithm and
FBP algorithm for both the phantom and the mouse image. Contrast-to-noise ratio (CNR) values for the image derived
from TV minimization algorithm with 80-view was approximately 45 % higher than FBP with 400-view up to 10 %
iodine solution material. UQI values for the phantom and the mouse images were measured with 0.974 and 0.999,
respectively. Low dose image using TV minimization algorithm was proved to be advantageous for micro-CT system.
Contrast and noise properties on image from TV minimization at few-view were higher than FBP 80-view and full-view
scan. However, CNR from 15 % iodine solution to 20 % iodine solution were lower than FBP due to the lower-intensity
of the x-rays. The reconstructed images for both phantom and mouse from TV minimization algorithm were well
matched to FBP-reference images.
Low-dose CT reconstruction based on multiscale dictionary
Author(s):
Ti Bai;
Xuanqin Mou;
Qiong Xu;
Yanbo Zhang
Show Abstract
Statistical CT reconstruction using penalized weighted least-squares(PWLS) criteria can improve image-quality in low-dose CT reconstruction. A suitable design of regularization term can benefit it very much. Recently, sparse representation based on dictionary learning has been treated as the regularization term and results in a high quality reconstruction. In this paper, we incorporated a multiscale dictionary into statistical CT reconstruction, which can keep more details compared with the reconstruction based on singlescale dictionary. Further more, we
exploited reweigted l1 norm minimization for sparse coding, which performs better than I norm minimization
in locating the sparse solution of underdetermined linear systems of equations. To mitigate the time consuming process that computing the gradiant of regularization term, we adopted the so-called double surrogates method to accelerate ordered-subsets image reconstruction. Experiments showed that combining multiscale dictionary and reweighted l1 norm minimization can result in a reconstruction superior to that bases on singlescale dictionary and l1 norm minimization.
Detection of low-dose CT reconstruction artifacts using a bi-modal approach
Author(s):
Salman Mahmood;
Klaus Mueller
Show Abstract
Low-dose Computed Tomography (CT) has the benefit of exposing patients to less radiation. However,
low dose CT requires special reconstruction techniques to improve the clarity of the image. Unfortunately,
these special reconstruction techniques often cannot remove all of the low-dose artifacts. It is important to
recognize these artifacts else we run the risk of obscuring important detail or adding false features. In this
work, we present a simple scheme which allows us to detect these artifacts. Our technique applies to the
specific low-dose CT strategy in which the number of X-ray views taken from the patient is reduced. The
first step uses directional interpolation in the low dose sinogram to add more views. While the image
created from this interpolated sinogram does not have any artifacts it lacks significantly in clarity due to
blurring. Our scheme then compares this image with the image created directly with a low-dose CT
reconstruction technique which has better detail but also some remaining artifacts. The comparison
reveals these artifacts which we then remove by simple pixel replacement.
Truncation artifact correction by support recovery
Author(s):
Scott S. Hsieh;
Guangzhi Cao;
Brian E. Nett;
Norbert J. Pelc
Show Abstract
Truncation artifacts arise when the object being imaged extends past the scanned field of view (SFOV). The line
integrals which lie beyond the SFOV are unmeasured, and reconstruction with traditional filtered backprojection (FBP)
produces bright signal artifacts at the edge of the SFOV and little useful information outside the SFOV. A variety of
techniques have been proposed to correct for truncation artifacts by estimating the unmeasured rays. We explore an
alternative, iterative correction technique that reduces the artifacts and recovers the support (or outline) of the object that is consistent with the measured rays. We assume that the support is filled uniformly with tissue of a given CT number (for example, water-equivalent soft tissue) and segment the region outside the SFOV in a dichotomous fashion into tissue and air. In general, any choice for the object support will not be consistent with the measured rays in that a
forward projection of the image containing the proposed support will not match the measured rays. The proposed
algorithm reduces this inconsistency by deforming the object support to better match the measured rays. We initialize the reconstruction using the water cylinder extrapolation algorithm, an existing truncation artifact correction technique, but other starting algorithms can be used. The estimate of the object support is then iteratively deformed to reduce the inconsistency with the measured rays. After several iterations, forward projection is used to estimate the missing rays. Preliminary results indicate that this iterative, support recovery technique is able to produce superior reconstructions in the case of significant truncation compared to water cylinder extrapolation.
User-friendly, ultra-fast simulation of detector DQE(f)
Author(s):
Eric Abel;
Mingshan Sun;
Dragos Constanin;
Rebecca Fahrig;
Josh Star-Lack
Show Abstract
Development of the indirect scintillating detector is hindered not only by the cost and lead-time of manufacturing
but also the computational resources required for numerical modeling. The simulation is bogged down by
the number of x-ray photons (gammas) required to duplicate the experimental flood image ensemble necessary
to characterize the noise power spectrum (NPS), a key input into the detective quantum efficiency (DQE). The
simulation approach presented in this work exploits our previously reported procedure named Fujita-Lubberts-
Swank (FLS)6 . This novel technique computes the Lubberts NPS from an ensemble of single gamma point spread
functions (PSF) and, as a result, allows for a significant reduction in the number of simulated particles, enabling
full DQE(f) simulations with optical transport in less than one CPU-hour. For a given detector and spectrum,
the FLS execution time is determined primarily by the number of gamma and optical photons initiated. The optimal
number of each varies with the detector specifics. In this work, we present a different simulation paradigm
in which Geant4 was customized to allow for the user to specify the quantities of detected gammas, and detected
opticals per gamma. These quantities were empirically shown to be constant over a small selection of different
detector types. While work still needs to be done to explore the range of detectors for which this technique will
work, we demonstrate a concept which brings added convenience and efficiency to FLS detector simulations.
Quantitative breast imaging using photon counting detector
Author(s):
Seokmin Han;
Dong-Goo Kang;
Sunghoon Kang;
Younghun Sung
Show Abstract
Possible limitations of current dual energy Contrast Enhanced Digital Marnmography(CEDM) are that over lapping normal breast tissue structures can obscure the visualization of iodine, and that the only two images acquired provide solution for two variable equations while three variables are required as the breast consists of three materials - adipose and glandular tissues and iodine. To solve this problem with dual energy CEDM, it requires knowledge of the breast thickness at each pixel. However, in many clinical mammography systems employing a spring-loaded paddle, the physical thickness of the breast may not be uniform due to deformation and tilt of the compression paddle. Therefore, we chose to use triple energy CEDM to overcome these limitations, which can provide a third image. However, the radiation dose can remain a major concern due to three exposures. Photon counting detector(PCD) can provide triple energy radiography without mentioned extra exposures. For triple energy CEDM, an iodine quantification method for breast imaging was suggested in this research. We acquired triple energy images of calibration phantom of different iodine concentrations first, using PCD. Then, intensity values at each energy of imaging object could be mapped pixel by pixel to different locations on the calibration phantom images of different iodine thicknesses(concentrations). Interpolation surface of iodine con centration was constructed from the mapped locations at each energy. Resultant triple surfaces were combined to find out the intersection of the three iodine thickness surfaces from the three energy images, which tells the estimated iodine thickness from the input intensity value. The result shows that the proposed method could quantify iodine inserts in breast phantom accurately, which simulate lesions in breast filled with different iodine concentrations.
Application of organic semiconductors in amorphous selenium based photodetectors for high performance X-ray imaging
Author(s):
Shiva Abbaszadeh;
Zhechen Du;
Nicholas Allec;
Karim S. Karim
Show Abstract
In order to improve the performance of amorphous selenium (a-Se) based detectors, it is beneficial to operate the device
at high electric field (≥10 V/μm). Increasing the electric field reduces the ionization energy and increases the hole
mobility within the a-Se detector. In order for a practical a-Se detector to be capable of working at a high electric field,
injection of holes from the positively biased electrode and injection of electrons from the negatively biased electrode
should be prevented. We have investigated different organic materials with high ionization potential as hole-blocking
contacts for a-Se based photodetectors. The effect of the organic layer thickness on the dark current and photocurrent
performance of the detector was examined. It was found that the injection of holes could be reduced at high electric
fields by increasing the thickness of the organic layer.
Spatial resolution characteristics of a-Se imaging detectors using
Monte Carlo methods with detailed spatiotemporal transport of x-rays,
electrons, and electron-hole pairs under applied bias
Author(s):
Yuan Fang;
Andreu Badal;
Aldo Badano;
Karim S. Karim
Show Abstract
Detectability of microcalcifications and small lesions in mammography has driven the development of high spatial
resolution imagers with small pixel pitch. In this work, we study the detector resolution limits of amorphous selenium (a-
Se) with a detailed Monte Carlo transport code for simulation of direct x-ray detectors. The model takes into account
generation and re-absorption of characteristic x rays, spreading due to Compton scattering and high-energy secondary
electron transport, and drift and diffusion of electron-hole pairs under the applied external electric field. The transport of
electron-hole pairs is achieved with a spatiotemporal model that accounts for recombination and trapping of carriers and
Coulombic effects of 3D spatial charge distribution. The location information for each detected electron and hole over
millions of simulation histories are used to build the detector point response. A range of incident x-ray energies are
simulated from 10 to 100 keV. The simulated detector point response can be used to study the spatial resolution
characteristics of detectors at different energies ranges and for calculation of the modulation transfer function and image
quality metrics.
Fabrication and characterization of a novel x-ray silicon detector
Author(s):
Kyung-Wook Shin;
Karim S. Karim
Show Abstract
Protein crystallography is a key method for protein structure investigation in modern medicine and
X-ray diffraction detectors are key to performance. We introduced a silicon detector, based on an
active-pixel readout of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) for
protein crystallography. In this work, we present the fabrication process of the detector array,
performance of the first fabricated TFT arrays, and the performance of the TFTs in terms of fieldeffect
mobility, gate material quality, and stability under long stress using a Fe-55 (50 μCi)gamma
ray source (6 to 10 keV photon energies). Device fabrication was performed in an in-house facility,
Giga-to-Nano microfabrication facility, at the University of Waterloo, and involved plasma
enhanced chemical vapor deposition (PECVD) and wet and dry etch techniques with a simple two
mask process. The TFT test results promise higher effective field effect mobility of 16.49 cm2/V·s
due to the presence of silicon substrate contacting the a-Si:H channel layer along with a compromise
in leakage current, yielding a 104 ON/OFF ratio. Meanwhile, the threshold voltage shift is
manageable by applying a negative voltage of a duration less than 1/10 of the duty cycle. From the
detector leakage test, the leakage current through the TFT gate was acceptable range while the
photo-generated current needs to be suppressed with positive voltage bias at the gate electrode. Thus,
minimizing the negative gate bias in readout operation is crucial. Finally, TFT readout current under
the same Fe-55 X-ray source shows that optimal operation range can be determined when bulk bias
is higher than TFT operation bias.
High performance microstructured Lu2O3:Eu thin film scintillator for X-ray computed tomography
Author(s):
Zsolt Marton;
Harish B. Bhandari;
Charles Brecher;
Stuart R. Miller;
Bipin Singh;
Vivek V. Nagarkar
Show Abstract
Large penetration depth and weak interaction of high energy X-rays in living organisms provide a non-destructive
way to study entire volumes of organs without the need for sophisticated preparation (injection of contrast material,
radiotracer labels etc.). X-ray computed tomography (CT) is a powerful diagnostic tool allowing 3D image
reconstruction of the complete structure. Using hard X-rays in medical imaging leads to reduced dose received by
the patient. At higher energies, however, the conventional scintillators quickly become the limiting factor. They
must be thin in order to provide reasonable spatial resolution and preserve image quality. Nevertheless, insufficient
thickness introduces the need for long acquisition times due to low stopping power. To address these issues, we
synthesized a new structured scintillator to be integrated into CCD- or photodiode-based CT systems. Europiumdoped
Lu2O3 (Lu2O3:Eu) has the highest density among all known scintillators, very high absorption coefficient for X-rays and a bright red emission matching well to the quantum efficiency of the underlying CCD- and photodiode arrays. When coupled to a suitable detector, this microcolumnar scintillator significantly improves the overall
detective quantum efficiency of the detector. For the first time ever, structured and scintillating film of Lu2O3:Eu
was grown by electron-beam physical vapor deposition. A prototype sensor was produced and evaluated using both
laboratory X-ray sources as well as synchrotron radiation. Comparative performance evaluations of the newly
developed sensor versus commercial grade scintillators were conducted. Such synthesis of high density, microstructured,
scintillating coatings enables the development of high sensitivity X-ray detectors for CT applications.
Low dark current and high dynamic range a-Si:H MSM photodetector for large area medical imaging
Author(s):
Sina Ghanbarzadeh;
Shiva Abbaszadeh;
Michael Adachi;
Karim S. Karim
Show Abstract
Previously a-Si:H metal-semiconductor-metal (MSM) lateral detectors for indirect medical imaging applications had been proposed by our research group. These lateral detectors are attractive due to their ease of fabrication primarily because there is no p+ doped semiconductor layer, thus making it compatible with industry standard amorphous silicon thin film transistor electronics processing. However the earlier devices exhibited high dark current which is problematic for integration mode imaging. In the other words, they were limited in term of dynamic range. In this study, we demonstrate an a-Si:H MSM lateral structure with low dark current, high dynamic range and comparable sensitivity and quantum efficiency to conventional p-i-n photodiodes. These improvements are achieved by the introduction of a thin polymer layer as a blocking contact. The fabricated amorphous silicon based MSM detector exhibits a photo-response of more than 3 orders of magnitude to a green light source (λ = 525nm). In comparison to vertical p-i-n structures, the reported MSM lateral devices show gains in terms of dynamic range, ease of fabrication (no p+ layer), faster speed at the cost of a slightly reduced quantum efficiency. The experimental results of dark and photocurrent measurements as well as the responsivity for two in-house fabricated MSM structures at different bias voltages and light intensity are presented. This results are promising and encourage the development of a-Si:H lateral MSM devices for indirect conversion large area medical imaging applications and especially low cost flat panel computed tomography.
