Photoacoustic imaging is a hybrid imaging modality that is based on the detection of acoustic waves generated by absorption of pulsed light by tissue chromophores such as hemoglobin in blood. Serial photoacoustic imaging has been performed over a 10-day period after subcutaneous inoculation of pancreatic tumor cells in a rat. The images were obtained from ultrasound generated by absorption in hemoglobin of short laser pulses at a wavelength of 1064 nm. The ultrasound signals were measured in reflection mode using a double-ring photoacoustic detector. A correction algorithm has been developed to correct for scanning and movement artifacts during the measurements. Three-dimensional data visualize the development and quantify the extent of individual blood vessels around the growing tumor, blood concentration changes inside the tumor and growth in depth of the neovascularized region.

We developed an improved signal and image processing of optoacoustic data collected by our laser optoacoustic imaging system designed for breast cancer detection (LOIS-B). The implemented wavelet-based signal processing allowed significant reduction of the low-frequency acoustic noise, improved contrast and localization of the optoacoustic sources of interest. The system was able to differentiate phantoms mimicking breast tumors based on the contrast and morphology of their images. The implemented wavelet-based signal processing also facilitated high (0.5 mm) resolution of the phantoms mimicking parallel blood vessels in the presence of large-amplitude low-frequency acoustic artifacts. The application of the 3D radial back projection image reconstruction algorithm allowed visulaization of the tumor phantoms located beyond the imaging slice of the arc-shaped array of transducers. The visualization of slices parallel to the array of transducers with individual adjustment of the image palette for each particular slice eliminated the image artifacts caused by the large gradient of the laser fluence orthogonal to the array of transducers.

As an emerging imaging technology that combines the merits of both light and ultrasound, photoacoustic tomography (PAT) holds promise for screening and diagnosis of inflammatory joint diseases such as rheumatoid arthritis. In this study, the feasibility of PAT in imaging small-animal joints and human peripheral joints in a noninvasive manner was explored. Ex vivo rat tail and fresh cadaveric human finger joints were imaged. Based on the intrinsic optical contrast, intra- and extra-articular tissue structures in the joints were visualized successfully. Using light in the near-infrared region, the imaging depth of PAT is sufficient for cross-sectional imaging of a human peripheral joint as a whole organ. PAT, as a novel imaging modality with unique advantages, may contribute significantly to the early diagnosis of inflammatory joint disorders and accurate monitoring of disease progression and response to therapy.

We attempted to monitor the healing process of burn injuries by multiwavelength photoacoustic (PA) measurement. Deep dermal burn with 20% total body surface area was made in the dorsal skins of rats. The wavelengths of 532 nm, 556 nm, 576 nm and 600 nm were used: 532 nm is isosbestic point for oxyhemoglobin (HbO₂) and deoxyhemoglobin (HHb); 576 nm is HbO₂ absorption dominant; and 556 nm and 600 nm are HHb absorption dominant. At 532 nm, 556 nm and 576 nm, the depths of PA signal peak were shifted to the shallower region of the wound with the elapse of time, which was found to reflect angiogenesis due to wound healing by histological analysis. The amplitudes of PA signals increased at all the wavelengths until 24 h postburn time. At 48 h postburn time, the signal amplitude continued to increase at 532 nm and 576 nm, while it decreased at 556 nm and 600 nm. This is attributable to the change from a shock phase to the phase of hyperdynamic state, which is accompanied by increases in cardiac output and oxygen consumption. These results suggest that multiwavelength photoacoustic measurement is useful for monitoring healing process of burn injuries.

Detection of sperm cells in dilute samples may have application in forensic testing and diagnosis of male reproductive health. Due to the optically dense subcellular structures in sperm cells, irradiation by nanosecond laser pulses induces a photoacoustic response detectable using a custom flow cytometer. We determined the detection threshold of bull sperm using various concentrations, from 200 to 1,000,000 sperm cells per milliliter. Using a tunable laser system set to 450nm with a 5 ns pulse duration and 11-12 mJ/pulse, we obtained a detection threshold of 3 sperm cells. The flow rate was 4 ml/minute through the flow chamber. The acoustic sensor was a 100 μm PVDF film attached to the glass flow chamber. The acoustic signal was preamplified and sent to an oscilloscope. The threshold signal indicated a signal to noise ratio of approximately 6 to 1. Improved system design may decrease the threshold to single sperm cells.

Photoacoustic tomography (PAT) is adopted to image the brain cortex of monkeys through the intact scalp and skull ex vivo. The reconstructed PAT image shows the main structure of the blood vessels on the monkey brain cortex. For comparison, the brain cortex is imaged without the scalp then imaged again without the scalp and skull. Ultrasound attenuation through the skull is also measured at various angles of incidence to illuminate the effect of the incident angle. This study demonstrates that PAT of the brain cortex is capable of surviving the ultrasound signal attenuation and distortion caused by a considerably thick skull.

Detection of breast cancer cells in human blood may provide early determination of metastasis, enabling aggressive treatment prior to detection by conventional radiographic methods. We developed a photoacoustic flowmetry system in which we irradiated breast cancer cells in suspension to simulate metastatic breast cancer cells derived from human blood. In order to provide optical discrimination between the breast cancer cells and lymphocytes, we attached antibody labeled latex microspheres and gold nanoparticles to breast cancer cells. The breast cancer cells were derived from an estrogen receptor (ER) positive cell line, MCF-7. The particles were conjugated to ER antibodies. We irradiated the cell suspension using the photoacoustic flowmeter consisting of a glass flow chamber with a piezoelectric sensor. We irradiated the suspension at 422 and 530nm and solved a linear system of equations in two variables to separate the contribution of the photoacoustic wave from the breast cancer cells and possible erythrocytes that may be present in a patient blood draw. We found a detection threshold of 10 breast cancer cells using this flowmeter. Future optimization of the system may decrease the detection threshold to single breast cancer cells.

Photothermal radiometry (PTR) and modulated luminescence (LUM) were applied to detect and monitor the demineralization of root and enamel surfaces of human teeth to produce caries lesions and the subsequent remineralization of the produced lesions. The experimental set-up consisted of a semiconductor laser (659 nm, 120 mW), a mercury-cadmium-telluride IR detector for PTR, a photodiode for LUM, and two lock-in amplifiers. A lesion was created on a 1-mm × 4-mm rectangular window, spanning root to enamel surface, using an artificial caries lesion gel to demineralize the tooth surface and create small carious lesions. The samples were subsequently immersed in a remineralization solution. Each sample was examined with PTR/LUM on root and enamel before and after treatment at times from 1 to 10 days of demineralization and 2 to 10 days of remineralization. PTR/LUM signals showed gradual and consistent changes with treatment time. At the completion of the experiments, transverse micro-radiography (TMR) analysis was performed to correlate the PTR/LUM signals to depth of the carious lesions and mineral losses. In this study, TMR showed good correlation with PTR/LUM. It was also found that treatment duration did not correlate well to any technique, PTR/LUM, or TMR, which is indicative of significant variations in demineralization - remineralization rates among different teeth.

One of the applications of the photoacoustic effect in biomedical research is generation of ultra-short acoustic pressure pulses in tissue. An acoustic wave is generated directly in tissue or in an acoustically well coupled immersion liquid, thus avoiding mechanical resonances of the piezoelectric ultrasonic transducer. Although laser generation of the unipolar pressure pulses has been proposed and used before, little attention was paid to the change of the temporal shape of the pulse when it propagates from a transducer. Here we derive simple mathematical solution which helps to predict the pulse shape in the focal region of the transducer and to experimentally verify theoretical calculations showing generation of short quasi-unipolar pressure pulses.

We have developed a new method to perform local measurements of fluorophore excited state lifetimes in turbid media without collecting the fluorescence emission. The method is based on a double pulse illumination where a first laser pulse excites the dye and then a second laser is used for photoacoustic probing of the transient absorption. The photoacoustic response generated by the probe pulse is recorded by an ultrasound receiver. Varying the time delay between excitation and probing allows for tracking the relaxation dynamics of the excited state. The method was validated by measuring the lifetime of an oxygen sensitive dye (Pt(II) octaethylporphine) solution at different concentrations of dissolved oxygen. The dye was excited with a 532 nm pulsed laser and the transient absorption at 740 nm was probed using a second pulsed laser system. The photoacoustics based results coincide with those obtained from simultaneous time-resolved fluorescent measurements. The method can be extended to photoacoustic lifetime imaging by using a receiver array instead of a single receiver. This opens unique possibilities for non-invasive, clinical functional imaging. For example, combined with oxygen sensitive dye, 3D imaging of tissue oxygenation could be developed for accurate diagnosis of cancer tumors, better planning of radiation therapy, and monitoring efficacy of treatment. Other potential applications include: in-vivo mapping of ion (e.g. Ca) concentration and dynamics and imaging of enzymes activity and metabolic functions, as well as environmental studies in turbid media.

