We show that the radiation pressure exerted by a beam of ultrasound can be used for contrast enhancement in high resolution x-ray imaging of tissue. Interfacial features of objects are highlighted as a result of both the displacement introduced by the ultrasound and the inherent sensitivity of x-ray phase contrast imaging to density variations. The potential of the method is demonstrated by imaging various tumor phantoms and tumors from mice. The directionality of the acoustic radiation force and its localization in space permits the imaging of ultrasound-selected tissue volumes. In a related effort we report progress on development of an imaging technique using and electrokinetic effect known as the ultrasonic vibration potential. The ultrasonic vibration potential refers to the voltage generated when ultrasound traverses a colloidal or ionic fluid. The theory of imaging based on the vibration potential is reviewed, and an expression given that describes the signal from an arbitrary object. The experimental apparatus consists of a pair of parallel plates connected to the irradiated body, a low noise preamplifier, a radio frequency lock-in amplifier, translation stages for the ultrasonic transducer that generates the ultrasound, and a computer for data storage and image formation. Experiments are reported where bursts of ultrasound are directed onto colloidal silica objects placed within inert bodies.

In photoacoustic (optoacoustic) medical imaging, short laser pulses irradiate absorbing structures found in tissue, such as blood vessels, causing brief thermal expansions that in turn generate ultrasound waves. These ultrasound waves which correspond to the optical absorption distribution were imaged using a two dimensional array of capacitive micromachined ultrasonic transducers (CMUTs). Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large two dimensional arrays, arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. In this study, a phantom consisting of three 0.86-mm inner diameter polyethylene tubes inside a tissue mimicking material was imaged using a 16 x 16 element CMUT array. The center tube was filled with India-ink to provide optical contrast. Traditional pulse-echo data as well as photoacoustic image data were taken. 2D cross-sectional slices and 3D volume rendered images are shown. Simple array tiling was attempted, whereby a 48 x 48 element array was simulated, to illustrate the advantages of larger arrays. Finally, the sensitivity of the photoacoustics setup to the concentration of ink in the tube was also explored. For the sensitivity experiment a different phantom consisting of only one 1.14-mm inner diameter polyethylene tube inside a tissue mimicking material was used. The concentration of the ink inside the tube was varied and images were taken.

Multi-dimensional, high frequency ultrasound arrays are extremely difficult to fabricate from conventional piezoelectrics. For over a decade, our lab has explored optical detection as an alternate technology for high frequency applications. We have developed several different types of acoustically coupled optical resonators to provide the sensitivity and bandwidth required for biomedical imaging. Waveguide and fiber lasers, thin Fabry-Perot etalons constructed from polymers, and thin microring resonators imprinted into polymers have all been used as ultrasound transducer arrays. Their performance rivals the theoretical conversion efficiency of piezoelectric devices but with bandwidths approaching 100 MHz, array element dimensions approaching 10 um, and no electrical interconnects. In this paper we present results on several resonant optical ultrasound transducer (ROUT) arrays, emphasizing their potential use in photoacoustic imaging. These results strongly suggest that a high resolution photoacoustic microscope can be constructed using a ROUT in a footprint appropriate for endoscopic and minimally invasive applications.

Purpose. To evaluate photoacoustic spectroscopy as a potential imaging modality capable of measuring intra-tumor heterogeneity and spectral features associated with hemoglobin and the molecular probe indocyanine green (ICG). Material and Methods. Immune deficient mice were injected with wildtype and VEGF enhanced MCF-7 breast cancer cells or SKOV3x ovarian cancer cells, which were allowed to grow to a size of 6-12 mm in diameter. Two mice were imaged alive and after euthanasia for (oxy/deoxy)-hemoglobin content. A 0.4 mL volume of 1 μg/mL concentration of ICG was injected into the tail veins of two mice prior to imaging using the photoacoustic computed tomography (PCT) spectrometer (Optosonics, Inc., Indianapolis, IN 46202) scanner. Mouse images were acquired for wavelengths spanning 700-920 nm, after which the major organs were excised, and similarly imaged. A histological study was performed by sectioning the organ and optically imaging the fluorescence distribution. Results. Calibration of PCT-spectroscopy with different samples of oxygenated blood reproduced a hemoglobin dissociation curve consistent with empirical formula with an average error of 5.6%. In vivo PCT determination of SaO₂ levels within the tumor vascular was measurably tracked, and spatially correlated to the periphery of the tumor. Statistical and systematic errors associated with hypoxia were estimated to be 10 and 13%, respectively. Measured ICG concentrations determined by contrast-differential PCT images in excised organs (tumor, liver) were approximately 0.8 μg/mL, consistent with fluorescent histological results. Also, the difference in the ratio of ICG concentration in the gall bladder-to-vasculature between the mice was consistent with excretion times between the two mice. Conclusion. PCT spectroscopic imaging has shown to be a noninvasive modality capable of imaging intra-tumor heterogeneity of (oxy/deoxy)-hemoglobin and ICG in vivo, with an estimated error in SaO₂ at 17% and in ICG at 0.8 μg/mL in excised tissue. Ongoing development of spectroscopic analysis techniques, probe development, and calibration techniques are being developed to improve sensitivity to both exogenous molecular probes and (oxy/deoxy)-hemoglobin fraction.

Molecular imaging is a newly emerging field in which the modern tools of molecular and cell biology have been married to state-of-the-art technologies for noninvasive imaging. The study of molecular imaging will lead to better methods for understanding biological processes as well as diagnosing and managing disease. Here we present noninvasive in vivo spectroscopic photoacoustic tomography (PAT)-based molecular imaging of αvβ3 integrin in a nude mouse U87 brain tumor. PAT combines high optical absorption contrast and high ultrasonic resolution by employing short laser pulses to generate acoustic waves in biological tissues through thermoelastic expansion. Spectroscopic PAT-based molecular imaging offers the separation of the contributions from different absorbers based on the differences in optical absorption spectra among those absorbers. In our case, in the near infrared (NIR) range, oxy-heamoglobin (O2Hb), deoxy-heamoglobin (HHb) and the injected αvβ3-targeted peptide-ICG conjugated NIR fluorescent contrast agent are the three main absorbers. Therefore, with the excitation by multiple wavelength laser pulses, spectroscopic PAT-based molecular imaging not only provides the level of the contrast agent accumulation in the U87 glioblastoma tumor, which is related to the metabolism and angiogenesis of the tumor, but also offers the information on tumor angiogenesis and tumor hypoxia.

Purpose: The purpose of this study is to evaluate PCT Imaging technique to classify tissue and extract kidney cysts in pcy mice model of human adolescent nephronophthisis. Method: Four mice with late stages of nephronophthisis with polycystic kidney disease-PKD and one normal mouse were scanned in the PCT Small Animal Scanner. Both vivo and ex-vivo images of mice kidney were taken at wavelength from 680 nm to 940 nm. The ex-vivo PCT images were compared with histology photographs to check the sensitivity of detecting cysts. Histograms of kidney images were generated over slices and fitted to Gaussian-curve model for volumetric analysis. The portions of cysts in kidneys were estimated and kidney images were segmented by three different colors to present the distribution of different tissues. Result: A good correspondence between PCT imaging findings and PKD histology result was observed. Histogram curves from images of pcy kidneys and normal kidneys were fitted to Gaussian-curve model. Portions of cysts, parenchyma and area of high level hemoglobin were estimated according to the curve fit result. A growth of cysts associated with relatively volume decrease of parenchyma and tissues with high perfusion of hemoglobin was observed. Conclusion: The PCT enabled visualization of renal cysts for mouse model and had the potential for volumetric measurements of kidney.

