This PDF file contains the front matter associated with SPIE Proceedings Volume 6913, including the Title Page, Copyright information, Table of Contents, In Memoriam Sam Dwyer, and the Conference Committee listing.

Imaging tumor angiogenesis in small animals is extremely challenging due to the size of the tumor vessels. Consequently, both dedicated small animal imaging systems and specialized intravascular contrast agents are required. The goal of this study was to investigate the use of a liposomal contrast agent for high-resolution micro-CT imaging of breast tumors in small animals. A liposomal blood pool agent encapsulating iodine with a concentration of 65.5 mg/ml was used with a Duke Center for In Vivo Microscopy (CIVM) prototype micro-computed tomography (micro-CT) system to image the R3230AC mammary carcinoma implanted in rats. The animals were injected with equivalent volume doses (0.02 ml/kg) of contrast agent. Micro-CT with the liposomal blood pool contrast agent ensured a signal difference between the blood and the muscle higher than 450 HU allowing the visualization of the tumors 3D vascular architecture in exquisite detail at 100-micron resolution. The micro-CT data correlated well with the histological examination of tumor tissue. We also studied the ability to detect vascular enhancement with limited angle based reconstruction, i.e. tomosynthesis. Tumor volumes and their regional vascular percentage were estimated. This imaging approach could be used to better understand tumor angiogenesis and be the basis for evaluating anti-angiogenic therapies.

A prototype physiologically gated micro-computed tomography (micro-CT) system based on a field emission micro-focus x-ray source has been developed for in vivo imaging of small animal models. The novel x-ray source can generate radiation with a programmable waveform that can be readily synchronized and gated with non-periodic physiological signals. The system performance is evaluated using phantoms and sacrificed and anesthetized mouse models. Prospective respiratory-gated CT images of anesthetized free-breathing mice are collected using this scanner at 100msec temporal resolution and 10 lp/mm of 10% system MTF.

During the last eight years our group has developed radial acquisitions with angular undersampling factors of several hundred that accelerate MRI in selected applications. As with all previous acceleration techniques, SNR typically falls as least as fast as the inverse square root of the undersampling factor. This limits the SNR available to support the small voxels that these methods can image over short time intervals in applications like time-resolved contrast-enhanced MR angiography (CE-MRA). Instead of processing each time interval independently, we have developed constrained reconstruction methods that exploit the significant correlation between temporal sampling points. A broad class of methods, termed HighlY Constrained Back PRojection (HYPR), generalizes this concept to other modalities and sampling dimensions.

Both mammography and ultrasound imaging have sub-optimal sensitivity for the detection of solid breast lesions. We investigate the feasibility of detecting occult solid breast lesions using ultrasound propagation velocity differences between malignant lesions and surrounding breast tissue in a phantom study using 3D ultrasound. In this technique, the breast is placed on a flat backplate and imaged using a 3D ultrasound system. The backplate serves as a reference plane in the resulting 3D image. The difference in ultrasound propagation velocity through solid lesions versus surrounding breast tissue produces a shift in the apparent elevation of the backplate in the resulting image. Visible backplate distortion may therefore indicate the presence of an occult solid breast lesion. We provide a mathematical model to predict backplate distortion as a function of lesion thickness, and validate this theory experimentally using multi-compartment agar phantoms with embedded lesions ranging from 2 mm to 16 mm in thickness. Two-dimensional backplate elevation maps were constructed, and visible backplate distortions were observed for lesions of thickness 4 mm and greater. Measured backplate distortions were in excellent agreement with our theoretical predictions for these lesions. This study suggests that ultrasound propagation velocity differences between solid lesions and surrounding tissue may be useful as an additional screening tool for detection and localization of otherwise occult solid breast lesions.

Preliminary results for a new CT scanning device with dose-reduction potential were presented at the SPIE Medical Imaging conference 2007. The new device acquires the Radon data after the X-ray beam is collimated through a special mask. This mask is combined with a new and efficient data collection geometry; thus the device has the potential of reducing the dose by a factor of two. In this work, we report the first complete proof of the idea using the same simplified mask of 197 detectors as last year, and a clinical C-arm with a flat panel detector to simulate the gantry. This addition enables the acquisition of two independent and complementary data sets for reconstruction. Moreover, this clinical set-up enables the acquisition of data for clinically relevant phantoms. Phantom data were acquired using both detector sets and were reconstructed with the robust algorithm OPED. The independent sinograms were matched to a single one, and from this a diagnostic image was reconstructed successfully. This image has improved resolution, as well as less noise and artifacts compared to each single independent reconstruction. The results obtained are highly promising, even though the current device acquires only 197 views. Dose comparisons can be carried out in the future with a more precise prototype, comparable to current clinical devices with respect to imaging performance.

The multi-prism lens (MPL) is a refractive x-ray lens consisting of two rows of prisms facing each other at an angle. Rays entering the lens at the periphery will encounter a larger number of prisms than will central ones, hence experiencing a greater refraction. The focusing effect of the MPL can be used to gather radiation from a large aperture onto a smaller detector, and accordingly to make better use of the available x-ray flux in medical x-ray imaging. Potential advantages of a better photon economy include shorter acquisition times, a reduced tube loading, or an improved resolution. Since the focusing effect is one-dimensional it matches the design of scanning systems. In this study we present the first images acquired with an MPL instead of the pre-breast slit collimator in a scanning mammography system. According to the measurements, the MPL is able to increase the flux 32% at equal resolution compared to the slit collimator, or to improve the resolution 2.4 mm^-1 at equal flux. If used with a custom-made absorption filter in a clinical set-up, the gain of flux of the MPL is expected to be at least 45%, and the corresponding improvement in resolution to be 3 mm^-1.

Neutron stimulated emission computed tomography (NSECT) is being proposed as a non-invasive technique to diagnose iron overload in humans. It uses inelastic scatter interactions between incident neutrons and iron nuclei to stimulate gamma-ray emission from iron. Tomographic detection of the emitted gamma-rays yields information about the concentration and spatial distribution of iron in the liver. Early proof-of-concept experiments have shown that NSECT has the potential to quantify clinical quantities of liver iron overload through single-position spectroscopy. However, a tomography application for patient diagnosis has never been tested. This work uses a Monte-Carlo simulation of a tomographic NSECT system to investigate the feasibility of imaging the spatial distribution of liver iron through tomography. A simulation of an NSECT system has been designed in GEANT4 and used to tomographically scan a simulated human liver phantom with high-concentration iron lesions. Images are reconstructed with the MLEM algorithm and analyzed for pixel values within iron regions to determine the statistical significance of detection. Analysis results indicate that a wet iron concentration of 3 mg/g can be detected in surrounding liver tissue with p-value ≤ 0.0001 for neutron exposure corresponding to a radiation dose of 0.72 mSv. The research performed here demonstrates that NSECT has the ability to image clinically relevant distributions of iron through tomographic scanning.

Most electronic portal imaging devices (EPIDs) developed so far use a Cu plate/phosphor screen to absorb x rays. The main problem with this approach is that the Cu plate/phosphor screen must be thin (~ 2 mm) in order to obtain a high spatial resolution, resulting in a low quantum efficiency (QE) for megavoltage (MV) × rays (typically 2-4%). In addition, the phosphor screen contains high atomic number (high-Z) materials, resulting in an over-response of the detector to low energy x rays in dosimetric verification. Our overall goal is to develop a new high QE MV x-ray detector made of a low-Z material for both geometric and dosimetric verification in radiotherapy. Our approach is based on radiation-induced light (Cherenkov radiation) in optical fibers to convert x-ray energy into light. With our approach, a thick (~ 10-30 cm) fiber-optic taper (FOT) consisting of a matrix of optical fibers aligned with the incident x rays is used to replace the thin Cu plate/phosphor screen to dramatically improve the QE. In this work, we demonstrated that the predominant light source in optical fibers under irradiation of a MV beam is indeed Cherenkov radiation, and thus validated the feasibility of using Cherenkov radiation as the primary light source in our proposed Cherenkov detector. A prototype Cherenkov detector array was also built and images were obtained.

Analytically predicting photon paths in real-high scattering-anisotropic tissues is extremely complex, due to the significant random scattering events that photons suffer as they traverse the tissue, especially at boundaries between areas with different optical properties. An statistically correct optical and anatomical model of photon trajectories inside laboratory animals will therefore improve considerably our understanding about how light diffuses within the animals, and therefore help us designing efficient experimental setups and reconstruction algorithms for fluorescence mediated tomography (FMT). Here, we present new simulations of photon propagation and fluorescence emission in anisotropic media using realistic models of laboratory animals and a Monte Carlo (MC) based approach. We compare the MC simulation results with an approximation of the solution of the diffusion equation using finite differences and discuss the different behaviour of the two methods.

The forward problem of optical bioluminescence and fluorescence tomography seeks to determine, for a given 3D source distribution, the photon density on the surface of an animal. Photon transport through tissues is commonly modeled by the diffusion equation. The challenge, then, is to accurately and efficiently solve the diffusion equation for a realistic animal geometry and heterogeneous tissue types. Fast analytical solvers are available that can be applied to arbitrary geometries but assume homogeneity of tissue optical properties and hence have limited accuracy. The finite element method (FEM) with volume tessellation allows reasonably accurate modeling of both animal geometry and tissue heterogeneity, but this approach is computationally intensive. The computational challenge is heightened when one is working with multispectral data to improve source localization and conditioning of the inverse problem. Here we present a fast forward model based on the Born approximation that falls in between these two approaches. Our model introduces tissue heterogeneity as perturbations in diffusion and absorption coefficients at rectangular grid points inside a mouse atlas. These reflect as a correction term added to the homogeneous forward model. We have tested our model by performing source localization studies first with a biolumnescence simulation setup and then with an experimental setup using a fluorescent source embedded in an inhomogeneous phantom that mimicks tissue optical properties.

In this paper, we describe a new 3D light propagation model aimed at understanding the effects of various physiological properties on subcutaneous vein imaging. In particular, we build upon the well known MCML (Monte Carlo Multi Layer) code and present a tissue model that improves upon the current state-of-the-art by: incorporating physiological variation, such as melanin concentration, fat content, and layer thickness; including veins of varying depth and diameter; using curved surfaces from real arm shapes; and modeling the vessel wall interface. We describe our model, present results from the Monte Carlo modeling, and compare these results with those obtained with other Monte Carlo methods.

In this study we apply a total variation (TV) minimization algorithm to image reconstruction in magnetic resonance imaging (MRI). This algorithm is particularly effective for underlying images that are approximately piecewise constant. While the underlying proton spin density in MRI can satisfy this condition under certain circumstances, it is often distorted by unavoidable physical factors that alter the phase of the complex image. In this work, we employ a known method of removing this slow phase variation resulting from magnetic field inhomogeneities to obtain a spin density distribution that is piecewise constant. After the phase removal, we apply the TV minimization algorithm to obtain images from 20% of the full MRI data set.

A novel method for highly-undersampled Magnetic Resonance Image (MRI) reconstruction is presented. One of the principal challenges faced in clinical MR imaging is the fundamental linear relation between net exam duration and admissible spatial resolution. Increased scan duration diminishes patient comfort while increasing the risk of susceptibility to motion artifact and limits the ability to depict many physiological events at high temporal rates. With the recent development of Compressive Sampling theory, several authors have successfully demonstrated that clinical MR images possessing a sparse representation in some transform domain can be accurately reconstructed even when sampled at rates well below the Nyquist limit by casting the recovery as a convex ℓ₁-minimization problem. While ℓ₁-based techniques offer a sizeable advantage over Nyquist-limited methods, they nonetheless require a modest degree of over-sampling above the true theoretical minimum sampling rate in order to guarantee the achievability of exact reconstruction. In this work, we present a reconstruction model based on homotopic approximation of the ℓ₀ quasi-norm and discuss the ability of this technique to reconstruct undersampled MR images at rates even lower than are achievable than with ℓ₁-minimization and arbitrarily close to the true minimum sampling rate. A semi-implicit numerical solver is presented for efficient numerical computation of the reconstruction process and several examples depicting the capability for accurate MRI reconstructions from highly-undersampled K-space data are presented.

Breathing motion causes severe artifacts in Magnetic Resonance Imaging (MRI), which are usually avoided using either breath-holding during the acquisition or gating or triggering techniques while the patient is breathing freely. In MRI with continuously moving table the patient is moved through the magnet during the data acquisition, thereby allowing for time efficient coverage of arbitrary large body regions. Due to the continuous table motion standard breathing motion compensation techniques are not applicable here. In this work a retrospective compensation technique for abdominal breathing motion in moving table MRI is proposed, which reconstructs artifact free images from data acquired during free breathing. For each slice position multiple motion consistent snapshots of arbitrary breathing states are acquired and subsequently combined consistently using a combination of rigid and deformable slice-to-volume registration. Typical breathing motion induced artifacts such as ghosting, blurring or misalignments of organs occurring if data from different breathing states is inconsistently combined are avoided. The proposed method significantly extends a previous approach which aimed only at the suppression of ghosting artifacts in MRI with continuously moving table.

Spin noise is inherent in magnetic resonance. It is caused by incomplete cancellation of spin moments when the external static magnetic field is absent or by their small but finite fluctuations when the magnetic field is applied. Spin noise is viewed as the variation of thermal equilibrium macroscopic magnetization (TEMM), and can be described statistically. For MRI, TEMM is shown to be characterized by a Binomial distribution and is well approximated by a Gaussian. Parameters of this Gaussian distribution are determined by the spin density and the ratio of population difference over the total population of spins in a unit volume of the sample. Statistics of spin noise not only confirm Bloch's prediction of spin noise in the absence of the external magnetic field, but also give a more accurate account of its behavior under various conditions. These statistics also provide a new insight into the limits of spatial resolution in magnetic resonance microscopy (MRM) and are consistent with Glover and Mansfield' corresponding conclusions based on their experiments.

Conventional active matrix flat-panel imagers (AMFPIs), employing amorphous silicon (a-Si:H) semiconductors, are based on a relatively simple pixel architecture, commonly taking the form of a single, thin-film transistor (TFT) coupled to a pixel storage capacitor. Although this semiconductor-architecture combination has led to the successful creation of x-ray imagers for many applications, a variety of significant performance limitations related to DQE, frame rate and charge trapping have also become apparent. While prospects for designing solutions to these restrictions based on a-Si:H TFTs are uncertain, progress in the development of high-quality polycrystalline silicon (poly-Si) TFTs is opening up new possibilities for large area x-ray imager design. Recently, initial prototype imagers have been developed using poly-Si TFTs in the form of 1-stage and 2-stage pixel amplifiers-ircuit architectures that can generally be referred to as active pixel sensors (APS). The insight gained from empirical evaluations of such prototypes, coupled with theoretical studies, can inspire increasingly sophisticated APS architectures that overcome the limitations, while preserving the advantages, of conventional AMFPIs. In this paper, cascaded systems analysis and circuit simulation are used to explore potential performance improvements enabled by APS architectures based on poly-Si TFTs. These studies suggest that it is possible to achieve significant improvements in DQE at low exposures or very small pixel sizes, higher maximum frame rates, and reduced charge trapping effects through implementation of such architectures.

We describe a new type of a sensitive semiconductor photodetector that could be used in medical imaging applications. The photodetector, based on the mechanism of discrete amplification, has performance parameters comparable to, and for some applications exceeding, those of the vacuum photomultiplier tubes. High amplification gain achieved at very low levels of excess noise is accompanied by the fast speed and high dynamic range of the photodetector. Comparison of the technology with classic arrays of Geiger-mode APD arrays is also performed.

The solid-state x-ray image intensifier (SSXII) is an EMCCD-based x-ray detector designed to satisfy an increasing need for high-resolution real-time images, while offering significant improvements over current flat panel detectors (FPDs) and x-ray image intensifiers (XIIs). FPDs are replacing XIIs because they reduce/eliminate veiling glare, pincushion or s-shaped distortions and are physically flat. However, FPDs suffer from excessive lag and ghosting and their performance has been disappointing for low-exposure-per-frame procedures due to excessive instrumentation-noise. XIIs and FPDs both have limited resolution capabilities of ~3 cycles/mm. To overcome these limitations a prototype SSXII module has been developed, consisting of a 1k x 1k, 8 μm pixel EMCCD with a fiber-optic input window, which views a 350 μm thick CsI(Tl) phosphor via a 4:1 magnifying fiber-optic-taper (FOT). Arrays of such modules will provide a larger field-of- view. Detector MTF, DQE, and instrumentation-noise equivalent exposure (INEE) were measured to evaluate the SSXIIs performance using a standard x-ray spectrum (IEC RQA5), allowing for comparison with current state-of-the-art detectors. The MTF was 0.20 at 3 cycles/mm, comparable to standard detectors, and better than 0.05 up to 7 cycles/mm, well beyond current capabilities. DQE curves indicate no degradation from high-angiographic to low-fluoroscopic exposures (< 2% deviation in overall DQE from 1.3 mR to 2.7 μR), demonstrating negligible instrumentation-noise, even with low input signal intensities. An INEE of < 0.2 μR was measured for the highest-resolution mode (32 μm effective pixel size). Comparison images between detector technologies qualitatively demonstrate these improved imaging capabilities provided by the SSXII.

Digital flat panel a-Si x-ray detectors can exhibit image lag of several percent. The image lag can limit the temporal resolution of the detector, and introduce artifacts into CT reconstructions. It is believed that the majority of image lag is due to defect states, or traps, in the a-Si layer. Software methods to characterize and correct for the image lag exist, but they may make assumptions such as the system behaves in a linear time-invariant manner. The proposed method of reducing lag is a hardware solution that makes few additional hardware changes. For pulsed irradiation, the proposed method inserts a new stage in between the readout of the detector and the data collection stages. During this stage the photodiode is operated in a forward bias mode, which fills the defect states with charge. Parameters of importance are current per diode and current duration, which were investigated under light illumination by the following design parameters: 1.) forward bias voltage across the photodiode and TFT switch, 2.) number of rows simultaneously forward biased, and 3.) duration of the forward bias current. From measurements, it appears that good design criteria for the particular imager used are 8 or fewer active rows, 2.9V (or greater) forward bias voltage, and a row frequency of 100 kHz or less. Overall, the forward bias method has been found to reduce first frame lag by as much as 95%. The panel was also tested under x-ray irradiation. Image lag improved (94% reduction), but the temporal response of the scintillator became evident in the turn-on step response.

An indirect flat-imager with programmable avalanche gain and field emitter array (FEA) readout is being investigated for low-dose x-ray imaging with high resolution. It is made by optically coupling a structured x-ray scintillator CsI (Tl) to an amorphous selenium (a-Se) avalanche photoconductor called HARP (high-gain avalanche rushing photoconductor). The charge image created by HARP is read out by electron beams generated by the FEA. The proposed detector is called SAPHIRE (Scintillator Avalanche Photoconductor with HIgh Resolution Emitter readout). The avalanche gain of HARP depends on both a-Se thickness and applied electric field E_Se. At E_Se of > 80 V/μm, the avalanche gain can enhance the signal at low dose (e.g. fluoroscopy) and make the detector x-ray quantum noise limited down to a single x-ray photon. At high exposure (e.g. radiography), the avalanche gain can be turned off by decreasing E_Se to < 70 V/μm. In this paper the imaging characteristics of the FEA readout method, including the spatial resolution and noise, were investigated experimentally using a prototype optical HARP-FEA image sensor. The potential x-ray imaging performance of SAPHIRE, especially the aspect of programmable gain to ensure wide dynamic range and x-ray quantum noise limited performance at the lowest exposure in fluoroscopy, was investigated.

The output response characteristics of an X-ray photon counting detector are measured experimentally and simulated using a Monte Carlo method in order to quantify the loss of statistical information due to pile-up. The analysis is applied to idealize counting detector models, but is adaptable to realistic event processing that is not amenable to analytic solution. In particular, the detective quantum efficiency (DQE) is calculated as a function of flux rate and shown to have an intermediate zero for the paralyzable case at the maximum periodic rate. The progressive degradation of the spectral response as a function of increasing flux rate is also modeled. Analogous metrics to DQE are defined in regards to the detector's ability to resolve atomic number and enhance image contrast based on atomic number differentiation. Analytic solutions are provided for the output and linearized response statistics and these interpolate well across the Monte Carlo and experimental results.

We developed an efficient, depth- and energy-dependent Monte Carlo model for columnar CsI detectors. The optical photon, electron/positron Monte Carlo package MANTIS developed by our group, was used to generate optical photon response and collection efficiency as a function of the x-ray/electron interaction depth for a realistic scintillator geometry. The detector geometry we used for the simulations was reported in the past and is based on a 500 μm thick columnar CsI scintilator. The resulting depth-dependent optical photon responses were fit to a parametrized Gaussian mixture model. The model parameters were the depth-dependent radial shift of the response peak, the depth dependent widths of the Gaussians, and the depth-dependent magnitude of the Gaussians in the mixture. The depth-dependent optical spread has a maximum spatial shift of 53 μm. The optical collection efficiency at the photo-diode layer followed a power law varying from 90% for interactions at the scintillator exit surface to 20% for interactions at the detector entrance. The responses were consequently incorporated into penMesh, a PENELOPE based Monte Carlo x-ray, electron/positron transport simulation package for generating clinically realistic images of triangular mesh phantoms. The resulting detector responses from this empirical model were compared against the full x-ray/electron/optical photon simulation using the package MANTIS, showing good agreement. The simulation speed, using the optical transport model in penMesh, increases by two orders of magnitude compared to MANTIS.

A hybrid pixel detector based on the concept of simultaneous charge integration and photon counting will be presented. The second generation of a counting and integrating X-ray prototype CMOS chip (CIX) has been operated with different direct converting sensor materials (CdZnTe and CdTe) bump bonded to its 8x8 pixel matrix. Photon counting devices give excellent results for low to medium X-ray fluxes but saturate at high rates while charge integration allows the detection of very high fluxes but is limited at low rates by the finite signal to noise ratio. The combination of both signal processing concepts therefore extends the resolvable dynamic range of the X-ray detector. In addition, for a large region of the dynamic range, where counter and integrator operate simultaneously, the mean energy of the detected X-ray spectrum can be calculated. This spectral information can be used to enhance the contrast of the X-ray image. The advantages of the counting and integrating signal processing concept and the performance of the imaging system will be reviewed. The properties of the system with respect to dynamic range and sensor response will be discussed and examples of imaging with additional spectral information will be presented.

Despite its obvious advantages, well known CsI:Tl scintillator has two characteristic properties that undermine its use in clinical and high speed imaging: the presence of an afterglow component in its scintillation decay, and a hysteresis effect that causes drift in the scintillation yield after exposure to high radiation doses. We have previously reported that the addition of a second dopant, Sm²⁺, to the CsI:Tl crystals, significantly suppresses both afterglow and hysteresis. Here we report on the fabrication and characterization of the Sm co-doped CsI:Tl microcolumnar films to examine if these properties are preserved in films as well. Our preliminary data suggests that the Sm co-doped CsI:Tl films significantly improve temporal response relative to their CsI:Tl counterpart, and that the newly developed films demonstrate excellent spatial resolution. Various aspects of these effects and their consequences for imaging performance are discussed in this paper.