Investigating the optical diffusion capabilities of nanophosphors for use in medical imaging
Author(s):
P. F. Liaparinos;
I. S. Kandarakis
Show Abstract
The quality of medical images can be characterized by the signal transfer properties of the x-ray converter. Various
studies have previously investigated the influence of the detector configuration on the optimization of medical imaging
systems. However, novel technologies related to new luminescent materials seem to be promising for further
improvements in medical imaging instrumentation technology. The aim of this study was to investigate and optimize
granular phosphor-based X-ray converters by examining different phosphor materials with grain size in the nano-scale
(four different categories). Optical diffusion was simulated based on Mie scattering theory and the imaging performance
was predicted using Monte Carlo simulation methods. The Modulation Transfer Function (MTF) of all cases were
evaluated and compared. Results showed and analyzed the relation between phosphor intrinsic properties on optical
diffusion. It was found that the resolution of the nanophosphor is directly affected by the optical parameters and becomes
better for high values of light extinction factor and light absorption probability. In particular, the present study showed
that the utilization of optimum optical parameters based on specific physical (refractive index, light wavelength) and
structural (grain size, packing density) parameters enhance manufacturers in nanophopshor synthesis and preparation to
follow particular configurations of nanophoshor composition. Finally, high optical modulation was accomplished
employing grains of high refractive index and size close to 200 nm.
Light emission efficiency of Lu2O3:Eu nanophosphor scintillating screen under x-ray radiographic conditions
Author(s):
I. E. Seferis;
N. I. Kalyvas;
I. G. Valais;
C. M. Michail;
P. F. Liaparinos;
G. P. Fountos;
E. Zych;
I. S. Kandarakis;
G. S. Panayiotakis
Show Abstract
Powder phosphors scintillators are used in indirect digital radiography as x-ray to light converters coupled to electronic
optical sensors (photodiodes, CCDs, CMOS). Recently, nanophosphors have been reported to have enhanced
luminescence efficiency. The purpose of the present study was to evaluate Lu2O3:Eu nanophosphor as a candidate for
digital medical imaging applications. Lu2O3:Eu was employed in the form of a 30.2 mg/cm2 powder screen with 50 nm
grain size and 5% Eu concentration. Both the nanophosphor material and the screen were prepared in our laboratories.
Parameters such as the Absolute Efficiency-AE (light energy flux over exposure rate), the Luminescence Efficiency-
XLE (Light energy flux over incident x-ray energy flux), Detector Quantum Gain-DQG (optical quanta emitted per
incident x-ray quantum) and the light spectral compatibility to electronic optical sensors (Effective Efficiency) were
investigated under x-ray excitation in the radiographic energy range. Results were compared with previously published
data for a 33.1 mg/cm2 Gd2O2S:Eu conventional phosphor screen. It was found that Lu2O3:Eu nanophosphor has higher
AE and XLE by a factor of 1.32 and 1.37 on average, respectively, in the whole radiographic energy range. DQG was
also found higher in the energy range from 50 kVp to 100 kVp and comparable thereafter. Effective efficiency was found
with high values for electronic optical sensors such as CCDs and CMOS, due to the high spectral compatibility with the
upper visible wavelength range. These results indicate that Lu2O3:Eu nanophosphor could potentially be considered for
applications in digital x-ray radiography detectors.
Expanded analysis of occupational dose in interventional and diagnostic fluoroscopy with the use of active dosimeters
Author(s):
R. Bujila;
C. Palmgren;
A. Omar;
A. Fransson
Show Abstract
Due to the relatively high occupational doses associated with interventional and diagnostic fluoroscopy procedures it is
important to create awareness about and to quantify the radiation environment that medical staff are exposed to. A
computer program was developed to analyze dose data collected from a dosimetry system that uses active personal
dosimeters to monitor staff dose in real-time, to obtain an expanded analysis of the radiation environment that clinical
staff are exposed to, on a procedural basis. The analyses that are made per procedure and staff member include:
accumulated dose μSv, maximum and median dose rate mSv/h, the amount of time a staff member has been exposed to
radiation compared to the total fluoroscopy time and the percentage of accumulated dose from 3 different dose rate
intervals, including < 0.3 mSv/h, 0.3 - 2.6 mSv/h, and < 2.6 mSv/h. The developed computer program was used to
analyze dose data collected from the dosimetry system at the Karolinska University Hospital to study the radiation
environment that different categories of staff are exposed to during interventional aorta aneurysm treatment procedures.
The analyses have provided the ability to know where to concentrate radiation safety training in interventional and
diagnostic fluoroscopy and to ensure that operating rooms are equipped with adequate radiation protection (e.g.,
radiation protection barriers etc.). The developed computer program and dose data collected from the dosimetry system
can be appropriated for other radiation environmental studies in diagnostic x-ray imaging.
Dose reduction in fluoroscopic interventions using a combination of a region of interest (ROI) x-ray attenuator and spatially different, temporally variable temporal filtering
Author(s):
S. N. Swetadri Vasan;
Liza Pope;
Ciprian N. Ionita;
A. H. Titus;
D. R. Bednarek;
S. Rudin
Show Abstract
A novel dose reduction technique for fluoroscopic interventions involving a combination of a material x-ray
region of interest (ROI) attenuator and spatially different, temporally variable ROI temporal recursive filter, was used to
guide the catheter to the ROI in three live animal studies, two involving rabbits and one involving a sheep. In the two
rabbit studies presented , a catheter was guided to the entrance of the carotid artery. With the added ROI attenuator the
image under the high attenuation region is very noisy. By using temporal filtering with a filter weight of 0.6 on previous
frames, the noise is reduced. In the sheep study the catheter was guided to the descending aorta of the animal. The sheep
offered a relatively higher attenuation to the incident x-rays and thus a higher temporal filter weight of 0.8 on previous
frames was used during the procedure to reduce the noise to levels acceptable by the interventionalist.
The image sequences from both studies show that significant dose reduction of 5-6 times can be achieved with
acceptable image quality outside the ROI by using the above mentioned technique. Even though the temporal filter
weighting outside the ROI is higher, the consequent lag does not prevent perception of catheter movement.
Updates in the real-time dose tracking system (DTS) to improve the accuracy in calculating the radiation dose to the
patients skin during fluoroscopic procedures
Author(s):
Vijay K. Rana;
Stephen Rudin;
Daniel R. Bednarek
Show Abstract
We have developed a dose-tracking system (DTS) to manage the risk of deterministic skin effects to the patient during fluoroscopic image-guided interventional cardiac procedures. The DTS calculates the radiation dose to the patient's skin in real-time by acquiring exposure parameters and imaging-system geometry from the digital bus on a Toshiba C-arm unit and displays the cumulative dose values as a color map on a 3D graphic of the patient for immediate feedback to the interventionalist. Several recent updates have been made to the software to improve its function and performance. Whereas the older system needed manual input of pulse rate for dose rate calculation and used the CPU clock with its potential latency to monitor exposure duration, each x-ray pulse is now individually processed to determine the skin-dose increment and to automatically measure the pulse rate. We also added a correction for the table pad which was found to reduce the beam intensity to the patient for under-table projections by an additional 5-12% over that of the table alone at 80 kVp for the x-ray filters on the Toshiba system. Furthermore, mismatch between the DTS graphic and the patient skin can result in inaccuracies in dose calculation because of inaccurate inverse-square-distance calculation. Therefore, a means for quantitative adjustment of the patient-graphic-model position and a parameterized patient-graphic library have been developed to allow the graphic to more closely match the patient. These changes provide more accurate estimation of the skin-dose which is critical for managing patient radiation risk.
Extraction of coronary angiographic information from low tube current HYPR-CT myocardial perfusion scans
Author(s):
Yinghua Tao;
Michael A. Speidel;
Michael Van Lysel;
Guang-Hong Chen
Show Abstract
Low tube current scanning in combination with HYPR (HighlY constrained backPRojection) noise reduction is a proposed method for low-dose time-resolved CT myocardial perfusion imaging. We report animal studies and simulations investigating the coronary angiographic information available in these scans. Four pigs were scanned at 100 rnA and 500 rnA. A HYPR coronary angiographic image was formed from each 100 rnA scan by producing a time-averaged composite image from cardiac cycles centered on the time of peak left ventricular intensity and then multiplying the composite by a weighting image to restore image intensities. Image noise, coronary artery cross sectional area, and coronary artery intensity were measured as a function of the number of beats in the composite, weighting image filtration, and coronary artery size. HYPR images maintained coronary artery area and intensity to within 6-8% of of filtered back projection (FBP) image values, on average, for vessels with cross
sectional area greater than 2 mm2. Vessel visibility in 100-mA HYPR images was improved relative to 100-mA
FBP, in cross sectional and multiplanar reformatted images.
Image extrapolation for patient-specific CT dose determination based on scout images
Author(s):
Qing Liang;
Larry A. DeWerd
Show Abstract
Monte Carlo (MC) simulation based on a patient’s computed tomography (CT) images is a promising way to
retroactively determine patient-specific dose from CT scans. CT scans generally include only a portion of the patient’s
body; however, photon interactions in regions adjacent to the scan region contribute to the overall scan dose. Thus, the
CT images alone, which do not include these adjacent regions, are insufficient for determining dose. In fact, dose
underestimation, especially at the scan edge, occurs when the adjacent regions are not accounted for. In this work, the
dose underestimation without the scatter region is demonstrated, and the size of the scatter region required to provide
sufficient simulated scatter was determined with mathematical phantom studies. For a simple cylindrical water phantom,
up to a 25% underestimation of dose was found when no scatter region was used, and a 40 mm scatter region was
determined to be required to eliminate this error for scans performed with both the 40 mm and 5 mm collimation sizes.
In addition, four different image extrapolation methods based on CT images and scout images were proposed for a chest
CT scan of an anthropomorphic phantom. The dose was calculated with the chest images only and the chest images plus
the four types of extrapolated images. The results were compared with the dose calculated using whole body images of
the anthropomorphic phantom. The image extrapolation methods, especially the ones based on scout images, were
shown to improve the dose calculation accuracy under both step-shoot scan mode and helical scan mode.
Keywords: CT, patient-specific dose, Monte Carlo, image extrapolation, scout images
An approach to correlate the CTDIvol to organ dose for thorax and
abdomen CT taking tube current modulation and patient size into
account
Author(s):
X. Lopez-Rendon;
F. Zanca;
R. Oyen;
H. Bosmans
Show Abstract
Purpose: To estimate conversion factors for calculating effective dose (E) and organ dose taking tube current modulation (TCM) and patient size into account in adult thorax and abdomen CT examinations.
Method: 99 consecutive adult patients were included in this study. All examinations were performed with TCM (CareDose 4D. Siemens Definition Flash) at 120 kVp and 110 (thorax) and 200 (abdomen) reference mAs. E and organ dose were estimated with PCXMC 2.0 (STUK. Helsinki. Finland). using an extension of the software from a planar geometry to spiral acquisitions of aCT scanner. This software accounts for patient size by rescaling the anthropomorphic phantom to actual patient weights and heights.
E and organ doses were normalized to the CTDivol as reported in the patient's report. These conversion factors (dE and dorgan were studied as a function of different patient metrics: lateral and anterior-posterior (AP) diameter. sum of the lateral and AP diameter, area of a cross section image and effective diameter.
Results:. No trend was found for any of the metrics neither forE nor for the organs investigated (lungs. breasts. stomach and liver). Average value ± 2 standard deviation were calculated. For a thorax examination, the average dE was 0.57 ± 0.14 mSv/mGy. dlungs was 1.26 ± 0.28 mGy/mGy and dbreasts was 1.29 ± 0.40 mGy/mGy. For an abdomen scan dE was
0.82 ± 0.18. mSv/mGy. d,tomooh was 1.42 ± 0.26 mGy/mGy. dliver was 1.42 ± 0.30 mGy/mGy.
Conclusion:. For the scanner studied, average conversion factors, which account for TCM and patient size, were
proposed. This is a first step towards patient-specific dosimetry.
Longitudinal study of radiation exposure in computed tomography with an in-house developed dose monitoring system
Author(s):
Bernhard Renger;
Ernst J. Rummeny;
Peter B. Noël
Show Abstract
During the last decades, the reduction of radiation exposure especially in diagnostic computed tomography is one of the
most explored topics. In the same time, it seems challenging to quantify the long-term clinical dose reduction with regard
to new hardware as well as software solutions. To overcome this challenge, we developed a Dose Monitoring System
(DMS), which collects information from PACS, RIS, MPPS and structured reports. The integration of all sources
overcomes the weaknesses of single systems. To gather all possible information, we integrated an optical character
recognition system to extract, for example, information from the CT-dose-report. All collected data are transferred to a
database for further evaluation, e.g., for calculations of effective as well as organ doses. The DMS provides a single
database for tracking all essential study and patient specific information across different modality as well as different
vendors. As an initial study, we longitudinally investigated the dose reduction in CT examination when employing a
noise-suppressing reconstruction algorithm. For this examination type a significant long-term reduction in radiation
exposure is reported, when comparing to a CT-system with standard reconstruction. In summary our DMS tool not only
enables us to track radiation exposure on daily bases but further enables to analyses the long term effect of new dose
saving strategies. In the future the statistical analyses of all retrospective data, which are available in a modern imaging department, will provide a unique overview of advances in reduction of radiation exposure.
Comparative dosimetry of radiography, tomosynthesis, and CT for chest imaging across 59 adult patients
Author(s):
Yakun Zhang;
Xiang Li;
W. Paul Segars;
Ehsan Samei
Show Abstract
There are three main x-ray based modalities for imaging the thorax: radiography, tomosynthesis, and CT. CT provides
perhaps the highest level of feature resolution but at notably higher radiation dose. To implement the ALARA (as low as
reasonable achievable) principle in making an appropriate choice between standard chest projection imaging,
tomosynthesis, and CT to achieve the lowest possible dose to patients, the effective doses and risk indices for each
modality should be accurately known. In this study, we employed 59 computational anthropomorphic male and female
extended cardiac-torso (XCAT) adult phantoms and a Monte Carlo simulation program (PENELOPE, version 2006,
Universitat de Barcelona, Spain). Effective dose and risk index was estimated for a clinical radiography system enabling
to conduct chest radiography and tomosynthesis sweep (Definium 8000, Volume RAD, GE Healthcare) and a clinical
CT system (LightSpeed VCT, GE Healthcare). It was found that the absolute effective dose and risk index increased
greatly with increasing patient size for CT, while these two dose metrics only increased slightly for radiography and
tomosynthesis. This suggests that it is important to specify patient size when comparing radiation dose across imaging
modalities.
Comparison of photon counting and conventional scintillation detectors in pinhole SPECT system for small animal imaging
Author(s):
Young-Jin Lee;
Hyun-Ju Ryu;
Su-Jin Park;
Hee-Joung Kim
Show Abstract
The photon counting detector using cadmium telluride (CdTe) or cadmium zinc telluride (CZT) is a promising imaging modality and provides many benefits compared to conventional scintillation detectors. When using the pinhole
collimator with the photon counting detector, we are able to improve both spatial resolution and sensitivity. The purpose of this study was to evaluate the photon counting and conventional scintillation detectors in a pinhole single photon emission computed tomography (SPECT) system. We designed five pinhole SPECT systems of two types. One was the
CdTe photon counting detector, and the other was the conventional NaI(Tl) scintillation detector. We conducted simulation studies and evaluated the imaging performance. The results showed that the spatial resolution of CdTe photon counting detector was 0.38 mm and the sensitivity in this detector was 1.40 times higher than conventional NaI(Tl)
scintillation detector in the same detector thickness condition. Also, the average scatter fraction of the CdTe photon counting and the conventional NaI(Tl) scintillation detectors were 1.93% and 2.44%, respectively. In conclusion, we
successfully evaluated various pinhole SPECT systems for small animal imaging.