In the post-genomic era, there is an increasing interest in visualizing the expression of functional genes in vivo. With the assistance of the reporter gene technique, various imaging modalities have been adopted for this purpose. In vivo gene expression imaging promises to provide biologists with a powerful tool for deepening our understanding of developmental biology, expanding our knowledge of the genetic basis of disease, and advancing the development of medicine. In this paper, we demonstrate the feasibility of imaging gene expression with photoacoustic imaging, which offers unique absorption contrast with ultrasonic resolution in vivo. We mark tumors in rats with the lacZ reporter gene. The lacZ gene encodes an enzyme β-galactosidase, which yields a dark blue product when acting on a colorimetric assay called X-gal. Photoacoustic tomography at 650nm clearly visualizes the presence of this blue product. The spectroscopic method can also potentially improve specificity. Considering how many staining methods are used in traditional biology, we believe that photoacoustic techniques will revolutionize the field of molecular imaging. The further development of reporter gene systems with high absorbing products in the NIR region is needed.

We have designed, fabricated and tested a new 128-channel laser optoacoustic imaging system (LOIS-128) for cancer diagnostics consisting of an acoustic probe with 128 PVDF transducers, a digital signal processor with 128 independent channels, and software for reconstruction of optoacoustic images. The system was capable of continuous planar imaging (at rates up to 1 Hz) of small (less than 1 cm) tumors at depths over 6 cm. The directivity of the optoacoustic transducers used in LOIS-128 assured that signal detection at all angles within a narrow imaging slice was at least 40% of the maximal signal. The signal detection was better than 70% of the maximum for about 75% of the image in the image slice and close to zero for signals coming from out of the image slice. LOIS-128 could image high-aspect-ratio objects with about 0.5 mm resolution. Finally, with the designed image reconstruction algorithm we were able to estimate absorption coefficients for test objects with accuracy of at least 5%.

We have developed and used a laser optoacoustic imaging system with transrectal probe (LOIS-P) for detection of mechanical lesions in canine prostates in vivo. LOIS images have been acquired with a 128-channel transrectal probe and a 32-channel data acquisition system. Optoacoustic images showed a strong contrast enhancement for a blood containing lesion, when compared with ultrasound images. Our studies demonstrated that sufficient optoacoustic contrast exists between blood containing lesion and prostate tissue, although the lesion has been undetectable with ultrasound. The imaging results have been compared with visual examination of surgically excised prostates. Although axial resolution of the wide-band transducers employed in the transrectal probe provides good axial resolution of 0.5 mm, the convex arc geometry of the this array of transducers provides lateral resolution degrading with depth in tissue. A two step algorithm has been developed to improve the lateral resolution of deeply located objects. This algorithm employs optoacoustic image reconstruction based on radial back-projection to determine location and shape of the target object, then a procedure, we call Maximum Angular Amplitude Probability (MAAP), to determine true brightness of the object and simultaneously remove arc-shaped artifacts associated with radial back-projection. A laser optoacoustic imaging system (LOIS-P) with transrectal probe operating in backward detection mode empowered with the new image reconstruction algorithm seems promising as a modality for detection of prostate cancer and guiding prostate biopsy.

We present a study of the functional photoacoustic imaging of tumor hypoxia in mice in vivo. Based on spectroscopic photoacoustic tomography that detects the optical absorption of oxy- and deoxy-hemoglobins, the blood oxygen saturation and the vascularization of brain tumors were visualized. U87 glioblastoma tumor cells were inoculated intracranially at a 3-mm depth from the surface of the nude mouse head seven days before the experiment. Increased blood content and hypoxia inside the tumor vasculature were detected through the intact skin and skull. This technique will be useful for future studies on tumor metabolic activities in the brain and hypoxia-related therapeutic resistance.

We present design and comprehensive characterization of a versatile, small-scale photoacoustic sensor stick. Due to its optimized forward-looking directional characteristic, it is a valuable tool for spatially resolved PA depth scanning and 3D imaging. The pencil-formed, optical fiber-coupled sensor has a diameter of only 6 mm, with a length of 15 cm. For characterization of its fundamental parameters, we applied a pulsed frequency-doubled Nd:YAG laser (532 nm) with a pulse repetition rate of 10 Hz. Different designs of the sensor tip are compared. We present a full characterization of the qualities of the system as imaging tool, i.e. lateral and depth resolution in dependence on light absorption and scattering properties of the samples as well as of the surrounding matrix. Specially tailored phantoms are introduced for these experiments. The phantoms in combination with a xy-scanning stage are applied to produce 2D and 3D images with the sensor. The imaging properties of the endoscope are explored by several methods of characterization. We test the sensitivity to absorbing structures of different size and absorptivity, which can be summarized as contrast. Finally, we present first tomographic images of tissue phantoms resembling the optical properties of human tissue.

Intravascular ultrasound (IVUS) imaging has emerged as an imaging technique to evaluate coronary artery diseases including vulnerable plaques. However, in addition to the morphological characteristics provided by IVUS imaging, there is a need for functional imaging capability that could identify the composition of vulnerable plaques. Intravascular photoacoustic (IVPA) imaging, in conjunction with clinically available IVUS imaging, may be such a technique allowing vulnerable plaque characterization and differentiation. We have developed an integrated intravascular ultrasound and photoacoustic imaging system to visualize clinically relevant structural and functional properties of the coronary arteries. The performance of the combined IVUS and IVPA imaging system was evaluated through images of arterial phantoms. Experiments were performed using high frequency IVUS imaging catheters operating at 20 MHz, 30 MHz and 40 MHz. The IVPA imaging was successful in highlighting inclusions based on differential optical absorption while these lesions did not have sufficient contrast in the IVUS images. Finally, initial IVUS and IVPA imaging studies were performed on ex vivo samples of a rabbit artery using the 40 MHz IVUS imaging catheter. Results of the above studies demonstrate the feasibility of combining intravascular ultrasound and photoacoustic imaging and suggest clinical utility of the developed imaging system in interventional cardiology.