Sophisticated radiotherapy techniques lead to more conformal dose distributions but increase treatment complexity. Image guidance allows for varying degrees of accuracy in patient set-up. However, the consequences of inaccurate set-up and/or patient motion during treatment become more serious when treatment doses are increased and treatment margins are decreased. Thus, the need to know if the dose has been delivered as planned has driven the development of plastic scintillation detector systems for accurate measurements in real time with high spatial resolution. We have developed a clinical prototype comprising 29 scintillating fiber detectors 1 mm in diameter and 2 mm in length. The detectors are coupled to clear optical fibers that collect the scintillation photons and transport them to a CCD for detection. Open field profiles and depth-dose profiles in water-equivalent phantoms were compared to ionization chamber measurements in water. The maximum relative in-field difference was 1.6%. With a standard deviation for in-field measurements smaller than 1%, this prototype array was found to be accurate, precise and practical. Monte Carlo simulations were also used to evaluate the response of the scintillation detector to proton beams and to optimize the light collection efficiency. The Monte Carlo code Geant4 was used to simulate dose deposition, the production of scintillation photons and the propagation of those photons inside the scintillation detector. Further development of the system will allow thousands of measurement points distributed in a three-dimensional volume per single irradiation, therefore producing a rapid evaluation of complex dose distributions emanating from these new complex treatment modalities.

Permanent breast seed implant (PBSI) brachytherapy technique was recently introduced as an alternative to high dose rate (HDR) brachytherapy and involves the permanent implantation of radioactive ¹⁰³Palladium seeds into the surgical cavity of the breast for cancer treatment. To enable accurate seed implantation, this research introduces a gamma camera based on a hybrid amorphous selenium detector and CMOS readout pixel architecture for real-time imaging of ¹⁰³Palladium seeds during the PBSI procedure. A prototype chip was designed and fabricated in 0.18-μm n-well CMOS process. We present the experimental results obtained from this integrated photon counting readout pixel.

This work aims at defining an information-theoretic quality assessment technique for cardiovascular X-ray images, using a full-reference scheme (relying on averaging a sequence to obtain a noiseless reference). With the growth of advanced signal processing in medical imaging, such an approach will enable objective comparisons of the quality of processed images. A concept for describing the quality of an image is to express it in terms of its information capacity. Shannon has derived this capacity for noisy channel coding. However, for X-ray images, the noise is signal-dependent and non-additive, so that Shannon's theorem is not directly applicable. To overcome this complication, we exploit the fact that any invertible mapping on a signal does not change its information content. We show that it is possible to transform the images in such a way that the Shannon theorem can be applied. A general method for calculating such a transformation is used, given a known relation between signal mean and noise standard deviation. After making the noise signal-independent, it is possible to assess the information content of an image and to calculate an overall quality metric (e.g. information capacity) which includes the effects of sharpness, contrast and noise. We have applied this method on phantom images under different acquisition conditions and computed the information capacity for those images. We aim to show that the results of this assessment are consistent with variations in noise, contrast and sharpness, introduced by system settings and image processing.

The objective performance evaluation metrics, termed Generalized Modulation Transfer Function (GMTF), Generalized Noise Power Spectrum (GNPS), Generalized Noise Equivalent Quanta (GNEQ), and Generalized Detective Quantum Efficiency (GDQE), have been developed to assess total imaging-system performance by including the effects of geometric unsharpness due to the finite size of the focal spot and scattered radiation in addition to the detector properties. These metrics were used to evaluate the performance of the HSMAF, a custom-built, high-resolution, real-time-acquisition detector with 35-μm pixels, in simulated neurovascular angiographic conditions using a uniform head-equivalent phantom. The HSMAF consists of a 300-μm-thick CsI(Tl) scintillator coupled to a 4 cm diameter, variable-gain, Gen2 light image intensifier with dual-stage microchannel plate, followed by direct fiber-optic coupling to a 30-fps CCD camera, and is capable of both fluoroscopy and angiography. Effects of focal-spot size, geometric magnification, irradiation field-of-view, and air-gap between the phantom and the detector were evaluated. The resulting plots of GMTF and GDQE showed that geometric blurring is the more dominant image degradation factor at high spatial frequencies, whereas scatter dominates at low spatial frequencies. For the standard image-geometry and scatter conditions used here, the HSMAF maintains substantial system imaging capabilities (GDQE>5%) at frequencies above 4 cycles/mm where conventional detectors cannot operate. The loss in image SNR due to scatter or focal-spot unsharpness could be compensated by increasing the exposure by a factor of 2 to 3. This generalized evaluation method may be used to more realistically evaluate and compare total system performance leading to improved system designs.

This paper presents an information-entropy based metric for combined evaluation of resolution and noise properties of radiological images. The metric is expressed by the amount of transmitted information (TI). It is a measure of how much information that one image contains about an object or an input. Merits of the proposed method are its simplicity of computation and the experimented setup. A computer-simulated step wedge was used for simulation study on the relationship of TI and the degree of blur as well as the noise. Three acrylic step wedges were also manufactured and used as test sample objects for experiments. Two imaging plates for computed radiography were employed as information detectors to record X-ray intensities. We investigated the effects of noise and resolution degradation on the amount of TI by varying exposure levels. Simulation and experimental results show that the TI value varies when the noise level or the degree of blur is changed. To validate the reasoning and usefulness of the proposed metric, we also calculated and compared the modulation transfer functions and noise power spectra for the employed imaging plates. Results show that the TI has close correlation with both image noise and image blurring, and it may offer the potential to become a simple and generally applicable measure for quality evaluation of medical images.

A perfusion phantom with unique features and a wide variety of applications in MR and other imaging modalities is presented. The phantom is especially suited for tissue perfusion simulation with diffusible and non-diffusible MR tracers. A network of micro-channels in the scale of actual capillaries replicates the blood flow in tissues. Using microfabrication techniques, networks with any desired pattern can be generated. Since the geometry of networks is known, flow rate, delay, dispersion and other fluid parameters can be exactly calculated using finite elements numerical methods. These calculated results can be used to investigate the accuracy of experimental measurements and the precision of mathematical models.

For optimisation in diagnostic medical imaging it is important to consider the relation between diagnostic image quality and patient dose. In the past, schematic representations of the human body were commonly used for dosimetric simulations together with Monte Carlo codes. During the last two decades, voxel models were introduced as an improvement to these body models. Studies performed by various research groups have shown that the more realistic organ topology of voxel models constructed from medical image data of real persons has an impact on calculated doses for external as well as internal exposures. As a consequence of these findings, the ICRP decided to use voxel models for the forthcoming update of organ dose conversion coefficients. These voxel models should be representative of an average population, i.e. they should resemble the ICRP reference anatomical data with respect to their external dimensions and their organ masses. To meet the ICRP requirements, our group at the Helmholtz Zentrum München (formerly known as GSF-National Research Center for Environment and Health) constructed voxel models of a male and female adult, based on the voxel models of two individuals whose body height and weight resembled those of the male and female ICRP reference adult. The organ masses of both models were adjusted to the ICRP reference anatomical data, without spoiling their realistic anatomy. The paper describes the method used for this process and the resulting voxel models.

Dual-energy contrast-enhanced breast tomosynthesis has been proposed as a technique to improve the detection of early-stage cancer in young, high-risk women. This study focused on optimizing this technique using computer simulations. The computer simulation used analytical calculations to optimize the signal difference to noise ratio (SdNR) of resulting images from such a technique at constant dose. The optimization included the optimal radiographic technique, optimal distribution of dose between the two single-energy projection images, and the optimal weighting factor for the dual energy subtraction. Importantly, the SdNR included both anatomical and quantum noise sources, as dual energy imaging reduces anatomical noise at the expense of increases in quantum noise. Assuming a tungsten anode, the maximum SdNR at constant dose was achieved for a high energy beam at 49 kVp with 92.5 μm copper filtration and a low energy beam at 49 kVp with 95 μm tin filtration. These analytical calculations were followed by Monte Carlo simulations that included the effects of scattered radiation and detector properties. Finally, the feasibility of this technique was tested in a small animal imaging experiment using a novel iodinated liposomal contrast agent. The results illustrated the utility of dual energy imaging and determined the optimal acquisition parameters for this technique. This work was supported in part by grants from the Komen Foundation (PDF55806), the Cancer Research and Prevention Foundation, and the NIH (NCI R21 CA124584-01). CIVM is a NCRR/NCI National Resource under P41-05959/U24-CA092656.

Dual-Energy Contrast Enhanced Digital Breast Tomosynthesis (DE CEDBT) is a promising technique for breast cancer detection, which combines the strengths of functional and 3D imaging. In the present study, we first focused on the optimization of the acquisition parameters for the low and high-energy projections, which leads to a trade-off between image quality in the recombined slices and the Average Glandular Dose (AGD) delivered to the patient. Optimized parameters were found and experimentally validated on phantom images. Then, we addressed the problem of iodine quantification in the recombined slices. In DE CEDBT, iodine quantification is limited by the z-resolution, due to the restricted angle acquisition inherent to tomosynthesis. We evaluated the lesion thickness above which determination of iodine volumetric concentration is possible. For lesions below this thickness, estimation of iodine concentration is possible if a priori information or a model on the shape of the lesion is available. Iodine quantification for lesions located near the breast boundary is also challenging, due to scatter border effects and variation of the breast thickness in this region. A scatter correction algorithm based on a deconvolution scheme and a thickness compensation algorithm were applied on the low and high-energy projections. Corrected images showed a more accurate quantification of iodine.

Dual-energy subtraction imaging (DES) is a method to improve the detectability of contrast agents over a lumpy background. Two images, acquired at x-ray energies above and below an absorption edge of the agent material, are logarithmically subtracted, resulting in suppression of the signal from the tissue background and a relative enhancement of the signal from the agent. Although promising, DES is still not widely used in clinical practice. One reason may be the need for two distinctly separated x-ray spectra that are still close to the absorption edge, realized through dual exposures which may introduce motion unsharpness. In this study, electronic spectrum-splitting with a silicon-strip detector is theoretically and experimentally investigated for a mammography model with iodinated contrast agent. Comparisons are made to absorption imaging and a near-ideal detector using a signal-to-noise ratio that includes both statistical and structural noise. Similar to previous studies, heavy absorption filtration was needed to narrow the spectra at the expense of a large reduction in x-ray flux. Therefore, potential improvements using a chromatic multi-prism x-ray lens (MPL) for filtering were evaluated theoretically. The MPL offers a narrow tunable spectrum, and we show that the image quality can be improved compared to conventional filtering methods.

The relationship between theoretical descriptions of imaging performance (Fourier-based cascaded systems analysis) and the performance of real human observers was investigated for various detection and discrimination tasks. Dual-energy (DE) imaging provided a useful basis for investigating this relationship, because it presents a host of acquisition and processing parameters that can significantly affect signal and noise transfer characteristics and, correspondingly, human observer performance. The detectability index was computed theoretically using: 1) cascaded systems analysis of the modulation transfer function (MTF), and noise-power spectrum (NPS) for DE imaging; 2) a Fourier description of imaging task; and 3.) integration of MTF, NPS, and task function according to various observer models, including Fisher-Hotelling and non-prewhitening with and without an eye filter and internal noise. Three idealized tasks were considered: sphere detection, shape discrimination (sphere vs. disk), and texture discrimination (uniform vs. textured disk). Using images of phantoms acquired on a prototype DE imaging system, human observer performance was assessed in multiple-alternative forced choice (MAFC) tests, giving an estimate of area under the ROC curve (A_Ζ). The degree to which the theoretical detectability index correlated with human observer performance was investigated, and results agreed well over a broad range of imaging conditions, depending on the choice of observer model. Results demonstrated that optimal DE image acquisition and decomposition parameters depend significantly on the imaging task. These studies provide important initial validation that the detectability index derived theoretically by Fourier-based cascaded systems analysis correlates well with actual human observer performance and represents a meaningful metric for system optimization.

Dual-energy (DE) X-ray computed tomography (CT) has been proposed as an useful tool in various applications. One promising application is DECT with low radiation doses used for attenuation correction in positron emission tomography (PET). In low-dose DECT, conventional methods for sinogram decomposition have been based on logarithmic transformations and ignored noise properties, leading to very noisy component sinogram estimates. In this paper, we propose two novel sinogram restoration methods that are statistically motivated; penalized weighted least square (PWLS) and penalized likelihood (PL), producing less noisy component sinogram estimates for low-dose DECT than the conventional approaches. The restored component sinograms can improve attenuation correction, thus allowing better image quality in PET. Experiments with a digital phantom indicate that the proposed methods produce less noisy sinograms, reconstructed images, and attenuation correction factors (ACF) than the conventional one, showing promise for CT-based attenuation correction in emission tomography.

Fast kV-switching is a dual energy acquisition technique in CT in which alternating views correspond to the low and high tube voltages. Its high temporal resolution and its suitability to a variety of source trajectories make it an attractive option for dual energy data acquisition. Its disadvantages include a one view mis-registration between the data for high and low voltages, the potential for poor spectrum separation because the fast kV-switching waveform may be more like a sine wave than the desired square wave, and the higher noise in the low voltage data because of the technical difficulty of swinging the tube current to counter the loss of x-ray production efficiency and loss of penetration at lower tube voltages. These issues are investigated with a recently developed pre-reconstruction decomposition method by the authors. Results include that symmetric view matching eliminates streaks from the view mis-registration, a sinusoidal waveform swinging between 80 and 135 kV gives sufficient spectrum separation, and that contrast-to-noise for the simulated imaging task maximizes at monochromatic energy of 75 keV.

The goal of this work is to create a detailed three-dimensional (3D) digital breast phantom based on empirical data and to incorporate it into the four-dimensional (4D) NCAT phantom, a computerized model of the human anatomy widely used in imaging research. Twenty sets of high-resolution breast CT data were used to create anatomically diverse models. The datasets were segmented using techniques developed in our laboratory and the breast structures will be defined using a combination of non-uniform rational b-splines (NURBS) and subdivision surfaces (SD). Imaging data from various modalities (x-ray and nuclear medicine) were simulated to demonstrate the utility of the new breast phantoms. As a proof of concept, a simple compression technique was used to deform the breast models while maintaining a constant volume to simulate modalities (mammography and tomosynthesis) that involve compression. Initial studies using one CT dataset indicate that the simulated breast phantom is capable of providing a realistic and flexible representation of breast tissue and can be used with different acquisition methods to test varying imaging parameters such as dose, resolution, and patient motion. The final model will have a more accurate depiction of the internal breast structures and will be scaleable in terms of size and density. Also, more realistic finite-element techniques will be used to simulate compression. With the ability to simulate realistic, predictive patient imaging data, we believe the phantom will provide a vital tool to investigate current and emerging breast imaging methods and techniques.

Dedicated x-ray computed tomography (CT) of the breast using a cone-beam flat-panel detector system is a modality under investigation by a number of research teams. As previously reported, we have fabricated a prototype, bench-top flat-panel CT breast imaging (CTBI) system and developed computer simulation software to model such a system. We are developing a methodology to use high resolution, low noise CT reconstructions of fresh mastectomy specimens for generating an ensemble of 3D digital breast phantoms that realistically model 3D compressed and uncompressed breast anatomy. These breast models can be used to simulate realistic projection data for both breast tomosynthesis (BT) and CT systems thereby providing a powerful evaluation and optimization mechanism.

The use of mammography techniques for the screening and diagnosis of breast cancers has been limited by the overlapping of cancer symptoms with normal tissue structures. To overcome this problem, two methods have been developed and actively investigated recently: digital tomosynthesis mammography and cone beam breast CT. Comparison study with these three techniques will be helpful to understand their difference and further might be supervise the direction of breast imaging. This paper describes and discusses about a technique using a general-purpose PC cluster to develop a parallel computer simulation model to simulate mammograms and tomosynthesis imaging with cone beam CT images of a mastectomy breast specimen. The breast model used in simulating mammography and tomosynthesis was developed by re-scaling the CT numbers of cone beam CT images from 80kVp to 20 kev. The compression of breast was simulated by deformation of the breast model. Re-projection software with parallel computation was developed and used to compute projection images of this simulated compressed breast for a stationary detector and a linearly shifted x-ray source. The resulting images were then used to reconstruct tomosynthesis mammograms using shift-and-add algorithms. It was found that MCs in cone beam CT images were not visible in regular mammograms but faintly visible in tomosynthesis images. The scatter signal and noise property needs to be simulated and incorporated in the future.

Breast density is an independent factor of breast cancer risk. In mammograms breast density is quantitatively measured as percent density (PD), the percentage of dense (non-fatty) tissue. To date, clinical estimates of PD have varied significantly, in part due to the projective nature of mammography. Digital breast tomosynthesis (DBT) is a 3D imaging modality in which cross-sectional images are reconstructed from a small number of projections acquired at different x-ray tube angles. Preliminary studies suggest that DBT is superior to mammography in tissue visualization, since superimposed anatomical structures present in mammograms are filtered out. We hypothesize that DBT could also provide a more accurate breast density estimation. In this paper, we propose to estimate PD from reconstructed DBT images using a semi-automated thresholding technique. Preprocessing is performed to exclude the image background and the area of the pectoral muscle. Threshold values are selected manually from a small number of reconstructed slices; a combination of these thresholds is applied to each slice throughout the entire reconstructed DBT volume. The proposed method was validated using images of women with recently detected abnormalities or with biopsy-proven cancers; only contralateral breasts were analyzed. The Pearson correlation and kappa coefficients between the breast density estimates from DBT and the corresponding digital mammogram indicate moderate agreement between the two modalities, comparable with our previous results from 2D DBT projections. Percent density appears to be a robust measure for breast density assessment in both 2D and 3D x-ray breast imaging modalities using thresholding.

The purpose of this study is to evaluate the recently proposed variable dose (VD) acquisition scheme that has been hypothesized to overcome the limitations of microcalcification detection in breast tomosynthesis. In this acquisition methodology, approximately half of the total dose is used for one central projection. This central projection view is similar to a conventional mammogram and used to detect microcalcifications. The other half of the total dose is split among the rest of the projection views. These variable dose projection data are then reconstructed and the 3D slices are used for detection of masses. This novel acquisition methodology can potentially overcome the current limitations with microcalcification detection in breast tomosynthesis (BT) and may result in faster and more accurate detection of both microcalcifications and masses. Having access to both a conventional mammogram (i.e., the central projection) and tomosynthesis slices would also act as a bridge for radiologists who are used to viewing single projection images. In the current study, a comparison of microcalcification detection accuracy obtained using VD and conventional BT was conducted. A realistic computer simulation was used to model the realistic noise and blur encountered in BT systems. The simulation used a compressed breast phantom, modeled using CT images of compressed mastectomy specimens. Localization receiver operating characteristic (LROC) analysis was performed for detecting microcalcifications of size ranging from 147 microns to 178 microns. The results suggested higher microcalcification detection and localization accuracy using the VD technique. The complete study will also consist of evaluating detection of masses for the two strategies.

A stationary digital breast tomosynthesis (DBT) system using a carbon nanotube based multi-beam field emission x-ray (MBFEX) source has been designed. The purpose is to investigate the feasibility of reducing the total imaging time, simplifying the system design, and potentially improving the image quality comparing to the conventional DBT scanners. The MBFEX source consists of 25 individually programmable x-ray pixels which are evenly angular spaced covering a 48° field of view. The device acquires the projection images by electronically switching on and off the individual x-ray pixels without mechanical motion of either the x-ray source or the detector. The designs of the x-ray source and the imaging system are presented. Some preliminary results are discussed.

We studied the use of the mammography contrast detail phantom (CDMAM) with tomosynthesis to evaluate the performance of our system as well as to explore the application of CDMAM in 3D breast imaging. The system was Hologic's 1st generation tomosynthesis machine. CDMAM phantom plus PMMA slabs were imaged at 3 cm, 5 cm, 7 cm, and 9 cm PMMA-equivalent thickness with 11 projections per scan and the scan angle selected from 0, 15 and 28 degrees. CDMAM images were reconstructed using the back projection method, and were scored with the CDCOM automatic analysis program. The threshold thickness of each disk size was obtained with psychometric curve fitting. We first studied errors and variability associated with the results when different numbers of images were used in contrast detail analysis, then studied factors that affected CDMAM results in tomosynthesis, including the x-ray dose, the scan angle, the in-plane reconstruction pixel size, the slice-to-slice step size, the location of the CDMAM inside the PMMA slabs, and the scatter effect. This paper will present results of CDMAM performance of our tomosynthesis system, as well as their dependence on the various factors, and the comparison with 2D mammography. Additionally we will discuss the novel processing and analysis methods developed during this study, and make proposals to modify the CDMAM phantom and the CDCOM analysis program to optimize the method for 3D tomosynthesis.

European Guidelines for quality control in digital mammography specify minimum and achievable standards of image quality in terms of threshold contrast, based on readings of images of the CDMAM test object by human observers. However this is time-consuming and has large inter- and intra-observer error. To overcome these problems a software program (CDCOM) is available to automatically read CDMAM images. After some further analysis the automated measurements can be used to predict the threshold contrast for a typical observer. The results of threshold contrast determination by human observers at three different centres were compared against automated readings. These data provide a means of predicting average human performance using the automated reading software. The coefficient of variation in automatically determined threshold gold thickness was about 4% for detail sizes from 0.2 to 1.0mm when 8 images were analysed. The coefficient of variation was about 10% at a detail size of 0.1mm. Using larger numbers of images improved reproducibility for all detail sizes. A change in phantom design could greatly improve reproducibility for the smallest detail sizes. Greater consistency of phantom construction would also be desirable as one of the four phantoms tested was significantly different from the other three. Despite some limitations automated reading of CDMAM images can provide a reproducible means of assessing digital mammography systems against European Guidelines.

Preliminary evidence has suggested that computerized tomographic (CT) imaging of the breast using a cone-beam, flat-panel detector system dedicated solely to breast imaging has potential for improving detection and diagnosis of early-stage breast cancer. Hypothetically, a powerful mechanism for assisting in early stage breast cancer detection from annual screening breast CT studies would be to examine temporal changes in the breast from year-to-year. We hypothesize that 3D image registration could be used to automatically register breast CT volumes scanned at different times (e.g., yearly screening exams). This would allow radiologists to quickly visualize small changes in the breast that have developed during the period since the last screening CT scan, and use this information to improve the diagnostic accuracy of early-stage breast cancer detection. To test our hypothesis, fresh mastectomy specimens were imaged with a flat-panel CT system at different time points, after moving the specimen to emulate the re-positioning motion of the breast between yearly screening exams. Synthetic tumors were then digitally inserted into the second CT scan at a clinically realistic location (to emulate tumor growth from year-to-year). An affine and a spline-based 3D image registration algorithm was implemented and applied to the CT reconstructions of the specimens acquired at different times. Subtraction of registered image volumes was then performed to better analyze temporal change. Results from this study suggests that temporal change analysis in 3D breast CT can potentially be a powerful tool in improving the visualization of small lesion growth.

The feasibility of using the dual-resolution cone beam breast CT technique to obtain high-resolution images inside a selected volume-of-interest (VOI). The spatial resolution improvement, dose saving and scatter reduction with this technique are studied and demonstrated with simulations. With the dual-resolution cone beam CT technique, the breast is first scanned with a low resolution detector at a lower exposure level. A selected volume-of-interest (VOI) in the breast is then scanned with a small field, high-resolution detector at a higher exposure level. The two image sets are then combined together to reconstruct high-resolution 3-D images for the VOI. The spatial resolution that can be achieved was estimated by obtaining 3-D images of point objects and use them to compute the MTFs for evaluation as a function of the geometric magnification, detector blurring function and focal spot size. Monte Carlo simulation based on the Geant4 package was used to estimate the degree of dose saving and scatter reduction for a cylinder shaped breast phantom. The VOI images generated with the dual-resolution cone beam CT technique demonstrated the same visibility of micro-calcifications as those generated with the full-breast, high resolution image acquisition. It has been shown that the spatial resolution can be increased by factor of 1.2 with smaller focal spot size and larger magnification. With the exposure level outside the VOI reduced by a factor of 15, scatter components can be reduced by a factor of 5.5 or greater in and outside the VOI. Dose can be reduced by a factor of 5.5 inside the VOI and up to 20 outside the VOI. We have demonstrated that high spatial resolution inside the VOI may be achieved with a high-resolution detector (e.g. CCD/CsI), reduced focal spot size (0.3 mm), and optimized geometrical magnification (e.g.1.63). Exposure reduction outside the VOI has been shown to reduce the scatter components in the high-resolution projection image data for the VOI. This has also led to significantly lowered doses inside and outside the VOI.