Non-invasive high-resolution tracking of human neuronal pathways: diffusion tensor imaging at 7T with 1.2 mm isotropic voxel size
Author(s):
Ralf Lützkendorf;
Frank Hertel;
Robin Heidemann;
Andreas Thiel;
Michael Luchtmann;
Markus Plaumann;
Jörg Stadler;
Sebastian Baecke;
Johannes Bernarding
Show Abstract
Diffusion tensor imaging (DTI) allows characterizing and exploiting diffusion anisotropy effects, thereby providing
important details about tissue microstructure. A major application in neuroimaging is the so-called fiber tracking where
neuronal connections between brain regions are determined non-invasively by DTI. Combining these neural pathways
within the human brain with the localization of activated brain areas provided by functional MRI offers important
information about functional connectivity of brain regions. However, DTI suffers from severe signal reduction due to the
diffusion-weighting. Ultra-high field (UHF) magnetic resonance imaging (MRI) should therefore be advantageous to
increase the intrinsic signal-to-noise ratio (SNR). This in turn enables to acquire high quality data with increased
resolution, which is beneficial for tracking more complex fiber structures. However, UHF MRI imposes some difficulties
mainly due to the larger B1 inhomogeneity compared to 3T MRI. We therefore optimized the parameters to perform DTI
at a 7 Tesla whole body MR scanner equipped with a high performance gradient system and a 32-channel head receive
coil. A Stesjkal Tanner spin-echo EPI sequence was used, to acquire 110 slices with an isotropic voxel-size of 1.2 mm
covering the whole brain. 60 diffusion directions were scanned which allows calculating the principal direction
components of the diffusion vector in each voxel. The results prove that DTI can be performed with high quality at UHF
and that it is possible to explore the SNT benefit of the higher field strength. Combining UHF fMRI data with UHF DTI
results will therefore be a major step towards better neuroimaging methods.
Motion correction of rodent thoracic PET image using radioactive bead and MRI image
Author(s):
Jung Woo Yu;
Sang-Keun Woo;
Yong Jin Lee;
In Ok Ko;
Ran Ji Yoo;
Joo Hyun Kang;
Byung Il Kim;
Yong Hyun Chung;
Sang Moo Lim;
Kyeong Min Kim
Show Abstract
PET image of tumor located in thoracic region was affected by various organ motions such as respiration and heartbeat.
Thoracic motion is difficult to estimate and correct accurately using external measurement or anatomical image solely.
The aim of this study was to compare the accuracy of motion correction using PET fiducial mark and 3D MRI image.
The radioactive bead for PET fiducial mark was realized from molecular sieve contained 0.37 MBq F-18 and placed in
thoracic region. PET study was performed using a small animal PET scanner after IV injection of FDG. MRI study was
performed using 3-T clinical MRI system with 3D T1-VIBE (TR/TE=5.67/1.42 ms) sequence. Motion corrected PET
image was created by mutual information registration with B-Spline interpolation to the mean image after first
realignment. FWHM of lung and liver region in static PET image was 4.77±0.87 and 4.81±0.45, respectively. Measured
FWHM of lung region in motion corrected PET image using PET fiducial mark and 3D VIBE MRI was measured
4.22±0.09 and 4.59±0.06, respectively. In case of liver region, FWHM was measured 4.47±0.16 and 4.65±0.25
respectively. The improvement of resolution was observed by proper correction method. In this study PET correction
was implemented by motion information extracted from various images. These results suggest motion correction would
be possible without external device or fiducial mark using MRI motion data. Motion correction using MRI should be
considered acquisition method and organ region in accordance with motion characteristics.
LASCA and PPG imaging for non-contact assessment of skin blood supply
Author(s):
Dainis Jakovels;
Uldis Rubins;
Janis Spigulis
Show Abstract
Laser speckle contrast analysis (LASCA) offers a non-contact, full-field, and real-time mapping of capillary blood flow
and can be considered as an alternative method to Laser Doppler perfusion imaging (LDPI). Photoplethysmography
(PPG) is well known technique for assessment of skin blood pulsations that can be related to blood flow. In recent years
several studies have been done on development of non-contact PPG imaging (PPGI).
LASCA and PPGI techniques are simpler and cheaper compared with LDPI. LASCA technique has been implemented in
several commercial instruments. However, these systems are still too expensive and bulky to be widely available.
Several optical techniques have found new implementations as connection kits for mobile phones thus offering low cost
screening device.
In this work we demonstrate simple implementation of LASCA and PPG imaging technique for primary low-cost
assessment of skin blood flow. Both devices comprise a widely available 1.3 mega pixel CMOS camera. Stabilized 650
nm laser diode module is used for LASCA illumination, and white LEDs are illuminators for PPG imaging device.
An arterial occlusion test was performed to test LASCA and PPGI imaging devices. An example of scratch color image
and corresponding blood flow map also was demonstrated. The results showed that both techniques can be used for fast
monitoring and mapping of skin blood flow and implemented as connection kits for smartphone.
Multispectral imaging for early diagnosis of melanoma
Author(s):
Anna Pelagotti;
Pasquale Ferrara;
Leonardo Pescitelli;
Chiara Delfino;
Gianni Gerlini;
Alessandro Piva;
Lorenzo Borgognoni
Show Abstract
Melanoma is a very aggressive cutaneous neoplasm, incidence and mortality of which continues to rise worldwide.
Identification of initial melanoma may be difficult because it may be clinically, and sometimes also dermoscopically,
indistinguishable from benign lesions. Currently definitive diagnosis is made only by histopathological observation of
the excised lesion. Several tools have been developed to help detecting malignant lesions. Dermoscopy highlights
numerous characteristic features of the lesion and of the pigmented network. The method we propose exploits a
multispectral imaging device to acquire a set of images in the visible and NIR range. Thanks to the fact that light propagates into the skin and reaches different depths depending on its wavelength, such a system is capable of imaging
layers of structures placed at increasing depths. Therefore a new semeiotics is proposed to describe the content of such images. Dermoscopic criteria can be easily applied to describe each image in the set, however inter-images correlation needs new suitable descriptors. The first group of new parameters describes how the dermoscopic ones, vary across the set of images. More features are then introduced. E.g. the longest wavelength where structures can be detected gives an
estimate of the maximum depth reached by the pigmented lesion. While the presence of a bright-to-dark transition
between the wavebands in the violet to blue range, reveals the presence of blue-whitish veil, which is a further
malignancy marker.
Improved DOT reconstruction by estimating the inclusion location using artificial neural network
Author(s):
Rusha Patra;
Pranab K. Dutta
Show Abstract
Diffuse optical tomography (DOT), a noninvasive imaging modality, uses near infrared light to illuminate the tissue and
reconstructs the optical parameters of the tissue from the intensity measurements at the surface. Here continuous wave
measurement with improved localization is proposed to make the overall instrument inexpensive. Due to the non-unique
solution of the inverse problem, prior information improves the resolution of the reconstructed image. An artificial neural
network (ANN) based approach is developed to obtain the location of the inclusion. The peak amplitude, 50% and 10%
bandwidth and their corresponding source-detector angles of the difference intensity plot with and without the inclusion
are taken as the input. The offset distance between the source and centre of inclusion, the angle with x-axis, sample and
inclusion radii are the output of the 2 layered error back propagation neural network. Least square optimization with
regularization term is used to minimize the mean squared error for image reconstruction. The optical parameters are
updated using the prior information from the ANN. The parameters present in double the region of detected area only are
updated. The performance of the proposed method has been assessed quantitatively by computing the mean square error,
object centroid error and misclassification ratio. The use of prior improves the convergence and reduces the presence of
ghost or noise. Hence the proposed method shows potential to improve DOT reconstruction.
Single-shot phase-shifting digital holography
Author(s):
Jing Zhang;
Yu Xie;
Guifang Li;
Yutang Ye;
Bahaa E. A. Saleh
Show Abstract
Single-shot three-dimensional biological imaging has broad applications in basic and clinical research as well as clinical
use. We propose a three-dimensional imaging system that can be configured in transmission, reflection or fluorescence
modes. This system is capable of recording/measuring the amplitude, phase and polarization of an optical wavefront in a
single shot and reconstructing numerically objects with three-dimensional volumetric information. Built upon the
principle of digital holography (DH), the proposed system uses a reference beam to interfere with the light field under
investigation and digitally records/measures the in-phase and quadrature interference patterns. Single-shot in-phase and
quadrature interference pattern recording is made possible by incorporating technologies widely used in coherent optical
communication. Specifically, a free-space optical 90° hybrid is employed to measure the complex (real and imaginary)
optical field from transmission through, reflection from or fluorescence of biological samples. We have constructed a
single-shot phase-shifting digital holography system and experimentally demonstrated its operation for the first time.
Pressure distribution in mammography: compression of breasts with malignant tumor masses
Author(s):
Daniel Förnvik;
Magnus Dustler;
Ingvar Andersson;
Håkan Brorson;
Pontus Timberg;
Sophia Zackrisson;
Anders Tingberg
Show Abstract
The pressure distribution over a compressed breast is in general heterogeneous. In this study we investigated the pressure
distribution over compressed breasts with tumor masses. Twenty-two women either recalled for work-up of findings suspicious for breast cancer in the screening program or with clinically suspected findings were included in the study.
Twenty-one lesions turned out to be malignant and one benign. The distribution of compression pressure was measured using thin FSR (Force Sensing Resistor) pressure sensors attached to the compression plate. The pressure over the breast
was ascertained by acquiring an x-ray image of the compressed breast with the pressure sensors present. The pressure
data and the mammogram were used to create a composite image with pressure data displayed as a color overlay. The
malignant tumor area generally matched an elevated pressure area and this pressure was generally higher than the
pressure over surrounding parenchyma. In 11 out of 22 (50%) subjects the maximum pressure over the breast was located over the tumor. Only 4 out of 22 (18%) masses had a lower tumor mean pressure compared to the mean pressure
over the breast (including one small < 10 mm tumor and one benign structure). The results suggest that tumors are
stiffer, thus, absorbing more pressure compared to the surrounding parenchyma and that this property can be quantified.
Refined pressure techniques could possibly be used to demonstrate the relative elasticity distribution in breast tissue, which might provide valuable differential diagnostic information.
Optimizing the acquisition parameters of a newly developed digital breast tomosynthesis system
Author(s):
Hye-Suk Park;
Ye-Seul Kim;
JaeGu Choi;
Young-Wook Choi;
Hee-Joung Kim
Show Abstract
The purpose of this study was to investigate the effect of different acquisition parameters and to characterize their
relationships in order to improve the detection of microcalcifications using digital breast tomosynthesis (DBT). DBT
imaging parameters were optimized using 32 different acquisition sets with six angular ranges (±5°, ±10°, ±13°, ±17°, ±21°, and ±25°) and eight projection views (5, 11, 15, 21, 25, 31, 41, and 51 projections). To investigate the effects of
variable angular dose distribution, the acquisition sets were evaluated with delivering more dose toward the central
views. Our results show that a wide angular range improved the reconstructed image quality in the z-direction. If a large
number of projections are acquired, then electronic noise may dominate the contrast-to-noise ratio (CNR) due to reduced
radiation dose per projection. With delivering more dose toward the central views, it was found that the vertical
resolution was reduced with increasing dose in the central PVs. On the other hand, the CNR clearly increased with
increasing concentration of dose distribution in central views. Although it was found that increasing angular range
improved the vertical resolution, it was also found that the image quality of microcalcifications in the in-focus plane did
not improve by increasing the noise due to greater effective breast thickness. Angular dose distributions, with more dose
delivered to the central views, generally yielded a higher quality factor (QF) than uniform dose distributions.
Energy dispersive X-ray diffraction computed tomography of breast-simulating phantoms and a tissue sample
Author(s):
Shyma M. Alkhateeb;
Mohamed H. Abdelkader;
David A. Bradley;
Paul Seller;
Matthew C. Veale;
Matt D. Wilson;
Silvia Pani
Show Abstract
Breast lesions and normal tissue have different characteristics of density and molecular arrangement that affect their
diffraction patterns. X-ray diffraction can be used to determine the spatial structure of such tissues at the atomic and
molecular level and Energy Dispersive X-Ray Diffraction Computed Tomography (EDXRDCT) can be used to produce
2-dimensional images of cross sections of the samples. The purpose of this work is to use an EDXRDCT system to find
the limiting visibility for details that simulate breast lesions. Results are presented for EDXRDCT images of samples of
different materials simulating breast tissue contrast and shapes. For simple circular details, the contrast between details and background in the images was measured with the goal of simulating the contrast between real breast tissue components. The limiting visible diameter was measured as a function of detail diameter for different combinations of scanning and geometrical parameters. Images of more complex test objects were assessed in terms of both contrast and accuracy of shape reproduction, evaluating the feasibility of using shape analysis as an additional parameter for lesion identification. The optimum combination of parameters are intended to be applied to the scanning of waxed breast tissue blocks.
Mask collimation meets high-efficient data acquisition: a novel design of a low-dose-CT-Scanner for breast-imaging
Author(s):
Claudia Braun;
Oleg Tischenko;
Roswitha Giedl-Wagner;
Helmut Schlattl;
Christoph Hoeschen
Show Abstract
A novel designed x-ray CT scanning geometry is proposed. Composed of a specially designed tungsten collimation mask
and a flat panel detector, which is placed inside the mask, this scanning geometry provides high efficient data acquisition
allowing dose reduction potential by a factor of two.
In recent years a first prototype of the CTDOR geometry (CT with Dual Optimal Reading) has been evaluated. It
consisted of a discontinuous ring of detectors fixated on X-Ray absorbing material. The source and an outer detector
were mounted on a gantry rotating around the inner static detector and the patient. Despite many drawbacks, resulting
images have shown promising potential of dual reading. Based on those results, the present work presents further
development and improvement of the recommended scanner geometry. The main idea consists of collimating the X-ray
beam through a specially designed shielding mask thereby reducing radiation dose and structuring data without
compromising image quality. An especially developed high precision laser-beam cutting process assures an accurate
mask crafting with tungsten shielding and window sizes of 300μm.