Photoacoustic tomography (PAT) with integrating line detectors is able to overcome resolution limitations that are caused by finite aperture sensors commonly used for acoustic wave detection. As integrating line detectors currently different types of sensors are used, e.g., a strip of PVDF film, a free propagating optical beam as a part of a Mach-Zehnder interferometer or an optical fiber interferometer. However, neither satisfies the conditions for an ideal sensor for PAT, which are high noise immunity, compactness, the possibility of parallel detection, and the sufficient resolution and sensitivity. Integrated waveguides (IWGs) in combination with some kind of an interferometer are less investigated for use as an acoustic sensor. An arriving acoustic pulse modifies not only the optical properties but also the dimensions of the waveguide. This leads to a change of phase of guided modes, which is converted into a modulation of light intensity by an interferometer. The topic of this study is the fabrication and testing of IWG sensors for their application in PAT. Free and guided beam detection methods in combination with a Mach-Zehnder interferometer are compared and the sensitivity of the sensors is derived theoretically and experimentally.

Both mechanical and optical imaging of biological tissue can provide relevant contrasts in terms of biomedical tissue characterization. While ultrasound imaging can easily be performed at depth thanks to the weak scattering of ultrasound in soft tissue, the optical spatial resolution is limited for thick tissue by the strong scattering of light. In this paper, we present a technique involving the optical detection of a transient displacement caused by the acoustic radiation force created at cm depth by a focused intense short ultrasound burst (typically ~millisecond). This localized displacement disturbs the optical paths and allows localizing the information with a resolution dictated by the ultrasound spatial distribution. Using a high-speed camera, our objective was to detect and time-resolve displacements in the focal region and the associated propagation shear waves. Experiments were carried out in attenuating tissue-like media illuminated by a continuous laser source. In this work, we refined the optical detection scheme in order to work with low photons flux, based on two-phase heterodyne interferometry setup. We used tissue-mimicking phantoms with different optical and shear mechanical contrast. We demonstrate that it is possible to detect both types of contrast, and moreover to discriminate between these two types of contrast.

While photoacoustic imaging has emerged as a promising modality in recent years, a key drawback of practical and widespread use of the technique has been slow imaging rates. We present a 30-MHz array-based photoacoustic imaging system that can acquire and display photoacoustic images in realtime. Realtime display is very helpful and provides the system operator the ability to better navigate and position the probe for selecting a desired anatomical field of view. The system is capable of imaging at 50 frames per second to depths of a few mm in tissue. We used this system to successfully image the beating hearts of young athymic nude mice in vivo. Also of interest was the ability to visualize microvascular changes during respiration.

Cardiovascular inflammatory activity was imaged in vivo. Inflammation is known to be a major cause of cardiovascular disease. Photoacoustic (PA) imaging was employed using bio-conjugated gold nanorods (GNR) as a contrast agent. A mouse model based on vascular endothelium injury by a photochemical reaction of Rose Bengal (RB) dye to green light laser was used. Following a mid-line laparotomy under an approved animal protocol, anti-ICAM-1 conjugated GNR was injected through the dorsal penile vein followed by RB injection through the same vein. The inferior vena cava immediately distal to the renal veins of a C57BL/6 mouse was exposed to the green light laser for 10 minutes. The peak absorption of GNR was tuned to be 700 nm to minimize possible background absorption by blood and RB. The stability of GNR in the blood plasma was tested in vitro. Photoacoustic images were obtained through an ultrasound gel pouch in the mouse abdomen using a commercial ultrasound probe to evaluate inflammatory changes to the vascular endothelium, confirmed by histology. Preliminary results demonstrate the feasibility of in vivo photoacoustic imaging by a commercial ultrasound scanner of inflammation using GNR as a contrast agent.

Poly(ethylene glycol)-coated Au nanocages have been evaluated as a potential near-infrared (NIR) contrast agent for photoacoustic tomography (PAT). Previously, Au nanoshells were found to be an effective NIR contrast agent for PAT; however, Au nanocages, with their more compact sizes (<50 nm compared to >100 nm for Au nanoshells) and larger optical absorption cross-sections, should be better suited for in vivo applications. In this study, we tested Au nanocages as a contrast agent for PAT. The result suggests that Au nanocages are promising contrast agents for our applications. We also present PAT results when novel, dye-containing nanoparticles are used as contrast agents.

In this study, photoacoustic imaging is utilized to probe information from oncogene surface molecules of cancer cell with the aid of specific targeting. The ultimate goal is to provide prediction of clinical outcome and treatment response of anti-cancer drugs. Different from single targeting in most research, we accomplished multiple targeting to obtain a molecular profile potentially representing tumor characteristics or to locate the heterogeneous population in one lesion. By conjugating different antibodies to gold nanorods corresponding to different peak absorption bands, multiple targeting and simultaneous detection with photoacoustic imaging can be achieved with laser irradiation at the respective peak optical absorption wavelength. Her2 and EGFR were chosen as our primary target molecules. The targeting complex was evaluated in two types of oral cancer cells, OECM1 and Cal27. The OECM1 cell line overexpresses Her2 but has low expression of EGFR, while Cal27 cell line expresses both antibodies. Also, the targeting efficacy to OECM1 can be further improved by using mixed nanoprobes. The cancer cells were induced on the back of the mice by subcutaneous injection. The captured images show that both cancer cells exhibit a higher photoacoustic response (maximum 3 dB) than control groups with specific targeting, thus demonstrating the feasibility of multiple selective targeting with bioconjugated gold nanorods. Images of multiple targeting with mixed nanoprobes of OECM1 cells also reveal further enhancement of targeting (4 dB). The results showed potential of in vivo photoacoustic molecular imaging, providing a better guidance for diagnosis and treatment of cancer.

In an effort of developing clinical LANTCET (laser-activated nano-thermolysis as cell elimination technology) we achieved selective destruction of individual tumor cells through laser generation of vapor microbubbles around clusters of light absorbing gold nanorods (GNR) selectively formed in target tumor cells. Among all gold nanoparticles, nanorods offer the highest optical absorption in the near-infrared. We applied covalent conjugates of gold nanorods with targeting vectors such as monoclonal antibodies CD33 (specific for Acute Myeloid Leukemia), while GNR conjugates with polyethylene-glycol (PEG) were used as nonspecific targeting control. GNR clusters were formed inside the tumor cells at 37 °C due to endocytosis of large concentration of nanorods accumulated on the surface of tumor cells targeted at 4 °C. Formation of GNR clusters significantly reduces the threshold of tumor cell damage making LANTCET safe for normal cells. Appearance of GNR clusters was verified directly with optical resonance scattering microscopy. LANTCET was performed in vitro with living cells of (1) model myeloid K562 cells (CD33 positive), (2) primary human bone marrow CD33-positive blast cells from patients diagnosed with acute myeloid leukemia. Laser-induced microbubbles were generated and detected with a photothermal microscope equipped with a tunable Ti-Sa pulsed laser. GNT cluster formation caused a 100-fold decrease in the threshold optical fluence for laser microbubble generation in tumor cells compared with that in normal cells under the same targeting and irradiation conditions. Combining imaging based on resonance optical scattering with photothermal imaging of microbubbles, we developed a method for detection, image-guided treatment and monitoring of LANTCET. Pilot experiments were performed in flow mode bringing LANTCET closer to reality of clinical procedure of purging tumor cells from bone marrow grafts.

Use of gold nanoparticles (GNPs) as a contrast agent for medical imaging is shown to improve the efficiency of optoacoustic signal generation; signal enhancement allows differentiation between different tissue types. This aspect of medical imaging is important when concerned with early cancer detection. The present paper presents a comparative analysis of two different optical techniques, optical transmission and optoacoustics, to define the different components associated with the attenuation of light in GNPs. This attenuation of light is first studied for a pure absorber where the results are shown to be in agreement for both optical methods, thus showing the effectiveness of the measurement technique. A comparative analysis is also carried out on spherical GNPs which have been synthesized to have peak absorption at the laser wavelength.

Plasmon-resonant gold nanorods show great potential as an agent for contrast-enhanced biomedical imaging or for phototherapeutics. This is primarily due to the high molar extinction coefficient at the absorption maximum and the dependence of the wavelength of the absorption maximum on the aspect ratio, which is tunable in the near-infrared (NIR) during synthesis. Although gold nanorods can be produced in high-yield through the seed-mediated growth technique, the presence of residual cetyltrimethylammonium bromide (CTAB), a stabilizing surfactant required for nanorod growth, interferes with cell function and causes cytotoxicity. To overcome this potential obstacle to in vivo use, we synthesized gold nanorods and conjugated them to a methoxy (polyethylene glycol)-thiol (mPEG ₍₅₀₀₀₎-SH). This approach yielded mPEG-SH modified gold nanorods with optical and morphometric properties that were similar to raw (CTAB) nanorods. Both the CTAB and mPEG-SH nanorods were tested for cytotoxicity against the HL-60 human leukemia cell line by trypan blue exclusion, and the mPEG-SH modified gold nanorods were also tested against a rat insulinoma (RIN-38) and squamous cell carcinoma (SCCVII) cell line. Cells incubated for 24 h with the mPEG-SH modified nanorods had little change in cell viability compared to cells incubated with vehicle alone. This was in contrast to cytotoxicity of CTAB nanorods on HL-60 cells. These results suggest that mPEG-SH modified gold nanorods are better suited for cell loading protocols and injection into animals and facilitate their use for imaging and phototherapeutic purposes.