For decades, visual, tactile and radiographic examinations have been the standard for diagnosing caries. Nonetheless, the extent of variation in the diagnosis of dental caries is substantial among dental practitioners using these traditional techniques. Therefore, a more reliable standard for detecting incipient caries would be desirable. Using photoacoustics, near-infrared (NIR) optical contrast between sound and carious dental tissues can be relatively easily and accurately detected at ultrasound resolution. In this paper, a pulsed laser (Nd:YAG, Quanta-Ray) was used to probe extracted human molars at different disease stages determined from periapical radiographs. Both fundamental (1064nm) and first harmonic (532nm) pulses (15ns pulse length, 100mJ at fundamental and 9mJ at first harmonic , 10Hz pulse repetition rate) were used to illuminate the occlusal surface of tooth samples placed in a water tank. The photoacoustic signal was recorded with an unfocused wideband single-element piezoelectric transducer (centered at 12 MHz, bandwidth 15 MHz) positioned at small angle (less than 30 degrees) to the laser beam close to the occlusal surface. At the fundamental wavelength, total photoacoustic energy increases from normal to incipient stage disease by as much as a factor of 10. Differences between photoacoustic energy at the fundamental and first harmonic wavelength further indicate spectral absorption changes of the underlying structure with disease progression. Using a focused laser beam, an extracted molar with suspected incipient caries was scanned along the occulusal surface to help localize the caries inside enamel and dentin. The significantly increasing photoacoustic signal at a specific scan line both at fundamental and first harmonic indicates the local development of the incipient caries. The photoacoustic results compare well with visual inspection after layer by layer dissection. Preliminary results demonstrate the feasibility of detecting incipient occlusal and proximal caries. This technique may ultimately allow for continuous monitoring of caries before and during treatment.

A 2D photoacoustic imaging system for spectroscopic biomedical applications is reported, based on a Fabry-Perot (FP) polymer film ultrasound sensor. A variety of broadband sensors have been developed with bandwidths from 20MHz to 50MHz. These ultrasound sensors have a unique dichroic design which has an optical transmission window from 650nm to 1200nm and can be interrogated in the 1520-1610nm wavelength region. This enables the system to operate in backward mode with a tunable Optical Parametric Oscillator (OPO) as the excitation source for near infrared (NIR) spectroscopic applications such as the measurement of blood oxygenation. The area over which the photoacoustic signals can be mapped is 4cm × 2.5cm with an optically defined element size of 64μm diameter. The system's noise-equivalent pressure (NEP) is 0.3kPa over a 20MHz bandwidth without signal averaging. The photoacoustic signals are mapped by rapidly scanning a focused laser beam over the surface of the sensor with a point to point acquisition time of 100ms. The spatial resolution of the imaging system, evaluated from tests on phantoms using a 50MHz FP sensor, is 37μm (lateral) × 27μm (vertical) FWHM. It is considered that this system has the potential to be used in applications that require high resolution 3D imaging of the structure and oxygenation status of the microvasculature.

We describe novel ex vivo method for elimination of tumor cells from bone marrow and blood, Laser Activated Nano-Thermolysis for Cell Elimination Technology (LANTCET) and propose this method for purging of transplants during treatment of leukemia. Human leukemic cells derived from real patients with different diagnoses (acute lymphoblastic leukemias) were selectively damaged by LANTCET in the experiments by laser-induced micro-bubbles that emerge inside individual specifically-targeted cells around the clusters of light-absorbing gold nanoparticles. Pretreatment of the transplants with diagnosis-specific primary monoclonal antibodies and gold nano-particles allowed the formation of nanoparticle clusters inside leukemic cells only. Electron microscopy found the nanoparticulate clusters inside the cells. Total (99.9%) elimination of leukemic cells targeted with specific antibodies and nanoparticles was achieved with single 10-ns laser pulses with optical fluence of 0.2 - 1.0 J/cm² at the wavelength of 532 nm without significant damage to normal bone marrow cells in the same transplant. All cells were studied for the damage/viability with several control methods after their irradiation by laser pulses. Presented results have proved potential applicability of developed LANTCET technology for efficient and safe purging (cleaning of residual tumor cells) of human bone marrow and blood transplants. Design of extra-corporeal system was proposed that can process the transplant for one patient for less than an hour with parallel detection and counting residual leukemic cells.

The photoacoustic (PA) technique has been employed to a number of new biomedical applications based of highly sensitive detection of laser-induced acoustic waves from individual live cells and single absorbing micro-particles or clusters of nanoparticles. These applications involve both linear and non-linear thermoacoustic phenomena initiated by focused nanosecond single laser pulse and detected with a fast PZT-ceramic acoustic transducer. Particularly, we present the following experimental results: 1) monitoring of linear and non-linear PA responses from red blood cells in suspensions in vitro; 2) detection of PA responses from breast cancer cell targeted with gold nanoparticles; 3) PA study of linear and non-linear interaction of laser with colored polystyrene micro-particles as model single absorbers; 4) monitoring of PA responses from moving absorbers in flow in vitro (PA flow cytometry in vitro); 5) recording of PA responses from blood flow in vivo on rat mesentery as animal model (PA flow cytometery in vivo); and 6) monitoring of sedimentation kinetics of particles and cells. The obtained results demonstrate the high sensitivity, low background, simple detection principle, easy data acquisition, and straightforward interpretation of the PA data.

A contrast agent for optoacoustic imaging and laser therapy of early tumors is being developed based on gold nanocolloids strongly absorbing visible and near-infrared light. The optoacoustic signals obtained from gold nanospheres and gold nanorods solutions are studied. In the case of 100 nm nanospheres as an example, a sharp increase in the total area under the curve of the optoacoustic signal is observed when the laser fluence is increased beyond a threshold value of about 0.1 J/cm². The change in the optoacoustic signal profile is attributed to the formation of water vapor bubbles around heated nanoparticles, as evidenced via thermoacoustic microscopy experiments. It has been determined that, surprisingly, gold nanoparticles fail to generate detectable nanobubbles upon irradiation at the laser fluence of ~2 mJ/cm2, which heats the nanoparticles up to 374°C, the critical temperature of water. Only when the estimated temperature of the particle reaches about 10,000°C, a marked increase of the optoacoustic pressure amplitude and a changed profile of the optoacoustic signals indicate nanobubble formation. A nanoparticle based contrast agent is the most effective if it can be activate by laser pulses with low fluence attainable in the depth of tissue. With this goal in mind, we develop targeting protocols that form clusters of gold nanocolloid in the target cells in order to lower the bubble formation threshold below the level of optical fluence allowed for safe laser illumination of skin. Experiments and modeling suggest that formation of clusters of nanocolloids may improve the sensitivity of optoacoustic imaging in the detection of early stage tumors.

A time-intensity based method for photoacoustic blood flow measurements was proposed in last year's meeting. The method made use of the strong photoacoustic response of gold nanospheres and the "wash-out" characteristics of the nanospheres were analyzed. In this paper, we develop a new quantitative technique for measuring blood flows based on the "wash-in" characteristics of the nanoparticles. In particular, the technique makes use of the shape dependence of the optical absorption of gold nanorods (i.e., cylindrical nanoparticles) and the transitions in their shape induced by pulsed laser irradiation. The photon-induced shape transition of gold nanorods involves mainly a rod-to-sphere conversion and a shift in the peak optical absorption wavelength. The application of a series of laser pulses with the same laser energy will induce shape changes in gold nanorods as they flow through a region of interest, with quantitative flow information being derived from the photoacoustic signals from the irradiated gold nanorods measured as a function of time. To demonstrate the feasibility of the technique, an Nd:YAG laser operating at 1064 nm was used for irradiation and a ultrasonic transducer with a center frequency of 1 MHz was used for acoustic detection. Excellent agreement between the measured velocities and the actual velocities was demonstrated, with a linear regression correlation coefficient higher than 0.9. Compared to the wash-out analysis, the wash-in analysis is more suitable for measuring flows in microcirculation.