Additionally, simulation data were obtained with Monte Carlo calculations to test the dose reduction potential of the
scanning device. Retaining advantages of the CTDOR geometry such as 3D-capability, built-in capacity of scatter
correction and radiation structuring, a high-precision manufactured collimation mask of novel designed CT-scanner
enables high resolution images for breast-imaging in low energy ranges.
The influence of position within the breast on microcalcification detectability in continuous tube motion digital breast tomosynthesis
Author(s):
Eman Shaheen;
Nicholas W. Marshall;
Hilde Bosmans
Show Abstract
In digital breast tomosynthesis (DBT), the detectability and characterization of all lesions, especially microcalcifications,
is still an issue under investigation. For DBT systems equipped with an x-ray tube that moves continuously during
exposure, theory predicts some influence of the focal spot motion blur on detectability and diagnosis of small lesions,
such as microcalcifications. Motion blur experienced by a lesion at some position in the breast is known to depend on the
height of the lesion above the table within the breast. In this study, we investigated the influence of position above the
table on microcalcification contrast and signal difference to noise ratio (SdNR) (as a surrogate for detectability) in
tomosynthesis images, by means of a hybrid simulation method. Microcalcifications, represented by spheres of calcium
with 400 μm diameter, were simulated into projection images of homogeneous objects and into anatomical backgrounds.
The influence of system sharpness was included via the modulation transfer function (MTF) model that included
detector, focus size, tube motion and x-ray oblique entry components. Results show contrast reductions for spheres at
increasing heights above the detector in all datasets. For example, contrast drops of 31.5% and 43.1% for a sphere
inserted at 1 mm height compared to insertions at 40 mm and 69 mm above the table, respectively, were found for
spheres simulated near the chest wall for homogeneous background. For the same cases, the corresponding drops in
SdNR were 30.6% and 40.3%, respectively. Similar trends were also seen for sphere contrasts measured in anatomical
backgrounds.
Breast image registration by using non-linear local affine transformation
Author(s):
Feiyu Chen;
Peng Zheng;
Penglong Xu;
Andrew D. A. Maidment;
Predrag R. Bakic;
David D. Pokrajac;
Fengshan Liu;
Xiquan Shi
Show Abstract
A novel breast image registration method is proposed to obtain a composite mammogram from several images with
partial breast coverage, for the purpose of accurate breast density estimation. The breast percent density estimated as a
fractional area occupied by fibroglandular tissue has been shown to be correlated with breast cancer risk. Some
mammograms, however, do not cover the whole breast area, which makes the interpretation of breast density estimates
ambiguous. One solution is to register and merge mammograms, yielding complete breast coverage. Due to elastic
properties of breast tissue and differences in breast positioning and deformation during the acquisition of individual
mammograms, the use of linear transformations does not seem appropriate for mammogram registration. Non-linear
transformations are limited by the changes in the mammographic projections pixel intensity with different positions of
the focal spot. We propose a novel method based upon non-linear local affine transformations. Initially, pairs of feature
points are manually selected and used to compute the best fit affine transformation in their small neighborhood. Finally, Shepherd interpolation is employed to compute affine transformations for the rest of the image area. The pixel values in the composite image are assigned using bilinear interpolation. Preliminary results with clinical images show a good match of breast boundaries, providing an increased coverage of breast tissue. The proposed transformation is continued and can be controlled locally. Moreover, the method is converging to the ground truth deformation if the paired feature points are evenly distributed and its number large enough.
Reduction of patient dose in digital mammography: simulation of low-dose image using computed radiography system and flat panel detector system
Author(s):
Yuki Saito;
Maki Sakai;
Naotoshi Fujita;
Yoshie Kodera
Show Abstract
To reduce the patients’ exposure, several low-dose images are necessary to obtain an image that can be used for
diagnosis. However, it is clinically undesirable to expose a patient to multiple exposures in order to obtain an optimal
image. The purpose of this study was to simulate a low-dose image from the image generated by a routine-dose. Images
of acrylic steps were obtained using multiple doses in digital mammography to generate additional noise. The additional
noise was calculated as three different noise sources. This study used the digital mammography system with different
detectors. It is computed radiography (CR) system and flat panel detector (FPD) system. This noise was added to take
into account the resolution of the X-ray detector using the following filters. The filters were designed based on the
presampled modulation transfer function (MTF) and digital MTF containing aliasing. The image simulated using the
presampled MTF filter was less similar to an actual low-dose image using the CR system. The image simulated using the
digital MTF filter was closer to an actual low-dose image compared to the image simulated using the presampled MTF
filter using the CR system. The image simulated using the digital MTF filter of the FPD system was similar to an actual
low-dose image. By using the proposed method, we were able to obtain a simulated low-dose image from an image
generated by a routine-dose.
Estimating breast density with dual energy mammography: a simple model based on calibration phantoms
Author(s):
Hyunkoo Chung;
Lynda Ikejimba;
Nooshin Kiarashi;
Ehsan Samei;
Mathias Hoernig;
Joseph Y. Lo
Show Abstract
Dual energy digital mammography has been used to suppress specific breast tissue, primarily for the purpose of iodine
contrast-enhanced imaging. Another application of dual energy digital mammography is to estimate breast density, as
defined by the fraction of glandular tissue, by suppressing adipose tissue. Adipose equivalent phantoms were used to
derive the weighting factor for dual energy subtraction at 2, 4, 6, and 8 cm thickness. For each thickness besides 8 cm,
measurements were taken over a range of densities (0, 50, and 100%) and used for calibration measurements to model a density map. Once the density map was verified with uniform slabs, the density map was evaluated with 50/50 CIRS 020 phantom at 2, 4, and 6 cm thickness and demonstrated the feasibility of using dual energy subtraction to estimate breast density on complex phantoms.
Are uniform phantoms sufficient to characterize the performance of iterative reconstruction in CT?
Author(s):
Justin Solomon;
Ehsan Samei
Show Abstract
The evaluation of dose reduction potential from iterative reconstruction (IR) algorithms is an area of ongoing research in
CT. The non-linearity of IR algorithms poses challenges to using traditional image quality metrics. Past attempts to
evaluate iterative algorithms have relied on measurements taken from uniform background phantoms. In this study, noise
is evaluated in CT images with no texture (water), fine texture (sponge + water), and gross texture (acrylic spheres +
water. Images were reconstructed with a commercially available IR algorithm (SAFIRE 5) and filtered back projection
(FBP). Noise was characterized in terms of its magnitude (pixel standard deviation) and stationarity across reconstruction
algorithms and background types using an image subtraction technique. The IR algorithm reduced noise magnitude
across all dose levels by 66 ±1%, 47 ±3%, and 29 ±4% in uniform, finely textured, and grossly textured backgrounds
respectively. Noise was reasonably stationary in uniform FBP and IR images. For IR images with gross texture, pixel
noise was 29 ±4% lower in acrylic sphere regions compared to water regions in the same slice. For FBP images, there
were negligible differences between acrylic sphere and water regions in terms of pixel noise. This object-dependent
noise is a feature of SAFIRE reconstruction that has not been previously reported.
Noise power spectrum and modulation transfer function analysis of breast tomosynthesis imaging
Author(s):
Weihua Zhou;
Linlin Cong;
Xin Qian;
Yueh Z. Lee;
Jianping Lu;
Otto Zhou;
Ying Chen
Show Abstract
The recent commercialization of digital breast tomosynthesis systems realizes the clinical applications of
this novel three-dimensional imaging technology. The total dosage of breast tomosynthesis for single
patient is comparable to that of the traditional mammography. This paper presents our continuous work
on image quality analysis for the optimization of a new multi-beam breast tomosynthesis system based on
carbon nanotube X-ray emission technology. Several tomosynthesis reconstruction algorithms were
implemented to reconstruct the phantom data. Noise power spectrum and modulation transfer function
were investigated to evaluate the image quality.
System sharpness (STF) analysis of HD-OCT in 3D space using standard MTF methods
Author(s):
Horst Scherer;
Rainer Nebosis;
Malte Schulz;
Marc Weber
Show Abstract
For many applications of optical coherence tomography systems (OCT) optical resolution is a key feature. We present a method to determine the system transfer function (STF) of a full field high definition OCT (HD OCT). The measurement of the system sharpness is performed using structured glass edge phantoms, with a reflectivity adapted to the system dynamic range. After aligning the phantom within the field of view, a 3D image is recorded. A polynomial fit is applied to the 3D cube to extract the surface of the edge in space. In that way the image field curvature as well as the orientation of the surface in space are obtained. In a second step the intensity distribution of that surface is projected onto a plane using the polynomial fit parameters. Such a reconstructed planar phantom image allows a true 3D sharpness evaluation using standard MTF analysis. In this way the technical image sharpness during production can be monitored. This procedure- to our knowledge
- was applied for the first time to a full 3D OCT (SKINTELL ). Such a HD-OCT is capable of producing a full 3D image with more than 325k parallel A-scans in only one fast sweep. The optimum sharpness in every depth position is ensured during the sweep by focus tracking.
Evaluation of nonlinear pre-sampled modulation transfer function in iterative reconstruction CT
Author(s):
Hyeong Min Jin;
Jong Hyo Kim
Show Abstract
Iterative reconstruction (IR) technique is growingly used in clinical CT imaging with an expectation for improved image
quality at lower patient doses. However, the nonlinear frequency response in different noise level and object contrast is
less explored. In this study, we evaluate object contrast and dose level-dependent behavior of modulation transfer
function in iterative reconstruction computed tomography imaging with a specially fabricated phantom. We created
multi-contrast edge phantom, which consists of acrylic panel and diluted iodine contrast agent with different
concentrations. Images were acquired with a multi-detector CT (Discovery CT750 HD: GE) at four dose levels (25, 50,
100 and 200mAs), and were reconstructed using FBP and two IR techniques (ASIR50 and VEO). Edge spread functions
were extracted across angled edges on image, and were differentiated to yield line spread function. LSF were Fourier
transformed to evaluate the presampled MTFs of IR and FBP reconstruction techniques. At same dose level (200mAs),
the MTFs with higher contrast showed higher response than that of lower contrast in VEO. A MTF50 of 200mAs showed
markedly higher responses up to 23% than that of 25mAs scan with VEO reconstruction for an edge phantom of 520HU
contrast. Our study revealed that MTF of IR technique degrades depending on noise level at low dose scan. Therefore,
we recommend that its characteristic should be considered in quantitative analysis such as lesion size measurement.
An experimental study on the shift-variant MTF of CT systems using a simple cylindrical phantom
Author(s):
Soohwa Kam;
Hanbean Youn;
Ho Kyung Kim;
Hosang Jeon
Show Abstract
The modulation transfer function (MTF) is a typical parameter to measure the spatial resolution, which is an essential
factor for evaluating the performance of computed tomography (CT) systems. It is known that the CT system does not
follow the shift-invariant manner because of the cone-beam geometry and the transformation from the cylindrical
coordinates to the axial coordinates when the image reconstruction is employed. Several studies reported that if the
position of impulse receded from the center of a region of interest (ROI), the MTF degraded continuously. In this study,
the trend of shift-variant characteristics of CT systems was measured and analyzed using a novel multi-cylindrical
phantom. This study used to determine a point spread function (PSF) and MTF of a CT system using a simple cylindrical
phantom. First of all, the optimal diameter of cylinder phantoms was experimentally determined as 70 mm to obtain
reliable PSFs. Two kinds of field of views (FOVs), 40 cm and 60 cm, were used to vary reconstructed pixel sizes. The
shift-variant MTF curves were acquired at five off-center positions per FOV. For the effective analysis of MTF shiftvariance,
the integrated MTF values were calculated and used. In the result, the MTF slightly decreased as diameter
increased from CT center in the central region within the distance of 10 cm. Moreover, a considerable MTF decrease
suddenly occurred around the distance of 15 cm in the actual FOVs. The decreasing trend of the off-center spatial
resolution of CT cannot be neglected in recent radiologic and radio-therapeutic fields requiring high degree of image
precision, especially in sub-mm images. It is recommended that the ROI is laid on the CT center as close as possible. A
novel cylindrical phantom was finally suggested to effectively measure PSFs with optimal diameters for clinical FOVs in
this study. This phantom is cheap and convenient to use because it was only made of acryl with simple geometry. It is
expected that the spatial resolution of CT can be easily monitored using our methodology in clinical CT sites.
Characterisation of a breast tomosynthesis unit to simulate images
Author(s):
Alistair Mackenzie;
Nicholas W. Marshall;
David R. Dance;
Hilde Bosmans;
Kenneth C. Young
Show Abstract
The aim of this work is to characterise the image quality of a mammography system in both planar and tomosynthesis
imaging modes for the purpose of realistic image simulation. The simulation technique will be applied to projected
images from voxelised breast phantoms to investigate the imaging properties of different configurations of tomosynthesis
systems. Methods: A Hologic Dimensions mammography system was characterised in terms of noise, sharpness, and lag
in both planar and tomosythesis modes. The noise power spectra (NPS) were measured for both imaging modes. The
contributions (noise coefficients) to the NPS from electronic, quantum and structure noise were calculated. Results: The
MTF was shown to be affected by the focal spot size and the movement of the X-ray tube. These effects can be modelled
in the tomosynthesis projection images using an extended source. The quantum noise coefficients for the two imaging
modes showed a close match up to the Nyquist frequency of the tomosynthesis mode, while the electronic and structure
noise showed differences. The effect of lag and ghosting caused the signal in the last projection of a tomosynthesis run to
be between 2 and 4% higher than the first. Conclusions: We have characterised the noise and resolution properties of
images from a Hologic Dimensions tomosynthesis system. This forms the basis of further work required to create a
model for applying imaging characteristics to mathematically produced images. More work is required on detector lag
and ghosting and the influence of oblique X-ray incidence.
Characterization of spectral x-ray imaging for dental cone-beam computed tomography
Author(s):
Radin Adi Aizudin Bin Radin Nasirudin;
Petar Penchev;
Kai Mei;
Ernst J. Rummeny M.D.;
Martin Fiebich;
Peter B. Noël
Show Abstract
The recent advancement in detector technology contributed towards the development of
photon counting detectors with the ability to discriminate photons according to their energy
on reaching the detector. This provides spectral information about the acquired object; thus,
giving additional data on the type of material as well as its density. In this paper, we
investigate possible reduction of dental artifacts in cone-beam CT (CBCT) via integration of
spectral information into a penalized maximum log-likelihood algorithm. For this
investigation we simulated (with Monte-Carlo CT simulator) a virtual jaw phantom, which
replicates components of a real jaw such as soft-tissue, bone, teeth and gold crowns. A
maximum-likelihood basis-component decomposition technique was used to calculate
sinograms of the individual materials. The decomposition revealed the spatial as well as
material density of the dental implant. This information was passed on as prior information
into the penalized maximum log-likelihood algorithm. The resulting reconstructions showed
significant reduced streaking artifacts. The overall image quality is improved such that the
contrast-to-noise ratio increased compared to the conventional FBP reconstruction. In this
work we presented a new algorithm that makes use of spectral information to provide a prior
for a penalized maximum log-likelihood algorithm.