In-vivo photoacoustic/ultrasound (PA/US) imaging of nude mice was investigated using a photoacoustic imaging system based on a commercial ultrasound scanner HDI-5000. Raw per-channel data was captured and beamformed to generate each individual photoacoustic image with a single laser shot. An ultra-broadband CL15-7 linear array with a center frequency of 8 MHz, combined with a Schott Glass fiber bundle, was used as a compact high resolution imaging probe, with lateral and axial PA resolutions of about 300µm and 200µm, respectively. The imaging system worked in a dual PA-US mode, with the ultrasound outlining the tissue structure and the photoacoustic image showing the blood vessels. PA signals were generated by exposing mice to ultra-short optical pulses from a Nd:YAG-pumped OPO laser operating in a wavelength range of 700-950nm. The corresponding ultrasound images were generated in the regular B-mode with standard delay-and-sum beamforming algorithm. The system resolution was sufficiently high to identify and clearly distinguish the dorsal artery and the two lateral veins in the mouse tail. Both the saphena artery and the ischiatic vein on the cross-section of the mouse leg were clearly outlined in the PA images and correctly overlaid on the ultrasound image of the tissue structure. Similarly, cross-section PA images of the mouse abdomen revealed mesenteric vasculatures located below the abdominal wall. Finally, a successful PA imaging of the mouse thoracic cavity unveiled the ascending and descending aorta. These initial results demonstrate a great potential for a dual photoacoustic/ultrasound imaging modality implemented on a commercial ultrasound imaging scanner.

Photoacoustic (PA) experiments were performed using a modified commercial ultrasound scanner equipped with an array transducer and a Nd:YAG pumped OPO laser. The contrast agent SIDAG (Bayer Schering Pharma AG, Germany), used to enhance the optical absorption, demonstrated an expected pharmacokinetic behavior of the dye in the tumor and in the bladder of the nude mice. A typical behavior in the tumor consisted of an initial linear increase in PA signal followed by an exponential decay. PA signal approached the pre-injection level after about one hour following the dye injection, which was consistent with the behavior for such contrast agents when used in other imaging modalities, such as fluorescence imaging. The in-vivo spectral PA data from the mouse bladder, conducted 1.5 hours after the dye injection, clearly demonstrated presence of the dye. The multi-spectral PA data was obtained at 760nm, 784nm and 850nm laser excitations. The PA intensities obtained at these three wavelengths accurately matched the dye absorption spectrum. In addition, in the kidney, a clearance organ for this contrast agent, both in-vivo and ex-vivo results demonstrated a significant increase (~ 40%) in the ratio of PA signal at 760nm (the peak of the dye absorption) relative to the signal at 850nm (<1% absorption), indicating significant amounts of the dye in this organ. Our initial results confirm the desired photoacoustic properties of the contrast agent, indicating its great potential to be used for imaging with a commercial array-based ultrasound scanner.

A 3D photoacoustic (PA) imaging system has been developed for the non-invasive, in vivo characterization of small animal models of human disease processes. The system utilizes a Fabry Perot polymer film sensing interferometer (FPI) for mapping the spatial-temporal distribution of the PA signals in 2D enabling a 3D PA image to be reconstructed. The mirrors of the sensing FPI are transparent between 590 and 1200nm and highly reflective between 1500 and 1600nm. This enables the transmission of excitation laser pulses from an OPO laser in the former wavelength range through the sensor into the tissue. The induced PA signals are then detected with a CW focused interrogating laser beam at 1550nm which is scanned across the surface of the sensor point by point. Hence, the system is capable of operating in backward mode. The operation of a two-channel interrogating scheme for the FPI sensor has been demonstrated, which will lead to multi-point simultaneous sampling of the PA signals and consequently significantly reduces the data acquisition time. Other measures for speeding up the imaging process and further enhancing image resolution are examined. The system was used to obtain 3D images of mouse tumours of various sizes and in vivo images of the superficial vasculature of the human palm illustrating the potential for characterising small animal cancer models.

We report experimental imaging results with mice using an array-based photoacoustic tomography system designed for small animal imaging. The system features a 128-element curved transducer array with stage rotation to enable complete two-dimensional tomographic imaging in less than 15 seconds. High fidelity imaging of ex vivo mouse brain vasculature was achieved with resolution of vessels less than 200 microns in diameter in the cortex as well as the cerebellum. Images obtained using varying measurement surface angular spans clearly illustrate the impact on feature definition with orientation. The high sensitivity of the system was demonstrated by images of the brain vasculature with an overlying turbid medium (μa=0.03 cm^-1 and μs'~7 cm^-1 at 780 nm) of over 2 cm depth. In phantom experiments, high-quality images of blood tubing in a turbid medium were achieved at depths greater than 3 cm for incident fluences of less than 15 mJ/cm². These results illustrate the suitability for near real-time small animal imaging of deep tissue with high definition.

A three-dimensional laser optoacoustic imaging system was developed, which combines the advantages of optical spectroscopy and high resolution ultrasonic detection, to produce high contrast maps of optical absorbance in tissues. This system was tested in a nude mouse model of breast cancer and produced tissue images of tumors and vasculature. The imaging can utilize either optical properties of hemoglobin and oxyhemoglobin, which are the main endogenous tissue chromophores in the red and near-infrared spectral ranges, or exogenous contrast agent based on gold nanorods. Visualization of tissue molecules targeted by the contrast agent provides molecular information. Visulization of blood at multiple colors of light provides functional information on blood concentration and oxygen saturation. Optoacoustic imaging, using two or more laser illumination wavelengths, permits an assessment of the angiogenesis-related microvasculature, and thereby, an evaluation of the tumor stage and its metastatic potential. The optoacoustic imaging system was also used to generate molecular images of the malignancy-related receptors induced by the xenografts of BT474 mammary adenocarcinoma cells in nude mice. The development of the latter images was facilitated by the use of an optoacoustic contrast agent that utilizes gold nanorods conjugated to monoclonal antibody raised against HER2/neu antigens. These nanorods possess a very strong optical absorption peak that can be tuned in the near-infrared by changing their aspect ratio. The effective conversion of the optical energy into heat by the gold nanorods, followed by the thermal expansion of the surrounding water, makes these nanoparticles an effective optoacoustic contrast agent. Optical scattering methods and x-ray tomography may also benefit from the application of this contrast agent. Administration of the gold nanorod bioconjugates to mice resulted in an enhanced contrast of breast tumors relative the background of normal tissues in the nude mouse model. The combination of this novel contrast agent and optoacoustic imaging has the potential to become a useful imaging modality, for preclinical research in murine models of cancer and other human diseases.

The OPUS (OPtoacoustic UltraSound) system combines a conventional ultrasound (US) system with a specially designed OPO (Optical Parametrical Oscillator) laser system to generate and detect optoacoustical (OA) signals at multiple wavelengths. The intention of this combination was to demonstrate that a conventional ultrasound system can be transformed into an optoacoustic module without major modifications. To offer operational ease of use similar to those of the conventional US instrumentation, i.e. slow moving of the US transducer over the examined tissue area, a high repetition rate of the laser is required. A repetition rate of 100 Hz of the laser system enables a fast image frame rate. Different approaches for the presentation of the two types of images to the operator are compared. For an optimum applicability of the system we found it essential to provide both, the well-known US image and the OA image of the same tissue section to the user. The operator has now the possibility to overlay both images on one screen and thus to extract the desired information from each imaging mode.

Ultrasound imaging is the current gold standard for guiding biopsy of prostate. Optoacoustic imaging yields higher contrast in detection of malignant tissues. The two techniques provide complementary information. We are currently developing a hybrid laser optoacoustic and ultrasound imaging system for prostate tumor detection (LOUIS-P). The optoacoustic part consists of a fiber-coupled Q-switched laser operating at either 757 nm or 1064 nm attached to a commercially-available 128-channel ultrasonic probe modified for optimal detection of optoacoustic signals, a digital signal processor with 128 independent channels, and software that uses the radial (filtered) backprojection algorithm to reconstruct tomographic images. We evaluated system-imaging performance using test objects submerged in milky water, and poly(vinyl-chloride) plastisol tissue phantoms simulating malignant lesions. LOUIS-P demonstrates potential as a clinical technique for minimally invasive imaging and diagnosis of prostate cancer.