Cancer cells presented altered surface molecules to encourage their growth and metastasis. Expression of oncogeneic surface molecules also play important roles in the prediction of clinical outcome and treatment response of anti-cancer drugs. It is thus conceivable that imaging of cancer lesions while simultaneously obtaining their pathogenic information at molecular level of as many oncogenic proteins as possible is of great clinical significance. Gold nanoparticles have been used as a contrast agent for photoacoustic imaging. In addition, gold nanoparticles can be bioconjugated to probe certain molecular processes. An intriguing property of gold nanoparticles is its ability to tailor its optical properties. For example, size effects on the surface plasmon absorption of spherical gold nanoparticles have shown that the peak optical absorption red-shifts with the increasing particle size. In addition, the optical absorption spectrum of cylindrical gold nanoparticles (i.e., gold nanorods) exhibits a strong absorption band that is directly related to the aspect ratio. With these unique characteristics, selective targeting can be achieved in photoacoustic molecular imaging. Specifically, gold nanorods with different aspect ratios can be bioconjugated to different antibodies. Multiple targeting and simultaneous detection can then be achieved by using laser irradiation at the respective peak optical absorption wavelength. In this study, photoacoustic multiple targeting using gold nanorods is experimentally demonstrated. We have chosen Her2 and CXCR4 as our primary target molecule as Her2 expression is associated with growth characteristics and sensitivity to Herceptin chemotherapy. On the other hand, CXCR4 expression predict the organ-specific metastatic potential of the cancer cells for clinical intervention in advance. Monoclonal antibody (mAb) against Her2/neu was conjugated to nanorods with several different aspect ratios. The agarose gel is suitable for photoacoustic signal acquisition. A wavelength tunable Ti-Sapphire laser was used for laser irradiation and a 1 MHz ultrasound transducer was used for acoustic detection. The optical wavelength of the laser was tuned between 800 nm and 940 nm, corresponding to gold nanorods of an aspect ratio ranging from 3.7 to 5.9. The results clearly show the potential of photoacoustic molecular imaging with multiple targeting in revealing different oncogene expression levels of the cancer cells.

Ultrasound-induced blood stasis was demonstrated thirty years ago. Most of the literature has been focused on methods employed to prevent stasis from occurring during ultrasound imaging. The current work discusses some of the theory behind this phenomenon. It also demonstrates ultrasound-induced blood stasis in murine tumor and muscle tissue, observed through noninvasive measurements of optical spectroscopy, and discusses possible diagnostic uses. We demonstrate that, using optical spectroscopy, effects of ultrasound can be used to noninvasively differentiate tumor from muscle tissue in mice, and that we can quantitatively differentiate tumor from muscle with maximum specificity 0.83, maximum sensitivity 0.79, and area under ROC curve 0.90, using a simple algorithm.

We report results of a feasibility study regarding the question whether or not venous valves can be imaged using photoacoustics, and how they will appear in the images. First an in vitro study was made on tissue phantoms consisting of blood filled rubber tubes with discontinuities in the inner tube wall. We also have studied superficial veins on the ventral side of the wrist. For excitation, an Nd:YAG laser at 1064 nm was used. Detection of acoustic signals was performed with a PVdF sensor consisting of two concentric rings. Measurements were performed on valves which where first localized by palpation. The phantom studies showed that irregular structures of the tube walls could clearly be identified from the photoacoustic images. Furthermore, in a photoacoustic image of a vein at the dorsal side of the wrist, the presence of a valve could be identified from a region of increased signal intensity within the vessel lumen.

In this study, we demonstrate the potential of photoacoustic tomography for the study of traumatic brain injury (TBI) in rats in vivo. Based on spectroscopic photoacoustic tomography that can detect the absorption rates of oxy- and deoxy-hemoglobins, the blood oxygen saturation and total blood volume in TBI rat brains were visualized. Reproducible cerebral trauma was induced using a fluid percussion TBI device. The time courses of the hemodynamic response following the trauma initiation were imaged with multi-wavelength photoacoustic tomography with bandwidth-limited spatial resolution through the intact skin and skull. In the pilot set of experiments, trauma induced hematomas and blood oxygen saturation level changes were detected, a finding consistent with the known physiological responses to TBI. This new imaging method will be useful for future studies on TBI-related metabolic activities and the effects of therapeutic agents.

A three-dimensional in vivo near-infrared photoacoustic tomography imaging system was newly designed and built to visualize the structure of a whole small animal head. For high sensitivity, a single flat 2.25MHz low frequency transducer, whose active element size is 6mm, was employed. To increase the penetration depth of light, a wavelength of 804nm in the NIR range, which matches the oxy- and deoxy-hemoglobin isosbestic point, was chosen. To avoid strong photoacoustic signal generation from the skin surface, we applied dark field illumination. To illuminate efficiently, we split the laser light into two beams, which were delivered to an animal by two mirrors and were finally homogenized by two ground glasses. To complete the dark field illumination, the transducer was located in the middle of two light sources. Two key devices for the in vivo imaging were rotating devices and animal holders. The rotating devices were composed of two parts, located at the top and bottom, which rotated at the same angular speed. The holders were composed of a head holder and a body holder. Both holders fixed the animal firmly to reduce motion artifacts. This system achieved radial resolution of up to 260μm. We accomplished successful in vivo imaging of arterial and venous vessels deeply, as well as superficially, with the animal head of up to 1.7cm diameter. The technique forms a basis for functional imaging, such as measurement of the oxygen consumption ratio in the brain, which is a vital parameter in a brain disease research.

Using peak amplitude spectral PA measurements in the range of 570 - 600 nm, we found it possible to quantify blood oxygenation levels. Visible light illumination minimizes the inversion error of the PA measurements. Owing to high blood absorption in this optical regime, there is also an improved signal-to-noise ratio and less influence from optical scattering. To arrive at correct, and vessel size independent, SO2 measurements, the central frequency of the ultrasonic transducer must be high enough to satisfy the relation, that is above 25 MHz for a chosen optical wavelength region, although lower frequency transducers may produce correct results after correction of the optical absorption spectra. However, additional efforts are needed to achieve accurate SO2 values for in vivo measurements.

A high frame rate photoacoustic imaging system is described. Applications of this system to perfusion measurements are also presented as a demonstration of its potential usage. The system consists of an ultrasound front-end sub-system for acquisition of acoustic array data. The ultrasound front-end sub-system is also known as the DiPhAS (digital phased array system) which is capable of simultaneously acquiring radio frequency data from 64 transducer channels at a rate up to 40 MSamples/sec per channel. In this study, an ultrasonic linear array with a 5 MHz center frequency was employed as part of the integrated photoacoustic probe. The photoacoustic probe also had two linear light guides mounted on the sides of the ultrasonic array for broad laser irradiation from a Q-switched Nd:YAG pulsed laser. After the acquired ultrasound array data were transferred to a personal computer via a high speed digital I/Q card, dynamic focusing and image reconstruction were done off-line. The 64-channel array data can be acquired and transferred every 4 milliseconds, thus making the frame rate of the system up to 250 Hz. The actual frame rate of the current system is limited by the pulse repetition frequency of the laser at 15 Hz. To demonstrate capabilities of the system, photoacoustic perfusion measurements with gold nanorods were performed. A previously proposed time-intensity based flow estimation technique utilizing the shape transitions of gold nanorods under laser irradiation was employed. Good estimation results were achieved and potential of this high frame rate photoacoustic imaging system is clearly demonstrated.