Image guidance during cardiac interventional procedures (IP) using cardiac C-arm CT systems is desirable for many procedures. Applying the concept of retrospective electrocardiogram gating (ECG) to the acquisition of multiple, ECG-triggered rotational acquisitions using a C-arm system allows the 3D+t reconstruction of the heart. The process of retrospective gating is a crucial component of 3-D reconstruction. The gold-standard in gating is still ECG based. However, the ECG signal does not directly reflect the mechanical situation of the heart. Therefore an alternative gating method, based on the acquired projection data is required. Our goal is to provide an image-based gating (IBG) method without ECG such that already acquired projection data from a multi-sweep acquisition can still be used for reconstruction. We formulate the gating problem as a shortest-path optimization problem. All acquired projection images build a directed graph and the path costs are defined by projection image similarities that are based on image metrics to measure the heart phase similarity. The optimization is additionally regularized to prefer solutions where the path segment of consecutive selected projections acquired along a particular forward or backward C-arm sweep is short. This regularization depends on an estimated average heart rate that is also estimated using an image-based method. First promising results using in-vivo data are presented and compared to standard ECG gating. We conclude that the presented IBG method provides a reliable gating.

Since the advent of multi-slice CT, helical scan has played an increasingly important role in cardiac imaging. With the availability of diagnostic volumetric CT, step-and-shoot scan has been becoming popular recently. Step-and-shoot scan decouples patient table motion from heart beating, and thus the temporal window for data acquisition and image reconstruction can be optimized, resulting in significantly reduced radiation dose, improved tolerance to heart beat rate variation and inter-cycle cardiac motion inconsistency. Multi-sector data acquisition and image reconstruction have been utilized in helical cardiac imaging to improve temporal resolution, but suffers from the coupling of heart beating and patient table motion. Recognizing the clinical demands, the multi-sector data acquisition scheme for step-and-shoot scan is investigated in this paper. The most outstanding feature of the multi-sector data acquisition combined with the stepand- shoot scan is the decoupling of patient table proceeding from heart beating, which offers the opportunities of employing prospective ECG-gating to improve dose efficiency and fine adjusting cardiac imaging phase to suppress artifacts caused by inter-cycle cardiac motion inconsistency. The improvement in temporal resolution and the resultant suppression of motion artifacts are evaluated via motion phantoms driven by artificial ECG signals. Both theoretical analysis and experimental evaluation show promising results for multi-sector data acquisition scheme to be employed with the step-and-shoot scan. With the ever-increasing gantry rotation speed and detector longitudinal coverage in stateof- the-art VCT scanners, it is expected that the step-and-shoot scan with multi-sector data acquisition scheme would play an increasingly important role in cardiac imaging using diagnostic VCT scanners.

The Scanning-Beam Digital X-ray (SBDX) system performs rapid scanning of a narrow x-ray beam using an electronically scanned focal spot and inverse beam geometry. SBDX's ability to perform real-time multi-plane tomosynthesis with high dose efficiency is well-suited to interventional procedures such as left atrial ablation, where precise knowledge of catheter positioning is desired and imaging times are long. We describe and evaluate techniques for frame-by-frame 3D localization of multiple catheter electrodes from the stacks of tomosynthetic images generated by SBDX. The localization algorithms operate on gradient-filtered versions of the tomosynthetic planes. Small high contrast objects are identified by thresholding the stack of images and applying connected component analysis. The 3D coordinate of each object is the center-of-mass of each connected component. Simulated scans of phantoms containing 1-mm platinum spheres were used to evaluate localization performance with the SBDX prototype (5.5 × 5.5 cm detector, 3° tomographic angle) and a with new SBDX detector under design (10-cm wide × 6 cm, 6° × 3°). Z-coordinate error with the SBDX prototype was -0.6 +/- 0.7 mm (mean+/-standard deviation) with 28 cm acrylic, 24.3 kWp source operation, and 12-mm plane spacing. Localization improved to -0.3 +/- 0.3 mm using the wider SBDX detector and a 3-mm plane spacing. The effects of tomographic angle, plane-to-plane spacing, and object velocity are evaluated, and a simulation demonstrating ablation catheter localization within a real anatomic background is presented. Results indicate that SBDX is capable of precise real-time 3D tracking of high contrast objects.

Rational and Objective: In CT systems, blurring is the main limiting factor for imaging in-stent restenosis. The aim of this study is to systematically analyze the effect of blurring related biases on the quantitative assessment of in-stent restenosis and to evaluate potential correction methods. Methods: 3D analytical models of a blurred, stented vessel are presented to quantify blurring related artifacts in the stent diameter measurement. Two correction methods are presented for an improved stent diameter measurement. We also examine the suitability of deconvolution techniques for correcting blurring artifacts. Results: Blurring results in a shift of the maximum of the signal intensity towards the center position of the stent, resulting in an underestimation of the stent diameter. This shift can be expressed as a function of the stent radius and width of the point spread function. The correction for this phenomenon reduces the error with 75 percent. Deconvolution reduces the blurring artifacts but introduces a ringing artifact. Conclusion: The analytical vessel models are well suited to study the influence of various parameters on blurring-induced artifacts. The blurring-related underestimation of the stent diameter can significantly be reduced using the presented corrections. Care should be taken into choosing suitable deconvolution filters since they may introduce new artifacts.

One of the key measures of response to treatment for patients in multicenter clinical trials is the lung density measured in Hounsfield Units (HU) from Computer Tomography (CT) scans. The purpose of this work is to determine the dependence of CT attenuation values on scanner type by using in vivo measurements made from homogeneous anatomic areas. In vivo measurements were made in areas within the trachea, aorta, fat and muscle regions of CT scans obtained from subjects scanned as part of a multicenter treatment trial. Scans were selected so that exams from all four major manufacturers were included in the study. For each anatomic region of interest, the mean and standard deviation values were computed to investigate attenuation dependence on scanners. For example, trachea mean (standard deviation) measurements for exams from GE, Siemens, Philips and Toshiba scanners were -986 HU(±15), - 993 HU(±9), -988HU(±8), -1046(±10) respectively. Inter-scanner variability was observed for each scanner showing significant differences (all p-values <0.005). Previous work in examining attenuation dependence on scanners has been performed using anthropomorphic phantoms. The novelty of this work is the use of in vivo measurements from homogeneous regions in order to examine scanner effects on CT attenuation values. Our results show that CT attenuation values for the anatomic regions vary between scanners and hence, dependence of CT attenuation values on scanners is observed.

The purpose of this study is to evaluate the effect of reduced tube current, as a surrogate for radiation dose, on lung nodule detection in pediatric chest multi-detector CT (MDCT). Normal chest MDCT images of 13 patients aged 1 to 7 years old were used as templates for this study. The original tube currents were between 70 mA and 180 mA. Using proprietary noise addition software, noise was added to the images to create 13 cases at the lowest common mA (i.e. 70 mA), 13 cases at 35 mA (50% reduction), and 13 cases at 17.5 mA (75% reduction). Three copies of each case were made for a total of 117 series for simulated nodule insertion. A technique for three-dimensional simulation of small lung nodules was developed, validated through an observer study, and used to add nodules to the series. Care was taken to ensure that each of three lung zones (upper, middle, lower) contained 0 or 1 nodule. The series were randomized and the presence of a nodule in each lung zone was rated independently and blindly by three pediatric radiologists on a continuous scale between 0 (definitely absent) and 100 (definitely present). Receiver operating characteristic analysis of the data showed no general significant difference in diagnostic accuracy between the reduced mA values and 70 mA, suggesting a potential for dose reduction with preserved diagnostic quality. To our knowledge, this study is the first controlled, systematic, and task-specific assessment of the influence of dose reduction in pediatric chest CT.

Computed tomography (CT) has been well established as a diagnostic tool through hardware optimization and sophisticated data calibration. For screening purposes, the associated X-ray exposure risk must be minimized. An effective way to minimize the risk is to deliver fewer X-rays to the subject or lower the mAs parameter in data acquisition. This will increase the data noise. This work aims to study the noise property of the calibrated or preprocessed sinogram data in Radon space as the mAs level decreases. An anthropomorphic torso phantom was scanned repeatedly by a commercial CT imager at five different mAs levels from 100 down to 17 (the lowest value provided by the scanner). The preprocessed sinogram datasets were extracted from the CT scanner to a laboratory computer for noise analysis. The repeated measurements at each mAs level were used to test the normality of the repeatedly measured samples for each data channel using the Shapiro-Wilk statistical test merit. We further studied the probability distribution of the repeated measures. Most importantly, we validated a theoretical relationship between the sample mean and variance at each channel. It is our intention that the statistical test and particularly the relationship between the first and second statistical moments will improve low-dose CT image reconstruction for screening applications.

Metallic objects severely limit diagnostic CT imaging because of their high X-ray attenuation in the diagnostic energy range. In contrast, radiation therapy linear accelerators now offer CT imaging with X-ray energies in the megavolt range, where the attenuation coefficients of metals are significantly lower. We hypothesized that Mega electron-Voltage Cone-Beam CT (MVCT) implemented on a radiation therapy linear accelerator can detect and quantify small features in the vicinity of metallic implants with accuracy comparable to clinical Kilo electron-Voltage CT (KVCT) for imaging. Our test application was detection of osteolytic lesions formed near the metallic stem of a hip prosthesis, a condition of severe concern in hip replacement surgery. Both MVCT and KVCT were used to image a phantom containing simulated osteolytic bone lesions centered around a Chrome-Cobalt hip prosthesis stem with hemispherical lesions with sizes and densities ranging from 0.5 to 4 mm radius and 0 to 500 mg•cm^-3, respectively. Images for both modalities were visually graded to establish lower limits of lesion visibility as a function of their size. Lesion volumes and mean density were determined and compared to reference values. Volume determination errors were reduced from 34%, on KVCT, to 20% for all lesions on MVCT, and density determination errors were reduced from 71% on KVCT to 10% on MVCT. Localization and quantification of lesions was improved with MVCT imaging. MVCT offers a viable alternative to clinical CT in cases where accurate 3D imaging of small features near metallic hardware is critical. These results need to be extended to other metallic objects of different composition and geometry.

The recent multidetector-row CT (MDCT) systems yield high spatial resolution in all directions of volumetric images in clinical routine. The quantitative characterization of the performance of MDCT systems is important for comparing the effects of different scan and reconstruction parameters, for comparing between different CT systems, and for evaluating the accuracy of size and density measurements of fine details in MDCT images. This paper presents a method to characterize the performance of MDCT using a three-dimensional (3D) point spread function (PSF) obtained from MDCT images. With separability assumed, the 3D PSF is decomposed into 2D PSF in the scan plane and SSP on the z-axis. The 2D PSF and SSP are modeled on the basis of the symmetric Lèvy function that generalizes Gaussian and Lorentzian functions. The descriptions of 3D PSF are defined from the Fourier analysis of MDCT images of wire (diameter 0.049mm) and micro-disk (thickness 0.05mm) phantoms. Experimenting with a MDCT system, we demonstrate the method for 3D PSF measurement.

Tumor motion induced by patient breathing decreases the effectiveness of radiation treatment. Image guided radiation treatment (IGRT) is an advanced approach for cancer radiation treatment. The success of IGRT is largely dependent on the accurate localization of tumor in real-time. There are two major imaging approaches currently in use to localize a tumor: internal imaging and external imaging. Internal imaging determines the tumor locations by directly x-ray of the tumor area. It is accurate however radiation dose is a big concern. External imaging derives the internal tumor locations through an external mark on the patient surface. It is radiation dose free however the insufficient accuracy limits its wide application. Integrating the internal and external signals together is necessary for reliable radiation treatment and acceptable patient radiation exposure. Our work tries to identify the correlation patterns between internal/external signals and the influential factors so that the hybrid signal will give desire accuracy in dose delivery while limiting radiation exposure to the patients. Both theoretical simulation based on sinusoidal functions and statistical analysis on real patient data are performed. The sinusoidal simulation will identify the potential influence factors of different correlation conditions. The results have demonstrated the various correlation patterns with amplitude various, frequency changes (duration changes), phase shifts, and baseline drift. The results will aid the statistical analytical on real-patients to identify the dominant factors of the internal/external motion signals for a specific patients. The described work is very useful in advanced IGRT to update the internal/external correlation in real-time for better cancer patient care.

The statistical properties of medical images are central in characterizing the performance of imaging systems. The noise in cone-beam CT (CBCT) is often characterized using Fourier-based metrics, such as the 3D noise-power spectrum (NPS). Under a stationarity assumption, the NPS provides a complete representation of the covariance of the images, since the covariance matrix of the Fourier transform of the image is diagonal. In practice, such assumptions are obeyed to varying degrees. The objective of this work is to investigate the degree to which such assumptions apply in CBCT and to experimentally characterize the NPS and off-diagonal elements under a range of experimental conditions. A benchtop CBCT system was used to acquire 3D image reconstructions of various objects (air and a water cylinder) across a range of experimental conditions that could affect stationarity (bowtie filter and dose). We test the stationarity assumption under such varying experimental conditions using both spatial and frequency domain measures of stationarity. The results indicate that experimental conditions affect the degree of stationarity and that under some imaging conditions, local descriptions of the noise need to be developed to appropriately describe CBCT images. The off-diagonal elements of the DFT covariance matrix may not always be ignored.

In the framework of Spectral Computed Tomography (Spectral CT), scattered X-ray radiation is examined for its spectral composition and spatial distribution by means of Monte Carlo simulations. A reliable material (e.g. bone / contrast agent) separation and quantification requires a precise knowledge of the transmitted X-ray spectrum especially for low energy photons. Unfortunately, for lower energies the primary intensity is increasingly covered by scattered radiation. The detected scattered radiation can be classified into two main categories with respect to their scattering history. The first category contains purely Rayleigh or one-time Compton scattered photons which typically have small scattering angles and an energy spectrum similar to that of the transmitted primary radiation. The second category comprises multiple Compton scattered photons with a spectral composition which is typically softer than that of the transmitted primary photons. In regions of strong beam attenuation (i.e. in the X-ray shadow of a scanned object), the scattered radiation is mainly composed of multiple Compton scattered photons. As a consequence, the spectrally resolved scatter-to-primary ratios strongly increase at low energies. High-quality anti-scatter grids can be used to reduce especially the detection of multiple Compton-scattered photons. A quantitative evaluation of measured photon energies below a certain limit between 30 keV and 50 keV (depending on the phantom geometry and the applied anti-scatter grid) is challenging, since primary photons are superposed by a significantly higher amount of scattered photons.

Crucial to understanding the factors that govern imaging performance is a rigorous analysis of signal and noise transfer characteristics (e.g., MTF, NPS, and NEQ) applied to a task-based performance metric (e.g., detectability index). This paper advances a theoretical framework for calculation of the NPS, NEQ, and DQE of cone-beam CT (CBCT) and tomosynthesis based on cascaded systems analysis. The model considers the 2D projection NPS propagated through a series of reconstruction stages to yield the 3D NPS, revealing a continuum (from 2D projection radiography to limited-angle tomosynthesis and fully 3D CBCT) for which NEQ and detectability index may be investigated as a function of any system parameter. Factors considered in the cascade include: system geometry; angular extent of source-detector orbit; finite number of views; log-scaling; application of ramp, apodization, and interpolation filters; back-projection; and 3D noise aliasing - all of which have a direct impact on the 3D NEQ and DQE. Calculations of the 3D NPS were found to agree with experimental measurements across a broad range of imaging conditions. The model presents a theoretical framework that unifies 3D Fourier-based performance metrology in tomosynthesis and CBCT, providing a guide to optimization that rigorously considers the system configuration, reconstruction parameters, and imaging task.

Currently all CT scanners collect the projection images sequentially, one at a time. The serial approach demands high x-ray power which in turn limits the scanning speed of the CT scanners. To overcome the limitations of the current CT scanners, the concept of stationary CT canners has been proposed to completely eliminate the need for gantry rotation. In such multi-pixel x-ray system, multiple x-ray sources and detectors are distributed around the scanning tunnel. Based on the multi-pixel x-ray system, we have recently demonstrated the feasibility of multiplexing radiography that enables simultaneous collection of multiple projection images through multiplexing. A drastic increase of the speed and reduction of the x-ray peak power can be potentially achieved without compromising the imaging quality. In this paper we demonstrated novel Hadamard multiplexing radiography based on Hadamard transform technique using a carbon nanotube based multi-pixel x-ray source. The combination of the multi-pixel x-ray and multiplexing technologies has the potential to lead to a new generation of stationary CT scanners that have drastically increased throughput at reduced cost.

Multiplexed x-ray imaging was recently proposed as a way to possibly reduce x-ray source power requirements while maintaining temporal resolution and imaging speed. Rather than measuring projections sequentially, with multiplexing multiple sources send photons to the same detector corresponding to different projections. Data for the multiple projections that are measured by the same detector are separated by energizing the sources with different temporal sequences. This concept could be used for radiography, tomosynthesis, or CT imaging. Multiplexed measurements are used very successfully in other modalities. For example, in magnetic resonance imaging (MRI) data from multiple voxels, perhaps even the entire object, can be measured through a single channel. In MRI, the simultaneous interrogation of multiple regions has no SNR penalty. It is important to examine the noise impact of multiplexing/demultiplexing in x-ray imaging. We examined the propagation of noise due to the quantum statistics of the measured x-rays. The analysis showed that for quantum limited acquisitions, the noise penalty of multiplexing can be severe. Indeed, if both the multiplexed and sequential methods are quantum limited, the simpler sequential method always outperforms multiplexing.

Image reconstruction of dynamically deforming objects from projections and known time-dependent motion field is of interest for x-ray computed tomography. Recently, three analytical exact methods have been developed based on DFBP or DBPF algorithms which compensates for time-dependent standard affine or relaxed affine transformation. In contrast, an empirical algorithm has been proposed by Schafer, et al., which merely "trace" the motion of each voxel during the backprojection process. The method is known to be an approximation; however, it has not been discussed how good or bad the level of approximation is. In this paper, we present that a slightly modified Schafer's method (FBPx) is exact if the motion of the object can be described by a class of time-dependent affine transformation - isotropic scaling (contraction and expansion), rotation, and translation. We show mathematically and experimentally that Schafer's method is a good approximation.

In dual source CT (DSCT) with two X-ray sources and two data measurement systems mounted on a CT gantry with a mechanical offset of 90 deg, cross scatter radiation, (essentially 90 deg Compton scatter) is added to the detector signals. In current DSCT scanners the cross scatter correction is model based: the idea is to describe the scattering surface in terms of its tangents. The positions of these tangent lines are used to characterize the shape of the scattering object. For future DSCT scanners with larger axial X-ray beams, the model based correction will not perfectly remove the scatter signal in certain clinical situations: for obese patients scatter artifacts in cardiac dual source scan modes might occur. These shortcomings can be circumvented by utilizing the non-diagnostic time windows in cardiac scan modes to detect cross scatter online. The X-ray generators of both systems have to be switched on and off alternating. If one X-ray source is switched off, cross scatter deposited in the respective other detector can be recorded and processed, to be used for efficient cross scatter correction. The procedure will be demonstrated for cardiac step&shoot as well as for spiral acquisitions. Full rotation reconstructions are less sensitive to cross scatter radiation; hence in non-cardiac case the model-based approach is sufficient. Based on measurements of physical and anthropomorphic phantoms we present image data for DSCT systems with various collimator openings demonstrating the efficacy of the proposed method. In addition, a thorough analysis of contrast-to-noise ratio (CNR) shows, that even for a X-ray beam corresponding to a 64x0.6 mm collimation, the maximum loss of CNR due to cross scatter is only about 7% in case of obese patients.

A common problem arising in medical imaging is the suppression of undesired image artifacts with the simultaneous preservation of salient clinical information. Often the proposed processing "cure" introduces its own artifacts in other parts of the image that confound reliable diagnosis. A canonical example is the suppression of artifacts from hyperdense objects, such as metal and calcium. In this paper we propose a new decomposition-based approach to the combined image formation and the suppression of localized image artifacts which is motivated by recent results on image inpainting. The approach, which we term Model-Based Algebraic Iteration (MBAI) processing, decomposes an image into a collection of homogeneous components, each of which can be reconstructed in the manner most appropriate to its underlying nature. Because each component is localized, the effects of processing on that component do not contaminate other areas of the image. Our specific motivation is the mitigation of artifacts in cardiac multi-detector computed tomography (MDCT) images. Recently, MDCT has offered the promise of a non-invasive alternative to invasive coronary angiography to evaluate coronary artery disease. An impediment preventing its utilization as a routine clinical replacement for angiography is the presence of image "blooming" artifacts due to the presence vascular calcium. We develop MBAI for the purpose of ameliorating artifacts in cardiac images and thus increase the applicability of MDCT for the evaluation of at-risk patient population. We demonstrate preliminary results in the reduction of the calcium blooming-effect in software simulation, phantom, ex-vivo, and in-vivo MDCT data.

In flat detector cone-beam computed tomography (CBCT), scattered radiation is a major source of image degradation, making accurate a posteriori scatter correction inevitable. A potential solution to this problem is provided by computerized scatter correction based on Monte-Carlo simulations. Using this technique, the detected distributions of X-ray scatter are estimated for various viewing directions using Monte-Carlo simulations of an intermediate reconstruction. However, as a major drawback, for standard CBCT geometries and with standard size flat detectors such as mounted on interventional C-arms, the scan field of view is too small to accommodate the human body without lateral truncations, and thus this technique cannot be readily applied. In this work, we present a novel method for constructing a model of the object in a laterally and possibly also axially extended field of view, which enables meaningful application of Monte-Carlo based scatter correction even in case of heavy truncations. Evaluation is based on simulations of a clinical CT data set of a human abdomen, which strongly exceeds the field of view of the simulated C-arm based CBCT imaging geometry. By using the proposed methodology, almost complete removal of scatter-caused inhomogeneities is demonstrated in reconstructed images.

X-ray in-line phase-contrast imaging is a technique that aims to reconstruct the projected absorption and refraction properties of an object. To achieve this, phase retrieval algorithms are employed. The statistical properties of the reconstructed images in phase-contrast imaging remain largely unexplored. In this work, the covariance structure of the absorption and refractive index images is derived analytically, to characterize the noise texture in quantitative in-line phase-contrast imaging. This information is utilized to investigate how object detectability is affected by specification of imaging geometry.

Metrics of system performance are used to assess the abilities and safety of x-ray imaging systems. The detective quantum efficiency (DQE) is used as a measure of "dose efficiency" but, when applied to fluoroscopic systems, requires a measurement of the temporal modulation transfer function (MTF) to account for the effects of system lag. It is shown that the temporal MTF is exposure-rate dependent, and hence must be measured under the specific exposure conditions of interest. We develop a small-signal approach to temporal MTF measurements using a semi-transparent moving slanted edge. Using an x-ray image intensifier-based bench-top system, we show that there is a 50% overstatement of the DQE when not properly accounting for lag. The small-signal approach is used to calculate a lag-free fluoroscopic DQE that agrees with a radiographic DQE measurement under the same exposure-rate conditions. It was found that the temporal MTF did not change within measured precision over normal fluoroscopic conditions, and the radiopaque falling-edge results were consistent with the small-signal temporal MTF. This approach could be implemented in a clinical setting with access to raw (linear or linearized) fluoroscopic image data and could be generalized for use on pulsed-exposure systems.