In photothermal therapy, a localized temperature increase is achieved by using a continuous wave laser and optically tuned metal nanoparticles. However, the successful outcome of therapy depends on identifying the presence of nanoparticles in the tumor before therapy and monitoring temperature rise during the photothermal procedure. In this paper, we investigate the utility of photoacoustic and ultrasound imaging to guide photothermal therapy. Differences in the optical properties of tissue, enhanced by the presence of nanoparticles, provide a contrast for photoacoustic imaging. Thus, an uptake of nanoparticles in the tumor can be detected by monitoring a photoacoustic image over time. A temperature rise causes the photoacoustic signal amplitude to increase. In addition, a temperature change also leads to time shifts in an ultrasound signal, primarily due to the change in speed of sound. Therefore, by measuring the change in the photoacoustic signal, and differential motion of ultrasound speckle, the temperature rise during photothermal therapy can be computed. Combined imaging was performed with a tunable pulsed laser and an array-based ultrasound transducer. Experiments were carried out on ex-vivo animal tissue injected with composite and broadly absorbing gold nanoparticles. The photoacoustic imaging identified the presence of nanoparticles in tissue. In addition, a localized temperature increase, obtained during therapy, was monitored using photoacoustic and ultrasound imaging. The temperature profiles, obtained by both imaging techniques, were spatially and temporally co-registered. Therefore, the experimental results suggest that photoacoustic and ultrasound imaging can be used to guide and monitor photothermal therapy.

We report the first experimental investigations of photoacoustic guidance of diffusive optical tomography for detection and characterization of optical contrast targets. The hybrid system combined an 8-source, 10-detector reflection mode frequency domain DOT imager with either orthogonal and reflection-geometry photoacoustic systems. The PAT subsystems imaged two-dimensional cross-sections to define centers and radii of regions of interest for a dual-zone mesh DOT imaging algorithm. Phantom absorbers, 1 cm in diameter, of high and low contrast, were spaced 1.5 to 2.5 cm apart at depths ranging from 1 to 2 cm in a turbid medium. Without PAT guidance, the absorber DOT images in many cases were merged and indistinguishable. With PAT guidance, the two targets were well resolved and the reconstructed absorption coefficients improved to 86-130% of the true values. In addition, using both pulse-echo and photoacoustic image detection, the photoacoustic guidance correctly distinguished mechanical from optical contrast providing more specific target information and reconstruction accuracy.

A coupled diffuse-photon-density-wave and thermal-wave theoretical model was developed to describe the biothermophotonic phenomena in a turbid medium under photothermal radiometry experimental conditions. The solution of the radiative transport equation in the limit of the diffuse-photon-density field was considered as a source term in the thermal-wave field equation. The model was used to analyze laser induced photothermal phenomena in a demineralized tooth sample as a function of depth. The analysis is based on a three-layer approach (demineralized enamel + healthy enamel + dentin) and considering the influence of thermal and optical properties of each layer on the resulting optical and thermal field.

Radiofrequency (RF) pulses used to generate thermoacoustic computerized tomography (TCT) signal couple directly into the pulser-receiver and oscilloscope, swamping true TCT signal. We use a standard RF enclosure housing both RF amplifier and object being imaged. This is similar to RF shielding of magnetic resonance imaging (MRI) suites and protects electronics outside from stray RF. Unlike MRI, TCT receivers are ultrasound transducers, which must also be shielded from RF. A transducer housing that simultaneously shields RF and permits acoustic transmission was developed specifically for TCT. We compare TCT signals measured with and without RF shielding.

In biomedical photoacoustic tomography of soft tissue, the initial acoustic pressure distribution following the absorption of a short excitation laser pulse, is recovered as a function of position. This initial pressure distribution is proportional to the absorbed optical energy density, and is thus related (albeit indirectly) to the tissue optical coefficients. When imaging soft tissue which contains several absorbing chromophores (such as oxy- and deoxy-haemoglobin, water, etc.), the primary quantity of interest is the concentrations of the chromophores at each point in the tissue, and not the absorbed optical energy density, which is nonlinearly related to the chromophore concentrations, and also depends on the distribution of scattering. Estimating the distribution of the concentration of a chromophore therefore requires the recovery of two unknown functions (chromophore concentration and scattering distributions) from measurements of one (absorbed energy density). For measurements made at a single optical wavelength, this problem suffers from nonuniqueness, and cannot be solved without additional information being incorporated. A simulated example is used here to demonstrate that, in principle, by using multi-wavelength data and incorporating the known wavelength dependence of the chromophore absorption and the scattering as prior information, a chromophore concentration and spatial dependence of the scattering can be recovered simultaneously. This step opens the way to physiological and molecular imaging using multispectral photoacoustic tomography.

Photoacoustic spectroscopy has been shown to be capable of making non-invasive, spatially resolved measurements of haemodynamic parameters, such as the concentrations of oxy- and deoxyhaemoglobin and blood oxygen saturation. The development of photoacoustic techniques for molecular imaging that go beyond the measurement of haemodynamic parameters has recently become an area of interest. These techniques are aimed at the detection and quantification of for example contrast agents targeted at pathologies such as tumours for diagnostic or therapeutic purposes. This study aimed to validate a model-based inversion scheme by recovering chromophore concentrations from 2D multiwavelength images obtained using a tissue phantom. The inversion scheme employed a complete photoacoustic forward model, which incorporates a model of light transport, a model of acoustic propagation and Fourier transform image reconstruction algorithm. Using the structural information from the measured images, the photoacoustic forward model was used to calculate theoretical multiwavelength photoacoustic images as a function of the concentrations of spatially distributed tissue chromophores and scatters. The chromophore concentrations were determined by fitting the model to the measured images. It was found that concentration ratios of reasonable accuracy were recovered while the absolute concentrations showed significant errors due to light-induced instabilities in the nearinfrared dyes used in the tissue phantom.

Many photoacoustic imaging systems use back projection-based image reconstruction algorithms to estimate photoacoustic source locations from the time-of-flight pressure information collected by either a scanned single transducer or an array of stationary transducer elements. Accurate image reconstruction requires that the transducer(s) locations relative to the imaged volume and the transducer(s) sensitivity distribution within the imaged volume be accurately known. The objective of this work was to develop a method for estimation of the sensitivity distribution of a transducer array. Our approach was to capture for each transducer element the response to a photoacoustic point source that was robotically scanned throughout the imaging volume. The temporally resolved photoacoustic signal was then processed to obtain the relative transducer sensitivity at the source location. We performed a scan over a transducer array fabricated in our laboratory that consisted of twelve 3 mm elements in a 30 mm annular ring arrangement. We measured both the sensitivity distribution of single elements, from which the spatial and angular sensitivity profiles were extracted, as well as the simultaneous response of all transducer elements, which allowed us to locate the effective position of each element, but also highlighted the non-uniformity between transducers' response of up to 20%. We present these and other measured parameters, and discuss their significance as well as their effect on our image reconstruction algorithm.

We have investigated if the application of microsecond length pulses of ultrasound and laser light for AO sensing could result in an improvement of the detection of changes of local absorbances in tissue-mimicking phantoms. An Intralipid-based phantom model, which mimics a blood vessel in human tissue, was used. The detection technique was based on homodyne parallel speckle detection and subsequent image contrast processing. This approach has proved that a spatial resolution of the system of a few millimeters can be obtained and thus, smaller changes in the absorber concentration can be measured. Based on a comparison of experimental data and Monte- Carlo simulations, the quantitative correlation between local absorbances of the phantom and the measured signal has been shown.

Because the wavelengths of radio-frequency (RF) waves used in thermoacoustic tomography (TAT) are comparable with the size of detected objects, RF diffraction plays important roles in TAT. The RF diffraction affects not only the global distribution of the RF field in the tissue, but also local RF energy deposition. In this paper, we discussed these two major effects. Both numerical simulations and phantom experiments are done to demonstrate these phenomena. We also provide a partial correction method for the image distortion and a calibration algorithm for image calibration.

Current non-invasive imaging methods of fluorescent molecular probes in the visible and near-infrared suffer from low spatial resolution as a result of rapid light diffusion in biological tissues. We show that three-dimensional distribution of fluorochromes deep in small animals can be resolved with below 25 femtomole sensitivity and 150 microns spatial resolution by means of multi-spectral photoacoustic molecular tomography. The low sensitivity limit of the method is enabled by using the highly resonant absorption spectrum of a commonly used near-infrared fluorescent molecular probe Alexa Fluor® 750 in order to acquire differential images at multiple wavelengths with tomographic topology suitable for whole-body small animal imaging.