We modeled three-dimensional light distribution in the radial artery using geometrical parameters of this blood vessel and optical parameters typical for blood in the near-infrared spectral range. The obtained light distributions allowed for calculation of optoacoustic signals detected by transducers on the skin surface. We then compared the calculated signals with optoacoustic signals experimentally measured from tissue phantoms at different effective attenuation coefficients and obtained good correlation. Our data suggest that the light distribution and optoacoustic signal modeling can be used in transducer design and in optoacoustic signal processing for accurate measurement of blood parameters.

The effects of acoustic heterogeneities on thermoacoustic tomography (TAT) are examined and corrected. One assumption made in the existing reconstruction algorithms for thermoacoustic tomography is that biological tissue is acoustically homogeneous. In medical imaging applications, this assumption can cause blurring and distortion in the reconstructed images. This degradation of image quality can be compensated for by using an approximate distribution of the acoustic speed in the tissue. A method based on transmission ultrasound tomography, which is compatible with our thermoacoustic imaging setup, is developed to correct those effects. Experiments verify the validity of this method. The technique can be used to improve the image quality of thermoacoustic tomography.

Ultrasound focusing through complex media can be achieved using time-reversal techniques. These techniques make use of back-propagating ultrasonic waves generated by localized sources. Such sources generally consist of high acoustic contrasts echoing ultrasonic waves generated by an incident ultrasonic field, or directly by point-like transducers inserted at the desired focusing location. In this work, we experimentally investigate time-reversal of acoustic waves generated by photo-acoustic emission. A frequency-doubled Q-switched Nd:YAG laser was used to illuminate phantom with 5-ns laser pulses. A 128-element ultrasonic transducer array, with a center frequency of 1.5 MHz, was used to detect acoustic waves generated by optically absorbing targets suspended in water. A dedicated 32-channel electronics was used to time-reverse and re-emit the detected ultrasonic field. Gel spheres dyed with India ink (diameter approximately 1-2 mm)illuminated by the laser beam were used to generate the photo-acoustic waves. Time-reversal of the detected field was performed to focus ultrasound in the presence of highly defocusing media in front of the transducer array. We demonstrate how this allows correcting for the aberration in order to provide good quality images in the isoplanetic region surrounding the photo-acoustic source.

Photoacoustic tomography is emerging as a promising imaging modality for various biomedical applications. Unlike traditional ultrasound imaging that is plagued by strong speckle artifacts, no obvious speckle has so far been observed in photoacoustic images. We systematically studied the reason for this lack of speckle in photoacoustic tomography based on speckle contrast. Theoretical explanations were validated by simulation. The results here can serve as a basis for developing specific applications, such as tissue characterization, using photoacoustic methods.

The concept of tagging photons with ultrasound for medical imaging has been under development by several groups since the early 1990's. All the early attempts were plagued by low signals. The problem is very fundamental because the goal of good spatial resolution requires sampling only that light which has gone through a very small volume. It is compounded by the fact that the diverse paths of the photons result in signals which are not coherent with each other. Thus averaging over a large number of paths is of limited benefit. Recently, some new techniques have been developed for enhancing the signals. We will discuss the fundamental limits on signal-to-noise ratio which apply across all techniques.

Photoacoustic tomography (PAT) is an inherently three-dimensional imaging technique with great potential for a wide range of biomedical imaging applications. In certain applications such as imaging of small animals or human limbs, it is natural to employ a measurement geometry in which the photoacoustic data are recorded on a cylindrical aperture. Analytic PAT reconstruction formulas are available for infinite cylindrical apertures, which are obviously not experimentally realizable. In this work, we investigate data suffciency conditions and iterative reconstruction algorithms for exact image reconstruction in PAT assuming various truncated cylindrical measurement apertures. We employ known results regarding singularity detection in PAT to achieve this. Three dimensional computer-simulation studies are conducted to corroborate the theoretical conjectures.

Photoacoustic tomography has captured a significant amount of attention recently. Although several commercially available optical imaging modalities, including confocal microscopy, two-photon microscopy, and optical coherence tomography have been highly successful, none of these technologies can penetrate beyond ~1 mm into scattering biological tissues because all of them are based on ballistic and quasi-ballistic photons. Consequently, until now, there has been a void in high-resolution optical imaging beyond this depth limit. Photoacoustic tomography has filled this void by combining high ultrasonic resolution and high optical contrast in a single modality. Up until now, however, free-space ultrasound propagation has always been assumed in photoacoustic tomography. In this paper, boundary conditions that should be considered in certain imaging configurations are presented and their associated inverse solutions for image reconstruction are provided.

Interferometric measurements for in-vivo imaging of biological tissues are strongly sensitive to the related speckle decorrelation time tc, whose effect is to reduce the contrast of the speckle pattern at the exit of the sample and thus blur detection. Though acousto-optic imaging is a well suited technique for the case of thick tissues, it has been shown that an acquisition rate in the 1-10kHz range is required for a good efficiency. We have previously built for this purpose an holographic setup that combines a fast but large area single photodetector and a photorefractive crystal, in order to measure a real-time acousto-optic signal by the so-called self-adaptive wavefront holography technique. In such a configuration, one critical point is the time response tPR of the photorefractive effect, which depends on the photorefractive configuration of the setup as well as the light intensity within the crystal. We have developed an original in situ method that determines this time in measuring the acousto-optic response through a combination of an amplitude modulation of the ultrasound and a frequency de-tuning of the reference beam. We can measure precisely this time but also monitor it according to a theoretical model that we have previously described. This offers the possibility to adapt the response of the setup to the decorrelation time of the medium under study, and also to have a measurement of τc.

The aim of this paper is to show that we can perform acousto-optical signal acquisition of one datapoint (or voxel of a 3D image) in a very short time (2 - 4 ms), in order to overcome the speckle decorrelation effect. To demonstrate this, we have performed experiments in in dynamic scattering media such as liquids. We will show that we can work with pulsed wave ultrasound, to reduce the sound irradiation duration in order to be compatible with safety limits. These are significant steps towards in-vivo experiments.

We explored the possibility of applying very high ultrasound frequencies to achieve very high resolution in ultrasound-modulated optical tomography of soft biological tissues. The ultrasound-modulated coherent light that traversed the scattering biological tissue was detected by a long-cavity and a large etendue confocal Fabry- Perot interferometer. We used various focused ultrasound transducers of 15 MHz, 30 MHz, and 50 MHz to obtain two dimensional images of optically absorbing objects positioned at a few millimeters depth below the surface of both optically scattering phantoms and soft biological tissue samples. This technology is complementary to other imaging technologies, such as confocal microscopy and optical-coherence tomography, and has potential for broad biomedical applications.