In this paper, we present shift-invariant filtered backprojection (FBP) cone-beam image reconstruction algorithms for a cone-beam CT system based on a clinical C-arm gantry. The source trajectory consists of two concentric arcs which is complete in the sense that the Tuy data sufficiency condition is satisfied. This scanning geometry is referred to here as a CC geometry (each arc is shaped like the letter "C"). The challenge for image reconstruction for the CC geometry is that the image volume is not well populated by the familiar doubly measured (DM) lines. Thus, the well-known DM-line based image reconstruction schemes are not appropriate for the CC geometry. Our starting point is a general reconstruction formula developed by Pack and Noo which is not dependent on the existence of DM-lines. For a specific scanning geometry, the filtering lines must be carefully selected to satisfy the Pack-Noo condition for mathematically exact reconstruction. The new points in this paper are summarized here. (1) A mathematically exact cone-beam reconstruction algorithm was formulated for the CC geometry by utilizing the Pack-Noo image reconstruction scheme. One drawback of the developed exact algorithm is that it does not solve the long-object problem. (2) We developed an approximate image reconstruction algorithm by deforming the filtering lines so that the long object problem is solved while the reconstruction accuracy is maintained. (3) In addition to numerical phantom experiments to validate the developed image reconstruction algorithms, we also validate our algorithms using physical phantom experiments on a clinical C-arm system.

In 3^rd generation CT systems projection data, generated by X-rays emitted from a single source and passing through the imaged object, are acquired by a single detector covering the entire field of view (FOV). Novel CT system architectures employing distributed sources [1,2] could extend the axial coverage, while removing cone-beam artifacts and improving spatial resolution and dose. The sources can be distributed in plane and/or in the longitudinal direction. We investigate statistical iterative reconstruction of multi-axial data, acquired with simulated CT systems with multiple sources distributed along the in-plane and longitudinal directions. The current study explores the feasibility of 3D iterative Full and Half Scan reconstruction methods for CT systems with two different architectures. In the first architecture the sources are distributed in the longitudinal direction, and in the second architecture the sources are distributed both longitudinally and trans-axially. We used Penalized Weighted Least Squares Transmission Reconstruction (PWLSTR) and incorporated a projector-backprojector model matching the simulated architectures. The proposed approaches minimize artifacts related to the proposed geometries. The reconstructed images show that the investigated architectures can achieve good image quality for very large coverage without severe cone-beam artifacts.

It is known that x-ray projections collected from a circular orbit of an x-ray source are insufficient for accurate reconstruction of a 3D object. For each local region of the object (except in the plane containing the source trajectory) there is a conical volume in the object's spatial frequency space that is unmeasured due to the circular geometry. The Feldkamp, Davis and Kress (FDK) algorithm based on filtered backprojection (FBP) involves a 3D backprojection step so that these unmeasured spatial frequencies are set to zero, resulting in cone beam artifacts for certain objects. We present a new type of cone beam CT reconstruction algorithm based on the Fourier rebinning (FORE) framework of Defrise et al. The cone beam x-ray projection data are rebinned into a set of in-plane sinograms using the FORE rebinning approximation, followed by 2D FBP to reconstruct each axial slice. The algorithm is able to extrapolate data into the missing region of the object's frequency space in a computationally efficient way, allowing for a reduction of cone beam artifacts for certain objects. Unlike FDK, the algorithm is exact for an impulse object located anywhere along the axis of rotation. Reconstruction errors are dependent on the radial distance, cone angle, and the second-derivative of the projection data in the longitudinal direction. Finally, an extension to the algorithm is presented that permits reconstruction in regions of the object that are not seen by the detector in every view.

Some of the recently developed image reconstruction algorithms for cone-beam computed tomography (CBCT) involve the computation of the finite Hilbert transform. We have previously studied noise property of the finite Hilbert transform and observed that it can be used for potentially improving the image noise property within a region of interest (ROI) in IGRT. Imaging radiation dose is one of the critical issues in IGRT, and in addition to existing dose-reduction schemes by use of ROI imaging, it is possible to achieve further patient dose reduction through modulating beam intensity so that a sub-ROI in the ROI be exposed by high flux of x-ray photons and the rest of the ROI be exposed by low flux of them. In this work, we investigate the technique for obtaining sub-ROI images, which is supposed to include the target under treatment, with high contrast-to-noise ratio (CNR) and the images within the rest of the ROI with low CNR. Numerical studies have been conducted as a preliminary in this work.

We present Computed Tomography (CT) acquisition and reconstruction schemes for low-dose neuro-angiography based on the method of HighlY constrained back PRojection (HYPR). Simulated and experimental low X-ray radiation dose scans were prepared using the techniques of interleaved view angle under-sampling and tube current reduction. Dynamic CT Angiograms (CTAs) were produced for both standard and low dose images sets. The spatial correlation coefficient, r, between the two reconstruction approaches was determined for each time frame and the SNR and CNR values in arterial ROIs were calculated. The undersampled HYPR reconstructions produced r values of > 0.95 at undersampling and dose reduction factors of 10 and SNR and CNR were more than doubled using HYPR techniques at a tube current of 25 mA. HYPR approaches to contrast enhanced neuro-imaging provide not only volumetric brain hemodynamics but also the ability to produce high quality maps of standard perfusion parameters. The synergy of volumetric hemodynamics and assessment of tissue function provides the medical imaging community with high quality diagnostic information at a fraction of the radiation dose in a single contrast-enhanced scan.

X-ray differential phase-contrast tomography (DPCT) is a method for reconstructing the spatial distribution of the X-ray refractive index within an object from knowledge of differential projection data. Assuming geometrical optics wave propagation, these data describe the angles by which the probing optical beams are deflected by the object due to refraction. Phase-sensitive X-ray imaging methods such as diffraction enhanced imaging can measure the required beam-deflection data, and are being actively developed for medical imaging applications. In this work, we investigate and demonstrate the applicability of algorithms recently developed for conventional tomography for obtaining region-of-interest images in DPCT from knowledge of truncated differential projection data. A preliminary numerical study is conducted to validate and demonstrate the proposed reconstruction algorithm.

Digital breast tomosynthesis (DBT) is a rapidly developing imaging modality that gives some tomographic information for breast cancer screening. The effectiveness of standard mammography can be limited by the presence of overlapping structures in the breast. A DBT scan, consisting of a limited number of views covering a limited arc projecting the breast onto a fixed flat-panel detector, involves only a small modification of digital mammography, yet DBT yields breast image slices with reduced interference from overlapping breast tissues. We have recently developed an iterative image reconstruction algorithm for DBT based on image total variation (TV) minimization that improves on EM in that the resulting images have fewer artifacts and there is no need for additional regularization. In this abstract, we present the total p-norm variation (TpV) image reconstruction algorithm. TpV has the advantages of our previous TV algorithm, while improving substantially on the efficiency. Results for the TpV on clinical data are shown and compared with EM.

Optimal noise control is critical for dose reduction in CT. In this work, we investigated the use of a locally-adaptive method for noise reduction in low-dose CT. This method is based upon bilateral filtering, which smoothes the projection data using a weighted average in a local neighborhood, where the weights are determined according to both the spatial proximity and intensity similarity between the center pixel and the neighboring pixels. This filtering is locally adaptive and can preserve important edge information in the sinogram, thus without significantly sacrificing the spatial resolution. It is closely related to anisotropic diffusion, but is significantly faster. More importantly, a CT noise model can be readily incorporated in the filtering and denoising process. We have evaluated the noise-resolution properties of the bilateral filtering in a phantom study and a preliminary patient study with contrast-enhanced abdominal CT exams. The results demonstrated that bilateral filtering can achieve a better noise-resolution tradeoff than a series of commercial reconstruction kernels. This improvement on noise-resolution properties can be used for improving the image quality in low-dose CT and can also be translated to substantial dose reduction.

In classical tomosynthesis, the x-ray source generally is moved along a curve segment, such as a circular trajectory, within a plane that is perpendicular to the detector plane. Studies suggest that when the angular coverage and number of projection views are limited, it can be difficult to reconstruct accurate images within planes perpendicular to the detector plane in classical tomosynthesis. In this work, we investigate imaging strategies in tomosynthesis using trajectories that are not confined within a plane perpendicular to the detector plane. We expect that such trajectories can increase data information and thus lead reconstructed images with improved quality. Numerical studies were conducted for evaluating the image-reconstruction quality in classical tomosynthesis and tomosynthesis with trajectories that are not confined within a plane perpendicular to the detector plane. The results of the studies indicated that, with the same number of views, (or equivalenntly, the same amount of image radiation), data acquired in tomosynthesis with the trajectories that are not confined within a plane perpendicluar to the detector plane generally contain more information than that acquired with classical tomosynthesis and can thus yield images with improved quality.

In this paper we propose a novel scatter correction methodology for X-ray based cone-beam CT that allows to combine the advantages of projection-based and volume-based correction approaches. The basic idea is to use a potentially non-optimal projection-based scatter correction method and to iteratively optimize its performance by repeatedly assessing remaining scatter-induced artifacts in intermediately reconstructed volumes. The novel approach exploits the fact that due to the flatness of the scatter-background, compensation itself is most easily performed in the projection-domain, while the scatter-induced artifacts can be better observed in the reconstructed volume. The presented method foresees to evaluate the scatter correction efficiency after each iteration by means of a quantitative measure characterizing the amount of residual cupping and to adjust the parameters of the projection-based scatter correction for the next iteration accordingly. The potential of this iterative scatter correction approach is demonstrated using voxelized Monte Carlo scatter simulations as ground truth. Using the proposed iterative scatter correction method, remarkable scatter correction performance was achieved both using simple parametric heuristic techniques as well as by optimizing previously published scatter estimation schemes. For the human head, scatter induced artifacts were reduced from initially 148 HU to less than 8.1 HU to 9.1 HU for different studied methods, corresponding to an artifact reduction exceeding 93%.

There exists a strong need to reconstruct computed tomographic (CT) images with practically useful quality from a small number of projections in image-guided radiation therapy: for lowering radiation dose delivered to the subject, for shortening the imaging time, and for reducing the imaging-configuration complexity. We have recently developed an iterative image reconstruction algorithm based on total-variation (TV) minimization from incomplete projection data in CT. In numerical studies with a variety of incomplete projection-data sets including truncated data, reduced scan range, and sparse sampling, the developed algorithm seems to yield reasonable reconstruction, as compared to some of the existing algorithms, such as algebraic reconstruction technique (ART) and expectation minimization (EM). The TV-based algorithm begins in general with a uniform image as an initial guess, and goes through iteration steps to minimize the image TV subject to satisfying the given incomplete projection data. In image-guided radiation therapy (IGRT), a patient usually undergoes CT scanning for treatment planning, which can provide the reference image for image guidance. Therefore, we propose a TV-based algorithm with a priori information in few-view CT for IGRT, in an attempt to further reduce the number of projections needed for image reconstruction from what the TV-based algorithm uses when no a priori information is included. In this work, we report the initial results of a preliminary numerical study that we have conducted to demonstrate this approach.

Recently, foundational mathematical theory, compressed sensing (CS), has been developed which enables accurate reconstruction from greatly undersampled frequency information (Candes et. al. and Donoho). Using numerical phantoms it has been demonstrated that CS reconstruction (e.g. minimizing the ℓ₁ norm of the discrete gradient of the image) offers promise for computed tomography. However, when using experimental CT projection data the undersampling factors enabled were smaller than in numerical simulations. An extension to CS has recently been proposed wherein a prior image is utilized as a constraint in the image reconstruction procedure (i.e. Prior Image Constrained Compressed Sensing - PICCS). Experimental results are demonstrated here from a clinical C-arm system, highlighting one application of PICCS in reducing radiation exposure during interventional procedures while preserving high image quality. In this study a range of view angles has been investigated from very limited angle aquisitions (e.g. tomosythesis) to undersampled CT acquisitions.

Truncation of CT projection data is always coupled with incomplete angular sampling and can lead to severe image artifacts in clinical CT. Extrapolation of projection data is needed to restore CT values inside and outside the scan field of view (SFOV). We present three types of extrapolation schemes. The first type (M1) is characterized by extrapolation of projection data using a virtual object of constant attenuation. For multi-slice helical CT this extrapolation scheme is applied in a row-wise manner. The second type (M2) utilizes consistency conditions of parallel projection data. The conservation of m^th order moments of non-truncated projections can be utilized for the extrapolation of truncated projection data by fitting extrapolation functions of variable length. The third method (M3) extrapolates truncated data by sinogram decomposition and completion. For each voxel in image space the corresponding trace in the 3D-sinogram is computed. The minimum signal within each trace is extrapolated to the extended sinogram parts, which represent the extended FOV. Based on the evaluation of both simulation data of an anthropomorphic thorax phantom and clinical data, we evaluate the three reconstruction techniques. Sinogram decomposition proofs to be better than the other techniques, but is computationally very demanding.

To improve the speed and quality of ordered-subsets expectation-maximization (OSEM) SPECT reconstruction, we have implemented a content-adaptive, singularity-based, mesh-domain, image model (CASMIM) with an accurate algorithm for estimation of the mesh-domain system matrix. A preliminary image, used to initialize CASMIM reconstruction, was obtained using pixel-domain OSEM. The mesh-domain representation of the image was produced by a 2D wavelet transform followed by Delaunay triangulation to obtain joint estimation of nodal locations and their activity values. A system matrix with attenuation compensation was investigated. Digital chest phantom SPECT was simulated and reconstructed. The quality of images reconstructed with OSEM-CASMIM is comparable to that from pixel-domain OSEM, but images are obtained five times faster by the CASMIM method.

Chord-based algorithms can eliminate cone-beam artifacts in images reconstructed from a clinical computed tomography (CT) scanner. The feasibility of using chord-based reconstruction algorithms was evaluated with three clinical CT projection data sets. The first projection data set was acquired using a clinical 64-channel CT scanner (Philips Brilliance 64) that consisted of an axial scan from a quality assurance phantom. Images were reconstructed using (1) a full-scan FDK algorithm, (2) a short-scan FDK algorithm, and (3) the chord-based backprojection filtration algorithm (BPF) using full-scan data. The BPF algorithm was capable of reproducing the morphology of the phantom quite well, but exhibited significantly less noise than the two FDK reconstructions as well as the reconstruction obtained from the clinical scanner. The second and third data sets were obtained from scans of a head phantom and a patient's thorax. For both of these data sets, the BPF reconstructions were comparable to the short-scan FDK reconstructions in terms of image quality, although sharper features were indistinct in the BPF reconstructions. This research demonstrates the feasibility of chord-based algorithms for reconstructing images from clinical CT projection data sets and provides a framework for implementing and testing algorithmic innovations.

Electronic noise compensation can be important for low-dose x-ray CT applications where severe photon starvation occurs. For clinical x-ray CT systems utilizing energy-integrating detectors, it has been shown that the detected x-ray intensity is compound Poisson distributed, instead of the Poisson distribution that is often studied in the literature. We model the electronic noise contaminated signal Z as the sum of a compound Poisson distributed random variable (r.v.) Y and a Gaussian distributed electronic noise N with known mean and variance. We formulate the iterative x-ray CT reconstruction problem with electronic noise compensation as a maximum-likelihood reconstruction problem. However the likelihood function of Z is not analytically trackable; instead of working with it directly, we formulate the problem in the expectation-maximization (EM) framework, and iteratively maximize the conditional expectation of the complete log-likelihood of Y. We further demonstrate that the conditional expectation of the surrogate function of the complete log-likelihood is a legitimate surrogate for the incomplete surrogate. Under certain linearity conditions on the surrogate function, a reconstruction algorithm with electronic noise compensation can be obtained by some modification of one previously derived without electronic noise considerations; the change incurred is simply replacing the unavailable, uncontaminated measurement Y by its conditional expectation E(Y|Z). The calculation of E(Y|Z) depends on the model of Y, N, and Z. We propose two methods for calculating this conditional expectation when Y follows a special compound Poisson distribution - the exponential dispersion model (ED). Our methods can be regarded as an extension of similar approaches using the Poisson model to the compound Poisson model.

The back-projection filtration (BPF)algorithm is capable of reconstructing ROI images from truncated data acquired with a wide class of general trajectories. However, it has been observed that, similar to other algorithms for convergent beam geometries, the BPF algorithm involves a spatially varying weighting factor in the backprojection step. This weighting factor can not only increase the computation load, but also amplify the noise in reconstructed images The weighting factor can be eliminated by appropriately rebinning the measured cone-beam data into fan-parallel-beam data. Such an appropriate data rebinning not only removes the weighting factor, but also retain other favorable properties of the BPF algorithm. In this work, we conduct a preliminary study of the rebinned BPF algorithm and its noise property. Specifically, we consider an application in which the detector and source can move in several directions for achieving ROI data acquisition. The combined motion of the detector and source generally forms a complex trajectory. We investigate in this work image reconstruction within an ROI from data acquired in this kind of applications.

Most recently, a novel data acquisition method has been proposed and experimentally implemented for x-ray differential phase contrast computed tomography (DPC-CT), in which a conventional x-ray tube and a Talbot-Lau type interferometer were utilized in data acquisition. The divergent nature of the data acquisition system requires a divergent-beam image reconstruction algorithm for DPC-CT. This paper focuses on addressing this image reconstruction issue. We developed a filtered backprojection algorithm to directly reconstruct the DPC-CT images from acquired projection data. The developed algorithm allows one to directly reconstruct the decrement of the real part of the refractive index from the measured data. In order to accurately reconstruct an image, the data need to be acquired over an angular range of at least 180° plus the fan-angle. Different from the parallel beam data acquisition and reconstruction methods, a 180° rotation angle for data acquisition system does not provide sufficient data for an accurate reconstruction of the entire field of view. Numerical simulations have been conducted to validate the image reconstruction algorithm.

In order to improve reconstructed image quality for cone-beam collimator SPECT, we have developed and implemented a fully 3D reconstruction, using an ordered subsets expectation maximization (OSEM) algorithm, along with a volumetric system model - cone-volume system model (CVSM), a modified attenuation compensation, and a 3D depth- and angle-dependent resolution and sensitivity correction. SPECT data were acquired in a 128×128 matrix, in 120 views with a single circular orbit. Two sets of numerical Defrise phantoms were used to simulate CBC SPECT scans, and low noise and scatter-free projection datasets were obtained using the SimSET Monte Carlo package. The reconstructed images, obtained using OSEM with a line-length system model (LLSM) and a 3D Gaussian post-filter, and OSEM with FVSM and a 3D Gaussian post-filter were quantitatively studied. Overall improvement in the image quality has been observed, including better transaxial resolution, higher contrast-to-noise ratio between hot and cold disks, and better accuracy and lower bias in OSEM-CVSM, compared with OSEM-LLSM.

In this paper we extend our previous evaluation of 2D tomographic image reconstruction using a content-adaptive mesh model (CAMM) for emission tomography (EM). First we present a model for the projection operator calculation using a ray-tracing algorithm based on mesh modeling for image representation. The proposed calculation allows us to incorporate a non-uniform attenuation and distance-dependent spatial resolution of the imaging system. Next the proposed approach was tested using realistic simulations with the NCAT phantom and SIMIND Monte Carlo simulation of a SPECT system in which the mesh model is calculated using images from pre reconstruction by filtered-back projection. Thereby this paper establishes a necessary step for the future development of 3D reconstruction images and spatio-temporal processing using deformable mesh modeling.

Single photon emission computerized tomographic (SPECT) images often suffer from low resolution and low count density. To improve spatial resolution of SPECT it is possible to use a pinhole collimator; however, this further reduces the system sensitivity. A potential solution to this problem is to use coded apertures, which offers increased sensitivity by using multiple pinholes, at the cost of increased image reconstruction time. A generic reconstruction algorithm has been developed which allows for arbitrary acquisition geometry via affine transforms (translation and rotation). The reconstruction process uses a (Siddon) ray projector, the expectation maximization (EM) algorithm and a 1 to n pinhole position matrix. Iteration times scale as a function of the number of pinholes in the collimator. Resolution recovery has also been incorporated into the reconstruction algorithm. The algorithm developed allows for the investigation of optimal imaging settings for small animal imaging. Simulated acquisitions of an ex-vivo rat heart with 1, 5 and 8 pinholes, over 360 degree acquisition, showing that multi-pinhole imaging can be successfully applied to rat cardiac imaging. Further refinement of the acquisition parameters, such as image overlap, collimator pinhole configuration and geometrical imaging configuration, will predict the theoretical settings for quantitative cardiac multi-pinhole SPECT imaging.

A method is proposed for a 3D reconstruction of coronary networks from rotational projections that departs from motion-compensated approaches. It deals with multiple views extracted from a time-stamped image sequence through ECG gating. This statistics-based vessel reconstruction method relies on a new imaging model by considering both the effect of background tissues and the image representation using spherically-symmetric basis functions, also called 'blobs'. These blobs have a closed analytical expression for the X-ray transform, which makes easier to compute a cone-beam projection than a voxel-based description. A Bayesian maximum a posteriori (MAP) estimation is used with a Poisson distributed projection data instead of the Gaussian approximation often used in tomography reconstruction. A heavy-tailed distribution is proposed as image prior to take into account the sparse nature of the object of interest. The optimization is performed by an expectation-maximization like (EM) block iterative algorithm which offers a fast convergence and a sound introduction of the non-negativity constraint for vessel attenuation coefficients. Simulations are performed using a model of coronary tree extracted from multidetector CT scanner and a performance study is conducted. They point out that, even with severe angular undersampling (6 projections over 110 degrees for instance) and without introducing a prior model of the object, significant results can be achieved.

Forward and Backward projections are two computational costly steps in tomography image reconstruction such as Positron Emission Tomography (PET). To speed-up reconstruction time, a hardware projection/backprojection pair has been built following algorithm architecture adequacy principles. Thanks to an original memory access strategy based on an 3D adaptive and predictive memory cache, the external memory wall has been overcome. Thus, for both projector architectures several units run efficiently. Each unit reaches a computational throughput close to 1 operation per cycle. In this paper, we present how from our hardware projection/backprojection pair, an analytic (3D-RP) and an iterative (3D-EM) reconstruction algorithms can be implemented on a System on Programmable Chip (SoPC). First, an hardware/software partitioning is done based on the different steps of each algorithm. Then the reconstruction system is composed of two hardware configurations of the programmable logic resources (FPGA). Each one corresponds mainly to the projection and backprojection step. Our projector/backprojector has been validated with a software 3D-RP and 3D-EM reconstruction on simulated PET-SORTEO data. A reconstruction time evaluation of these reconstruction systems are done based on the measured performances of our projectors IPs and the estimated performances of the additional simple hardware IPs. The expected reconstruction time is compared with the software tomography distribution STIR. A speed-up of 7 can be expected for the 3D-RP algorithm and a speed-up of 3.5 for the 3D-EM algorithm. For both algorithms, the architecture cycle efficiency expected is largely greater than the software implementation: 120 times for 3D-RP and 60 times for 3D-EM.