Photoacoustic (PA) imaging provides excellent optical contrast with decent penetration and high spatial resolution, making it attractive for a variety of neural applications. We evaluated optical contrast agents with high absorption in the near infrared (NIR) as potential enhancers for PA neuroimaging: optical dyes, gold nanorods (GNRs) and PEBBLEs loaded with indocyanine green. Two PA systems were developed to test these agents in excised neural tissue and in vivo mouse brain. Lobster nerves were stained with the agents for 30 minutes and placed in a hybrid nerve chamber capable of electrical stimulation and recording, optical spectroscopy and PA imaging. Contrast agents boosted the PA signal by at least 30 dB using NIR illumination from a tunable pulsed laser. Photobleaching may be a limiting factor for optical dyes-the PA signal decreased steadily with laser illumination. The second setup enabled in vivo transcranial imaging of the mouse brain. A custom clinical ultrasound scanner and a 10-MHz linear array provided near real-time images during and after an injection of 2 nM gold nanorods into the tail vein. The peak PA signal from the brain vasculature was enhanced by up to 2 dB at 710 nm. Temporal dynamics of the PA signal were also consistent with mixing of the GNRs in the blood. These studies provide a baseline for enhanced PA imaging in neural tissue. The smart contrast agents employed in this study can be further engineered for molecular targeting and controlled drug delivery with potential treatment for a myriad of neural disorders.

Bacterial contamination can be detected using a minimally invasive optical method, based on laser-induced optoacoustic spectroscopy, to probe for specific antigens associated with a specific infectious agent. As a model system, we have used a surface antigen (Ag), isolated from Chlamydia trachomatis, and a complementary antibody (Ab). A preparation of 0.2 mg/ml of monoclonal Ab specific to the C. trachomatis surface Ag was conjugated to gold nanorods using standard commercial reagents, in order to produce a targeted contrast agent with a strong optoacoustic signal. The C. trachomatis Ag was absorbed in standard plastic microwells, and the binding of the complementary Ab-nanorod conjugate was tested in an immunoaffinity assay. Optoacoustic signals were elicited from the bound nanorods, using an optical parametric oscillator (OPO) laser system as the optical pump. The wavelength tuneability of the OPO optimized the spectroscopic measurement by exciting the nanorods at their optical absorption maxima. Optoacoustic responses were measured in the microwells using a probe beam deflection technique. Immunoaffinity assays were performed on several dilutions of purified C. trachomatis antigen ranging from 50 μg/ml to 1 pg/ml, in order to determine the detection limit for the optoacoustic-based assay. Only when the antigen was present, and the complementary Ab-NR reagent was introduced into the microwell, was an enhanced optoacoustic signal obtained, which indicated specific binding of the Ab-NR complex. The limit of detection with the current system design is between 1 and 5 pg/ml of bacterial Ag.

Gold nanoparticles functionalized with anti-EGFR antibodies undergo molecular specific aggregation on the cellular membrane and later within the cell that leads to a red shift in the plasmon resonance frequency of the gold nanoparticles. Capitalizing on this effect, we previously demonstrated on tissue phantoms that highly sensitive and selective detection of cancer cells can be achieved using the combination of photoacoustic imaging and molecular specific gold nanoparticles. To further evaluate the efficacy of molecular specific photoacoustic imaging technique in detecting deeply situated tumors, small animal experiments were performed. In this study, two gelatin solutions mixed with cells labeled with gold nanoparticles and cells mixed with polyethylene glycol-thiol (mPEG-SH) coated gold nanoparticles were injected in a mouse abdomen ex-vivo. The photoacoustic and ultrasound images from the same crosssection of the region before and after the injections were obtained using a 25 MHz single element ultrasound transducer interfaced with pulsed laser system. The results of our study suggest that the molecular specific photoacoustic imaging with plasmonic nanosensors could be used to detect deeply embedded tumors.

We present the first hybrid photoacoutic fluorescence molecular tomography system, capable of three-dimensional imaging of both fluorochrome and chromophore distributions in highly scattering and absorbing tissues. Quantitative three-dimensional maps of optical absorption coefficient are acquired using photoacoustic tomography. The reconstructed absorption data is fed into the Fluorescence Molecular Tomography inversion scheme in order to improve its accuracy and quatification capabilities in the presence of strong and distributed absorbers that are expected to bias stand-alone fluorescence reconstructions. At all, having both techniques in one hybrid modality yields a system that combines high molecular specificity and targeting flexibility of fluorescent imaging and high spatial resolution functional information obtained via photo-acoustic images.

It has been known for a long time that, in order to reconstruct a streak-free image in tomography, the sampling of view angles should satisfy the Shannon/Nyquist criterion. When the number of view angles is less than the Shannon/Nyquist limit, view aliasing artifacts appear in the reconstructed images. Most recently, it was demonstrated that it is possible to accurately reconstruct a sparse image using highly undersampled projections provided that the samples are distributed at random. The image reconstruction is carried out via an l₁ norm minimization procedure. This new method is generally referred to as compressed sensing (CS) in literature. Specifically, for an N×N image with significant image pixels, the number of samples for an accurate reconstruction of the image is . In medical imaging, some prior images may be reconstructed from a different scan or from the same acquired time-resolved data set. In this case, a new image reconstruction method, Prior Image Constrained Compressed Sensing (PICCS), has been recently developed to reconstruct images using a vastly undersampled data set. In this paper, we introduce the PICCS algorithm and demonstrate how to use this new algorithm to solve problems in medical imaging.

Several photoacoustic (PA) techniques, such as photoacoustic imaging, spectroscopy, and parameter sensing, measure quantities that are closely related to optical absorption, position detection, and laser irradiation parameters. The photoacoustic waves in biomedical applications are usually generated by elastic thermal expansion, which has advantages of nondestructiveness and relatively high conversion efficiency from optical to acoustic energy. Most investigations describe this process using a heuristic approximation, which is invalid when the underlying assumptions are not met. This study developed a numerical solution of the general photoacoustic generation equations involving the heat conduction theorem and the state, continuity, and Navier-Stokes equations in 2.5D axis-symmetric cylindrical coordinates using a finite-difference time-domain (FDTD) scheme. The numerical techniques included staggered grids and Berenger's perfectly matched layers (PMLs), and linear-perturbation analytical solutions were used to validate the simulation results. The numerical results at different detection angles and durations of laser pulses agreed with the theoretical estimates to within an error of 3% in the absolute differences. In addition to accuracy, the flexibility of the FDTD method was demonstrated by simulating a photoacoustic wave in a homogeneous sphere. The performance of Berenger's PMLs was also assessed by comparisons with the traditional first-order Mur's boundary condition. At the edges of the simulation domain, a 10-layer PML medium with polynomial attenuation grading from zero to 5x10⁶ m³/kg/s was designed to reduce the reflection to as low as -60 and -32 dB in the axial and radial directions, respectively. The reflections at the axial and radial boundaries were 32 and 7 dB lower, respectively, for the 10-layer PML absorbing layer than for the first-order Mur's boundary condition.

In-situ experimental work on laser induced pressure waves in water is presented in this paper. A double frequency Nd:YAG laser(532 nm, 4 ns pulse width) was irradiated on a chromium thin film on quartz substrate in contact with water. A plane pressure wave with high temporal and spatial resolution was generated by the laser induced thermoelastic stress around 8~12 mJ/cm² below the regime of shock wave generation. The pressure wave was observed to propagate at the speed of sound in water. The plane acoustic wave could be interacted and focused with solid structures. FEM numerical simulations of the aforementioned phenomena are also carried out to solve the 2D transient wave equation and compared with the experimental results.

Photoacoustic tomography (PAT) is an emerging ultrasound-mediated biophotonic imaging modality that has great potential for many biomedical imaging applications. In many practical implementations of PAT, the photoacoustic signals are recorded over an aperture that does not enclose the object, which results in a limitedview tomographic reconstruction problem. When conventional reconstruction algorithms are applied to limitedview measurement data, the resulting images can contain severe image artifacts and distortions. To circumvent such artifacts, we exploit a priori information about the locations of boundaries within the object (optical absorption function) to improve the fidelity of the reconstructed images. Such boundary information can be inferred, for example, from a co-registered B-mode ultrasound image or other adjunct imaging study. We develop and implement an iterative reconstruction algorithm that exploits a priori object information in the form of support constraints. We demonstrate that the developed iterative reconstruction algorithm produces images with reduced artifact levels as compared to those produced by a conventional PAT reconstruction algorithm.

Recently, the influence of acoustic inhomogeneities on optoacoustic images has gained wide attention in biomedical optoacoustics. Resolution and accuracy of optoacoustic images was found to be improved when a model taking inhomogeneous speed of sound into account was included into the reconstruction algorithm. However, scattering of optoacoustic transients on inhomogeneities of the acoustic impedance was not yet paid much attention to. We show that the same inhomogeneities which are responsible for the contrast in echo ultrasound imaging reduce the contrast in optoacoustic imaging. Absorption of light below the tissue surface results in optoacoustic transients which propagate into the tissue and get backscattered from acoustic inhomogeneities. The echoes interfere with the direct optoacoustic signals and lead to a strong background if the optoacoustic image alone is reconstructed. We show that simultaneous reconstruction of an optoacoustic and an echo image allows to reduce the echo background in the optoacoustic image. For this purpose, we iteratively apply optoacoustic and echo ultrasound Fourier algorithms, together with a special regularization technique. Simulations and experimental results show the validity of the algorithm, and demonstrate the impact of this new method.