This paper demonstrates the performance of CMOS detection methods and the potential for custom made CMOS sensors to be used in ultrasound modulated optical tomography. The sensors developed within our group have been characterized and results of the typical performance levels are shown. A model of ultrasound modulated light propagation from a scattering medium, through a random phase screen and onto the detector is shown. Experimental results in different detection configuration validate the modeling approach. Finally a comparison between a single point detector and an imaging array demonstrate the value of array detection in ultrasound modulated optical tomography.

Ultrasound-modulated optical tomography (UOT) is a new technique that combines laser light and ultrasound to provide images with good optical contrast and good ultrasound resolution in soft biological tissue. We improve the method proposed by Murray et al to obtain UOT images in thick biological tissues with the use of photorefractive crystal based interferometers. It is found that a long ultrasound burst (on the order of a millisecond) can improve the signal-to-noise ratio dramatically. Also with a long ultrasound burst, the response of the acoustic radiation force impulses can be clearly observed in the UOT signal, which will help to acquire images that record both the optical and mechanical properties of biological soft tissues.

We present a novel signal-enhancement method for ultrasound modulated optical tomography that can increase the amount of modulated light, compared to previous methods. By applying intense acoustic bursts, particle displacements of several microns are possible at a very local scale. A CCD camera captured the speckle pattern emerging from a laser-illuminated tissue phantom and differences in laser speckle contrast were measured between ultrasound on and off states. When CCD triggering was synchronized with burst initiation, ultrasound radiation force-induced displacements were detected with our technique, and resulting shear waves were shown to degrade image contrast and spatial resolution. Deleterious effects of shear waves were minimized by delaying CCD camera acquisition several ms until shear waves are adequately attenuated. Our system signal-to-noise ratio (SNR) is sufficiently high to perform UOT scanning without signal averaging. Because of substantially improved SNR compared to previous techniques, the use of intense acoustic burst is a new promising method for ultrasound modulated optical tomography.

The absorption of electromagnetic energy causes thermal expansion and induces acoustic waves in biological tissues. Various tissues present particular characteristics in their absorption spectra. To acquire both photoacoustic and thermoacoustic images with multiple contrasts that reflect the absorption of electromagnetic energy, biological tissues are stimulated using laser and microwave pulses, respectively. Muscles with a rich blood supply strongly absorb green optical radiation, which provides excellent optical contrast. High water content tissues, such as connective tissue and muscle tissue, display high contrast to fatty tissues when imaged using microwave radiation. Most cancerous tissues have higher water and ionic concentrations, two characteristics that are also associated with angiogenesis and hemoglobin oxygen saturation. Therefore, cancer diagnosis based on information from tissue properties over an electromagnetic spectrum from microwave to optical bands can be more accurate than was previously available.

The conventional imaging model in photoacoustic tomography (PAT) assumes the object is acoustically homogeneous. In many practical applications of PAT this assumption may not be valid, resulting in image distortions and artifacts. Knowledge of the acoustic speed distribution of the object can be incorporated into the imaging model to mitigate such artifacts, but this information is not typically available. In this work, we propose a heuristic method for reconstructing both the acoustic speed and electromagnetic absorption distributions of a weakly scattering object. The method exploits a two-fold data redundancy that exists in a complete set of PAT measurement data. A preliminary computer-simulation study is presented to demonstrate the method.

A multimode imaging system, producing conventional ultrasound (US) and acousto-optic (AO) images, has been developed and used to detect optical absorbers buried in excised biological tissue. A commercially-available diagnostic ultrasound imaging transducer is used to both generate B-mode ultrasound images and as a pump for AO imaging. Due to the fact that the steered and focused beam used for US imaging and the US source for pumping the AO image are generated from the same ultrasound probe, the acoustical and optical images are intrinsically co-registered. AO imaging is performed using short ultrasound pulse trains at a frequency of 5 MHz. The phase-modulated light emitted from the interaction region is detected using a photorefractive-crystal based interferometry system. Experimental results have previously been presented for the two-dimensional imaging in tissue-mimicking phantoms. In this paper, we report further experimental developments demonstrating three-dimensional fusion of B-mode ultrasound imaging and pulsed acousto-optic imaging in excised biological tissue (~2 cm thick). By mechanically scanning the ultrasound transducer array in a direction perpendicular to its imaging plane, both the acoustical and optical properties of an embedded target are obtained in three dimensions. The results suggest that AO imaging could be used to supplement conventional B-mode ultrasound imaging with optical contrast, and the multimode imaging system may find application in the detection and diagnosis of cancer.

We proposed a dynamic near infrared/ultrasound dual modal imaging system (dNIRUS) for characterizing suspicious breast lesions non-invasively. dNIRUS measures the change of tissue mechanical and physiologic parameters in response to dynamic stimuli such as cyclic mechanical compression. It integrates near infrared imaging of tumor physiologic properties, ultrasound imaging of tumor deformation/displacement, and real time pressure monitoring under a designated cyclic compression load. The concept of dNIRUS was quantitatively verified on multi-layer tissue simulating phantoms under cyclic compression. A lumped visco-elastic model was used to characterize the phantom mechanical properties and to simulate the tissue deformation. The diffusion equations were solved analytically in Fourier domain with the moving boundary. The theoretical models were verified by a series of bench top tests where a sensor head integrating an ultrasound probe and a near infrared probe was installed on a load frame. A cyclic compression force was applied to a two-layer tissue simulating phantom. Phantom displacement, compressive pressure and diffuse optical reflectance were recorded simultaneously. Deformation of each layer of the phantom was reconstructed from ultrasound images and was consistent with the load frame measurements as well as the theoretical predictions. Diffuse reflectance amplitude showed corresponding fluctuation during compression, while the phase did not change significantly at the oscillation frequency of 0.5Hz. Further work is necessary to develop forward and inverse algorithms for dynamic characterization of suspicious breast lesions.

Three-dimensional photoacoustic imaging of subcutaneous microvasculature in small animals is realized in vivo using photoacoustic microscopy. Previously, degraded lateral resolution in the out-of-focus region made three-dimensional visualization impractical. With the help of a virtual-detector synthetic aperture focusing technique, depth-independent lateral resolution is achieved. Therefore, volumetric imaging of the vessels is realized with a lateral resolution of 45 μm and an axial resolution of 15 μm. Detailed structural information, such as vessel depth and spatial orientation, are revealed with new clarity.

In this study, we introduce a synthetic aperture focusing technique which employs a virtual detector concept, combined with coherence weighting, to extend the depth of focus for an in-vivo photoacoustic microscopy system. This technique treats the transducer's focal point as a virtual point detector of photoacoustic signals, delays adjacent scan lines relative to the virtual detector, and then sums the delayed signals to achieve focusing in the out-of-focus region. In addition, a coherence factor among the delayed signals for each synthesized imaging point is used as a weighting factor to further improve the focusing quality. Images of an Intralipid phantom containing a carbon fiber show how this technique improves the -6 dB lateral resolution from 49-379 μm to 46-53 μm and increases the SNR by 0-29 dB, depending on the distance from the ultrasonic focal point. In vivo experiments show that this technique also provides a clearer tumorassociated angiogenesis in the mouse's scalp. The extended depth of focus for the photoacoustic microscopy system enables 3D reconstruction of the vascular network for the study of tumor angiogenesis.