Tomography is an important technique for non-invasive imaging, with applications in medicine, materials research and industry. Tomographic reconstructions are typically gray-scale images, that can possibly contain a wide spectrum of grey levels. Segmentation of these grey level images is an important step to obtain quantitative information from tomographic datasets. Thresholding schemes are often used in practice, as they are easy to implement and use. However, if the tomogram exhibits variations in the intensity throughout the image, it is not possible to obtain an accurate segmentation using a single, global threshold. Instead, local thresholding schemes can be applied that use a varying threshold, depending on local characteristics of the tomogram. Selecting the best local thresholds is not a straightforward task, as local image features (such as the local histogram) often do not provide sufficient information for choosing a proper threshold. In this paper, we propose a new criterion for selecting local thresholds, based on the available projection data, from which the tomogram was initially computed. By reprojecting the segmented image, a comparison can be made with the measured projection data. This yields a quantitative measure of the quality of the segmentation. By minimizing the difference between the computed and measured projections, optimal local thresholds can be computed. Simulation experiments have been performed, comparing the result of our local thresholding approach with global thresholding. Our results demonstrate that the local thresholding approach yields segmentations that are significantly more accurate, in particular when the tomogram contains artifacts.

This work is a feasibility study of a phase-contrast cone-beam CT (CBCT) system for ROI (region of interest) reconstruction in breast imaging that incorporates the in-line holography technique into a cone-beam CT system. A conventional CBCT scan is done first to find any suspicious lesion, followed by a phase-contrast CBCT scan of the ROI for detailed characterization. The phase-contrast in-line holographic images are generated using Fresnel theory through computer simulation, and the projected phase maps, as line integrals of the phase coefficient of the scanned breast, are retrieved using phase-attenuation duality theory. In this way the object's phase coefficient, as the object function, can be reconstructed using these projected phase maps through FDK algorithm. The reconstruction error is calculated to evaluate the accuracy of this approach. The noise property of this approach is investigated as well by adding Poisson noise to the holographic images. The projected phase maps are retrieved and the object function is reconstructed in the presence of noise. The results show that the object's phase coefficient can be reconstructed with very small reconstruction error, and the noise level can be greatly reduced compared to the conventional CBCT system. In conclusion, the phase-contrast CBCT breast imaging approach is very promising to provide better image quality and to lower x-ray dose level for tumor characterization.

With the increasing reliance of doctors on imaging procedures, not only visualization needs to be optimized, but the reconstruction of the volumes from the scanner output is another bottleneck. Accelerating the computationally intensive reconstruction process improves the medical work flow, matches the reconstruction speed to the acquisition speed, and allows fast batch processing and interactive or near-interactive parameter tuning. Recently, much effort has been focused on using the computational power of graphics processing units (GPUs) for general purpose computations. This paper presents a GPU-accelerated implementation of single photon emission computed tomography (SPECT) reconstruction based on an ordered-subset expectation maximization algorithm. The algorithm uses models for the point-spread-function (PSF) to improve spatial resolution in the reconstruction images. Instead of computing the PSF directly, it is modeled as efficient blurring of slabs on the GPU in order to accelerate the process. The algorithm for the calculation of accumulated attenuation factors that allows correcting the generated volume according to the attenuation properties of the volume is optimized for processing on the GPU. Since these factors can be reused between different iterations, a cache is used that is adapted to different sizes of the video memory so that only those factors have to be recomputed that do not fit onto graphics memory. These improvements make the reconstruction of typical SPECT volume near interactive.

Tomographic acquisition uses projection angles evenly distributed around 2π. The Mojette transform and the discrete Finite Radon Transform (FRT) both use discrete geometry to overcome the ill-posedeness of the inverse Radon transform. This paper focuses on the transformation of acquired tomographic projections into suitable discrete projection forms. Discrete Mojette and FRT algorithms can then be used for image reconstruction. The impact of physical acquisition parameters (which produce uncertainties in the detected projection data) is also analysed to determine the possible useful interpolations according to the choice of angle acquisitions and the null space of the transform. The mean square error (MSE) reconstruction results obtained for data from analytical phantoms consistently shows the superiority of these discrete approaches when compared to the classical "continuous space" FBP reconstruction.

Helical cone-beam CT is widely used nowadays because of its rapid scan speed and efficient utilization of x-ray dose. HCT-FDK is an effective reconstruction algorithm on Helical CT. However, like other 3D reconstruction algorithms, HCT-FDK is time consuming because of its large amount of data processing including the convolution and 3D-3D back projection. Recently, GPU is widely used to parallel many reconstruction algorithms. The latest GPU has some nice features, such as large memory, lots of processors, fast 3D texture mapping, and flexible frame buffer object. All these features help reconstruction a lot. In this paper, we present a solution to this problem with GPU. First, we bring a lookup table into HCT-FDK. Then, both convolution and back projection are implemented on GPU. At last, the reconstruction result is directly smoothed and visualized by GPU. Experimental results are given to compare among CPU and two generations of GPU: Geforce 6800GT and Geforce 8800GTX. The comparison was applied both on simulation data and real data. We show that, GPU-accelerated HCT-FDK gets result with similar levels of noise and clarity but gains a speed increase of about 10-100 times faster than using CPU only. With its newer feature, Geforce 8800GTX can get a similar quality like Geforce 6800GT and about 20 times faster.

Modern CT systems have advanced at a dramatic rate. Algebraic iterative reconstruction techniques have shown promising and desirable image characteristics, but are seldom used due to their high computational cost for complete reconstruction of large volumetric datasets. In many cases, however, interest in high resolution reconstructions is restricted to smaller regions of interest within the complete volume. In this paper we present an implementation of a simple and practical method to produce iterative reconstructions of reduced-sized ROI from 3D helical tomographic data. We use the observation that the conventional filtered back-projection reconstruction is generally of high quality throughout the entire volume to predict the contributions to ROI-related projections arising from volumes outside the ROI. These predictions are then used to pre-correct the data to produce a tomographic inversion problem of substantially reduced size and memory demands. Our work expands on those of other researchers who have observed similar potential computational gains by exploiting FBP results. We demonstrate our approach using cardiac CT cone-beam imaging, illustrating our results with both ex vivo and in vivo multi-cycle EKG-gated examples.

Clinical demands of image-guided procedures present technical challenges in X-ray 1K×1K fluoroscopy and cone-beam CT on a mobile C-arm. Performance-per-watt and performance-per-dollar are other major considerations in a search for an optimal computational platform. Real-time constraints of processing high-resolution fluoroscopic images currently necessitate the use of highly specialized proprietary image processing hardware, which cannot be easily repurposed for acceleration of other computing tasks. In our previous studies, we were investigating heterogeneous computing architectures and suitable hardware/software components to assist in time-critical surgical applications. Through those studies, it has been shown that Graphics Processing Units (GPUs) can provide outstanding levels of computational power utilizing the Single Instruction Multiple Data (SIMD) programming model. In the present study, we expand our research in the domain of real-time processing and continue to explore the feasibility of GPU acceleration for both fluoroscopic and tomographic imaging. Current emphasis is being placed on applicability of NVIDIA's novel Tesla computing solutions and Compute Unified Device Architecture (CUDA). The results of this pilot project comprise the Cg/OpenGL and CUDA algorithm implementations, benchmark evaluations, and examples of processing image data acquired with use of anthropomorphic phantoms.

In today's tomographic imaging, there are more incomplete data systems, such as few-view system. The advantage of few-view tomography is less x-ray dose and reduced scanning time. In this work, we study the projection distribution in few-view fan-beam imaging. It is one of the fundamental problems in few-view imaging because of its severe lack of projection data. The aim is to reduce data redundancy and to improve the quality of reconstructed images by research on projection distribution schemes. The reconstruction algorithm for few-view imaging is based on algebraic reconstruction techniques (ART) and total variation (TV) constraint approached by E. Sidky et al in 2006. Study of few-view fan-beam projection distribution is performed mainly through comparison of several distribution types in projection space and reconstructed images. Results show that the distribution called short-scan type obtains the best image in five typical distributions.

The aim of the present study is to investigate a type of Bayesian reconstruction which utilizes partial differential equations (PDE) image models as regularization. PDE image models are widely used in image restoration and segmentation. In a PDE model, the image can be viewed as the solution of an evolutionary differential equation. The variation of the image can be regard as a descent of an energy function, which entitles us to use PDE models in Bayesian reconstruction. In this paper, two PDE models called anisotropic diffusion are studied. Both of them have the characteristics of edge-preserving and denoising like the popular median root prior (MRP). We use PDE regularization with an Ordered Subsets accelerated Bayesian one step late (OSL) reconstruction algorithm for emission tomography. The OS accelerated OSL algorithm is more practical than a non-accelerated one. The proposed algorithm is called OSEM-PDE. We validated the OSEM-PDE using a Zubal phantom in numerical experiments with attenuation correction and quantum noise considered, and the results are compared with OSEM and an OS version of MRP (OSEM-MRP) reconstruction. OSEM-PDE shows better results both in bias and variance. The reconstruction images are smoother and have sharper edges, thus are more applicable for post processing such as segmentation. We validate this using a k-means segmentation algorithm. The classic OSEM is not convergent especially in noisy condition. However, in our experiment, OSEM-PDE can benefit from OS acceleration and keep stable and convergent while OSEM-MRP failed to converge.

Digital Tomosynthesis Mammography (DTM) is an emerging technique that has the potential to improve breast cancer detection. DTM acquires low-dose mammograms at a number of projection angles over a limited angular range and reconstructs the 3D breast volume. Due to the limited number of projections within a limited angular range and the finite size of the detector, DTM reconstruction contains boundary and truncation artifacts that degrade the image quality of the tomosynthesized slices, especially that of the boundary and truncated regions. In this work, we developed artifact reduction methods that make use of both 2D and 3D breast boundary information and local intensity-equalization and tissue-compensation techniques. A breast phantom containing test objects and a selected DTM patient case were used to evaluate the effects of artifact reduction. The contrast-to-noise ratio (CNR), the normalized profiles of test objects, and a non-uniformity error index were used as performance measures. A GE prototype DTM system was used to acquire 21 PVs in 3° increments over a ±30° angular range. The Simultaneous Algebraic Reconstruction Technique (SART) was used for DTM reconstruction. Our results demonstrated that the proposed methods can improve the image quality both qualitatively and quantitatively, resulting in increased CNR value, background uniformity and an overall reconstruction quality comparable to that without truncation. For the selected DTM patient case, the obscured breast structural information near the truncated regions was essentially recovered. In addition, restricting SART reconstruction to be performed within the estimated 3D breast volume increased the computation efficiency.

The inverse-geometry CT(IGCT) system uses a large-area scanned source and a 2D detector array with a smaller extent in the transverse direction. The acquired IGCT data can be significantly oversampled and the samples are not equally spaced in radial distance or angle. A previously proposed reconstruction algorithm used a modified gridding method to rebin the normalized and logged projection data into parallel projections. This approach can be suboptimal if the measured rays contributing to an output sample do not have the same signal-to-noise ratio (SNR) due to each ray having a different detected number of photons (due to different incident intensities). Reconstructed images may have better SNR if we consider the SNR of each ray in rebinning step. In this paper, we propose a new method to improve the SNR in the reconstructed image. In this method, input rays with different SNR were combined in the rebinning step by using weighted-least square fitting to produce SNR efficient output projection data. We simulated two cases : uniform, and triangular profiles of the detected number of photons across the detector array. For single slice 2D imaging, SNR improvements of 1% (uniform) and 21%(triangular) were observed. Experiments were also performed with air scan data acquired from a scanned source C-arm system (NovaRay, Inc., Palo Alto, CA). In this case, we observed SNR improvement as high as 20 %, depending on the intensity non-uniformity across the detector.

In this work, we investigate the parallel-beam projection and reconstruction of band-limited Hermite expansions. Using a recently developed coordinate conversion technique, we show how the Fourier slice theorem can be directly applied. In our new approach, we do not introduce a non-integrable filter that appears in the filtered back projection method. Since a projection of a 2D band-limited Hermite expansion is a 1D band-limited Hermite expansion and the coordinate conversion technique is lossless with this special expansion, we can avoid a series of approximations that the classical tomography techniques make.

Cosmic ray muon radiography which has a good penetrability and sensitivity to high-Z materials is an effective way for detecting shielded nuclear materials. Reconstruction algorithm is the key point of this technique. Currently, there are two main algorithms about this technique. One is the Point of Closest Approach (POCA) reconstruction algorithm which uses the track information to reconstruct; the other is the Maximum Likelihood estimation, such as the Maximum Likelihood Scattering (MLS) and the Maximum Likelihood Scattering and Displacement (MLSD) reconstruction algorithms which are proposed by the Los Alamos National Laboratory (LANL). The performance of MLSD is better than MLS. Since MLSD reconstruction algorithm includes scattering and displacement information while MLS reconstruction algorithm only includes scattering information. In order to get this Maximum Likelihood estimation, in this paper, we propose to use EM method to get the estimation (MLS-EM and MLSD-EM). Then, in order to saving reconstruction time we use the OS technique to accelerate MLS and MLSD reconstruction algorithm with the initial value set to be the result of the POCA reconstruction algorithm. That is, the Maximum Likelihood Scattering-OSEM (MLS-OSEM) and the Maximum Likelihood Scattering and Displacement-OSEM (MLSD-OSEM). Numerical simulations show that the MLSD-OSEM is an effective algorithm and the performance of MLSD-OSEM is better than MLS-OSEM.

The data rate requirements for raw data transmission through the slip ring of a CT scanner can be quite challenging. While lossy compression can offer a significant reduction in data set size, it introduces errors to the measurements. The design and evaluation of compression methods need to take into account the nature of the sinogram. In this paper, we present two compression noise shaping methods for fixed rate lossy compression of CT raw data that achieve a lower error level in the center region of the reconstructed image: error feedback filters and sub-band coding with bit allocation.

In this paper, we propose a novel method for beam hardening correction in polychromatic transmission tomography. A family of polynomials is firstly determined in a training phase, which forms a complete set in the sense of X-ray physics of medical diagnostic imaging. In particular, every polynomial in the set is indexed by a beam hardening factor, i.e. effective atomic number, which is further assigned to specific X-ray penetrating path. In order to successfully accomplish the assignation in an imaging phase, another polynomial is adopted to formulize the mapping relationship between the index of polynomial family and the area density ratio of bone tissue. Here, the area density ratio of bone tissue is calculated after the pre reconstructed image being segmented into soft tissue and bone regions. The mapping polynomial is iteratively approximated by a dedicated HL Consistency (HLC) based nonlinear algorithm. The characteristics of this method include that the polynomial family can cover the variations of both high potential and effective filter of X-ray tube, the beam hardening correction in the imaging phase can adapt the content variations of objects being imaged, and the correction effect is also sophisticated even bowtie filter exists. Performance analysis and related computer simulation show that our HLC based correction is much robust than traditional bone correction to the variants of scale factor lambda₀.

As CT scanners continue to increase in speed and to collect more data per rotation, transmitting raw data across the slip ring and storing raw data can be very challenging. While lossy compression can offer a significant reduction in data set size, it introduces errors to the measurements. We examined the effect of noise of different frequencies in the view (temporal) direction as well as at different locations in the detector arc. Our results showed that only low temporal frequency errors in the center detectors can contribute to errors in the center of the reconstructed field-of-view (FOV). On the other hand, high temporal frequency errors only contribute to errors in the periphery of the reconstructed FOV. Whether image errors arise from compression or electronic noise, their relative sensitivity to different frequencies and detectors is an important consideration for applications such as cardiac CT, where the center of the FOV may be considered the most critical region. Therefore, when limiting data rate is essential, detectors could be allocated different bit-rates for compression based on the tolerable frequency content of their errors and their spatial location.

Cone-beam CT (CBCT) technique has been used by orthopedists to monitor bone graft growth after orthopedic surgery. In order to correct severe metal artifacts in reconstructed images caused by metal implants used in bone grafting, a three-dimensional metal artifact correction method has been previously proposed. The implants' mathematic boundaries were generated to help to segment metal from reconstructed images. The segmented metal implants were forward-projected onto the detector to create metal-only projections to compensate for beam-hardening effect. This method was proved effective with the metal implants of regular shape which can be simulated by simple 3D primitives, such as cuboid, cylinder and cone. But for metal implants of arbitrary shape, their boundaries are difficult to define mathematically. To solve this problem, this paper proposed a method by setting up an implant image library and using the implants' a priori shape information from the library during the artifact correction. The implants were acquired and scanned before the surgery and their a priori information were stored in a library. During the artifact correction, the library was called to provide the shape information of the implants to help to do the implant segmentation. The segmented implants were forward-projected onto the detector to generate implant-only projections by a cone-beam forward-projection technique. Beam-hardening effect in the original projections was then compensated by high polynomial orders of implant projections. Finally, the corrected projections were back-projected to produce artifacts-reduced images. Both phantom studies and patient studies were conducted to test this correction method. Results from both studies show the artifacts have been greatly reduced and the accuracy of bone volume measurement has been increased.

Airway wall thickness (AWT) is an important bio-marker for evaluation of pulmonary diseases such as chronic bronchitis, bronchiectasis. While an image-based analysis of the airway tree can provide precise and valuable airway size information, quantitative measurement of AWT in Multidetector-Row Computed Tomography (MDCT) images involves various sources of error and uncertainty. So we have developed an accurate AWT measurement technique for small airways with three-dimensional (3-D) approach. To evaluate performance of these techniques, we used a set of acryl tube phantom was made to mimic small airways to have three different sizes of wall diameter (4.20, 1.79, 1.24 mm) and wall thickness (1.84, 1.22, 0.67 mm). The phantom was imaged with MDCT using standard reconstruction kernel (Sensation 16, Siemens, Erlangen). The pixel size was 0.488 mm × 0.488 mm × 0.75 mm in x, y, and z direction respectively. The images were magnified in 5 times using cubic B-spline interpolation, and line profiles were obtained for each tube. To recover faithful line profile from the blurred images, the line profiles were deconvolved with a point spread kernel of the MDCT which was estimated using the ideal tube profile and image line profile. The inner diameter, outer diameter, and wall thickness of each tube were obtained with full-width-half-maximum (FWHM) method for the line profiles before and after deconvolution processing. Results show that significant improvement was achieved over the conventional FWHM method in the measurement of AWT.

Variable Resolution X-ray (VRX) CT scanners allow imaging of different sized anatomy at the same level of detail using the same device. This is achieved by tilting the x-ray detectors so that the projected size of the detecting elements is varied to produce reconstructions of smaller fields of view with higher spatial resolution. As with regular CT scanners, the images obtained with VRX scanners are affected by different kinds of artifacts of various origins. This work studies some of these artifacts and the impact that the VRX effect has on them. For this, computational models of single-arm single-slice VRX scanners are used to produce images with artifacts commonly found in routine use. These images and artifacts are produced using our VRX CT scanner simulator, which allows us to isolate the system parameters that have a greater effect on the artifacts. A study of the behavior of the artifacts at varying VRX opening angles is presented for scanners implemented using two different detectors. The results show that, although varying the VRX angle will have an effect on the severity of each of the artifacts studied, for some of these artifacts the effect of other factors (such as the distribution of the detector cells and the position of the phantom in the reconstruction grid) is overwhelmingly more significant. This is shown to be the case for streak artifacts produced by thin metallic objects. For some artifacts related to beam hardening, their severity was found to decrease along with the VRX angle. These observations allow us to infer that in regular use the effect of the VRX angle artifacts similar to the ones studied here will not be noticeable as it will be overshadowed by parameters that cannot be easily controlled outside of a computational model.

In multi-slice cone beam CT imaging, there are artifacts known as windmill artifacts. These artifacts are due to not satisfying the Nyquist criteria in the patient longitudinal direction. This paper quantifies and compares these artifacts as a function of the number of rows, pitch, collimation, and image thickness of the CT scanner. Scanners with rows of 16, 64 and 128 are measured and compared with simulated data, using both Helical and Axial scanning modes. In addition three focal spot switching modes are compared: the traditional within image plane mode; diagonal mode; and quad mode. All images are compared via four criteria: artifacts, MTF, SSP and noise. Results show that the frequency of the artifact, or number of blades on the windmill and magnitude of each blade, is dependent on the rate at which the rows are crossed for an image. For example, for a given pitch, doubling the rows doubles the frequency of the artifact, with each artifact approximately the magnitude. A similar result can be obtained by keeping the number of rows constant and varying the pitch. The artifact disappears as the Nyquist criteria is satisfied by either increasing the slice thickness or incorporating one of the focal spot switching modes that switch in the patient longitudinal direction. For a given MTF and SSP, the diagonal focal spot switching mode has slightly more noise while the other two are approximately equal. The artifact varies with the quad mode being the best and traditional mode being the worse.

Cone-beam computed tomography (CBCT) provides a real volume imaging modality, which can reconstruct a digital volume with an isotropic grid resolution. However, in practical CT applications, because of the errors associated with machine manufacturing and system integration, the circular scanning orbit is not a perfect circle, and the detector plane may not be perpendicular to the isoray of the cone-beam projection. As a result, the cone-beam projection images acquired under the wobbled scanning orbit and tilted detector will deteriorate the cone-beam volume reconstruction. Specifically, the non-circular scanning orbit will cause severe spatial blurring and geometrical distortion, especially for the regions away from the midplane; the detector tilting will cause the asymmetrical distortion and spatial blurring. These spatial blurring and shape distortion effects can be characterized through the use of three-dimensional (3D) point spread function (PSF) measurement. In general, the orbit wobbling boosts the spatial variance of the cone-beam volume imaging in terms of blurring. The detector tilting causes asymmetry of 3D PSF because of the fact that: one detector side locates away from the source further than the other side due to the obliqueness. By computer simulations with bead phantom scanning and reconstruction, we demonstrate the effects of scanning-orbit wobbling and detector tilting on cone-beam computed tomography, under a variety of parameter settings. The findings in the simulation study are useful for constructing a cone-beam tomographic system.

Dedicated CT breast imaging using a flat-panel detector system holds great promise for improving the detection and diagnosis of early stage breast cancer. It is currently unclear whether dedicated CTBI systems will be useful for screening of the general population. Possibly a more realistic goal will be contrast-enhanced, flat-panel CTBI to assist in the diagnostic workup of suspected breast cancer patients. It has been suggested that the specificity of CE-CTBI can be improved by acquiring a dynamic sequence of CT images, characterizing the lesion enhancement pattern. To make dynamic CE-CTBI feasible, it is important to perform very fast CT acquisitions, with minimal radiation dose. One technique for reducing the time required for CT acquisitions, is to use a half-scan cone-beam acquisition, requiring a scan of 180° plus the detector width. In addition to achieving a shorter CT scan, half-scan acquisition can provide a number of benefits in CTBI system design. This study compares different half-scan reconstruction methods with a focus on evaluating the quantitative performance in estimating the CT number of iodinated contrast enhanced lesions. Results indicate that half-scan cone-beam acquisition can be used with little loss in quantitative accuracy.

We propose a new method for compensating patient motion in computed tomography. The method consists of analyzing the frequency spectrum of tracked landmark points in the acquired sinogram. These landmarks are either physically attached to the patient prior scanning, or coincide with anatomical points traceable in the sinogram. Without motion present, these extracted landmark curves represent one period of a sine wave in full-rotation tomography. Motion compensation is achieved by calculating the fundamental frequency component of the extracted landmark curves in order to obtain the motion compensated landmark curve. The extracted and compensated landmark curve pairs serve for constructing a motion map, and these are utilized to re-sort the acquired sinogram to obtain a motion compensated sinogram. Reconstructing the motion compensated sinogram using a standard reconstruction algorithm yields the motion compensated image. The proposed method is compared to a previously published motion artifact reduction method. The results show equal or improved motion compensation capabilities of the proposed method for three different types of computer simulated in-plane motion in synthetic 2D parallel-beam sinograms. The influence of inaccurate curve extraction has been simulated and results are included. Experiments aimed at demonstrating the compensation of motion artifacts in micro-computed tomography patient images are currently under way. In addition to compensation, future work will explore detection and quantification of patient motion based on frequency spectrum analysis of landmark curves, potentially providing a comprehensive tool for identifying, quantifying and correcting motion artifacts.