The goal of this paper is to compare and contrast various image reconstruction algorithms for tomography (OAT) assuming a finite linear aperture of the kind that arises when using a linear-array transducer. Because such transducers generally have tall, narrow elements, they are essentially insensitive to out of plane acoustic waves, and the usually 3D OAT problem reduces to a 2D problem. Algorithms developed for the 3D problem may not perform optimally. We have implemented and evaluated a number of previously described OAT algorithms, including an exact (in 3D) Fourier-based algorithm and a synthetic aperture based algorithm. We have also implemented an exact 2D algorithm developed by Norton for reflection mode tomography that has not, to the best of our knowledge, been applied to OAT before. Our simulation studies of resolution, contrast, noise properties and signal detectability measures suggest that Norton's approach based algorithm has the best contrast, resolution and signal detectability.

Photoacoustic tomography (PAT), also known as thermoacoustic or optoacoustic tomography, is a hybrid imaging modality that reconstructs the electromagnetic absorption properties of biological tissue from knowledge of acoustic signals produced by the thermoacoustic effect. Because the propagation of acoustic signals is most generally described by the 3D wave equation, PAT is an inherently 3D imaging modality. Due to the the limited penetration depth of the probing electromagnetic fields and the limited availability of 3D ultrasound detector arrays, a simplified two-dimensional (2D) PAT measurement geometry is used in many current experimental implementations. However, in this case, when unfocused transducers are employed, the acquired data are not sufficient to invert the 3D imaging model and ad hoc reconstruction procedures are employed. In this work we numerically investigate 2D and 3D PAT assuming an ultrasound transducer having an anisotropic detection response. The uncompensated effects of an anisotropic detection response on images reconstructed using a point-detector assumption are demonstrated.

Photoacoustic tomography (PAT) is a hybrid imaging modality that combines the advantages of both optical imaging and ultrasound imaging techniques. Most existing reconstruction algorithms assume the speed-of-sound distribution within the object is homogeneous. In certain practical applications, this assumption may not be valid and will result in conspicuous image artifacts. In this work, we investigate the possibility of simultaneously estimating the speed-of-sound and optical absorption properties from data acquired in a PAT experiment. We propose and numerically implement a time-domain iterative algorithm that can reconstruct both the speed-of-sound and optical absorption distribution, by use of a priori information regarding the geometry of the speed-of-sound map. Computer-simulation results are presented to demonstrate the efficacy of the proposed reconstruction method.

Photoacoustic tomography (PAT), also referred to optoacoustic tomography, is a hybrid imaging technique that combines an optical contrast mechanism and ultrasonic detection principles. The laser-induced photoacoustic signals in PAT are broadband in nature, but only a bandpass approximation of the signal is recorded by use of a conventional ultrasonic transducers due to its limited bandwidth. To circumvent this, a PAT system has been developed that records photoacoustic signals by use of multiple ultrasonic transducers that possess different central frequencies. In this work, we investigate a sensor fusion methodology for combining the multiple measurements to obtain an estimate of the true photoacoustic signal that is superior to that obtainable by use of any single transducer measurement. From the estimated photoacoustic signals, three-dimensional images of the optical absorption distribution are reconstructed and are found to possess improved accuracy and statistical properties as compared to the single transducer case. Preliminary computer-simulation studies are presented to demonstrate and investigate the proposed method.

Ideal transducers have perfectly uniform response to incoming pressure waves, regardless of frequency. In practice, piezoelectric transducers are designed with a particular center frequency (CF) and are most sensitive to signals with strong frequency content near CF, bandpass filtering signal - and reconstructed images. We characterized the frequency dependent receive sensitivity of three single-element transducers with CFs ranging from 1 to 3.5 MHz. The resulting sensitivity response curves are applied to ideal thermo/photo/opto-acoustic (TPOAT) signals generated by ideal spherical absorbers to show the impact of transducer frequency response on measured TPOAT data and reconstructed images.

A confocal photoacoustic microscope with improved lateral resolution has been developed by employing stronglyfocused bright field optical illumination and spherically focused 75-MHz ultrasonic detection. The lateral resolution was experimentally measured to be 5 μm and the axial resolution was estimated to be 15 μm. The maximum imaging depth was demonstrated to be greater than 0.7 mm. In in vivo experiments, microvessels with a diameter of ~ 5 μm are imaged in small animals.

Photoacoustic imaging techniques possess high optical contrast with ultrasonic resolution while exceeding imaging depths of pure optical techniques, affording high resolution images deep within scattering biological tissues. In this work, we employ reflection-mode photoacoustic microscopy to non-invasively monitor hemodynamic contrasts and map brain activity. Changes in vascular dynamics of the mouse somatosensory cortex were evoked through electrical stimulation of the hindpaw, resulting in increased photoacoustic intensities spatially correlated with contra-lateral vasculature. Results demonstrate the ability to map brain activation with vascular resolution in three-dimensions, as well as monitor single-vessel hemodynamics with millisecond temporal resolution. Furthermore, these results implicate the feasibility of photoacoustic microscopy to probe intra-cortical single-vessel hemodynamics and pave the way for more extensive functional brain imaging studies.

Here we present an ultrasound detection system with an optical end capable of parallel probe. An erbium-doped fiber amplifier, driven by a tunable laser, outputs light at 27 dBm. A lens collimates the light to probe a 6-μm thick SU-8 etalon and controls the parallel detection area (total array size). A two-lens system guides the reflected light into a photodetector and controls the active area (array element size) on the etalon surface. A translation stage carries the photodetector to detect signals from different array elements. The output of the photodetector is recorded using an oscilloscope. The system's noise equivalent pressure was estimated to be 6.5 kPa over 10~50 MHz using a calibrated piezoelectric transducer when the -3 dB parallel detection area was 1.8 mm in diameter. The detection bandwidth was estimate to exceed 70 MHz using a focused 50 MHz piezoelectric transducer. Using a single probe wavelength, a 1D array with 41 elements and a 1.06 mm aperture length was formed to image a 49 μm black bead photoacoustically. The final image shows an object size of about 95 μm in diameter. According to the results, realizing high-frequency 2D optoacoustic arrays using an etalon is possible.

Diagnostic ultrasound imaging traditionally uses piezoelectric transducers for transmission and reception of ultrasound pulses. As the elements in the imaging array are reduced in size, however, the sensitivity will inherently decrease. We have developed a new, optically-based ultrasound sensor using polymer microring resonators. The device consists of a 100μm-diameter polystyrene ring waveguide coupled to an input/output bus waveguide, and is fabricated by nanoimprint lithography. Acoustic pressure causes change in the waveguide cross-section dimension and strain in the polystyrene material, resulting in a change in the effective refractive index and a shift in resonant wavelength. The ultrasonic waveform can be recovered from this modulation of optical output. The dynamic range and sensitivity of each microring can be tuned appropriately by adjusting the Q during fabrication. Our experiments show a low noise-equivalent pressure on the order of 1 kPa. Sensitivity has been measured by the application of known static pressure and a calibrated 20 MHz ultrasound transducer. A simple 1D array is demonstrated using wavelength multiplexing. The angular response is determined by sensing the optoacoustic excitation of a 49μm polyester microsphere and shows wide-angle sensitivity, making the sensors useful for beamforming. The frequency response is relatively flat between DC and 40 MHz, and can be extended further by choice of substrate material, limited only by the electrical bandwidth of the photodetector. The high sensitivity, bandwidth, and angular response make it a potentially useful sensor platform for applications in ultrasound imaging, dosimetry, and non-destructive testing.

We report flow measurements based on the photoacoustic Doppler effect. We have performed flow experiments with a suspension of micrometer carbon particles and have detected the photoacoustic Doppler shift at various average flow speeds. We have also observed the directional dependence of the photoacoustic Doppler shift. Our experiment is based on the continuous wave (cw) photoacoustic generation. It is the goal of noninvasively monitoring hemodynamics in functional photoacoustic imaging that motivates our study.