Functional photoacoustic microscopy is a hybrid imaging technique that detects laser induced photoacoustic waves to image biological tissues in three dimensions. Its imaging depth exceeds the fundamental depth limit of the existing high resolution optical imaging modalities while maintaining a comparable ratio of imaging depth to axial resolution. The amplitude of photoacoutic waves is related to tissue's optical absorption and, therefore, functional imaging can be achieved by acquiring spectroscopic information. We demonstrate here the capabilities of functional photoacoustic microscopy by volumetric imaging a skin melanoma tumor and functional imaging of hemoglobin oxygen saturation in single vessels in vivo.

An optoacoustic detector denotes the detection of acoustic signals by optical devices. Recent advances in fabrication techniques and the availability of high power tunable laser sources have greatly accelerated the development of efficient optoacoustic detectors. The unique advantages of optoacoustic technology are of special interest in applications that require high resolution imaging. For these applications optoacoustic technology enables high frequency transducer arrays with element size on the order of 10 μm. Laser generated ultrasound (photoacoustic effect) has been studied since the early observations of A.G. Bell (1880) of audible sound generated by light absorption . Modern studies have demonstrated the use of the photoacoustic effect to form a versatile imaging modality for medical and biological applications. A short laser pulse illuminates a tissue creating rapid thermal expansion and acoustic emission. Detection of the resulting acoustic field by an array enables the imaging of the tissue optical absorption using ultrasonic imaging methods. We present an integrated imaging system that employs photoacoustic sound generation and 2D optoacoustic reception. The optoacoustic receiver consists of a thin polymer Fabry-Perot etalon. The etalon is an optical resonator of a high quality factor (Q = 750). The relatively low elasticity modulus of the polymer and the high Q-factor of the resonator combine to yield high ultrasound sensitivity. The etalon thickness (10 μm) was optimized for wide bandwidth (typically above 50 MHz). An optical scanning and focusing system is used to create a large aperture and high density 2D ultrasonic receiver array. High resolution 3D images of phantom targets and biological tissue samples were obtained.

In this work, we continue to explore a method we have developed for modeling and correcting the effects of acoustic attenuation in optoacoustic tomography. We have shown previously that in the temporal frequency domain, the attenuated optoacoustic imaging equation is equivalent to an inhomogeneous Helmholtz equation with a complex wave number. We have also developed a numerical method for correcting for these attenuation effects simply by pre-processing of the one-dimensional time signal measured at each transducer location employed in the acquisition. We demonstrate through simulation studies that ignoring ultrasonic attenuation can lead to resolution degradation and distortion in reconstructed images because optoacoustic tomography relies on broadband detection and ultrasonic attenuation is frequency-dependent. We demonstrate that the proposed approach is able to compensate for the attenuation and improve the quality of the images. We also find that the effect of frequency-dependent attenuation remains significant even when narrow-band transducers, be they lowpass or bandpass, are used for detection.

Photoacoustic spectroscopy has the potential to make non-invasive, spatially resolved measurements of absolute chromophore concentrations. This has a wide range of possible applications, for example the mapping of endogenous chromophores such as oxy- (HbO₂) and deoxyhaemoglobin (HHb) or externally administered contrast agents designed to target specific tissues or molecular processes. In this study we used near-infrared photoacoustic spectroscopy to determine the absolute concentrations of HbO₂ and HHb in a tissue phantom. The phantom consisted of three blood filled capillaries (Ø460microns) suspended at depths between 3mm and 9mm in a 2.5% Intralipid solution which also contained 2% blood in order to simulate the background optical attenuation in biological tissue. The blood oxygen saturation (SO₂) of the blood circulating in the capillaries was varied using a membrane oxygenator. At each SO₂ level, nanosecond pulses emitted by an OPO laser system that was tuneable over the wavelength range from 740nm to 1040nm illuminated the phantom. The generated photoacoustic waves were recorded using a single Fabry-Perot ultrasound detector and used to obtain a depth profile of the location of the tubes. The amplitudes of the part of the photoacoustic signal that corresponded to the capillaries and the surface of the Intralipid/blood mixture were plotted as a function of wavelength. The output of a diffusion theory based model of the wavelength dependence of the photoacoustic signal amplitude was then fitted to these spectra. This enabled the quantitative determination of absolute HbO₂ and HHb concentrations in the capillaries and the Intralipid/blood mixture from which the total haemoglobin concentrations and blood SO₂ were calculated. Based on these measurements, the smallest chromophore concentrations that can be detected in biological tissue were estimated.

Optoacoustic Imaging is based on the thermal expansion of tissue caused by a temperature rise due to absorption of short laser pulses. At constant laser fluence, optoacoustic image contrast is proportional to differences in optical absorption and the thermoacoustic efficiency, expressed by the Grueuneisen parameter, Γ. Γ is proportional to the thermal expansion coefficient, the sound velocity squared and the inverse heat capacity at constant pressure. In thermal therapies, these parameters may be modified in the treated area. In this work experiments were performed to examine the influence of these parameters on image contrast. A Laser Optoacoustic Imaging System (LOIS, Fairway Medical Technologies, Houston, Texas) was used to image tissue phantoms comprised of cylindrical Polyvinyl Chloride Plastisol (PVCP) optical absorbing targets imbedded in either gelatin or PVCP as the background medium. Varying concentrations of Black Plastic Color (BPC) and titanium dioxide (TiO₂) were added to targets and background to yield desired tissue relevant optical absorption and effective scattering coefficients, respectively. In thermal therapy experiments, ex-vivo bovine liver was heated with laser fibres (805nm laser at 5 W for 600s) to create regions of tissue coagulation. Lesions formed in the liver tissue were visible using the LOIS system with reasonable correspondence to the actual region of tissue coagulation. In the phantom experiments, contrast could be seen with low optical absorbing targets (μ_a of 0.50cm^-1 down to 0.13cm-1) embedded in a gelatin background (see manuscript for formula). Therefore, the data suggest that small objects (< 5mm) with low absorption coefficients (in the range < 1cm^-1) can be imaged using LOIS. PVCP-targets in gelatin were visible, even with the same optical properties as the gelatin, but different Γ. The enhanced contrast may also be caused by differences in the mechanical properties between the target and the surrounding medium. PVCP-targets imbedded in PVCP produced poorer image contrast than PVCP-targets in gelatin with comparable optical properties. The preliminary investigation in tissue equivalent phantoms indicates that in addition to tissue optical properties, differences in mechanical properties between heated and unheated tissues may be responsible for image contrast. Furthermore, thermal lesions in liver tissue, ex-vivo, can be visualized using an optoacoustic system.