We have used a CMOS camera as multi-purpose instrument for measuring image quality of high-resolution LCD displays for use in medical diagnosis. The camera uses a Foveon sensor so that the RGB pixels are co-located making for simplified spatial sampling. In this presentation we examine the temporal-noise characteristics of the CMOS camera. We are using one of several LED's in an integrating sphere as the light source to provide the nominally uniform illumination for each experiment. In first experiments we place the camera sensor near to the opening of the integrating sphere without introducing an imaging lens in order to initially eliminate the characteristics of the lens. The capture parameters for the camera control software are set to give us as nearly raw data as possible. We suppress the native processing because our application is much different than the usual image-reproduction function. We capture sequences of images with the same illumination and same capture parameters and write them to TIFF files. The Matlab-based software we've written first pre-processes the images then calculates the pixel-wise mean and standard deviation over the sequence of frames. The calculation is repeated for each of the three (RGB) channels of the CMOS camera. These statistics are displayed as graytone images in three windows with the maxima and minima displayed in the title-bars. The short-term goal of the experiments is to prepare the way to compare the performance of cooled and un-cooled versions of the camera.

Quality control of external conformal radiotherapy treatment planning systems softwares is a crucial issue. The treatment quality depends directly on the quality of treatment planning systems (TPS). Radiotherapists need to be sure that softwares compute accurately each parameter of the treatment. This paper focuses on the quality control of geometrical tools of the treatment planning systems, i.e. the virtual simulation software. These TPS compute the geometrical part of the treatment. They define the targets and shapes of the irradiation beams. Four operations done by these TPS are examined in this work. The quality control of the auto-contouring, auto-margin, isocenter computation and collimator conformation tools is treated with a new method based on Digital Test Objects (DTO). Standard methods for this quality control have been set up from the development of some Physical Test Objects (PTO). These methods are time-consuming, incomplete and inaccurate. Results are biased by the CT-scanner acquisition of PTOs and error evaluation is done with the graphic tools of the TPS. Our method uses DTOs and allows for an automated qualitative error evaluation. DTOs present many advantages for TPS quality control. They lead to a fast, accurate, complete and automatic quality assessment. Special DTOs have been developed to control the TPS tools mentioned previously as well as their automatic result analysis methods. A TPS has been controlled with these test objects. The quality assessment shows some errors and highlights some particularities in the TPS tools functioning. This quality control was then compared with the standard quality control.

Accurate isocentre positioning of the treatment machine is essential for the radiation therapy process, especially in stereotactic radio surgery and in image guided radiation therapy. We present in this paper a new method to evaluate a software which is used to perform an automatic analysis of the Winston-Lutz test used in order to determine position and size of the isocentre. The method consists of developing digital phantoms that simulate mechanical distortions of the treatment machine as well as misalignments of the positioning laser targeting the isocentre. These Digital Test Objects (DTOs) offer a detailed and profound evaluation of the software and allow determining necessary adjustments which lead to high precision and therefore contributes to a better treatment targeting.

Purpose: The purposes of the study were to set-up and validate a simulation framework for dose and image quality optimization studies. In a first phase we have evaluated whether CDRAD images as obtained with computed radiography plates could be simulated. Material and Methods: The Monte Carlo method is a numerical method that can be used to simulate radiation transport. It is in diagnostic radiology often used in dosimetry, but in present study it is used to simulate X-ray images. With the Monte Carlo software, MCNPX, the successive steps in the imaging chain were simulated: the X-ray beam, the attenuation and scatter process in a test object and image generation by an ideal detector. Those simulated images were further modified for specific properties of CR imaging systems. The signal-transfer-properties were used to convert the simulated images into the proper grey scale. To account for resolution properties the simulated images were convolved with the point spread function of the CR systems. In a last phase, noise, based on noise power spectrum (NPS) measurements, was added to the image. In this study, we simulated X-ray images of the CDRAD contrast-detail phantom. Those simulated images, modified for the CR-system, were compared with real X-ray images of the CDRAD phantom. All images were scored by computer readings. Results: First results confirm that realistic CDRAD images can be simulated and that reading results of series of simulated and real images have the same tendency. The simulations also show that white noise has a large influence on image quality and CDRAD analyses.

Neutron stimulated emission computed tomography (NSECT) is being proposed as a non-invasive technique to detect concentrations of elements in the body for diagnosis of liver iron overload. Several experiments have been conducted to investigate NSECT's ability to determine iron concentration in liver tissue and evaluate the accuracy and sensitivity of the system. While these experiments have been successful in demonstrating NSECT's capability of quantifying iron and other tissue elements in-vivo, they have been prohibitively time consuming, often requiring as much as 24 hour acquisitions for accurate quantification. Such extensive scan times limit the use of the experimental system for initial feasibility testing and optimization. As a practical alternative, GEANT4 simulations are being developed to investigate system optimization and aid further progress of the experimental technique. This work presents results of a validation study comparing the results of a GEANT4 simulation with experimental data obtained from a sample of iron. A simulation of the NSECT system is implemented in GEANT4 and used to acquire a spectrum from a simulated iron sample. Scanning is performed with a 7.5 MeV neutron beam to stimulate gamma emission from iron nuclei. The resulting gamma spectrum is acquired and reconstructed using high-purity germanium (HPGe) detectors and analyzed for energy peaks corresponding to iron. The simulated spectrum is compared with a corresponding experimental spectrum acquired with an identical source-detector-sample configuration. Five peaks are detected corresponding to gamma transitions from iron in both spectra with relative errors ranging from 4.5% to 17% for different peaks. The result validates the GEANT4 simulation as a feasible alternative to perform simulated NSECT experiments using only computational resources.

Radiology quality assurance programs are designed to ensure certain levels of image quality are maintained with imaging equipment. The detective quantum efficiency (DQE), expressed as a function of spatial frequency, is a direct measure of system performance and "dose efficiency" that is objective, quantitative and widely accepted by the scientific community. We have implemented a QA program in a tertiary care hospital in which both the DQE and modulation transfer function (MTF) are measured as part of a routine QA program. The DQE, MTF and system gain were measured bi-monthly over a 12-month evaluation period. Measurements of DQE were compliant with IEC62220-1 recommendations. In the past year, no significant deterioration in DQE or MTF of any system was observed. However, large differences in DQE and MTF were observed between different detector technologies. It is anticipated that routine monitoring of DQE could provide early warning of system failures or problems requiring service intervention, but no problems were experienced during the evaluation period.

The MDCT parameters affecting radiation dose include tube voltage, tube current, change of beam collimation, and size of the human body. The purpose of this study was to measure and evaluate radiation dose for MDCT parameters. A comparative analysis of the radiation dose according to before and after the calibration of the ionization chamber was performed. The ionization chamber was used for measuring radiation dose in the MDCT, as well as of CTDI_W according to temperature and pressure correction factors in the CT room. As a result, the patient dose of CTDI_W values linearly increased as tube voltage and current were increased, and nonlinearly decreased as beam collimation was increased. And the CTDI_W value which was reflected calibration factors, as well as correction factors of temperature and pressure, was found to be greater by the range of 0.479 ~ 3.162 mGy in effective radiation dose than the uncorrected value. Also, Under the abdomen routine CT conditions used in hospitals, patient exposure dose showed a difference of a maximum of 0.7 mSv between before and after the application of such factors. These results imply that the calibration of the ion chamber, and the application of temperature and pressure of the CT room are crucial in measuring and calculating patient exposure dose.

Number of radiologic exams using digital imaging systems has rapidly increased with advanced imaging technologies. However, it has not been paid attention to the radiation dose in clinical situations. It was the motivation to study radiation dosimetry in the DR system. The objective of this study was to measure beam quality and patient's dose using DR system and to compare them to both IEC standard and IAEA guidelines. The measured average dose for chest and abdomen was 1.376 mGy and 9.501 mGy, respectively, compared to 0.4 mGy and 10.0 mGy in IAEA guidelines. The results also indicated that the DR system has a lower radiation beam quality than that of the IEC standard. The results showed that the patients may be exposed higher radiation for chest exams and lower radiation for abdomen exams using DR system. IAEA Guidelines were prepared based on western people which may be different weight and height for patients compared them to Korean. In conclusion, a new guideline for acceptable DR dosimetry for Korean patients may need to be developed with further studies for large populations. We believe that this research greatly help to introduce the importance of the dosimetry in diagnostic radiology in Korea. And, a development of database for dosimetry in diagnostic radiology will become an opportunity of making aware of radiation safety of medical examination to patient.

Coronary angiography is the primary technique for diagnosing coronary abnormalities as it is able to locate precisely the coronary artery lesions. However, the clinical relevance of an appearing stenosis is not that easy to assess. In previous work we have analyzed the myocardial perfusion by comparing basal and hyperemic coronary flow. This comparison is the basis of a Relative Coronary Flow Reserve (RCFR) measure. In a Region-of-Interest (ROI) on the angiogram the contrast is measured as a function of time (the so-called time-density curve). The required hyperemic state of exercise is induced artificially by the injection of a vasodilator drug e.g. papaverine. In previous work we have presented the results of a small study of 20 patients. In this paper we present an analysis of the sensitivity of the method for variations in X-ray exposure between the two runs due to the Automatic Exposure Control (AEC) unit. The AEC is a system unit with the task to ensure a constant dose rate at the entrance of the detector by making the appropriate adaptations in X-ray factor settings for patients which range from slim to more obese. We have setup a phantom study to reveal the expected exposure variations. We present several of the developed phantoms together with a compensation strategy.

This work presents a computational method for automatic determination of the anode angle of any radiographic equipment using a non-invasive method. The anode angle is a significant parameter of radiographic equipment, as it determines the focal spot size and the magnitude of the heel effect, which is directly related to the image quality. Even though it is an important parameter, it is not always provided by the x-ray equipment manufacturer. Besides, it is very difficult to be measured in practical quality assurance procedures. First, a pinhole matrix made of lead was built, which contains 33 radial 50μm diameter holes. This pinhole matrix is used to obtain a radiographic image of focal spot projections along the radiation field. The computer algorithm calculates the point spread function of each focal spot projection as well as the distance between them. Thus, the anode angle can be determined automatically by using the field characteristic, as the geometric unsharpness at any arbitrary field position can be derived from those at the central beam position. Results showed good accuracy compared to nominal values, and also a methodology was developed to validate the computational algorithm. Determination of anode angle of any radiographic equipment (including mammographic ones) with great precision can easily be done by using the method proposed in this work.

An x-ray anti-scatter grid alignment aid has been developed and its performance evaluated for portable chest radiographic exams. The grid alignment aid consists of one line laser and one cross-hair laser mounted on a supporting frame. During an x-ray exam, the two lasers generate a predetermined light pattern on the x-ray collimator to indicate the optimal x-ray source position in the three-dimensional (3D) space. Two grids commonly used in portable chest exams (40 lp/cm resolution, 6:1 and 8:1 ratio, and 130 cm focal distance) were characterized in terms of the signal-to-noise ratio improvement factor (SIF), as a function of the source image distance (SID), and x-ray tube lateral displacement from which the optimal grid operation range was determined. Data indicated the grid alignment aid was able to provide sufficient accuracy for positioning the x-ray tube to achieve optimal grid usage.

Generally, the modulation transfer function (MTF) of a computed tomography (CT) scanner is calculated based on the CT value. However, it is impossible to measure the MTF directly because the CT value is defined as a nonlinear function of the X-ray intensity. Due to this characteristic, the MTF varies with the subject's contrast. Therefore, we measured the MTFs of a CT scanner using high- and low-contrast wire phantoms. We selected thick copper wire in water as the high-contrast subject and thin copper wire in water as the low-contrast subject. The MTF measured with the low-contrast subject was decreased relative to that measured with the high-contrast subject because the CT value was nonlinear. Thus, to evaluate the spatial resolution in a low-contrast subject such as the human body, we should measure the MTF with a low-contrast wire phantom. In addition, by using low-contrast subjects, we can approximately determine the CT value using a linear function.

Factors like, (i) noise and (ii) artifacts, that occur depending on acoustical properties of tissues, (iii) wrong selection of system variables, like (a) wrong operation frequency, (b) poor calibration, and (c) improper location of focal points, may cause high amount of image degradation during ultrasound imaging. This, in return, may lead to misdiagnoses, making correct diagnosis of uncommon cases impossible. These misdiagnoses may be avoided by enhanced training of physicians. Commercially available phantoms are limited in content and relatively expensive, which makes the simulation of ultrasound imaging a mandatory component in diagnostic ultrasound training. The aim of this study was to investigate the feasibility of the simulation of ultrasound imaging. Under the scope of this work, ultrasound imaging was simulated by using FIELD II program set developed by J.A. Jensen by for various settings. In order to compare the results a selected cyst phantom was used and the effects of simulation frequency and sampling frequency on visibility and simulation times were observed. The quality of generated images was evaluated by measuring the visibility of the cyst phantom. Identification of cysts was accomplished by detection of the cysts with an algorithm to perform a series of image processing operations. Located objects were classified manually and errors (with respect to size and position of cysts) were calculated. Our observations indicated that to obtain a good image quality, interdependent simulation and sampling frequencies should be selected carefully, which in return requires longer simulation times at higher frequencies.

Fast and accurate modeling of cone-beam CT (CBCT) x-ray projection data can improve CBCT image quality either by linearizing projection data for each patient prior to image reconstruction (thereby mitigating detector blur/lag, spectral hardening, and scatter artifacts) or indirectly by supporting rigorous comparative simulation studies of competing image reconstruction and processing algorithms. In this study, we compare Monte Carlo-computed x-ray projections with projections experimentally acquired from our Varian Trilogy CBCT imaging system for phantoms of known design. Our recently developed Monte Carlo photon-transport code, PTRAN, was used to compute primary and scatter projections for cylindrical phantom of known diameter (NA model 76-410) with and without bow-tie filter and antiscatter grid for both full- and half-fan geometries. These simulations were based upon measured 120 kVp spectra, beam profiles, and flat-panel detector (4030CB) point-spread function. Compound Poisson- process noise was simulated based upon measured beam output. Computed projections were compared to flat- and dark-field corrected 4030CB images where scatter profiles were estimated by subtracting narrow axial-from full axial width 4030CB profiles. In agreement with the literature, the difference between simulated and measured projection data is of the order of 6-8%. The measurement of the scatter profiles is affected by the long tails of the detector PSF. Higher accuracy can be achieved mainly by improving the beam modeling and correcting the non linearities induced by the detector PSF.

Amplified Pixel Sensor (APS) architectures using two transistors per pixel are introduced in this research for digital mammography tomosynthesis that requires high resolution and low noise imaging capability. The fewer number of on-pixel elements and reduced pixel complexity result in a smaller pixel pitch and higher gain, which makes the two-transistor (2T) APS architectures promising for high resolution, low noise and high speed digital imaging including medical imaging modalities such as tomosynthesis and cone beam computed tomography. Measured results from in-house fabricated test arrays using amorphous silicon (a-Si) thin film transistor (TFTs) are presented as well as driving schemes for minimizing the threshold voltage metastability problem and increasing frame rate. The results indicate that a pixel input referred noise value of down to 220 electrons is achievable with a 50μm pixel pitch a-Si 2T APS.

Lag and sensitivity modulation are well known temporal artifacts of a-Si photodiode based flat panel detectors. Both effects are caused by charge carriers being trapped in the semiconductor. Trapping and releasing of these carriers is a statistical process with time constants much longer than the frame time of flat panel detectors. One way to reduce these temporal artifacts is to keep the traps filled by applying a pulse of light over the entire detector area every frame before the x-ray exposure. This paper describes an alternative method, forward biasing the a-Si photodiodes and supplying free carriers to fill the traps. The array photodiodes are forward biased and then reversed biased again every frame between the panel readout and x-ray exposure. The method requires no change to the mechanical construction of the detector, only minor modifications of the detector electronics and no image post processing. An existing flat panel detector was modified and evaluated for lag and sensitivity modulation. The required changes of the panel configuration, readout scheme and readout timing are presented in this paper. The results of applying the new technique are presented and compared to the standard mode of operation. The improvements are better than an order of magnitude for both sensitivity modulation and lag; lowering their values to levels comparable to the scintillator afterglow. To differentiate the contribution of the a-Si array, from that of the scintillator, a large area light source was used. Possible implementations and applications of the method are discussed.

We report on the technology of imaging corrections for a new solid state x-ray image intensifier (SSXII) with enhanced resolution and fluoroscopic imaging capabilities, made of a mosaic of modules (tiled-array) each consisting of CsI(Tl) phosphor coupled using a fiber-optic taper or minifier to an electron multiplier charge coupled device (EMCCD). Generating high quality images using this EMCCD tiled-array system requires the determination and correction of the individual EMCCD sub-images with respect to relative rotations and translations as well as optical distortions due to the fiber optic tapers. The image corrections procedure is based on comparison of resulting (distorted) images with the known square pattern of a wire mesh phantom. The mesh crossing point positions in each sub-image are automatically identified. With the crossing points identified, the mapping between distorted and an undistorted array is determined. For each pixel in a distorted sub-image, the corresponding location in the corrected sub-image is calculated using bilinear interpolation. For the rotation corrections between sub-images, the orientation of the vectors between respective mesh crossing points in the various sub-images are determined and each sub-image is appropriately rotated with the pixel values again determined using bilinear interpolation. Image translation corrections are performed using reference structures at known locations. According to our estimations, the distortion corrections are accurate to within 1%; the rotations are determined to within 0.1 degree, and translation corrections are accurate to well within 1 pixel. This technology will provide the basis for generating single composite images from tiled-image configurations of the SSXII regardless of how many modules are used to form the images.

A theoretical model for describing the bias-dependent transient behavior of dark current in multilayer amorphous selenium (a-Se) detectors has been developed by solving the trapping rate equations and Poisson's equation in the a-Se layer. The transient dark currents in these detectors are measured and the proposed dark model is compared with the measured data. The model shows a very good agreement with the experimental results. It has been found that the dark current is mainly controlled by the Schottky emission of holes from the metal/a-Se contact. The space charge build-up due to the hole injection and trapping in the blocking layer reduces the internal field at the metal/a-Se interface of positive side and thus the dark current eventually is limited by the space charge. It has been found that the electric fields at the metal contacts reduce to 20-30% of the applied field (applied voltage/thickness). The comparison of the model with the experimental data estimates some important properties (e.g., trap center concentrations, space charges, and effective barrier heights) of the blocking layers of the multilayer detectors. The dependence of the X-ray sensitivity of multilayer a-Se X-ray imaging detectors on repeated X-ray exposures is studied by considering accumulated trapped charges and their effects (trap filling, recombination, electric field profile, electric field dependent electron-hole pair creation), the carrier transport in the blocking layers, X-ray induced metastable deep trap center generations, and the effects of dark current. We simultaneously solve the continuity equations for both holes and electrons, trapping rate equations, and the Poisson's equation across the photoconductor for a step X-ray exposure by the Backward Euler finite difference method. The theoretical model shows a very good agreement with the experimental relative sensitivity versus cumulative X-ray exposure characteristics. The electric field distribution across the multilayer detector and the dark current density under repeated exposures are also estimated.

To assess the feasibility of small soft tissue avascular tumor micro-CT imaging with x-ray phase-contrast in-line holography, we have studied micro-CT imaging with in-line geometry of small spheroidal avascular tumor models with quiescent cell core (< 250 μm) and various distributions of the proliferating cell density (PCD) forming the outer shell. We have simulated imaging with an ultrafast laser-based x-ray source with a Mo target. We observe phase-contrast enhancement of the tumor boundaries in the reconstructed transaxial images, resulting in improved detection of small soft tissue tumors, providing that the PCD density gradient is sufficiently large.

A clear understanding of the first pass dynamics of contrast agents in the vascular system is crucial in synchronizing data acquisition of 3D MR angiography (MRA) with arrival of the contrast bolus in the vessels of interest. We implemented a computational model to simulate contrast dynamics in the vessels using the theory of linear time-invariant systems. The algorithm calculates a patient-specific impulse response for the contrast concentration from time-resolved images following a small test bolus injection. This is performed for a specific region of interest and through deconvolution of the intensity curve using the long division method. Since high spatial resolution 3D MRA is not time-resolved, the method was validated on time-resolved arterial contrast enhancement in Multi Slice CT angiography. For 20 patients, the timing of the contrast enhancement of the main bolus was predicted by our algorithm from the response to the test bolus, and then for each case the predicted time of maximum intensity was compared to the corresponding time in the actual scan which resulted in an acceptable agreement. Furthermore, as a qualitative validation, the algorithm's predictions of the timing of the carotid MRA in 20 patients with high quality MRA were correlated with the actual timing of those studies. We conclude that the above algorithm can be used as a practical clinical tool to eliminate guesswork and to replace empiric formulae by a priori computation of patient-specific timing of data acquisition for MR angiography.

The simultaneous imaging system of PET and Fluorescent CT is being developed in the National Institute of Radiological Science in Japan. For the simultaneous system, we are considering using the DOI-PET detectors as simultaneous detectors of the gamma ray for PET and NIR light for fluorescence by changing the upper reflectors to the dichroic mirrors. Here, DOI-PET detector has very low spatial resolution to the NIR light compared to basically used CCD cameras. However, since the NIR light is scattered by biological tissues, it can be possible to reconstruct valuable image from the data which acquired from low resolution devices. In this study, we show feasibility for the Fluorescent CT imaging using the DOI-PET detector by computer simulations. In the simulations, we used a cubic phantom and square shaped detector geometry and a diffusion equation to approximate the light propagation. The system matrices of the Fluorescent CT geometries having different detector resolutions are calculated and we evaluated the singular values of the matrices. Using the system matrices, we simulated the image reconstruction from observed data which is generated by simulation and noise added. As a result, it is confirmed the reconstructed image from low resolution detectors is as same level as one from higher resolution detectors.

Micro-CT is a non-invasive imaging modality usually used to assess morphology in small animals. In our previous work, we have demonstrated that functional micro-CT imaging is also possible. This paper describes a dual micro-CT system with two fixed x-ray/detectors developed to address such challenging tasks as cardiac or perfusion studies in small animals. A two-tube/detector system ensures simultaneous acquisition of two projections, thus reducing scanning time and the number of contrast injections in perfusion studies by a factor of two. The system is integrated with software developed in-house for cardio-respiratory monitoring and gating. The sampling geometry was optimized for 88 microns in such a way that the geometric blur of the focal spot matches the Nyquist sample at the detector. A geometric calibration procedure allows one to combine projection data from the two chains into a single reconstructed volume. Image quality was measured in terms of spatial resolution, uniformity, noise, and linearity. The modulation transfer function (MTF) at 10% is 3.4 lp/mm for single detector reconstructions and 2.3 lp/mm for dual tube/detector reconstructions. We attribute this loss in spatial resolution to the compounding of slight errors in the separate single chain calibrations. The dual micro-CT system is currently used in studies for morphological and functional imaging of both rats and mice.