Acousto-optical imaging is based on the detection of strongly scattered light which is in part modulated by its interaction with an ultrasonic wave. This method benefits from the acoustic uniformity (low acoustic scattering and absorption) of an optically diffusive medium and the spectrally selective absorption of photons. In this work, we consider the use a pulsed single-frequency laser to increase the instantaneous optical power applied to the diffusive medium while maintaining the average power below the maximum permissible exposure. Such a laser source concentrates the illumination of the diffusive medium during the transit time of the ultrasonic toneburst. This allows collecting more ultrasound-modulated photons for a given ultrasonic wave amplitude. We found, however, that a pulsed laser of this kind generates additional noise which limits the sensitivity gain expected from its high peak power. Progress toward sensitive imaging was achieved by developing methods to reduce the impact of this additional noise. Results obtained with differential detection, laser beam spatio-temporal homogenization and variable delay synchronization are presented. With such measures, the use of a pulsed laser appears a promising solution for enhancing the sensitivity in acousto-optical imaging.

Using a CCD-based speckle contrast detection scheme of ultrasound-modulated optical tomography (UOT), we show the feasibility of imaging objects having different optical scattering coefficients relative to the surrounding scattering medium. Our results show that the spatial resolution depends on the ultrasound parameters and the image contrast depends on the difference in scattering coefficient between the object and the surrounding medium. Experimental measurements are in agreement with Monte Carlo simulations and analytical calculations. This study complements previous UOT experiments that demonstrated optical absorption contrast. It also demonstrates that UOT complements photoacoustic tomography, which is sensitive to optical absorption contrast.

While phase variation due to ultrasonic modulation of coherent light has been extensively studied in acousto-optical imaging, fewer groups have studied non-phase mechanisms of ultrasonic modulation, which may be important in exploring ultrasonic modulation of incoherent light for imaging. We have developed a versatile Monte Carlo based method that can model not only phase variation due to refractive index changes and scatterer displacement, but also amplitude and exit location variations due to the changes in optical properties and refractive index under ultrasonic modulation, in which the exit location variation has not been modeled previously to our knowledge. Our results show that the modulation depth due to the exit location variation is one to two orders of magnitude higher than that due to amplitude variation, but two to three orders of magnitude lower than that due to phase variation for monochromatic light. Furthermore, it is found that the modulation depth in reflectance due to the exit location variation is larger than that in transmittance for small source-detector separations.

We present a novel optical quantum sensor using spectral hole-burning for detecting signals in ultrasound-modulated optical tomography. In this technique, we utilize the capability of sub-MHz spectral filtering afforded by a spectral hole burning crystal to select the desired spectral component from the ultrasound-modulated diffuse light. This technique is capable of providing a large etendue, processing a large number of speckles in parallel, tolerating speckle decorrelation, and imaging in real-time. Experimental results are presented.

Ultrasound-modulated optical tomography uses a well focused ultrasound beam to modulate diffuse light inside soft biological tissues. This modality combines the advantages of ultrasound resolution with optical contrast. However, because of the low ultrasound modulation efficiency, the large background of un-modulated photons gives a low signal-to-noise ratio. Here we report a technique for detection of ultrasound-modulated light using a phase conjugated signal generated by four-wave mixing in a photorefractive polymer. The experimental results demonstrate the potential of this method to detect ultrasound-modulated optical signals in a highly scattering media with an excellent signal-to-noise ratio.

Due to wavelength-dependent optical attenuation in the skin, the local fluence at a subcutaneous vessel varies with the optical wavelength in a spectral measurement. Hence compensation for such a spectral attenuation is necessary in quantitative measurements of the oxygen saturation of hemoglobin (sO₂) in blood vessels in vivo using photoacoustic (PA)imaging. Here, by employing a simplified double-layer skin model, we find that although the absolute value of sO₂ in a vessel is seriously affected by the volume fraction of blood and the spatially averaged sO2 in the dermis, the difference of sO₂ between neighboring vessels is minimally affected. Based on in vivo experiments, we demonstrate that the difference in sO₂ between a typical artery and a typical vein is conserved before and after an experimentally acquired spectral compensation. This conservation holds regardless of the animal's systemic physiological state.

We present the feasibility of functional ultrasound-modulated optical tomography (UOT) in tissue phantoms with two optical wavelengths. By using intense acoustic bursts and a CCD camera-based speckle contrast detection technique, we observe variations of UOT signal at different optical absorptions. In addition, the results from Monte Carlo simulations highly correlate with the experimental outcomes. By irradiating the sample at two optical wavelengths, we quantitatively estimate the total concentration and the concentration ratio of double dyes in inclusions inside tissue phantoms. Therefore, UOT is potentially able to supply functional imaging of the total concentration and oxygen saturation of hemoglobin non-invasively in biological tissues.

Important to the laser based diagnosis of burn depth is an adequate understanding of the optical properties of thermally coagulated blood. Although the optical properties of photocoagulated blood have been studied in some detail, they are inadequate at completely characterizing the optical properties associated with the thermally coagulated blood of burn injuries. Using a photoacoustic method involving the addition of an absorber to thermally coagulated blood, we obtained data that will be used to derive a spectrum for the optical absorption coefficient, a, of thermally coagulated blood within the wavelength range from 580 to 700 nm. Before implementing this method, the stability of photoacoustic measurements within the diffusion theory realm was tested on two weakly absorbing, highly scattering Intralipid solutions. In addition, the absorber, Chlorazol Black, was tested for resistance to change caused by photobleaching and heating.

A deep reflection-mode photoacoustic imaging system was developed and demonstrated to possess a maximum imaging depth up to 38 mm in chicken breast tissue. Using this system, structures in the thoracic cavity and vasculature in cervical area of rats were clearly imaged. Particularly, part of the heart was imaged. In the thoracic cavity, the right atrium imaged, which is one of deepest, was situated ~7 mm deep. In the cervical area, common carotid artery and jugular vein were imaged, which are appropriate for the study of oxygenation between artery and vein. In the abdominal cavity, the embedded structures of a kidney, spinal cord, and vena cava inferior were also clearly imaged in situ and in vivo. The depth of the vena cava inferior was as deep as ~15 mm in vivo. This study shows the depth capability of the system in animals. This imaging modality can be a useful tool to diagnose the disease of organs by assessing the morphological and functional changes in the blood vessels and the organs.

In biomedical photoacoustic tomography (PAT), ultrasonic pulses generated by the absorption of near-infrared light are recorded over an array of detectors, and the measured pressure time series are used to recover an image of the initial acoustic pressure distribution within the tissue, which is related to the tissue optical coefficients and therefore to tissue physiology. For high resolution imaging, large-area detector arrays with a high density of sensitive, small elements are required. Such arrays can be expensive, so reverberant-field PAT has been suggested as a means of obtaining PAT images using arrays with a smaller number of detectors or even a single detector. We propose that by recording the reflections from a reverberant cavity in addition to the primary acoustic waves, sufficient information can be captured to allow a PAT image to be reconstructed, without the requirement for a large-area array. A pilot study using simple 2D simulations, backprojections and modal inversions was undertaken to assess the feasibility of this approach to PAT.

Optoacoustic images from rather large tissue samples, such as the human extremities, the breast, or large organs, are preferably obtained in reflection mode. In the past it has been assumed that irradiating the tissue directly below or even better through an acoustic receiver results in an optimum image contrast. Our theoretical and experimental results however show that when a linear array transducer is used, this is not always true. The optimum location of irradiation depends on the depth of the tissue structures to be imaged and on various sources of image background, namely random optical absorption in the bulk tissue surrounding the region of interest, reconstruction artifacts, and acoustic backscattering. It turns out that the influence of absorption in the bulk tissue becomes minimal when irradiating close to the transducer aperture, the opposite however is the case for image artefact background. Its influence becomes minimal if the fluence in the tissue is homogeneously distributed obtained for an irradiation far away from the transducer. Echo background, which results from backscattered optoacoustic transients, additionally limits the imaging depth in reflection mode optoacoustic imaging. Therefore, the irradiation geometry when using a linear array transducer has to be adapted to the depth of the imaged structures.

Source localization by photoacoustic tomography is dependent on time-of-flight pressure data collected by one or more transducers at multiple positions about the imaged object. Errors in transducer position lead directly to errors in source localization. The objective of this work was to develop a method for experimental determination of transducer position for the purpose of (i) comparison of the measured to the expected transducer position, and (ii) automated calibration of transducer position in scanning and array setups. Our approach was to acquire the time of arrival data at each transducer using a small, point-like photoacoustic source from many locations in the imaged volume. Source placement was controlled with a 3D robotic gantry (accuracy ±0.01 mm). Time of arrival data for all source locations was used to compute a vector of source-transducer distances. The coordinates of each transducer location were then found by nonlinear parameter estimation for a function that related the source distance to the known source location and the unknown transducer location. Application of the method to a planar array of 14 transducers resulted in identification of the position of each element in the transducer array. This finding suggested that the method may be useful for (i) mapping transducer positions during validation and calibration studies, (ii) measuring the effective position of transducers that are asymmetric or have fabrication errors, and (iii) obtaining the mapping relationship between the imaging system and the imaging volume in situations where coregistration of image data from other modalities is desired.