The effect of cross-scatter correction on the performance of dual energy micro-CT
Author(s):
D. Clark;
S. M. Johnston;
G. A. Johnson;
C. T. Badea
Show Abstract
Dual energy (DE) CT imaging is expected to play a major role in the diagnostic arena as it provides a quantitative
decomposition of basis materials, opening the door for new clinical applications without significantly increasing dose to
the patient. DE-CT provides a particularly unique opportunity in preclinical CT where new elemental contrast agents are
providing novel approaches for quantitative tissue characterization. We have implemented DE-CT imaging with a
preclinical dual source micro-CT scanner. With this configuration, both forward and cross-scatter can substantially
degrade image quality. This work investigated the effect of scatter correction on the accuracy of post-reconstruction
iodine and calcium decomposition. Scatter has been estimated using a lead beam stop technique. Our approach involves
noise reduction in the scatter corrected images using bilateral filtering. The scatter correction has been quantitatively
evaluated using phantom experiments and in vivo cancer imaging. As shown by our measurements, the dual source
scanning is affected more by the cross-scatter from the high energy to the low energy imaging chain. The scatter
correction reduced the presence of cupping artifacts and increased both the accuracy and precision of dual energy
decompositions of calcium and iodine. On average, the root mean square errors in retrieving true iodine and calcium
concentrations via dual energy were reduced by 32%. As a result of scatter corrections, we expect more accurate
quantification of important vascular biomarkers such as fractional blood volume and vascular permeability in preclinical
cancer studies.
Resonance-frequency based electrical impedance spectroscopy and its detection sensitivity to breast lesions
Author(s):
Sreeram Dhurjaty;
Bin Zheng;
David Gur
Show Abstract
Electrical impedance spectroscopy (EIS) has been investigated and emerged as a potential non-invasive, low cost,
and convenient tool for prescreening and detecting breast abnormalities that could lead to developing breast cancers.
However, the performance of conventional EIS is unacceptable in clinical practice. In our laboratory, we developed a
new EIS approach based on resonance frequency measurements. This system relies on parameters generated by
resonating breast capacitance with a fixed inductor in six different directions using the nipple as a reference electrode.
The system detects breast tissue abnormalities due to capacitance changes caused by angiogenesis. Although preliminary
testing results from a prospective clinical study were encouraging, we found that detection results were not robust. One
of the primary reasons is that the measured EIS signals, in particular, resonance frequencies vary with lesion-depth.
Using circuit theory we investigated and derived analytical expressions between the sensitivity of capacitance changes
and parallel resistances to pathologies with respect to distances of the lesions from the nipple electrode. The resistance
shorts the measured EIS signal thereby decreasing amplitudes of waveforms at resonance frequency. The theoretical
analysis is consistent with our experimental observation, which provides valuable data and guidelines for us to develop
and construct a new resonance-frequency based EIS system using a lumped parameter (resistance and multi-layer
capacitance) based breast model, resulting in an optimal electrical circuit for future studies.
TestDose: a SPECT image generator for clinical dosimetry studies
Author(s):
M.-P. Garcia;
H. Der Sarkissian;
E. McKay;
L. Ferrer;
Manuel Bardiès;
Daphné Villoing;
H. Batatia;
A. Basarab;
J.-Y. Tourneret;
D. Kouamé
Show Abstract
Patient-specific dosimetry in nuclear medicine relies on activity quantification in volumes of interest from scintigraphic imaging. Clinical dosimetry protocols have to be benchmarked against results computed from test phantoms. The design of an adequate model is a crucial step for the validation of image-based activ ity quantification. We propose a computing platform to automatically generate simulated SPECT images from a dynamic phantom for arbitrary scintigraphic image protocols. As regards the image generation, we first use the open-source NCAT phantom code to generate an anatomical model and 3D activity maps for different source compartments. This information is used as input for an image simulator and each source is modelled separately. Then, a compartmental model is designed, which describes interactions between dif ferent functional compartments. As a result, we can derive time-activity curves for each compartment with sampling time determined from real image acquisition protocols. Finally, to get an image at a given time after radionuclide injection, the resulting projections are aggregated by scaling the compartment contribution using the specific pharmacokinetics and corrupted by Poisson noise. Our platform consists of many software packages, either in-house developments or open-source codes. In particular, an important part of our work has been to integrate the GATE simulator in our platform, in order to generate automatically the command files needed to run a simulation. Furthermore, some developments were added in the GATE code, to optimize the generation of projections with multiple energy windows in a minimum computation time.
Comparison of correction methods for bronchial lumen and wall thickness measurement using a physical tube array phantom
Author(s):
Rafael Wiemker;
Udo van Stevendaal;
Holger Schmitt;
Amnon Steinberg;
Ekta Dharaiya;
Mark Rabotnikov;
Tobias Klinder
Show Abstract
To facilitate systematic calibration and validation of quantitative airway measurements on CT for COPD diagnosis, an
acrylic plastic phantom has been designed with an array of cylindrical tubes varying lumen diameter and wall thickness
in a systematic way, which can be manufactured by inexpensive 3D-printing. Accuracy and reproducibility of the 3Dprinting
have been confirmed by CT measurements. The multipliable, unobtrusive phantom can be scanned simultaneously
with the patient for each exam, allows scan-specific calibration, and can thus improve multicenter study
comparability between differing clinical imaging protocols.
Three different methods to correct the bronchial measurements for partial volume effect and scanner blur were tested.
The correction methods were variants of the physically motivated method suggested by Weinheimer et al. which
integrates the Hounsfield densities in a certain wall area, and derives the corrected values assuming a characteristic
constant Hounsfield density of the bronchial wall. The alternative methods compared here differ in the choice of the
integration boundaries. Analysis of CT scans showed high agreement and good noise robustness of all correction
methods on the one hand, but significant dependency on the choice of the CT reconstruction filter on the other hand,
which emphasizes the benefits of scan-specific calibration.
A statistical image reconstruction method to reduce small angle scattering induced streaking artifacts in differential
phase contrast CT
Author(s):
Kai Niu;
Ke Li;
Zhihua Qi;
Nicholas Bevins;
Joseph Zambelli;
Guang-Hong Chen
Show Abstract
Statistical iterative reconstruction methods have come to the forefront of CT research in recent years, as they
have the ability to incorporate the statistical fluctuations in CT measurements into the image reconstruction
process. While statistical iterative reconstruction methods have been found to be beneficial in CT imaging, they
have not been extensively investigated or applied in other new and promising CT imaging techniques, such as
x-ray differential phase contrast computed tomography (DPC-CT). The purpose of this study is to investigate
and apply statistical image reconstruction to DPC-CT to reduce streaking artifacts caused by strong small-angle
scattering objects.
Feasibility study of spectral imaging for differential phase contrast cone beam CT: computer simulations
Author(s):
Weixing Cai;
Ruola Ning;
Jiangkun Liu
Show Abstract
In principle, differential phase contrast (DPC) imaging allows the use of a hospital grade x-ray tube that has a large focal
spot size and a wide polychromatic spectrum. It should be noted that due to the integration of interference patterns over
the entire spectrum, the fringe contrast in the final intensity image is lower than that from a monochromatic spectrum.
Therefore better image quality should be potentially obtained if the energy-dependent interference patterns can be
analyzed separately. The key idea of the proposed spectral DPC imaging approach is to acquire DPC images for each
photon energy channel, which is named spectral DPC images. The final DPC image can be computed by summing up
these spectral DPC images or just computed using certain 'color' representation algorithms to enhance desired features.
This research is a feasibility study based on computer simulations to investigate how the spectral DPC method works for
a DPC-based cone beam CT (DPC-CBCT) system. The spectral DPC imaging approach is applied to an x-ray spectral
centered at 30keV, which is divided into four energy channels in simulation. A simple numerical phantom with low
contrast inserts is used and the entire imaging process is simulated using Fresnel diffraction theory. Phase stepping
approach is used to manifest and retrieve phase information. The phantom is scanned over a full circular trajectory and
the Hilbert filter-based FBP algorithm is used to compute the DPC-CBCT reconstruction. The reconstruction from the
proposed spectral DPC-CBCT is compared to that from the conventional DPC-CBCT that only takes detector images for
the integrated polychromatic spectrum. The uniformity, noise level and contrast of the inserts in the reconstruction are
measured and compared. Simulation results indicate that the spectral DPC imaging approach can improve object contrast and reduce noise for DPC-CBCT.
Phantom study for volume-of-interest breast imaging using differential phase contrast cone beam CT (DPC-CBCT)
Author(s):
Jiangkun Liu;
Ruola Ning;
Weixing Cai
Show Abstract
Differential phase contrast (DPC) imaging is reported to be able to deliver higher contrast-to-noise ratio (CNR)
compared to attenuation-based x-ray imaging technologies. Due to the nature of attenuation contrast, the conventional
cone beam CT (CBCT) technology has limitations in characterizing breast lesions with sufficiently high contrast and
spatial resolution. As an alternative, the grating-based DPC-CBCT technology is potentially a powerful tool for breast
imaging. However, limited by current grating fabrication techniques, a full field-of-view (FOV) that covers the whole
breast is not practical at present. Previously by our group, a volume-of-interest (VOI) imaging method, which
incorporates DPC-CBCT into a dedicated attenuation-based CBCT imaging system, was presented. In the method, the
CBCT scan was performed to localize the suspicious volume and then a VOI scan by DPC-CBCT characterized the
suspicious volume with higher contrast and resolution. In this work, we investigated the performance of DPC-CBCT
VOI imaging by performing a phantom study using our bench-top DPC-CBCT system with a hospital-grade X-ray tube.
A cylinder water phantom with a size of over twice of the FOV of our DPC-CBCT system was designed. The phantom
contains four different materials and it was scanned at four different dose levels. In thick object scanning, phase
wrapping errors cause artifacts for DPC-CBCT VOI imaging. A low-pass filter was designed to reduce the artifacts. In
order to compare the DPC-CBCT VOI with attenuation-based CBCT, the scanning data were used to reconstruct both
phase coefficient image and attenuation coefficient image. The reconstructed images will be quantitatively and visually
evaluated with regards to contrast, noise level and artifacts.
Energy-resolved interferometric x-ray imaging
Author(s):
Georg Pelzer;
Florian Bayer;
Karl Gödel;
Wilhelm Haas;
Florian Horn;
Jens Rieger;
André Ritter;
Peter Sievers;
Thomas Weber;
Andrea Zang;
Jürgen Durst;
Thilo Michel;
Gisela Anton
Show Abstract
Interferometric X-ray imaging becomes more and more attractive for applications such as medical imaging or non-destructive testing, where a compact setup is needed. Therefore a so-called Talbot-Lau interferometer in combination with a conventional X-ray tube is used.
Thereby, three different kinds of images can be obtained. An attenuation image like in conventional X-ray
imaging, an image of the differential phase-shifts caused by the object and the so-called dark-field image. The dark-field image shows information about the object's granularity even in sub-pixel dimensions what especially seems very promising for applications like mammography.
With respect to optimizing the output of interferometric X-ray imaging in any application, it is inevitable to
know the energy response of the interferometer as well as the energy dependence of the interactions of X- rays with matter.
In this contribution, simulations and measurements using a Medipix 2 and a Timepix detector are presented.
Preliminary study on phase-contrast digital tomosynthesis:
development and evaluation of experimental system
Author(s):
Ai Ikeya;
Atsushi Teramoto;
Kenji Noguchi;
Hiroshi Fujita
Show Abstract
The advantage of X-ray phase imaging is its ability to obtain information on soft tissues, which is difficult
using conventional X-ray imaging. Moreover, a sharp X-ray image can be obtained from the edge effect
resulting from phase contrast. Digital tomosynthesis is an imaging technique used to reconstruct multiple
planes in a single scan. In this study, we developed an experimental system that combines the
phase-contrast and digital tomosynthesis techniques. Our experimental system consists of a
transmission-type micro-focus X-ray source (minimum focus size: 1 μm). We also introduced an indirect
conversion-type flat panel detector (pixel pitch: 50 μm, matrix size: 2366 × 2368) as an imaging device.
The sample is placed on a computer-controlled rotation table, and projection images are captured from
various angles. The images are then reconstructed using the filtered back projection method. In the
experiments, a tomosynthesis image of an acrylic phantom was obtained at a tube voltage of 40 kV and at
a maximum projection angle of ±20°. To evaluate the edge enhancement effect by phase contrast, the
resolution, degree of edge enhancement, and image contrast were measured using the acrylic phantom. A
good edge enhancement effect was confirmed under the specified conditions. Furthermore, we compared
to the shape between the projection image and the tomosynthesis image and found that the tomosynthesis
image showed high shape reproducibility compared to the conventional projection image. These results
indicate that phase-contrast digital tomosynthesis may be useful for the three-dimensional imaging of
low-contrast material.
Detectability index of differential phase contrast CT compared with conventional CT: a preliminary channelized Hotelling observer study
Author(s):
Xiangyang Tang;
Yi Yang;
Shaojie Tang
Show Abstract
Under the framework of model observer with signal and background exactly known (SKE/BKE), we investigate the
detectability of differential phase contrast CT compared with that of the conventional attenuation-based CT. Using the
channelized Hotelling observer and the radially symmetric difference-of-Gaussians channel template , we investigate the
detectability index and its variation over the dimension of object and detector cells. The preliminary data show that the
differential phase contrast CT outperforms the conventional attenuation-based CT significantly in the detectability index
while both the object to be detected and the cell of detector used for data acquisition are relatively small. However, the
differential phase contrast CT’s dominance in the detectability index diminishes with increasing dimension of either
object or detector cell, and virtually disappears while the dimension of object or detector cell approaches a threshold,
respectively. It is hoped that the preliminary data reported in this paper may provide insightful understanding of the
differential phase contrast CT’s characteristic in the detectability index and its comparison with that of the conventional
attenuation-based CT.