CT numbers can be directly computed from the linear attenuation coefficients in the reconstructed CT images and are correlated to the electron densities of the chemical elements with specific atomic numbers. However, the computed CT numbers can be varied when different imaging parameters are used. Phantoms composed of clinically relevant and tissue-equivalent materials (lung, bone, muscle, and adipose) were scanned with a commercial circular-scanning micro CT imager. This imaging system is composed with a micro-focused x-ray tube and charged-coupled device (CCD) camera as the detector. The mean CT numbers and the corresponding standard deviations in terms of Hounsfield units were then computed from a pre-defined region of interest located within the reconstructed volumetric images. The variations of CT number were then identified from a series of imaging parameters. Those parameters include imaging acquisition modes (e.g., the metal filter used in the x-ray tube), reconstruction methods (e.g., Feldkamp and iterative algorithm), and post-image processing techniques (e.g., ring artifact, beam-hardening artifact, and smoothing processing). These variations of CT numbers are useful and important in tissue characterization, quantitative bone structure analysis, bone marrow density evaluation, and Monte Carlo dose calculations for the pilot small animal study when micro CT imaging systems are employed. Also these variations can be used as the quantification for the performance of the micro CT imaging systems.

We are currently developing a monochromatic x-ray source for small animal tomographic imaging. This source consists of a conventional cone beam microfocus x-ray tube with a tungsten target coupled to a filter that uses Bragg diffraction to transmit only x-rays within a narrow energy range (~3 keV FWHM). A tissue-equivalent mouse phantom was used to a) evaluate how clearly CT imaging using the quasi-monoenergetic beam is able to differentiate tissue types compared to conventional polyenergetic CT, and b) to test the ability of the source and Bragg filter combination to perform dual energy, iodine contrast enhanced imaging. Single slice CT scans of the phantom were obtained both with polyenergetic (1.8 mm Al filtration) and quasi-monoenergetic beams. Region of interest analysis showed that pixel value variance was signifcantly reduced in the quasi-monochromatic case compared to the polyenergetic case, suggesting a reduction in the variance of the linear attenuation coefficients of the tissue equivalent materials due to the narrower energy spectrum. To test dual energy iodine K-edge imaging, vials containing solutions with a range of iodine contrasts were added to the phantom. Single-slice CT scans were obtained using spectra with maximum values at 30 and 35 keV, respectively. Analysis of the resulting difference images (35 keV image - 30 keV image) shows that the magnitude of the difference signal produced by iodine exceeds that of bone for iodine concentrations above ~20 mg/ml, and that of muscle and fat tissues for iodine concentrations above ~5 mg/ml.

A new way of performing contrast enhanced magnetic resonance angiography (CE-MRA) is presented, in which the entire k-space is decomposed into interlaced subsets that are acquired sequentially. Based on a new parallel imaging technique, Generalized Unaliasing Incorporating object Support constraint and sensitivity Encoding (GUISE), reconstructions can be made using different subsets of k-space to reveal the level of contrast agent in the corresponding data acquisition time period. A proof-of-concept study using a custom made phantom was carried out to examine the utility of the new method. A quantity of contrast agent (copper sulfate solution) was injected into water flowing within a tube while data was acquired using an 8-coil receiver and the modified MRI sequence. A sequence of images was successfully reconstructed at high temporal resolution. This eliminated the need to precisely synchronize data acquisition with contrast arrival. Furthermore, subtraction of a pre-contrast data set prior to reconstruction, which eliminates the need for recovering the static background signal, has proven to be an effective way to improve the SNR and allow a higher temporal resolution to be achieved in recovering the dynamic signal containing contrast level change. Acceptably good reconstruction results were obtained at a temporal resolution equivalent to a 16-fold speed up compared to the time taken to fully sample k-space.

In order to achieve a truly hybrid, high quality X-ray/MR system one must have a rotating anode x-ray source as close as possible to the bore of the high-field MR magnet. Full integration between a closed bore MR system and an x-ray fluoroscopy system presents two main challenges that must be addressed: x-ray tube motor operation and efficiency in an external field, and focal spot deflection. Regarding the first challenge our results have shown that an AC induction motor operating in external fields will experience a drop off in efficiency. Specifically, fields on the order of 100 Gauss perpendicular to the rotor decrease the rotation speed from 2450 RPM to below 1800 RPM. We are currently developing an alternate brushless DC motor design that would exploit the presence of the external MR fringe field and our initial finite element results indicate that the necessary amount of torque is produced. Regarding the second challenge our results show that an external field of 195 Gauss perpendicular to the anode-cathode axis (B_R direction) produces a focal spot deflection of 5 mm. For the fields at which we want to operate the x-ray tube (~to 1000 Gauss along B_R) this deflection will be significantly larger than 5 mm and must be corrected for. We propose a design that includes active deflection coils which serve to counteract the presence of the external field and reduce the focal spot deflection to less than 1 mm in our simulations.

Differential Phase Contrast Imaging (DPCI) has the potential to vastly increase soft tissue contrast. DPCI requires spatial and temporal coherence as generated by a synchrotron or a micro-focus X-ray source; however, recent research demonstrates DPCI can be implemented using a conventional X-ray source with three transmission gratings (Pfeiffer et al., Nature 2006). This paper describes the optimization of the essential system parameters (system size, delivered dose, spatial resolution) of this implementation from a theoretical perspective. The optimization of these parameters is an essential step in practical application of DPCI. We conclude that the minimum size of the system is approximately 700 mm, the minimum resolution is 100 um, and the dose is 1/1000 that of conventional absorption CT.

The differential interference contrast (DIC) microscope is commonly used for the visualization of live biological specimens. It enables the view of the transparent specimens while preserving their viability, being a non-invasive modality. Fertility clinics often use the DIC microscope for evaluation of human embryos quality. Towards quantification and reconstruction of the visualized specimens, an image formation model for DIC imaging is sought and the interaction of light waves with biological matter is examined. In many image formation models the light-matter interaction is expressed via the first Born approximation. The validity region of this approximation is defined in a theoretical bound which limits its use to very small specimens with low dielectric contrast. In this work the Born approximation is investigated via the Helmholtz equation, which describes the interaction between the specimen and light. A solution on the lens field is derived using the Gaussian Legendre quadrature formulation. This numerical scheme is considered both accurate and efficient and has shortened significantly the computation time as compared to integration methods that required a great amount of sampling for satisfying the Whittaker - Shannon sampling theorem. By comparing the numerical results with the theoretical values it is shown that the theoretical bound is not directly relevant to microscopic imaging and is far too limiting. The numerical exhaustive experiments show that the Born approximation is inappropriate for modeling the visualization of thick human embryos.

In recent years, the clinical status of positron emission tomography(PET)/computed tomography(CT) in achieving more accurate staging of lung cancer has been established and the technology has been enthusiastically accepted by the medical community. However, its capability in chest imaging is still limited by several physical factors. As a result of typical PET/CT imaging protocol, respiration-averaged PET data and free of respiration-averaged CT data are collected in a PET/CT scanning. In this work, we investigate the effects of respiration motion. We employ mathematical and Monte-Carlo simulations for generating PET/CT data. We scale a Zubal phantom to generate 30 phantoms having various sizes in order to represent different torso anatomic states during respiration. Images reconstructed from selected scaling PET data using the respective scaling PET attenuation maps serve as baseline results. PET/CT imaging protocol is simulated by reconstruction from respiration-averaged PET data with the selected PET attenuation maps. We also reconstruct PET images from respiratory-averaged PET data with respiration-averaged PET attenuation maps, which simulates conventional PET imaging protocol. We will compare the resulting images reconstructed from the above-mentioned approaches to evaluate the effects of respiration motion in PET/CT.

We present our work toward implementing all-digital signal processing for Positron Emission Tomography (PET) event detection. In the conventional PET system, proper calibration and extending event processing are challenging tasks due to the huge number of channels and multiplexing of input signals in the mixed-signal front-end. To alleviate such limitations, we have proposed a simple all-digital PET system utilizing digital signal processing (DSP) technologies for analyzing event pulses generated in PET. In this work, we implement a Gaussian shaper circuit for scintillation pulses, which followed by a moderate sampling rate Analog-to-Digital Converter (ADC). We also evaluate two DSP algorithms for extracting time information from the digitized pulse samples, and the two algorithms examined could generate a coincidence timing resolution of ~ 2.4ns FWHM, by using a 125MSps sampling rate ADC.

The higher signal-to-noise ratio (SNR) of 3-Tesla magnetic resonance imaging (3T MRI) contributes to an improvement in the spatial and temporal resolution. However, T1-weighted images of the brain obtained by the spin-echo (SE) method using 3T MRI are unsuitable for clinical use because of the inhomogeneity of the radio frequency (RF) field B1 non-uniformity. And it is clear by SE method. In addition, the prolongation of the longitudinal relaxation time (T1) of most tissues leads to a decrease in the T1 contrast. Therefore, many hospitals that utilize 3TMRI use the GRE method instead of the SE method in order to obtain an adequate T1 contrast, as can be obtained using FLASH (fast low angle shot), and high uniformity of images. Further, many studies have been performed to improve the non uniformity using techniques such as spatial presaturation. However, when filters are used, the high intensity of the influence in susceptible regions, signal deficits, and original contrast are lost, and a distortion can be clearly observed when the GRE method is used. Therefore, we obtained the T1-weighted images by using the partial flip angle SE method instead of the GRE method or SE method. We attempted to improve the image non-uniformity by using the partial flip angle SE method. Using this method, we could improve the image uniformity and also realize an adequate T1 contrast. As a result, the uniformity was found to improve by 6% and it became 82.6% at 110°. These results indicate that the use of the partial flip angle SE method is effective for obtaining adequate uniformity in the T1-weighted images of the brain.

Small animal CT gains increasing interest in preclinical research. However, physiological motion compensation like in clinical CT has seldom been employed so far. We present different methods of retrospective motion correction for small animal imaging despite their high respiratory and heart rate. Beside respiratory gating alone the combination of respiratory and simultaneous cardiac gating is shown. In vivo data are acquired with an experimental flat-panel based CT scanner*(Siemens Healthcare, Forchheim Germany). Whole mice or rats fit in the available FOV of 25 * 25 * 4 cm³, while acquisition rate is 100fps. Extrinsic gating is realized by tracing the physiological motion from a small animal monitoring system with a pneumatic pillow for respiratory motion and ECG for heart motion. At the alternative intrinsic method, the lung motion is directly correlated to the movement of the center of gravity in the acquired projection data. As an advantage of the second method the even low preparation effort per scan is reduced. As long as the rotation time of the gantry is far below the cycle time of heart or the lung a multi-segment reconstruction is used in both methods. Motion artifacts are largely suppressed after gating. While in non gated images, the diaphragm, heart contours, bronchi and lung vessels are already visible, they are more sharply defined in the gated datasets. Four-dimensional assessment of lung motion is possible and lung volume in several phases such as peak inspiration and expiration could be segmented, quantified and compared.

The development of ultrasound tomography for the detection of breast cancer could have a major impact on the effectiveness of current diagnostic tools. Here, the potential of ultrasound tomography is investigated by means of a new generation of toroidal ultrasound arrays that can measure both the signals reflected and transmitted through human breast, simultaneously. Experiments performed on phantoms and human breast in vivo are used to compare continuous wave (CW) insonification versus wideband (WB) excitation. It is shown that while transmission diffraction tomography has little benefit from WB excitation, reflection tomography is greatly improved due to the low signal-to-noise ratio of reflection measurements.

The effect of screen optics on the spatial-frequency-dependent DQE of digital radiographic systems, expressed as the Lubberts factor L(f), is studied for both isotropic granular and structured columnar (CsI) x-ray screens. Analytical models for L(f) are derived from diffusion theory for isotropic screens, and optical ray tracing methods are also used to obtain depth-dependent line spread functions, MTF's, and Lubberts factors for both isotropic and columnar screens. The theoretical methods are used to investigate the dependence of L(f) on backing reflectance, thickness, x-ray absorption coefficient, and screen type. An experimental method to estimate L(f), in which an electron multiplying charge coupled device (EMCCD) is used to image individual x-ray bursts, is described. Preliminary results are obtained from data on sample screens, and are compared with the theory.

Current dedicated, cone-beam breast CT scanners generally use a circular scanning configuration largely because it is relatively easy to implement mechanically. It is also well-known, however, that a circular scanning configuration produces insufficient cone-beam data for reconstrucing accurate 3D breast images. Approximate algorithms, such as FDK has been widely applied to reconstruct images from circular cone-beam data. In the FDK reconstruction, it is possible to observe artifacts such as intensity decay for locations that are not within the plane containing the circular source trajectory. Such artifacts may potentially lead to false positive and/or false negative diagnosis of breast cancer. Non-circular imaging configurations may provide data sufficient for accurate image reconstruction. In this work, we implement, investigate innovative, non-circular scanning configurations such as helical and saddle configurations for data acquisition on a dedicated, cone-beam breast CT scanner, and develop novel algorithms to reconstruct accurate 3D images from these data. A dedicated, cone-beam breast CT scanner capable of performing non-circular scanning configurations was used in this research. We have investigated different scanning configurations, including helical and saddle configurations. A Defrise disk phantom and a dead mouse were scanned by use of these configurations. For each configuration, cone-beam data were acquired at 501 views over each turn. We have reconstructed images using our BPF algorithm from data acquired with the helical scanning configuration.

The quality and realism of simulated images is currently limited by the quality of the digital phantoms used for the simulations. The transition from simple raster based phantoms to more detailed geometric (mesh) based phantoms has the potential to increase the usefulness of the simulated data. A preliminary breast phantom which contains 12 distinct tissue classes along with the tissue properties necessary for the simulation of dynamic positron emission tomography scans was created (activity and attenuation). The phantom contains multiple components which can be separately manipulated, utilizing geometric transformations, to represent populations or a single individual being imaged in multiple positions. A new relational descriptive language is presented which conveys the relationships between individual mesh components. This language, which defines how the individual mesh components are composed into the phantom, aids in phantom development by enabling the addition and removal of components without modification of the other components, and simplifying the definition of complex interfaces. Results obtained when testing the phantom using the SimSET PET/SPECT simulator are very encouraging.

The European Guidelines for quality control in digital mammography specify a procedure for measuring contrast-to-noise ratio (CNR) using a 0.2mm thickness of aluminium with different thicknesses of Plexiglas. The relationship between ROI size and heel effect and how this affects CNR measurement is investigated in this work for DR and CR systems. The measured relative noise for the CR images was found to be strongly dependant on the ROI size due to the heel effect. After applying heel effect correction there was very little dependence on ROI size. The relative noise in the images from the DR system showed very little dependence on ROI size. The heel effect also distorted the CNR measurement on CR images when larger ROI is used. However the use of multiple small ROIs led to a result that was essentially the same as if a heel effect correction had been applied. The appropriate ROI size which should be used for CNR measurement was found to be 0.25 × 0.25 cm. Using this size the heel effect had an insignificant impact on the measurement of relative noise and CNR. This approach has the advantage that only a single image is required for each measurement. The application of heel effect correction with CR systems requires two images and complex image processing. The current suggestion in the European guidelines to use a 2 × 2cm ROI is inappropriate for CR systems and leads to an error of 8% to 18% in CNR determination due to the heel effect.

The purpose of this study was to investigate the effect of dose on lesion detection and characterization in breast tomosynthesis (BT), using human breast specimens. Images of 27 lesions in breast specimens were acquired on a BT prototype based on a Mammomat Novation (Siemens) full-field digital mammography (FFDM) system. Two detector modes - binned (2×1 in the scan direction) and full resolution - and four BT exposure levels - approximately 2×, 1.5×, 1×, and 0.5× the total mAs at the same beam quality as used in a single FFDM view with a Mammomat Novation unit under automatic exposure control (AEC) conditions - were examined. The exposure for all BT scans was equally divided among 25 projections. An enhanced filtered back projection reconstruction method was applied with a constant filter setting. A human observer performance study was conducted in which the observers were forced to select the minimum (threshold) exposure level at which each lesion could be both detected and characterized for assessment of recall or not in a screening situation. The median threshold exposure level for all observers and all lesions corresponded to approximately 1×, which is half the exposure of what we currently use for BT. A substantial variation in exposure thresholds was noticed for different lesion types. For low contrast lesions with diffuse borders, an exposure threshold of approximately 2× was required, whereas for spiculated high contrast lesions and lesions with well defined borders, the exposure threshold was lower than 0.5×. The use of binned mode had no statistically significant impact on observer performance compared to full resolution mode. There was no substantial difference between the modes for the detection and characterization of the lesion types.

This paper presents the optimized image quality and average glandular dose in digital mammography, and provides recommendations concerning anode-filter combinations in digital mammography, which is based on amorphous selenium (a-Se) detector technology. The full field digital mammography (FFDM) system based on a-Se technology, which is also a platform of tomosynthesis prototype, was used in this study. X-ray tube anode-filter combinations, which we studied, were tungsten (W) - rhodium (Rh) and tungsten (W) - silver (Ag). Anatomically adaptable fully automatic exposure control (AAEC) was used. The average glandular doses (AGD) were calculated using a specific program developed by Planmed, which automates the method described by Dance et al. Image quality was evaluated in two different ways: a subjective image quality evaluation, and contrast and noise analysis. By using W-Rh and W-Ag anode-filter combinations can be achieved a significantly lower average glandular dose compared with molybdenum (Mo) - molybdenum (Mo) or Mo-Rh. The average glandular dose reduction was achieved from 25 % to 60 %. In the future, the evaluation will concentrate to study more filter combinations and the effect of higher kV (>35 kV) values, which seems be useful while optimizing the dose in digital mammography.

Microcalcifications (MCs) are an important early sign of breast cancer. In conventional mammography, MC detectability is limited primarily due to quantum noise. In tomosynthesis, a dose comparable to that delivered in one projection mammogram is divided across a number of projection views (typically ranging between 10 and 30). This potentially will reduce the detectability of MCs, if detector noise is not very low. The purpose of this study is to explore the relationship between MC detectability in the projection views and in the reconstructed image. The effect of angular range and number of angles on detectability will also be evaluated for an ideal detector. Microcalcification detectability is shown to be greater in the sinogram than in the reconstructed images. Further, the detectability is reduced when the MC is located far from the center of the breast. Also, the detectability in the projection images is dependent on the projection angle.

The image quality of three experimental computed radiography (CR) mammography systems was compared through the measurement of commonly accepted image-quality metrics such as modulation transfer function (MTF) and detective quantum efficiency (DQE). The design and configuration of the scanners in the three systems were different in that they had different signal extraction strategies for each storage phosphor screen. Efforts were also made to improve the image quality through changes in phosphor layers, phosphor particle morphology, particle size distribution, and phosphor binder ratio. The effects on overall image quality as a result of these improvements were demonstrated on these systems. It was found that there were significant variations in system MTF and DQE, depending on how the CR system was configured. Higher system MTF does not always lead to higher DQE. Screen designs as well as scanning strategies need to be taken into consideration in order to achieve image quality improvements for the application of mammography.

Due to the high prevalence of breast cancer among women, much is being done to detect breast cancer earlier and more accurately. In current clinical practice, the most widely-used mode of breast imaging is mammography. Its main advantages are high sensitivity and low patient dose, although it is still merely a two-dimensional projection of a three-dimensional object. In digital breast tomosynthesis, a three-dimensional image of the breast can be reconstructed, but x-ray projection images of the breast are taken over a limited angular span. However, the breast tomosynthesis device itself is more similar to a digital mammography system and thus is a feasible replacement for mammography. Because of the angular undersampling in breast tomosynthesis, the reconstructed images are not considered quantitative, so a worthwhile question to answer would be whether the voxel values (VVs) in breast tomosynthesis images can be made to indicate tissue type as Hounsfield units do in CT. through some image processing scheme. To investigate this, simple phantoms were imaged consisting of layers of uniform, tissue-equivalent plastic for the background sandwiching a layer of interest containing multiple, small cuboids of tissue-equivalent plastic. After analyzing the reconstructed tomosynthesis images, it was found that the VV in each lesion increases linearly with tissue glandularity. However, for the two different x-ray tube energies and for the two different beam exposure levels tested, the trend-lines all have different slopes and y-intercepts. Thus, breast tomosynthesis has a definite potential to be quantitative, and it would be worthwhile to study other possible dependent parameters (phantom thickness, overall density, etc.) as well as alternative reconstruction algorithms.

One major image quality problem in digital tomosynthesis mammography (DTM) is the poor depth-resolution caused by the inherent incomplete sampling. This problem is more pronounced if high-attenuation objects, such as metallic markers and dense calcifications, are present in the breast. Strong ghosting artifacts will be generated in the depth direction in the reconstructed volume. Incomplete sampling of DTM can also cause visible ghosting artifacts in the x-ray source motion direction on the off-focus planes of the objects. These artifacts may interfere with radiologists' visual assessment and computerized analysis of subtle mammographic features. We previously developed an artifact reduction method by using 3D geometrical information of the objects estimated from the reconstructed slices. In this study, we examined the effect of imaging system blur in DTM caused by the focal spot and the detector modulation transformation function (MTF). The focal spot was simulated as a 0.3 mm square array of x-ray point sources. The detector MTF was simulated using the Burgess model with parameters derived from published data of a GE FFDM detector. The spatial-variant impulse responses for the DTM imaging system, which are required in our artifact reduction method, were then computed from the DTM imaging model and a given reconstruction technique. Our results demonstrated that inclusion of imaging system blur improved the performance of our artifact reduction method in terms of the visual quality of the corrected objects. The detector MTF had stronger effects than focal spot blur on artifact reduction under the imaging geometry used. Further work is underway to investigate the effects from other DTM imaging parameters, such as x-ray scattering, different polyenergetic x-ray spectra, and different configurations of angular range and angular sampling interval.

The angular probability distribution of x-rays from a single interaction of a high energy electron with a target atom is a function of the incident-electron energy, direction and target material. This distribution can be quite directional, which suggests that x-ray tube efficiency might be increased if this effect was used. This can be important for novel tubes that use scanning electron beams or carbon nanotubes that have low output flux. We performed Monte Carlo(MC) simulations for studying how this angular distribution is affected by the interactions in thick targets. The theoretical distribution for single-atom interaction was verified using a 4nm tungsten (W) sphere. Contributions of the various processes undergone by the electrons and x-ray photons were analyzed individually. The angular distributions of x-rays generated by electrons incident normally and at a grazing angle to a 4mm thick target were calculated. The results for a 12μm transmission target were also simulated. For single interactions, the theoretical peak for 120keV electron at 28° was measured to be 29° for the MC simulations. The transmission target was found to have 26% higher x-ray output in the forward direction compared to a conventional tube for E≥30keV. When x-ray flux per unit heat was considered for E≥30, grazing incidence of electrons and the associated reflection beam was found to be 41% more efficient than a conventional tube.

This study aims to evaluate the separability of bone from iodine- and gadolinium-based intravenous contrast agents using dual energy CT techniques in a phantom. The phantom was prepared containing varying concentrations of iodine-based contrast, gadolinium-based contrast, and calcium hydroxyapatite (to simulate bone). Thirteen iodine concentrations from 0.1 to 12 mg/mL, twelve gadolinium concentrations from 0.72 to 34.42 mg/mL, and four calcium concentrations from 0 to 200 mg/mL were used. These phantoms were scanned on a dual source CT using two different source spectra, producing one set of data at 80 kVp and another at 140 kVp. On each resulting image, the mean HU was measured at every concentration level for iodine, gadolinium, and calcium, and plotted on a graph of HU value at 80 versus 140 kVp. Linear regression was used to produce a best-fit line for each material. These lines were compared to test for a difference of slopes between calcium and iodine as well as between calcium and gadolinium. Each material exhibited a linear relationship between the HU values at 140 and 80 kVp (R² = 0.99) and demonstrated a unique slope to this line. The slope for iodine was 2.00, for gadolinium was 1.63, and for calcium was 1.55. The slopes of the calcium and iodine lines were significantly different (p < 0.05), while the slopes of the calcium and gadolinium lines were not significantly different (p > 0.05). Our results suggest that while it is technically feasible to separate iodine from bone, gadolinium-based contrast does not appear to be as readily separable from bone as iodine. This result is surprising as the atomic number and k-edge of calcium (Z = 20, k-edge = 4 keV) are closer to iodine (Z = 53, k-edge = 33 keV) than to gadolinium (Z = 64, k-edge = 50 keV).