In photoacoustic (also called optoacoustic or thermoacoustic) tomography acoustic pressure waves are generated by illumination of a semitransparent sample with pulsed electromagnetic radiation. Subsequently the waves propagate toward the detection surface enclosing the sample. The inverse problem consists of reconstructing the initial pressure sources from those measurements. By combining the high spatial resolution of ultrasonic imaging with the high contrast of optical imaging it offers new potentials in medical diagnostics. In certain applications of photoacoustic imaging one has to deal with media with spatially varying sound velocity, e.g. bones in soft tissue. These inhomogeneities have a strong influence on the propagation of photoacoustically generated sound waves. Image reconstruction without any compensation of this effect leads to a poor image quality. It is therefore essential to develop reconstruction algorithms that take spatially varying sound velocity into account and are able to reveal small structures in acoustically heterogeneous media. A model-based time reversal reconstruction method is presented that is capable of reconstructing the initial pressure distribution despite variations of sound speed. This reconstruction method calculates the time reversed field directly with a second order embedded boundary method by retransmitting the measured pressure on the detector positions in reversed temporal order. With numerical simulations the effect of heterogenous media on sound propagation and the consequences for image reconstruction without compensation are shown. It is demonstrated how time reversal can lead to a correct reconstruction if the distribution of sound speed is known. Corresponding experiments with phantoms consisting of areas with spatially varying sound velocity are carried out and the algorithm is applied to the measured signals.

Photoacoustic tomography (PAT) with line detectors is based on line integrals of the acoustic pressure field generated by a photoacoustic source. From these line integrals, which are measured around the source, a two-dimensional (2D) projection image of the initial pressure is reconstructed. From many such projections in different directions finally a three-dimensional (3D) image is obtained by applying the inverse Radon transform. In this study the use of an optical beam as line detector is demonstrated. The beam is part of an optical interferometer. To optimize the image resolution the beam is focused in the vicinity of the object. The influence of finite beam length, finite width and varying width on the measured signal is investigated using simulations and experiments. It is found that although the finite beam diameter limits the temporal resolution, the beam can be treated as almost perfect line detector within the focal range of the optical lens. An image of a phantom reveals an achievable resolution on the order of about 100 μm or less.

A one of a kind photoacoustic system has been built around a Philips iU22 ultrasound scanner. The modified channel board architecture allows access to the raw per-channel photoacoustic data, while keeping all of the imaging capabilities of an actual commercial ultrasound scanner. A captured photoacoustic data frame is Fourier beamformed to generate a single laser shot photoacoustic image. In addition to the photoacoustic data, the system supplies the beamformed ultrasound data, providing a truly dual-modality imaging capability. A tunable OPO laser system (700-900nm), pumped by an Nd:YAG solid state laser, is used as an illumination source with 5ns long pulses. An FPGA-based electronic board synchronizes the iU22 start of frame with the laser firing, currently permitting photoacoustic imaging at a rate of 10 Hz (laser repetition rate limit). At that imaging frame rate the photoacoustic system, consisting of a PC modified with 32 Gbytes of acquisition memory and an FPGA array, is able to store several minutes of continuously captured data, enabling monitoring and off-line analysis of dynamic photoacoustic events and/or fast scanning for performing pseudo-3D imaging. The system can use all of the standard iU22 array transducers both for photoacoustic imaging, and in all of the ultrasound imaging modes.

Although a small point ultrasound transducer has a wide acceptance angle, its signal-to-noise (SNR) is low due to the high thermal-noise-induced electric voltages in the transducer, which is a result of its small active area. By contrast, a finite size flat transducer has high sensitivity (good SNR), but the acceptance angle is generally small, which limits its application in reconstruction-based photoacoustic tomography (PAT). In this paper, we report a negative lens concept to increase the acceptance angle for a flat transducer. We also provide phantom experiments that demonstrate this concept can greatly increase the detection region for PAT and without losing sensitivity.

We studied the nature of photoacoustic signals that were generated under a variety of conditions from vessel-mimicking polyethylene tubes. The vessels, filled with a range of contrast agents, were buried in tissue-like phantoms that possessed low to high optical absorption and scattering properties. In a photoacoustic image, we observed that either a single spot or two distinct spots could represent a single vessel depending on the strength of the infused contrast agent and on the size of the vessels. We typically found linear increase of the photoacoustic intensity with laser excitation power as well as with absorption coefficient of the contrast agent. However, we found that there is an optimum excitation power for achieving the best photoacoustic signal. If a vessel is buried in a highly absorbing background, increasing the laser power beyond a certain limit reversibly reduces the photoacoustic signal from the vessel, eventually decreasing it to zero. We also studied the blood-to-tissue absorption contrast requirement for observing the photoacoustic signal from a vessel buried in an absorbing and scattering tissue. We find that, in order to distinguish the photoacoustic signal from its background, the absorption coefficient of contrast agent in the vessel must be at least 2.5 times larger than that of the surroundings.

Osteoporosis is a skeletal disorder characterized by a compromised bone strength predisposing a person to an increased risk of fracture. The early detection of osteoporosis is important to a successful treatment. Current prominent bone densitometry techniques include, among others, Dual Energy X-Ray Absorptiometry (DEXA) and Mechanical Response Tissue Analysis (MRTA). However, DEXA uses ionizing radiation and MRTA results are often unreliable. Simultaneous Photothermal Radiometry (PTR) and Modulated Luminescence (LUM) measurements can be a non-ionizing, noninvasive and reliable alternative to the aforementioned diagnostics techniques. Controlled mineral loss was simulated with sequential etching of a human skull bone. During the experiments, a low-power modulated laser illuminated the sample surface. The absorbed incident optical energy was then re-emitted either non-radiatively, in the form of thermal waves (PTR), or radiatively as lumimescence light emission (LUM). The experimental setup consisted of a semiconductor laser (635 nm, 20 mW), two lock-in amplifiers, a mercury-cadmium-telluride IR detector for PTR, a photodiode for LUM and a computer. A one-dimensional, one-layer theoretical model for LUM and PTR was developed to analyze the experimental data and extract optical and thermal properties of the sample.

Photoacoustic tomography provides good optical contrast with high spatial resolution making it an attractive tool for noninvasive imaging. While the mechanical parameters of tissue affect the photoacoustic signal, the differences in optical absorption mainly determines the contrast between different media. In this work we investigate how the variation in optical and mechanical properties during laser-induced coagulation can be detected by changes in the amplitude and temporal characteristics of photoacoustic signals. Photoacoustic pressure profiles are investigated for tissue equivalent albumen phantoms exposed to varying thermal doses, simulating thermal coagulation. Illumination is performed using an optical parametric oscillator (OPO) fed by a Q-switched Nd:YAG pulsed laser to illuminate at multiple wavelengths. The results of the study demonstrate that photoacoustic signals are sensitive to changes in delivered thermal dose and, hence, photoacoustic imaging has potential as a non-invasive monitoring tool for thermal therapy.

Focusing errors caused by sound velocity heterogeneities widen the mainlobe and elevate the sidelobes, thus degrading both spatial and contrast resolutions in photoacoustic imaging. We propose an adaptive array-based photoacoustic imaging technique that uses the Mallart-Fink (MF) focusing factor weighting to reduce the effect of such focusing errors. The definition of the MF focusing factor indicates that the MF focusing factor at the main lobe of the point-spread function is high (close to 1, without speckle noise being present, which is the case in photoacoustic imaging), whereas it is low at the sidelobes. Based on this property, the elevated sidelobes caused by sound velocity heterogeneities in the tissue can be suppressed after being multiplied by the corresponding map of the MF focusing factor on each imaging point; thus the focusing quality can be improved. This technique makes no assumption of sources of focusing errors and directly suppresses the unwanted sidelobe contributions. Numerical experiments with near field phase screen and displaced phase screen models were performed here to verify the proposed adaptive weighting technique. The effect of the signal-to-noise ratio on the MF focusing factor is also discussed.

We propose and demonstrate a new photoacoustic method to calculate the absolute optical absorption coefficient of a sample. An exact solution of the wave equation is employed to iteratively fit the detected photoacoustic signals. We fit the ratio of the amplitudes of the characteristic peaks of photoacoustic signals in order to increase fitting speed and reduce the influences of background signals. This method is tested by both numerical simulations and experimental results.

Three-dimensional imaging is very valuable in detecting and visualizing lesions from multiple viewing angles. In addition, co-registered 3D imaging combining conventional ultrasound and photoacoustic tomography allows visualization of tissue structures with simultaneous structural and functional information. We have developed a 1280 element 3D ultrasound imaging system based on a 1.75D acoustic array. Complete volumetric images over the full scanning range can be achieved in a few minutes. In conjunction with a Ti:Sapphire laser, the system has been used for photoacoustic imaging. We present 3D co-registered images obtained with the system. Ultrasound and photoacoustic co-registered images of phantoms with different optical and acoustical properties are shown to demonstrate its advantage in cancer detection.