Artifacts in X-ray dark-field measurements
Author(s):
Florian Horn;
Florian Bayer;
Karl Gödel;
Wilhelm Haas;
Georg Pelzer;
Jens Rieger;
André Ritter;
Thomas Weber;
Andrea Zang;
Jürgen Durst;
Thilo Michel;
Gisela Anton
Show Abstract
Grating-based X-ray phase-contrast imaging with a Talbot-Lau interferometer is a promising method which
might be able to increase soft tissue contrast and to gain additional information in comparison to attenuationbased
imaging. The method provides an attenuation image, a differential phase image and a dark-field image.
A conventional polychromatic X-ray tube can be used together with a Talbot-Lau interferometer consisting of
a source grating, a phase grating and an absorption grating. The dark-field image shows information about the
sub-pixel-size granularity of the measured object. This supplemental information is supposed to be suitable in
applications, such as mammography or nondestructive testing.
In this contribution we present results of measurements investigating the thickness-dependent behavior of
dark-field imaging. The measurements are performed with a wedge-shaped, granular object with our X-ray
phase-contrast imaging set-up and calculating the dark-field image. Measurements with this special phantom
show a resurgence of visibility contrast with increasing thickness of the object after passing a minimum. The
reason of this artifact is not completely clear up to now, but might be found in attenuation effects in the object
in combination with the polychromatic X-ray spectrum or in residual amplitudes in our fitting algorithm for low
visibilities and low intensities at large thicknesses. Understandig the thickness-dependent behavior of the X-ray
dark-field advances the understanding of the formation of the dark-field image.
Experimental measurement of the modulation transfer function of differential phase contrast CT systems
Author(s):
Ke Li;
Nicholas Bevins;
Joseph Zambelli;
Guang-Hong Chen
Show Abstract
This paper concerns an experimental method to quantify the spatial resolution of an experimental DPC-CT
system via modulation transfer function (MTF) measurement. Note that conventional metal wire-based MTF
measurement methods are no long applicable to DPC-CT, because: (i) The refractive signal generated by the
high density metal is out of the dynamic range of DPC-CT, and (ii) A DPC-CT system is not sensitive to
input pulse finer than a detector pixel. These technical challenges were overcome by a new experimental design,
in which a graphite rod was used as the probe to simultaneously measure the MTFs of the DPC-CT and the
associated absorption CT. An appreciable difference in MTFs between DPC-CT and absorption CT was observed
in our experimental benchtop system: At the 10% MTF level, the MTFs of the DPC-CT and the absorption
CT are 4.7 cycles/mm and 5.2 cycles/mm, respectively. Such a difference in MTF leads to the conclusion that
the grating interferometer used by the DPC-CT system has a frequency-dependent response to input DPC-CT
signals.
Single-step phase contrast x-ray imaging using photon counting detectors
Author(s):
Doga Gürsoy;
Mini Das
Show Abstract
Using solutions of spectral transport-of-intensity equations, we have demonstrated a single step method to
retrieve absorption and phase changes for a wide range of x-ray imaging energies and material composition. We
simulated a Cadmium-Zinc-Telluride based spectral detection system using a cascade model for investigations of
breast mass and microcalcification detectability when using both absorption and phase images simultaneously.
A compendium of publicly available Monte Carlo transport codes (including new tools) for the simulation of radiation imaging detectors
Author(s):
Diksha Sharma;
Han Dong;
Yuan Fang;
Aldo Badano
Show Abstract
Simulations play a vital role in the understanding and analysis of existing and emerging medical imaging tech nologies. Over the last years, Monte Carlo simulations have become increasingly necessary tools for studying the fundamental limitations and for the design and optimization of medical imaging systems. We compare available open-source software packages from the Division of Imaging and Applied Mathematics at the FDA for modeling scintillator- and semiconductor-based radiation imaging detectors for applications in x-ray and nuclear imaging including MANTIS, hybridMANTIS, cartesianDETECT2, and ARTEMIS. We describe the significant features of these packages and discuss their advantages or disadvantages. We also introduce a graphical user interface which greatly facilitates the set up of simple experiments involving scintillator structures with columnar geometries.
Evaluating radiation damage to scintillating plastic fibers with Monte Carlo simulations
Author(s):
Aimee L. McNamara;
Samuel J. Blake;
Philip Vial;
Lois Holloway;
Peter B. Greer;
Zdenka Kuncic
Show Abstract
Current electronic portal imaging devices (EPIDs) are generally used for megavoltage imaging in radiotherapy and employ a thin Cu plate/ phosphor screen to convert x-ray energies into optical photons . In order to achieve a high spatial resolution, thin screens are used which subsequently results in low x-ray absorption and thus a low detective quantum efficiency (DQE) for megavoltage x-rays. Additionally, the high atomic number Cu/ phosphor screen materials is not ideal for dosimetric applications. To improve the imaging and dosimetry dual-functionality of EPIDs, water equivalent plastic scintillators have been proposed. Plastic scintillator fibers may however be susceptible to radiation damage caused primarily by ionizations from low energy secondary electrons. An accumulation or clustering of these ionization events, within regions corresponding to the volume of the plastic polymer chains, may lead to chain breaks. This could result in changes to the optical photon absorption properties and optical yield of the fiber, affecting the overall imaging performance of the detector. Here we used Monte Carlo radiation transport simulations for a preliminary investigation into the distribution of ionizations within a single plastic fiber. We find a large number of ionization events can accumulate along the fiber length, which over repeated exposures could lead to damage. To determine the effect of damage on the imaging performance, two fiber arrays were modeled with and without areas of damage. The damaged fiber array was found to produce approximately half the number of counts as the undamaged array.
A 2.5 dimensional vein imaging system for venipuncture
Author(s):
Xiaoming Hu;
Ya Zhou;
Zhaoguo Wu
Show Abstract
Imaging of subcutaneous veins is important in many applications, such as gaining venous access, vascular surgery and
venipuncture. Traditional vein imaging system can only obtain the two dimensional information of the vein which loses
the depth information of the vein. It may cause the errors of judgment and increase the venipuncture failure.
On the basis of previous research, a new system was proposed to acquire the three dimensional of the vein. In this paper,
the infrared absorption characteristics of the vein and the principle of binocular vision were combined to obtain infrared
images of subcutaneous veins and recovery the three dimensional information. The binocular vision system was consists
of several 850 nm near-infrared LEDs to illuminate the back of the hand and two near-infrared CCD devices to obtain
the transmission of IR image.
The couple of CCDs will get IR images of the hand which contain the disparity information. The principle of stereo
vision was used to recover the three dimensional structure. The algorithm processes includes camera calibration, image
preprocessing, epipolar rectification, stereo correspondence and three dimensional reconstructions. Experimental result
shows that it can acquire a good three dimensional structure. Since the new system can recover the depth of the vein, it
can be applied as the venipuncture auxiliary equipment to improve the success rate of venipuncture.
Noise reduction for cone-beam SPECT by penalized reweighted least-squares projection restoration
Author(s):
Hao Zhang;
Junhai Wen;
Donghao Shi;
Rui Yang;
Jing Wang;
Zhengrong Liang
Show Abstract
In single photon emission computed tomography(SPECT), the non-stationary Poisson noise in the projection data is
one of the major degrading factors that jeopardize the quality of reconstructed images. In our previous researches for
low-dose CT reconstruction, based on the noise properties of the log-transformed projection data, a penalized
weighted least-squares (PWLS) cost function was constructed and the ideal projection data(i.e., line integral) was
then estimated by minimizing the PWLS cost function. The experimental results showed the method could effectively suppress the noise without noticeable sacrifice of the spatial resolution for both fan- and cone-beam
low-dose CT reconstruction. In this work, we tried to extend the PWLS projection restoration method to SPECT by redefining the weight term in PWLS cost function, because the weight is proportional to measured photon counts for transmission tomography (i.e., CT) while inversely proportional to measured photon counts for emission tomography (i.e., SPECT and PET). The iterative Gauss-Seidel algorithm was then used to minimize the cost function, and since
the weight term was updated in each iteration, we refer our implementation as penalized reweighted least-squares
(PRWLS) approach. The restorated projection data was then reconstructed by an analytical cone-beam SPECT reconstruction algorithm with compensation for non-uniform attenuation. Both high and low level Poisson noise was
simulated in the cone-beam SPECT projection data, and the reconstruction results showed feasibility and efficacy of
our proposed method on SPECT.
Iterative image reconstruction for sparse-view CT using normal-dose image induced total variation prior
Author(s):
Yunwan Zhang;
Jianhua Ma;
Jing Huang;
Hua Zhang;
Zhaoying Bian;
Dong Zeng;
Qianjin Feng;
Zhengrong Liang;
Wufan Chen
Show Abstract
Sparse-view x-ray computed tomography (CT) imaging still is an interesting topic in CT field. In this paper, a new
iterative image reconstruction approach for sparse-view CT with a normal-dose image was presented. The proposed
cost-function which is under the criteria of penalized weighed least-square (PWLS) for CT image reconstruction mainly
contains two terms, i.e., fidelity term and prior term. For the fidelity term, the weights of weighed least-square term are
determined by considering the relationship between the variance and mean of the projection data in the presence of
electronic background noise. For the prior term, a normal-dose image induced total variation (ndiTV) prior is proposed
as an extension of the PICCS algorithm introduced by Chen et al 2008, which can relieve the requirement of
misalignment reduction of the PICCS algorithm. For simplicity, the present approach is referred to as “PWLS-ndiTV”.
Qualitative and quantitative evaluations were carried out on the present PWLS-ndiTV approach. Experimental results
show that the present PWLS-ndiTV approach can achieve significant gains than the existing similar methods in noise
and artifacts suppression.
Semi-dynamic preconditioned alternating projection MAP ECT reconstruction from low-dose ECT
Author(s):
Andrzej Krol;
Si Li;
Yuesheng Xu;
Jiahan Zhang;
Levon O. Vogelsang;
Lixin Shen;
David H. Feiglin
Show Abstract
The objective of this study was to develop very low noise and high-contrast-to-noise ratio fast proximity algorithm for
MAP ECT reconstruction that would allow significant (factor of two or more) patient’s dose reduction, as compared to
conventional OSEM algorithm. We proposed a semi-dynamic Preconditioned Alternating Projection Algorithm (PAPA)
for solving the maximum a posteriori (MAP) emission computed tomography (ECT) reconstruction problem.
Specifically, we formulated the reconstruction problem as a constrained convex optimization problem with the total
variation (TV) regularization. We characterized the solution of the constrained convex optimization problem and showed
that it satisfies a system of fixed-point equations defined in terms of two proximity operators arising from the convex
functions that define the TV-norm and the constrain involved in the problem. We proved theoretically the convergence
of the proposed algorithm. For efficient numerical computation, we introduced to the alternating projection algorithm a
preconditioning matrix: the EM-preconditioner. In numerical experiments using Monte Carlo simulated SPECT data
performance of our algorithms was compared with performance of the conventional EM algorithm with Gaussian postfilter.
Based on the results of these experiments, we observed that PAPA algorithm with the EM-preconditioner
outperforms very significantly the benchmark EM in terms of contrast-to-noise ratio and the noise characteristics of the
reconstructed images.
A new imaging method for real-time 3D x-ray reconstruction
Author(s):
Murat Tahtali;
Sajib K. Saha;
Andrew J. Lambert;
Mark R. Pickering
Show Abstract
Existing Computed Tomography (CT) systems are vulnerable to internal organ movements. This drawback is
compensated by extra exposures and digital processing. CT being a radiation dose intensive modality, it is imperative to
limit the patient’s exposure to X-ray radiation, if only by removing the necessity to take extra exposures. A multiple
pinhole camera, akin to optical lightfield imaging, to acquire simultaneously multiple X-ray projections is presented.
This new method allows a single snapshot acquisition of all necessary projections for 3D reconstruction. It will also
allow the real-time dynamic 3D X-ray reconstruction of moving organs, as it requires no scanning and no moving parts
in its final implementation. A proof-of-concept apparatus that simulates the intended process was built and parallaxed
images were obtained with minor processing. Synthetic 3D reconstruction tests are also presented.
Characterization of a digital x-ray detector for region of interest tuberculosis screening
Author(s):
Ryan Mann;
Ian A. Cunningham;
Karim S. Karim
Show Abstract
Cost and accessibility are major barriers to x-ray medical diagnostic screening in low- to mid-income countries. The cost
of traditional medical-grade x-ray imaging systems is prohibitively large except to major hospitals in urban centers,
preventing the early diagnosis of many curable diseases. Sputum, blood and urine tests are slower, and difficult to
administer in remote locations, having high associated transportation and storage costs. A low-cost, tuberculosis-specific,
teleradiology-enabled, digital x-ray imaging system is proposed that will make diagnosis fast, accessible and
inexpensive. A system of this type would ideally cost below $10,000 and is achievable today using a combination of
commodity and industrial products, region-of-interest imaging, and an optimization of resolution and sensitivity
requirements for the task of screening pulmonary tuberculosis.
In this research, we report preliminary investigations we have carried out on the x-ray detector, a PerkinElmer (XRD
0820) industrial-grade, 8”x8” flat-panel, amorphous silicon array with 200 micron pixels. Detective quantum efficiency
(DQE), modulation transfer function (MTF) and noise power spectrum (NPS) measurements are taken at a typical chest
radiography exposure that allow a direct comparison with high-end chest x-ray detectors and existing CR systems.
Image acquisition, geometric correction and display of images from a 2x2 x-ray detector array based on electron multiplying charge coupled device (EMCCD) technology
Author(s):
S. N. Swetadri Vasan;
P. Sharma;
Ciprian N. Ionita;
A. H. Titus;
A. N. Cartwright;
D. R. Bednarek;
S. Rudin
Show Abstract
A high resolution (up to 11.2 lp/mm) x-ray detector with larger field of view (8.5 cm x 8.5 cm) has been
developed. The detector is a 2 x 2 array of individual imaging modules based on EMCCD technology. Each module
outputs a frame of size 1088 x 1037 pixels, each 12 bits. The frames from the 4 modules are acquired into the processing
computer using one of two techniques. The first uses 2 CameraLink communication channels with each carrying
information from two modules, the second uses a application specific custom integrated circuits, the Multiple Module
Multiplexer Integrated Circuit (MMMIC), 3 of which are used to multiplex the data from 4 modules into one
CameraLink channel. Once the data is acquired using either of the above mentioned techniques, it is decoded in the
graphics processing unit (GPU) to form one single frame of size 2176 x 2074 pixels each 16 bits. Each imaging module
uses a fiber optic taper coupled to the EMCCD sensor. To correct for mechanical misalignment between the sensors and
the fiber optic tapers and produce a single seamless image, the images in each module may be rotated and translated
slightly in the x-y plane with respect to each other.