Purpose: To investigate the influence of physician-selectable equipment variables on image quality for a cardiac X-ray system equipped with flat panel detector. Materials and Methods: Two contrast phantoms (Leeds TO.10 and CDRAD) were imaged in fluorography and fluoroscopy mode. Three variables are studied: the detector entrance dose, patient thickness and antiscatter grid. In fluorography mode, the detector entrance dose was 100, 120, 140, 170, 200 and 240 nGy/image. Patient thickness was simulated with Perspex blocks of 8, 12, 16 and 20cm. The detectability of contrast details was visually evaluated by five observers (subjective method). An alternative objective method of image quality evaluation was used. It consists on determining a simple "figure of merit" parameter based on signal-to-noise and dose measurements. Results: The threshold contrast values were determined for different settings. Contrast-detail curves are presented. Conversion of curve data in single numerical values and comparison with the "figure of merit" are discussed. Conclusion: Contrast detail objects are sensitive to variables changed and can be used in optimization process of new systems. The change of detector entrance dose from a superior to a next inferior setting does not change dramatically the image quality. Consequently, a saving of about 15% in patient "skin" dose is achievable.

A novel low-dose ECG-gated helical scan method to investigate coronary artery diseases was developed. This method uses a high pitch for scanning (based on the patient's heart rate) and X-rays are generated only during the optimal cardiac phases. The dose reduction was obtained using a two-level approach: 1) To use a 64-slice CT scanner (Aquilion, Toshiba, Otawara, Tochigi, Japan) with a scan speed of 0.35 s/rot. to helically scan the heart at a high pitch based on the patient's heart rate. By changing the pitch from the conventional 0.175 to 0.271 for a heart rate of 60 bpm, the exposure dose was reduced to 65%. 2) To employ tube current gating that predicts the timing of optimal cardiac phases from the previous cardiac cycle and generates X-rays only during the required cardiac phases. The combination of high speed scanning with a high pitch and appropriate X-ray generation only in the cardiac phases from 60% to 90% allows the exposure dose to be reduced to 5.6 mSv for patients with a heart rate lower than 65 bpm. This is a dose reduction of approximately 70% compared to the conventional scanning method recommended by the manufacturer when segmental reconstruction is considered. This low-dose protocol seamlessly allows for wide scan ranges (e.g., aortic dissection) with the benefits of ECG-gated helical scanning: smooth continuity for longitudinal direction and utilization of data from all cardiac cycles.

To investigate and compare the nodule detection in digital chest imaging between anti-scatter grid and slot scanning methods. Anthropomorphic chest phantom was imaged with a flat-panel based digital radiography system. The system was operated in both the slot scanning and full-field modes with and without anti-scatter grid. Imaging technique was 120 kVp and 1 to 16 mAs for both modes. 10-mm in diameter computer-simulated nodules with a nominal peak contrast ratio of 5% were inserted at hilum and mediastinum locations by applying SPR values. 4-AFC experiment was conducted to measure the ratio of correct observations as a function of the exposure level for various imaging conditions and locations. These images were displayed randomly on a review workstation and reviewed by three observers. The average ratios of correct observations were computed across over the readers. The statistical significance of the differences in fractions between imaging techniques was computed by the Student t-test. Nodule detection was not significantly improved by raising the exposure level in the hilum and mediastinum regions. Slot scan without grid and with grid received the highest and next highest fractions of correct response, followed in order by full-field without and with grid for the hilum region, and full-field with and without grid for the mediastinum region. Statistical significant difference was found for most comparisons between slot scan with or without grid and full-field with or without grid.

The current x-ray source trajectory for C-arm based cone-beam CT is a single arc. Reconstruction from data acquired with this trajectory yields cone-beam artifacts for regions other than the central slice. In this work we present the preliminary evaluation of reconstruction from a source trajectory of two concentric arcs using a flat-panel detector equipped C-arm gantry (GE Healthcare Innova 4100 system, Waukesha, Wisconsin). The reconstruction method employed is a summation of FDK-type reconstructions from the two individual arcs. For the angle between arcs studied here, 30°, this method offers a significant reduction in the visibility of cone-beam artifacts, with the additional advantages of simplicity and ease of implementation due to the fact that it is a direct extension of the reconstruction method currently implemented on commercial systems. Reconstructed images from data acquired from the two arc trajectory are compared to those reconstructed from a single arc trajectory and evaluated in terms of spatial resolution, low contrast resolution, noise, and artifact level.

In this study, we investigated the magnitude of scattered radiation and beam quality on the low contrast performance in cone beam breast CT imaging with applying volume-of-interest (VOI) imaging technique. For experiments, we used our bench-top cone beam CT (CBCT) system with a flat-panel digital detector. A cylindrical polycarbonate phantom of 11 cm in diameter was used to simulate breasts to measure radiation dose, scatter-to-primary-ratio (SPR), and contrast-to-noise ratio. To implement the VOI scanning technique, a lead filter with a rectangular opening was placed between the x-ray source and the breast phantom. The x-ray tube voltage setting was 80 kVp. The breast phantom was imaged without and with the VOI filter for open filed and VOI field, respectively. Dose measurement was performed using TLD dosimeters. Slot scanning technique with varying slot width was used to measure SPR values. The image quality assessment was performed based on figure of merit (FOM). The results showed that dose can be reduced by a factor of 3 or more outside the VOI and by a factor of 1.6 at the center of the phantom. The SPR value could be reduced by a factor of 9 inside the VOI, and the FOM was improved by a factor of 1.8 at the center of the phantom.

BRAGA (Bedside Radiography Automatic Grid Alignment) is a means of easily and automatically aligning the x-ray tube and grid in bedside radiography. BRAGA permits the use of high ratio (12:1 or 15:1) grids and produces portable radiographs with markedly improved image quality compared to conventional techniques, with no increase in patient dose. BRAGA's design and operational principles are described. Virtually perfect focal spot grid alignment is achieved in less than ten seconds with little additional effort on the part of the technologist. Presented are comparison conventional (non grid) and BRAGA (15:1 grid) chest portable radiographs obtained employing the same kVp, mAs and patient dose. The BRAGA X-ray has markedly better image quality than the conventional portable X-ray.

The spatial characteristics of noise in X-ray CT can influence object detectability. Now, as three dimensional image reformations become more clinically common, it has become vital to understand the structure of noise in the x, y, and z directions independently. The purpose of this paper is to study noise structure in the radial direction and tangential direction, under varying conditions, including a wedge filter and acquisition techniques (i.e. half scan vs full scan). Because the effect of the reconstruction algorithm on an image is highly dependent on spatial location within the field of view, the effect of off-center vs centered positioning in each direction is also examined. The noise spatial frequency distribution was investigated via calculation of the noise power spectrum (NPS) through Fourier methods on simulated water images. As expected, noise structure at center was equivalent in both the radial and tangential directions. Towards the periphery, overall noise power was muted. However, in the tangential direction high frequency noise power was preserved more than twice as much as in the radial direction. Towards the periphery noise becomes more low-frequency in the radial direction, while in the tangential direction it becomes more high-frequency. The half scan increased both noise magnitude and low-mid spatial correlation in the NPS compared to full scan. In conclusion, noise spatial structure is directionally dependent off-center and this may have an impact on object detectability in directional reformations.

The Variable Resolution X-ray (VRX) technique has been successfully used in a Cone-Beam CT (CBCT) system to increase the spatial resolution of CT images in the transverse plane. This was achieved by tilting the Flat Panel Detector (FPD) to smaller vrx_y angles in a VRX Cone Beam CT (VRX-CBCT) system. In this paper, the effect on the axial spatial resolution of CT images created by the VRX-CBCT system is examined at different vrx_x angles, where vrx_x is the tilting angle of the FPD about its x-axis. An amorphous silicon FPD with a CsI scintillator is coupled with a micro-focus x-ray tube to form a CBCT. The FPD is installed on a rotating frame that allows rotation of up to 90° about x and y axes of the FPD. There is no rotation about the z-axis (i.e. normal to the imaging surface). Tilting the FPD about its x-axis (i.e. decreasing the vrx_x angle) reduces both the width of the line-spread function and the sampling distance by a factor of sin vrx_x, thereby increasing the theoretical detector pre-sampling spatial resolution proportionately. This results in thinner CT slices that in turn help increase the axial spatial resolution of the CT images. An in-house phantom is used to measure the MTF of the reconstructed CT images at different vrx_x angles.

Dual-energy x-ray material decomposition has been proposed as a noninvasive quantitative imaging technique for more than 20 years. In this paper, we summarize previously developed dual-energy material decomposition methods and propose a simple yet accurate method for quantitatively measuring chemical composition in vivo. In order to take advantage of the newly developed dual-source CT, the proposed method is based upon post reconstruction (image space) data. Different from other post reconstruction methods, this method is designed to directly measure element composition (mass fraction) in a tissue by a simple table lookup procedure. The method has been tested in phantom studies and also applied to a clinical case. The results showed that this method is capable of accurately measuring elemental concentrations, such as iron in tissue, under low noise imaging conditions. The advantage of this method lies in its simplicity and fast processing times. We believe that this method can be applied clinically to measure the mass fraction of any chemical element in a two-material object, such as to quantify the iron overload in the liver (hemochromatosis). Further investigations on de-noising techniques, as well as clinical validation, are merited.

The three-dimensional point spread function (3D PSF) of cone-beam computed tomography (CBCT) can be measured through the use of point phantom or edge phantom. In cone-beam tomography theory, an input of delta impulse function will produce an output of a delta function (except a scale factor) under the assumption of continuous geometry, thus manifesting spatial shift invariance over the scan field of view (SFOV). For a practical CBCT system, its 3D PSF is of spatial variant distribution over the SFOV, due to the digital geometry in discrete projection detection and gridded volume reconstruction. In principle, the 3D PSF of a CBCT system can be measured either by a micro point-like phantom (<1mm) by approaching a delta impulse function, or by a macro edge phantom (>1mm) by analyzing the edge blurring mechanism. We found that there exists ambiguity and controversy among the 3D PSF measurement and characterization, varying with the size of the phantom. In this work, we will investigate the effects of digital geometry and phantom size on the 3D PSF measurement. In the results, we will propose an experimental protocol for eliminating the uncertainty associated with the 3D PSF characterization of a CBCT system.

We evaluated three strategies for optimizing the x-ray tube voltage in chest CT examinations: (1) keeping patient dose constant and maximizing contrast to noise ratios (CNR); (2) keeping CNR constant and minimizing patient effective dose (E); (3) maximizing CNR²/E. Lung and soft tissue Hounsfield unit values, together with the corresponding image noise, were measured in a Rando phantom at x-ray tube voltages between 80 and 140 kV. A CT dosimetry software package (ImPACT) was used to compute effective doses as a function of CT x-ray tube voltage for adult patients undergoing chest CT examinations. CNR and patient dose in chest CT examinations both increase with increasing x-ray tube voltage at a fixed mAs. All optimization strategies provided similar qualitative results, which showed the best imaging performance was achieved at the lowest x-ray tube voltage (80 kV). Optimization using constant CNR or effective dose is preferred since these methods provide explicit choices of optimal kV/mAs combinations, as well as quantitative data on how changing kV would modify CNR and/or patient dose. The CNR²/Dose figure of merit does not offer explicit choices of kV/mAs for performing CT examinations, and changes in FOM value are more difficult to relate to changes in imaging performance or patient dose.

The new scanner geometry CT D'OR ("CT with Dual Optimal Reading"), developed at the Helmholtz Zentrum München (former GSF-National Research Center for Environment and Health), consists of a discontinuous ring of detectors facing toward the ring center, which are fixated on an x-ray absorbing material. The x-ray source and an additional outer detector are mounted on a gantry which rotates around the inner static detector and thus the patient. When the source is moving, the detectors are alternately exposed and shielded from the source. Data recorded during periods of direct exposure can be combined and are used for the reconstruction of the image. When the detectors are shielded, their signal is solely caused by scatter. Therefore, direct scatter correction is possible. This can be used to considerably improve the image quality, when scatter radiation yields a strong deterioration of the reconstructed image. The advantage of CT D'OR is thus, that information about scatter radiation is obtained without additional effort or exposure. This property of CT D'OR is investigated and its feasibility is demonstrated by the use of Monte Carlo simulations.

The purposes of this study are to analyze signal-to-noise ratio (SNR) changes for in-plane (axial plane) position and in-plane direction in X-ray computed tomography (CT) system and to verify those visual effects by using simulated small low-contrast disc objects. Three-models of multi detector-row CT were employed. Modulation transfer function (MTF) was obtained using a thin metal wire. Noise power spectrum (NPSs) was obtained using a cylindrical water phantom. The measurement positions were set to center and off-centered positions of 64mm, 128mm and 192mm. One-dimensional MTFs and NPSs for the x- and y-direction were calculated by means of a numerical slit scanning method. SNRs were then calculated from MTFs and NPSs. The simulated low-contrast disc objects with diameter of 2 to 10mm and contrast to background of 3.0%, 4.5% and 6.0% were superimposed on the water phantom images. Respective simulated objects in the images are then visually evaluated in degree of their recognition, and then the validity of the resultant SNRs are examined. Resultant in-plane SNRs differed between the center and peripheries and indicated a trend that the SNR values increase in accordance with distance from the center. The increasing degree differed between x- and y-direction, and also changed by the CT systems. These results suggested that the peripheries region has higher low-contrast detectability than the center. The properties derived in this study indicated that the depiction abilities at various in-plane positions are not uniform in clinical CT images, and detectability of the low contrast lesion may be influenced.

Physiological motions can affect Computed Tomography (CT) exam. While the impact of some motions on CT imaging can be reduced, other physiological motions are unavoidable. To attempt correcting the resulting images, it is necessary to understand how the artifacts are formed and their influence on the image quality. Using a cardiac phantom and a dynamic platform, we have studied the influence of a translation in the z-axis associated with a rotation in the z-axis (at different speeds) on the quality of axial images using a 64-channel scanner. The results show that, the deformation, the detectability and the contrast of the calcifications are of course dependent on the density and size of the calcification but also on the movement they undergo. The noise in CT imaging is also affected by motion. The influence of motion on the image quality depends on the examined object and unfortunately cannot be predicted. The corruption of the data results in the loss of information about the form, the contrast and/or the size of the scanned object. This corruption can lead to diagnosis errors by mimicking diseases or by masking physiologic details.

Positioning a patient accurately in treatment devices is crucial for radiological treatment, especially if accuracy vantages of particle beam treatment are exploited. To avoid sub-millimeter misalignments, X-ray images acquired from within the device are compared to a CT to compute respective alignment corrections. Unfortunately, deviations of the underlying geometry model for the imaging system degrade the achievable accuracy. We propose an automatic calibration routine, which bases on the geometry of a phantom and its automatic detection in digital radiographs acquired for various geometric device settings during the calibration. The results from the registration of the phantom's X-ray projections and its known geometry are used to update the model of the respective beamlines, which is used to compute the patient alignment correction. The geometric calibration of a beamline takes all nine relevant degrees of freedom into account, including detector translations in three directions, detector tilt by three axes and three possible translations for the X-ray tube. Introducing a stochastic model for the calibration we are able to predict the patient alignment deviations resulting from inaccuracies inherent to the phantom design and the calibration. Comparisons of the alignment results for a treatment device without calibrated imaging systems and a calibrated device show that an accurate calibration can enhance alignment accuracy.

Routine quality control assessments of medical equipment are crucial for an accurate patient medical treatment as well as for the safety of the patient and staff involved. These regular evaluations become especially important when dealing with radiation-emitting equipment. Therefore, a quality control (QC) program has been developed to quantitatively evaluate imaging systems by measuring standard parameters related to image quality such as the Modulation Transfer Function (MTF), the Noise Power Spectrum (NPS), uniformity, linearity and noise level among others. First, the methods of evaluating the aforementioned parameters have been investigated using a cone beam CT imaging system. Different exposure techniques, phantoms, acquisition modes of the flat panel detector (FPD) and reconstruction algorithms relevant to a clinical environment were all included in this investigation. Second, using the results of the first part of this study, a set of parameters for the QC program was established that yields both, an accurate depiction of the system image quality and an integrated program for easy and practical implementation. Lastly, this QC program will be implemented and practiced in our cone beam CT imaging system. The results using our available phantoms demonstrate that the QC program is adequate to evaluate stability and image quality of this system since it provides comparable parameters to other QC programs.

Dual-energy imaging increases the possibility of pulmonary nodule detection by reducing the bone structure noise. Dual-shot techniques are limited by structural artefacts due to patient and natural movement during the switch of voltage between energies. A new acquisition approach for dual-energy imaging was envisioned in order to reduce this inter-exposure time. The idea is to keep the tube voltage constant, switch a filter in front of the patient and thus modulate the outgoing x-ray spectrum. The drawback of this method is a poorer spectral separation between low and high energy images leading to a higher sensitivity to noise. On the other hand, noise in the reconstructed image is mainly controlled by high-energy image noise, allowing the use of noise suppression algorithms without loosing high-frequency information present in the low-energy image. The first part of this paper is a simulation study presenting system optimisation that includes noise reduction in the HE image. Exposure times and filter thickness are chosen when optimising the signal difference to noise ratio (SDNR) and dose. Results show better SDNR (9 %) for similar dose than state-of-art dual-shot switching voltage technique. Thicker filters could lead to better results, but would demand more tube charge. In the second part is presented experimental validation and implemented noise suppression algorithm. As radiographs of anatomical phantoms are structured, anisotropic algorithm have been considered. Nodule and anthropomorphic phantoms were used to measure detail suppression after image processing. Results are shown in terms of noise suppression in the reconstructed image as well as in detail preservation.

The measurement of bone mineral content is important for diagnosis of demineralization diseases such as osteoporosis. A reliable method of obtaining bone mineral images using a digital magnification mammography system has been developed. The full-field digital phase contrast mammography (PCM) system, which has a molybdenum target of 0.1mm focal spot size, was used with 1.75 x magnification. We have performed several phantom experiments using aluminum step wedges (0.2 mm - 6.0 mm in thickness) and a bone mineral standard phantom composed of calcium carbonate and polyurethane (CaCO₃ concentration: 26.7 - 939.0 mg/cm³) within a water or Lucite phantom. X-ray spectra on the exposure field are measured using a CdTe detector for evaluation of heel effect. From the equations of x-ray attenuation and the thickness of the subjects, quantitative images of both components were obtained. The quantitative images of the two components were obtained for different tube voltages of 24 kV to 39 kV. The relative accuracy was less than 2.5% for the entire aluminum thickness of 0.5 to 6.0 mm at 5 cm water thickness. Accuracy of bone mineral thickness was within 3.5% for 5cm water phantom. The magnified quantitative images of a hand phantom significantly increased the visibility of fine structures of bones. The digital magnification mammography system is useful not only for measurement of bone mineral content, but also high-resolution quantitative imaging of trabecular structure.

The use of two-dimensional, focused, anti-scatter-grids (ASGs) in computed tomography is one essential solution to reduce the scatter radiation for large area detectors. A detailed analysis of the requirements and related image quality aspects lead to the specification of the two-dimensional focused geometry of the X-ray absorbing grids. Scatter simulations indicated trade-off conditions and provided estimations for the expected scatter reduction performance. Different production technologies for focused two-dimensional structures have been evaluated. The presented technology of Tomo Lithographic Molding (Tomo^TM) shows good fulfilment of the specifications. Tomo^TM is a synthesis of lithographic micromachining, precision stack lamination, molding, and casting processes with application-specific material systems. Geometry, material properties, and scatter performance have been investigated. Different analysis methods will be presented and results of the investigations demonstrate the performance capability of this two-dimensional grid technology. Material composition of the tungsten-polymer composite, homogeneity of wall thickness, and precision of the focusing have the biggest influence on the X-ray behavior. Dynamic forces on the anti-scatter-grid during CT operations should not lead to dynamic shadowing or intensity modulation on the active pixel area. Simulations of the wall deformation have been done to estimate the maximum position deviation. Prototype two-dimensional ASGs have been characterized and show promising results.

A graphical user interface based on LabVIEW software was developed to enable clinical evaluation of a new High-Sensitivity Micro-Angio-Fluoroscopic (HSMAF) system for real-time acquisition, display and rapid frame transfer of high-resolution region-of-interest images. The HSMAF detector consists of a CsI(Tl) phosphor, a light image intensifier (LII), and a fiber-optic taper coupled to a progressive scan, frame-transfer, charged-coupled device (CCD) camera which provides real-time 12 bit, 1k × 1k images capable of greater than 10 lp/mm resolution. Images can be captured in continuous or triggered mode, and the camera can be programmed by a computer using Camera Link serial communication. A graphical user interface was developed to control the camera modes such as gain and pixel binning as well as to acquire, store, display, and process the images. The program, written in LabVIEW, has the following capabilities: camera initialization, synchronized image acquisition with the x-ray pulses, roadmap and digital subtraction angiography acquisition (DSA), flat field correction, brightness and contrast control, last frame hold in fluoroscopy, looped play-back of the acquired images in angiography, recursive temporal filtering and LII gain control. Frame rates can be up to 30 fps in full-resolution mode. The user friendly implementation of the interface along with the high frame-rate acquisition and display for this unique high-resolution detector should provide angiographers and interventionalists with a new capability for visualizing details of small vessels and endovascular devices such as stents and hence enable more accurate diagnoses and image guided interventions.

The unavoidable distance between the cervical spine and the image receptor presents measurable levels of geometric unsharpness, which hinders arthritic scoring. The current work explores the impact on the visualisation of important arthritic indicators by increasing the distance between the X-ray source and image detector (SID) from the commonly employed 150cm. Lateral cervical spine images were acquired of an osteoarthritic human cadaver using a DR imaging system. All exposures were taken at 65kVp using automatic exposure control and various SID distances from 150 to 210cm. Four experienced clinicians assessed the images by means of visual grading analysis, using objective criteria based on normal anatomic features and arthritic indicators. A statistically significant improvement in image quality was observed with images acquired at 210cm compared with those acquired at 150cm and 180cm (p<0.05), with values of 56.0 (SE=1.105), 50.85 (SE=1.415) and 65.35 (SE=0.737) respectively. All images with a SID of 210cm scored higher for visually sharp reproduction of the spinous processes, facet joints, intervertebral disc spaces and trabecular bone pattern compared with both 180cm and 150cm. Results indicate that total image quality and visualisation of specific anatomical features is improved in cervical spine radiographs when traditionally employed SID distances are increased.

Mobile X-ray imagery is an omnipresent tool in conventional musculoskeletal and soft tissue applications. The next generation of mobile C-arm systems can provide clinicians of minimally-invasive surgery and pain management procedures with both real-time high-resolution fluoroscopy and intra-operative CT imaging modalities. In this study, we research two C-arm CT experimental system configurations and evaluate their imaging capabilities. In a non-destructive evaluation configuration, the X-ray Tube - Detector assembly is stationary while an imaging object is placed on a rotating table. In a medical imaging configuration, the C-arm gantry moves around the patient and the table. In our research setting, we connect the participating devices through a Mobile X-Ray Imaging Environment known as MOXIE. MOXIE is a set of software applications for internal research at GE Healthcare - Surgery and used to examine imaging performance of experimental systems. Anthropomorphic phantom volume renderings and orthogonal slices of reconstructed images are obtained and displayed. The experimental C-arm CT results show CT-like image quality that may be suitable for interventional procedures, real-time data management, and, therefore, have great potential for effective use on the clinical floor.