To evaluate the detector acquisition and correction techniques, an aneurysm model was placed over an anthropomorphic
head phantom and a coil was guided into the aneurysm under fluoroscopic guidance using the detector array. Image
sequences before and after correction are presented which show near-seamless boundary matching and are well suited for
fluoroscopic imaging.
Rectangular computed tomography using a stationary array of CNT emitters: initial experimental results
Author(s):
Brian Gonzales;
Derrek Spronk;
Yuan Cheng;
Zheng Zhang;
Xiaochuan Pan;
Moritz Beckmann;
Otto Zhou;
Jianping Lu
Show Abstract
XinRay Systems Inc has a rectangular x-ray computed tomography (CT) imaging setup using multibeam x-ray tubes.
These multibeam x-ray tubes are based on cold cathodes using carbon nanotube (CNT) field emitters. Due to their
unique design, a CNT x-ray tube can contain a dense array of independently controlled electron emitters which generate a linear array of x-ray focal spots. XinRay uses a set of linear CNT x-ray tubes to design and construct a stationary CT setup which achieves sufficient CT coverage from a fixed set of views. The CT system has no moving gantry, enabling it to be enclosed in a compact rectangular tunnel. The fixed locations of the x-ray focal spots were optimized through simulations. The rectangular shape creates significant variation in path length from the focal spots to the detector for different x-ray views. The shape also results in unequal x-ray coverage in the imaged space. We discuss the impact of this variation on the reconstruction. XinRay uses an iterative reconstruction algorithm to account for this unique geometry, which is implemented on a graphics processing unit (GPU). The fixed focal spots prohibit the use of an antiscatter grid. Quantitative measure of the scatter and its impact on the reconstruction will be discussed. These results represent the first known implementation of a completely stationary CT setup using CNT x-ray emitter arrays.
Multi-resolution analysis of scatter in digital breast tomosynthesis imaging
Author(s):
Da Zhang;
Xinhua Li;
Bob Liu
Show Abstract
The influence of large x-ray scatter components in projection images remains a problem for digital breast tomosynthesis,
especially when anti-scatter grids may not be used because of dose limitation and possible source/detector
geometric limitations. Monte-Carlo simulation of scatter fits better in this situation, but the heavy computational
cost hinders its clinical application. To simplify scatter estimation, scatter is often assumed to be smooth.
However, scatter is not spatial invariant across a projection image, and where and to what degree the smoothness
could be claimed and utilized is unclear. In this study, we investigated this question via multi-resolution
analysis based on two experiments: one with direct measurements of scatter profiles in the projection images
of an anthropomorphic breast phantom; the other with scatter map obtained from Monte-Carlo simulation that
used a voxelized breast model as input. We applied 1D and 2D wavelet-based multi-resolution analyses to the
scatter profiles and maps. The first experiment indicated that a reduced number of scatter data points that
matches the true data can be extracted from densely sampled but noisy scatter profiles: a data reduction rate
of 64-128 was achieved at the inner region of the phantom, suggesting that the slowly changing scatter may be
obtained at lower sampling distances of 9.0-17.9 mm. Near the edge of the phantom a data reduction rate of 8
was achieved, corresponding to a sampling distance of 2.2 mm. Similar observations were made from the second
experiment.
Metal artifact reduction in tomosynthesis by metal extraction and ordered subset-expectation maximization (OS-EM) reconstruction
Author(s):
Tomonori Sakimoto;
Kazuyoshi Nishino
Show Abstract
Tomosynthesis is a useful imaging tool for breast, lung, and orthopedic diagnostics. Compared with computed
tomography (CT), fewer artifacts are caused by metal components (metal artifacts). This advantage makes tomosynthesis
particularly useful for orthopedics. Implementing filtered back projection (FBP) with a modified kernel leads to an
increase in the low-frequency components of reconstructed images and reduces metal artifacts in tomosynthesis.
However, even using this reconstruction method, metal artifacts are present in the region very close to any piece of
metal. Due to the modified kernel, the observation of fine structures is difficult. We developed a new reconstruction
algorithm to provide fewer metal artifacts in tomosynthesis images than in conventional images without filtering. Our
new algorithm consists of four steps: 1) automatically extracting metal components from projection images using a novel
method that we developed; 2) dividing projection images into metal-free projection images and metal-only projection
images; 3) reconstructing these two projection images using the ordered subset-expectation maximization (OS-EM)
method to create metal-free tomosynthesis images and metal-only tomosynthesis images; and 4) combining the
tomosynthesis images and thereby obtaining metal artifact-reduced tomosynthesis images. Our new metal extraction
method in step 1 is based on the graph cuts algorithm. We compared four image reconstruction algorithms: (a) FBP, (b)
FBP with a modified kernel, (c) simple OS-EM and (d) the proposed method. The results demonstrate that the proposed
method significantly reduces metal artifacts when compared with the other methods.
Feasibility of stationary digital breast tomosynthesis as an effective screening tool for patients with augmentation mammoplasty
Author(s):
Andrew W. Tucker;
Cherie M. Kuzmiak M.D.;
Christy Inscoe;
Yueh Z. Lee M.D.;
Jianping Lu;
Otto Zhou
Show Abstract
Conventional mammography techniques used for imaging patients that have undergone augmentation mammoplasty
produce substandard images, incomplete evaluation of breast tissue, and can cause discomfort to the patient. Typically,
four images of each breast are acquired (double the amount of a patient without implants), two with the implant in view
and two "pushback" views, which moves the implant posteriorly out of the field of view. The "pushback" view can be
difficult to perform when there is encapsulation of the breast tissue around the implant. In severe cases, performing the
technique can cause unwarranted pain to the patient. This technique can also result in up to a three times increase in
procedure time which leads to lower patient throughput. Using 2D mammography, it is difficult to interpret tissue above
and below implants due to the overlap of breast tissue and implant. Recently, Digital Breast Tomosynthesis (DBT) has
been shown to aid in the localization and diagnosis of breast masses by removing underlying and overlying tissue from
the plane of interest in 3D space. However, commercial DBT systems have motion blurring of the projection images
associated with x-ray source motion. To overcome this limitation, stationary DBT (s-DBT) has been developed. Here we
report the feasibility of using s-DBT as an effective screening tool for patients who have undergone augmentation
mammoplasty. Qualitative image analysis is completed on reconstruction images of tomosynthesis phantoms combined
with implants. Reconstruction images show that it is possible to locate lesions above and below the implant using a
clinically relevant entrance dose.
Impact of subtraction and reconstruction strategies on dual-energy contrast enhanced breast tomosynthesis with interleaved acquisition
Author(s):
Lin Chen;
Yihuan Lu;
Yue-Houng Hu;
Wei Zhao;
Gene Gindi
Show Abstract
Contrast enhanced digital breast tomosynthesis can yield superior visualization of tumors relative to conventional
tomosynthesis and can provide the contrast uptake kinetics available in breast MR while maintaining a higher image
spatial resolution. Conventional dual-energy (DE) acquisition protocols for contrast enhancement at a given time point
often involve two separate continuous motion sweeps of the X-ray tube (one per energy) followed by weighted
subtraction of the HE (high energy)and LE (low energy) projection data. This subtracted data is then reconstructed.
Relative to two-sweep acquisition, interleaved acquisition suffers from a lesser degree of patient motion artifacts and
entails less time spent under uncomfortable breast compression. These advantages for DE interleaved acquisition are
reduced by subtraction artifacts due to the fact that each HE, LE acquisition pair is offset in angle for the usual case of
continuous tube motion. These subtraction artifacts propagate into the reconstruction and are present even in the absence
of patient motion. To reduce these artifacts, we advocate a strategy in which the HE and LE projection data are
separately reconstructed then undergo weighted subtraction in the reconstruction domain. We compare the SDNR of
masses in a phantom for the subtract-then-reconstruct vs. reconstruct-then-subtract strategies and evaluate each strategy
for two algorithms, FBP and SART. We also compare the interleave SDNR results with those obtained with the
conventional dual-energy double-sweep method. For interleave scans and for either algorithm the reconstruct-thensubtract
strategy yields higher SDNR than the subtract-then-reconstruct strategy. For any of the three acquisition modes,
SART reconstruction yields better SDNR than FBP reconstruction. Finally the interleave reconstruct-then-subtract
method using SART yields higher SDNR than any of the double-sweep conventional acquisitions.
A simulation based image reconstruction strategy with predictable image quality in limited-angle x-ray tomography
Author(s):
Shiyu Xu;
Ying Chen
Show Abstract
This paper presents a Pre-computed BackProjection (BP) based Penalized-likelihood (PPL) method for limited angle X-ray tomography based on the theory of resolution properties of regularized image reconstruction. Pre computed BP based penalty is a simplified version of the modified quadratic penalty proposed in the literature. 1
By inserting a BP equivalent estimation into a quadratic penalty, the data-related terms in the impulse response and noise reconstructed by PPL are absorbed, such that the effects of smoothing parameter of the penalty can be evaluated in advance through the simulated data. A simulation based two-step procedure is proposed to apply PPL method in real applications. It reconstructs images with predictable resolution properties by choosing a corresponding smoothing parameter. The effectiveness and robustness of the two-step strategy is validated through simulation based experiments.
Comparison of the diagnostic accuracy of stationary digital breast tomosynthesis to digital mammography with respect to lesion characterization in breast tissue biopsy specimens: a preliminary study
Author(s):
Andrew W. Tucker;
Yueh Z. Lee M.D.;
Cherie M. Kuzmiak M.D.;
Emily Gidcumb;
Jianping Lu;
Otto Zhou
Show Abstract
Current practice for imaging surgical breast specimens is a single 2D magnification view on a mammography system,
but 2D imaging overlaps the tissue in different planes causing distortion of lesion margins. Digital breast tomosynthesis
(DBT) could be used as an alternative imaging modality for imaging breast specimens. DBT systems acquire multiple
low dose projection images, over a small angular span, which are then reconstructed into a partial 3D volume. The
reconstructed images can be used to increase visualization of lesion margins and extent of microcalcifications (MCs).
Current commercial DBT systems use a single rotating X-ray source, the movement of which produces motion blur.
Motion blur reduces visualization of small objects such as MCs. MCs, depending on size and structure, can be
implicative of breast cancer. We have developed a stationary DBT (s-DBT) system using a linearly distributed, CNT Xray
source array. S-DBT allows for rapid acquisition of projection images with no image degradation from X-ray source
motion. Full tomosynthesis datasets can be acquired, allowing visualize of both masses and microcalcifications. Here we
report the preliminary results of a reader study comparing breast specimen images from a 2D commercial mammography
system and an s-DBT system. Preliminary results show that s-DBT is capable of producing equivalent image quality to
2D mammography, and in some cases is superior.
Preliminary evaluation of transmission x-ray tube system with a flat-detector DR system: image quality and dose reduction
Author(s):
Leonard Berliner M.D.;
Kui-Ming Chen;
Shenq-Rong Hwang;
Alfonso Buffa;
Martin Darms;
Andrew Jeffries
Show Abstract
Ionizing radiation from medical imaging now accounts for over 95% of all man-made radiation exposures and is the
single largest radiation source after natural background radiation. As a result, new techniques are under development for
reducing radiation exposure incurred in diagnostic radiography.
It was the purpose of this study to determine if a transmission X-ray tube and generator system in conjunction with a
flat-panel detector is capable of achieving diagnostic quality radiographic images at reduced radiation doses.
It was found that transmission tube technology, in combination with a flat-detector system, is capable of producing
radiographic images of sufficient quality for diagnostic medical imaging within certain parameters. It is postulated at this
time that when low mAs is required, as in imaging of neonatal and young pediatric patients, the transmission tube may
prove to be very effective in obtaining diagnostic images at reduced radiation dose.
X-ray tube focal spot size, digital detectors, imaging system aperture and spatial resolution
Author(s):
Edward L. Nickoloff
Show Abstract
Most modern radiology facilities are all digital, and film is not available for physics QC testing. An important test is
the measurement of the effective focal spot sizes of the x-ray tube which has an impact on overall image system spatial
resolution. Focal spot size measurements typically utilize a star pattern which is magnified and recorded on plain film.
Plain film is ideal because it has a spatial resolution of 50-75 LP/mm. Utilizing digital image receptors to record the
star pattern images does degrade the measurement because of the size of the individual detector elements (DEL). This
article presents a method for the determination of focal spot size using the digital image receptor (DIR) to record the
image of the star pattern. An equation which corrects for the image blur introduced by the finite size of the digital
detector is given. Measurements of effective focal spot sizes are still important to the assessment of radiographic
spatial resolution.
Physical model-based metal artifact reduction (MAR) scheme for a 3D cone-beam CT extremity imaging system
Author(s):
D. Yang;
R. A. Senn;
N. Packard;
S. Richard;
J. Yorkston
Show Abstract
In Cone Beam CT Imaging, metallic and other dense objects, such as implantable orthopedic appliances, surgical clips
and staples, and dental fillings, are often acquired as part of the image dataset. These high-density, high atomic mass
objects attenuate X-rays in the diagnostic energy range much more strongly than soft tissue or bony structures, resulting
in photon starvation at the detector. In addition, signal behind the metal objects suffer from increased quantum noise,
scattered radiation, and beam hardening. All of these effects combine to create nonlinearities which are further amplified
by the reconstruction algorithm, such as conventional filtered back-projection (FBP), producing strong artifacts in the
form of streaking. They reduce image quality by masking soft tissue structures, not only in the immediate vicinity of the
dense object, but also throughout the entire image volume. A novel, physical-model-based, metal-artifact reduction
scheme (MAR) is proposed to mitigate the metal-induced artifacts. The metal objects are segmented in the projection
domain, and a physical model based method is adopted to fill in the segmented area. The FDK1 reconstruction algorithm
is then used for the final reconstruction.
Theoretical performance analysis for CMOS based high resolution detectors
Author(s):
Amit Jain;
Daniel R. Bednarek;
Stephen Rudin
Show Abstract
High resolution imaging capabilities are essential for accurately guiding successful endovascular
interventional procedures. Present x-ray imaging detectors are not always adequate due to their
inherent limitations. The newly-developed high-resolution micro-angiographic fluoroscope
(MAF-CCD) detector has demonstrated excellent clinical image quality; however, further
improvement in performance and physical design may be possible using CMOS sensors. We
have thus calculated the theoretical performance of two proposed CMOS detectors which may be
used as a successor to the MAF.
The proposed detectors have a 300 μm thick HL-type CsI phosphor, a 50 μm-pixel CMOS
sensor with and without a variable gain light image intensifier (LII), and are designated MAFCMOS-
LII and MAF-CMOS, respectively. For the performance evaluation, linear cascade
modeling was used. The detector imaging chains were divided into individual stages
characterized by one of the basic processes (quantum gain, binomial selection, stochastic and
deterministic blurring, additive noise). Ranges of readout noise and exposure were used to
calculate the detectors' MTF and DQE.
The MAF-CMOS showed slightly better MTF than the MAF-CMOS-LII, but the MAF-CMOSLII
showed far better DQE, especially for lower exposures.
The proposed detectors can have improved MTF and DQE compared with the present high
resolution MAF detector. The performance of the MAF-CMOS is excellent for the angiography
exposure range; however it is limited at fluoroscopic levels due to additive instrumentation noise.
The MAF-CMOS-LII, having the advantage of the variable LII gain, can overcome the noise
limitation and hence may perform exceptionally for the full range of required exposures;
however, it is more complex and hence more expensive.
A newly developed a-Se mammography flat panel detector with high-sensitivity and low image artifact
Author(s):
Yoshihiro Okada;
Keiichiro Sato;
Takaaki Ito;
Yuichi Hosoi;
Toshiro Hayakawa
Show Abstract
A novel amorphous selenium (a-Se) detector with the hexagonal pixel has been developed for full-field digital
mammography. The pixel area of the detector was designed to be same as that of the 68 μm square pixel detector, while
the pixel pitch between neighboring pixels was set to be 73-75 μm in six directions. By applying the hexagonal pixels,
the sensitivity of the detector has improved by 18% compared with a conventional square pixel. A simulation showed
that the hexagonal pixel provided a more uniform electric field in the a-Se layer than the square pixel, which has lead to
higher sensitivity. The modulation transfer function of the detector was 92 % at 2 mm-1 and 62 % at 5 mm-1. These
values were as high as that of a conventional a-Se detector with 50 μm square pixels. As a result, the detective quantum
efficiency of this detector achieved 75 % with 5 mR and 72 % with 2.5mR at 2 mm-1. The exposure conditions were 28
kV W/Rh with a 2 mm aluminum filter. Therefore, the new detector can reduce the exposure dose while maintaining a
high image quality.
Monte Carlo simulation of bowtie filter scatter on a wide-cone low-dose CT system
Author(s):
Xin Liu;
Anjali Srivastava;
Hyoung-Koo Lee;
Jiang Hsieh
Show Abstract
Knowledge of scatter generated by bowtie filter is crucial for providing artifact free images on the wide-cone low-dose
CT scanners. We investigate and determine the scatter level and artifact generated by the widely used bowtie filter in a
wide-cone low-dose CT system. Our approach is to use Monte Carlo simulation to estimate the scatter level generated by
a bowtie filter made of a material with low atomic number. First, major components of CT systems, such as source, prepatient
collimator, flat filter, bowtie filter, body phantom, and an optional post patient collimator (anti-scatter grid), are
built into a 3D model. The scattered photon fluence and the primary transmitted photon fluence are simulated by
MCNP5 - a Monte Carlo simulation toolkit. With the increased interests in the low dose and wide coverage CT
technology, a tube potential of 80 kVp with more than 10 degree of cone angle is selected. The biased sinogram is
created by superimposing scatter signal generated by the bowtie filter onto the primary x-ray beam signal. Finally,
images with artifacts are reconstructed with the biased signal. Methods to reduce bowtie filter scatter are also discussed
and demonstrated.
Quantifying cross scatter in biplane fluoroscopy motion analysis systems
Author(s):
Janelle A. Cross;
Ben McHenry;
Taly Gilat Schmidt
Show Abstract
Biplane fluoroscopy is currently used for dynamic, in vivo three-dimensional motion analysis of various joints of the body. The benefits of fluoroscopy compared to conventional optical marker tracking methods are the elimination of marker skin motion artifacts, and the ability to directly quantify in vivo skeletal motion that is not optically accessible while wearing orthotic devices and footwear. One potential drawback for biplane fluoroscopy is the cross-scatter contamination between two gantries, as the acquisitions are typically synchronized to facilitate motion tracking. The purpose of this study was to experimentally measure the magnitude and effects of cross-scatter in biplane fluoroscopic images acquired over a range of gantry angles (45-90°) and kV settings (60-110 kV). Four cylindrical water phantoms of 4, 6, 8, and 10-in diameter were imaged, each containing a 1-in diameter Teflon sphere. The cross-scatter fraction and the relative change in contrast-to-noise ratio due to cross scatter were calculated. Results demonstrated that the crossscatter fraction varied from 0.051 for the 4-in cylinder to 1.326 for the 10-in cylinder at 60 kV, and from 0.010 to 0.832 at 110 kV. The reduction in ΔCNR ranged from 0.974 (110 kV, 75°) for the 4-in cylinder to 0.618 (60 kV, 60°) for the 10-in cylinder. The results suggest that cross-scatter contamination during biplane fluoroscopy is relatively low when imaging distal extremities, and would not likely require antiscatter grids or asynchronous timing circuits. Analyzing joints with more soft tissue may introduce cross scatter that could reduce accuracy and may require additional scatter reduction hardware.
Full-field digital mammography with grid-less acquisition and software-based scatter correction: investigation of dose saving and image quality
Author(s):
Andreas Fieselmann;
Daniel Fischer;
Ghani Hilal;
Frank Dennerlein;
Thomas Mertelmeier;
Detlev Uhlenbrock
Show Abstract
Anti-scatter grids used in full-field digital mammography not only attenuate scattered radiation but also attenuate
primary radiation. Dose saving could be achieved if the effect of scattered radiation is compensated with a software-based scatter correction not attenuating the primary radiation. In this work, we have carried out phantom studies in order to investigate dose saving and image quality of grid-less acquisition in combination with software-based scatter correction. The results show that similar image quality (contrast-to-noise ratio and contrast-detail visibility) can be obtained with this alternative acquisition and post-processing scheme at reduced dose. The relative dose reduction is breast-thickness-dependent and is >20% for typical breast thicknesses. We have carried out a clinical study with 75 patients that showed non-inferior image quality at reduced dose with our novel approach compared to the standard method.
Characterization of Varian on-board imaging systems for use in automatic exposure control software
Author(s):
D. Morton;
R. Rajapakshe;
C. Araujo
Show Abstract
Modern image guided radiation therapy involves the use of an isocentrically mounted on-board imager (OBI) to take kV
images of a patient’s position. Orthogonal OBI images are used with 2D-2D match software to determine the treatment
couch shifts required for ideal alignment based on digitally reconstructed radiographs created in treatment planning. The
lack of an automatic exposure control (AEC) on Varian OBI systems requires x-ray techniques to be selected manually
which may result in over or under exposed images and compromise the accuracy of the image matching. A software
based AEC system is being developed in order to predict the optimal, patient specific exposure factors. This software
requires that each OBI system be uniquely characterized in terms of both the x-ray tube output and the detector response
for a clinical range of energies (kVp). Characteristic curves show that the detector is highly energy dependent at low
energies and increasingly energy independent with increasing energy. The detector response (per unit exposure) was
determined as a function of the beam quality and the level of detector saturation due to scattered radiation was modeled
based on patient thickness, kVp, and field size. Using this model, the optimal exposure can be determined to produce the
highest quality image.
Carbon nanotube field emission x-ray system for computed tomography
Author(s):
Je Hwang Ryu;
Wan Sun Kim;
Seung Ho Lee;
Jung Su Kang;
Jae Gon Kim;
Soo Yeol Lee;
Kyu Chang Park;
Hun Kuk Park
Show Abstract
Computed Tomography (CT) using Carbon Nanotube (CNT) x-ray source is a technique of generating reconstruction
images of the structure of teeth sample. A proto type CNT x-ray CT was designed for medical imaging to examine
whether it could be used to analysis the equipment of medical and industrial application. The CNT field emitter array was grown on silicon substrate through a resist-assisted patterning (RAP) process. The field emission properties showed a gate turn-on field of 3.8 V/μm at an anode emission current of 0.5 mA. The author demonstrated the x-ray source with four electrode structures utilizing the CNT emitter. The acquisitioned images were reconstructed by filtered back projection (FBP) method.
A digital compact x-ray tube with carbon nanotube field emitters for advanced imaging systems
Author(s):
Jae-woo Kim;
Jin-Woo Jeong;
Jun-Tae Kang;
Sungyoul Choi;
Jeongyong Choi;
Seungjoon Ahn;
Yoon-ho Song
Show Abstract
We have successfully developed a fully vacuum-sealed CNT x-ray source in a very compact tube without any active
vacuum pump. The brazing process was specially designed and adopted for the vacuum sealing of the x-ray tube at an
elevated temperature. This method enables us to obtain and maintain a desired vacuum level for the reliable electron
emission from the CNT emitters after the vacuum packaging. The CNT x-ray tube also had a novel focusing electrode to
effectively focus electron beams from the CNT emitters on a small area of the anode target, giving a small focal spot of
below 0.3 mm with a large tube current of above 50 mA. The active-current control modulated the CNT x-ray source
digitally with a low voltage of below 5 V and enhanced its stability further. Also, the pull-up circuit positioned at the
cathode node of the x-ray tube shortened the response time down to several micro second. The developed CNT x-ray tube can open up new applications in medical imaging like a stationary tomosynthesis or pulsed fluoroscopy over conventional hot-cathode x-ray sources.
Quantification of a silver contrast agent in dual-energy breast x-ray imaging
Author(s):
Roshan Karunamuni;
Andrew D. A. Maidment
Show Abstract
Dual-energy (DE) breast x-ray imaging involves acquiring images using a low- and high-energy x-ray spectral pair.
These images are then subtracted with a weighting factor that eliminates the soft-tissue signal variation present in
the breast leaving only contrast that is attributed to an exogenous imaging agent. We have previously demonstrated
the potential for silver (Ag) as a contrast material for DE breast imaging. Theoretical analysis shows that silver can
provide better contrast to clinically-used iodine. Here, we present the subtraction method developed to eliminate the
contrast between adipose and glandular tissue; the two major component materials in the breast. The weighting
factor is calculated from the attenuation coefficients of the two tissue types and varies between values of 0 and 1 for
the energy combinations studied. A spectral search was performed to identify the set of clinically-feasible imaging
parameters that will optimize the contrast of silver using our subtraction technique. The subtraction methodology
was tested experimentally using step-phantoms and demonstrated that we are able to a) nullify the soft-tissue
contrast that arises from differences in glandularity, and b) preserve an image contrast for silver that is independent
of the underlying soft-tissue composition. By applying the DE subtraction proposed, a silver-based agent will
outperform an iodinated contrast agent on a commercially-available CEDE breast x-ray imaging system.
Exploring the relationship between SDNR and detectability in dual-energy breast x-ray imaging
Author(s):
Roshan Karunamuni;
Swathiu Kanamaluru;
Kristen Lau;
Sara Gavenonis;
Predrag R. Bakic;
Andrew D. A. Maidment
Show Abstract
Contrast-enhanced (CE) digital breast tomosynthesis (DBT) provides a technique to increase the contrast of radiographic
imaging agents by suppressing soft-tissue signal variation. By reducing the effect of the soft-tissue anatomical noise, it is
then possible to quantify the signal from an iodinated contrast agent. The combination of dual-energy and tomographic
acquisitions allows for both the accurate quantification and localization of an iodinated lesion. Here, we present our
findings demonstrating the relationship that exists between the signal difference to noise ratio (SDNR) and reader
detectability of iodinated lesions in a physical anthropomorphic phantom. The observer study was conducted using the
ViewDEX software platform with a total of nine readers. The readers were asked to score each of the iodinated lesions
on a scale from 1 (entire boundary and area are visible) to 5 (not visible). Both SDNR and lesion detectability were
found to improve as the concentration of the iodine increases, and the thickness of the phantom decreases. Lesion
detectability was better in the tomographic slice that best matches the focal plane of the imaged object. However, SDNR
does not significantly change with focal plane. Our results demonstrated that observer lesion detectability correlated well
with SDNR. Lesions whose SDNR fell below 1 were difficult to distinguish from the background and were in general
not visible. Lesions that were rated entirely visible corresponded to those with SDNR values above 3. Lesions with
intermediate SDNR values were visualized but not confidently from the surrounding background. These threshold SDNR
values can be used to optimize the imaging parameters in CE-DBT.
Rat coronary microangiography system for preclinical imaging using synchrotron radiation
Author(s):
Keiji Umetani;
James T. Pearson;
Daryl O. Schwenke;
Mikiyasu Shirai
Show Abstract
A rat microangiography system using a synchrotron radiation source was developed at SPring-8 to enable in vivo
visualization of coronary arteries for vascular-based functional imaging. Angiographic images of beating hearts were
obtained with spatial resolution in the micrometer range and temporal resolution in the millisecond range using a new rotating-disk X-ray shutter and an X-ray direct-conversion type detector. Quantitative evaluations of the imaging system were extracted from measurements of the smallest-detectable vessel size and detection of the vessel function. Small blood vessels of around 50 μm diameter were visualized clearly at heart rates of 350–400 per minute. Apparent vasodilation compared to baseline vessel diameters was observed by infusion of vasoactive agents. The
microangiography system enables direct investigation of the mechanisms of vascular dysfunction.
Development of a line electron focusing lens for carbon nanotube field emission based microbeam radiation device
Author(s):
Jian Zhang;
Michael Hadsell;
Jianping Lu;
Otto Zhou;
Sha Chang
Show Abstract
Microbeam radiation therapy (MRT) is an experimental and preclinical technique with demonstrated capability of
eradicating tumors while sparing normal tissues from radiation damage. We have proposed the design of a microbeam
radiation device using a carbon nanotube (CNT) field emission x-ray source. The key enabling technology is the CNT
based spatially distributed x-ray technology. The proposed MRT system has a unique square system geometry with the
radiations shining on the targeting tumor positioned at the center of the square. A high microbeam dose rate is achieved
by distributing the electron energy over multiple elongated focal tracks with a significantly larger area and therefore
higher heat capacity compared to a conventional x-ray source with a point focal spot. Meanwhile the efficiency of the xray
photons going through the narrow microbeam collimator, thus the dose rate, is greatly increased by making the
effective width of the focal track comparable to that of the microbeam collimator opening. In order to achieve the desired
focal track on the anode, a commercial software package (Opera 3D, Cobham plc) was used to simulate and design the
optimal line focusing lens. The finalized design was based on a two-electrode Einzel focusing lens configuration. The
simulation shows the two-stage electrostatic focusing lens is capable of providing the 100μm effective focal spot size
required for the proposed microbeam x-ray with 100μm beam width. The recent focal spot size measurement performed
using a testing x-ray chamber has also verified the simulation results.