Photoacoustic technology offers great promise for molecular imaging in vivo since it offers significant penetration, and optical contrast with ultrasonic spatial resolution. In this article we examine fundamental technical issues impacting capabilities of photoacoustic tomography for molecular imaging. First we examine how reconstructed photoacoustic tomography images are related to true absorber distributions by studying the modulation transfer function of a circular scanning tomographic system employing a modified filtered backprojection algorithm. We then study factors influencing quantitative estimation by developing a forward model of photoacoustic signal generation, and show conditions for which the system of equations can be inverted. Errors in the estimated optical fluence are shown to be a source of bias in estimates of molecular agent concentration. Finally we discuss noise propagation through the matrix inversion procedure and discuss implications for molecular imaging sensitivity and system design.

Biomedical photoacoustic imaging produces a map of the initial acoustic pressure distribution, or absorbed energy density, in tissue following a short laser pulse. Quantitative photoacoustic imaging (QPI) takes the reconstruction process one stage further to produce a map of the tissue optical coefficients. This has two important advantages. Firstly, it removes the distorting effect of the internal light distribution on image contrast. Secondly, by obtaining images at multiple wavelengths, it enables standard spectroscopic techniques to be used to quantify the concentrations of specific chromophores, for instance, oxy and deoxy haemoglobin for the measurement of blood oxygenation - applying such techniques directly to "conventionally" reconstructed absorbed energy maps is problematic due to the spectroscopic 'spatial crosstalk' effects between different tissue chromophores. As well as naturally-occurring chromophores, dye-labelled molecular markers can be used to tag specific molecules, such as cell surface receptors, enzymes or pharmaceutical agents. In QPI, a diffusion-based finite element model of light transport in scattering media, with δ-Eddington scattering coefficients, is fitted to the absorbed energy distribution to estimate the optical coefficient maps. The approach described here uses a recursive algorithm and converges quickly on the absorption coefficient distribution, when the scattering is known. By adding an area of known absorption, an unknown constant scattering coefficient may also be recovered. With optical coefficient maps estimated in this way, QPI has the potential to be a powerful tool for quantifying the concentration of molecular markers in photoacoustic molecular imaging.

Thermoacoustic tomography is based on the generation of acoustic waves by a bulk illumination of a sample with a short electromagnetic pulse. The thermal expansion of the sample generates acoustic waves. The absorption density inside the sample is reconstructed from the acoustic pressure signals measured outside of the sample. So far signals have been collected with small detectors that approximate point-detectors. In the present study we report on a novel measurement setup applying integrating detectors. With these detectors the pressure is integrated along one or two dimensions. This enables the use of numerically efficient algorithms, such as the inverse Radon transformation, for thermoacoustic tomography. To reconstruct a three-dimensional image, either a two-dimensional integrating detector has to be moved tangentially around a sphere enclosing the object or an array of line-detectors has to be rotated around a single axis. Implementations of line-detectors are demonstrated that are adaptations of a Fabry--Perot interferometer. As a novel approach for the implementation a Fabry-Perot interferometer consisting of a single mode fiber is demonstrated.

Laser optoacoustics is based on the interaction of light with materials producing the thermoelastic effect forming acoustic waves which are characteristic of the medium in which they traverse. This technique is currently on trial for use in biomedical imaging applications and is achieving great success. The work presented here develops an innovative technique for wideband acoustic detection using a fibre optic sensor in a high sensitive multi-coil Mach-Zehnder interferometric configuration. A comparative analysis is performed using both electrical and optical detection techniques on gels which are commonly used to mimic human soft tissue. Indications of future work in this area will be presented throughout this paper.

An important capability of photoacoustic methods is the ability to make spatially resolved spectroscopic measurements of blood oxygenation by imaging at multiple NIR excitation wavelengths, usually sourced from Q-switched Nd:YAG pumped OPO based systems. These excitation sources are usually bulky, expensive and with limited scope for varying the pulse repetition rate and pulse width. An alternative would be to use pulsed laser diodes as excitation sources. To evaluate the possibility of developing a multiwavelength excitation system composed of three wavelengths 810, 850 and 905nm, a single wavelength (905nm) system was built. To achieve a sufficient SNR, four high peak power pulsed laser diodes were combined. The design of the laser drivers provided variable pulse duration (65-500ns) and repetition rates of up to 2.5KHz. This allowed the pulse duration to be optimised in order to (a) maximise the energy in the generated photoacoustic signal and (b) reduce the effects of frequency dependent acoustic attenuation of tissue on the propagating acoustic wave by avoiding the generation of excessively high frequency components. It also enabled the high repetition rate of laser diodes to be exploited in order to average a large number of acquisitions over a short time period to increase SNR. Preliminary measurements of SNR were made in phantoms using the single wavelength excitation system, to demonstrate the practical biomedical utility of the system. A tissue phantom consisting of two capillaries (null set 460μm)filled with an absorbing dye of similar optical properties to blood (μa ≈ 1mm-1), immersed at different depths in a 1% solution of intralipid (μs ≈ 1mm-1) was used. To further demonstrate the capability of the system it was combined with a cylindrical scanning system to image a strongly absorbing cylinder immersed to a depth of 1cm in 1% solution of intralipid (μs ≈ 1mm-1). This study demonstrated the potential for using laser diodes as excitation sources for pulsed photoacoustic spectroscopic biomedical applications.

Real-time photoacoustic imaging requires ultrasonic array receivers and parallel data acquisition systems for the simultaneous detection of weak photoacoustic signals. In this paper, we introduce a newly completed ultrasonic receiving array system and report preliminary results of our measured point spread function. The system employs a curved ultrasonic phased array consisting of 128-elements, which span a quarter of a complete circle. The center frequency of the array is 5 MHz and the bandwidth is greater than 60%. In order to maximize the signal-to-noise ratio for photoacoustic signal detection, we utilized special designs for the analog front-end electronics. First, the 128 transducer-element signals were routed out using a 50-Ohm impedance matching PCB board to sustain signal integrity. We also utilize 128 low-noise pre-amplifiers, connected directly to the ultrasonic transducer, to amplify the weak photoacoustic signals before they were multiplexed to a variable-gain multi-stage amplifier chain. All front-end circuits were placed close to the transducer array to minimize signal lose due to cables and therefore improve the signal-to-noise ratio. Sixteen analog-to-digital converters were used to sample signals at a rate of 40 mega-samples per second with a resolution of 10-bits per sample. This allows us to perform a complete electronic scan of all 128 elements using just eight laser pulses.

As a promising and noninvasive method for biomedical imaging, the advantage of ultrasound-modulated optical tomography is its combination of optical contrast and ultrasonic resolution. In order to reconstruct the tissue imaging effectively and reliably, the propagation of the diffused light modulated by ultrasound in the tissue should be understood extensively. In the process, the propagation of the diffused light modulated by ultrasound play an important role in particularly because it reflects some information about the optical and ultrasonic properties of tissue. Based on the diffuse theory, the relations to the modulation depth contributed by the tissue thickness, optical properties, irradiation configuration, etc. are figured out at an extended condition.

We develop a temporal correlation transfer equation (CTE) and a temporal correlation diffusion equation (CDE) for ultrasound-modulated multiply scattered light, which can be used to calculate the ultrasound-modulated optical intensity in an optically scattering medium with a nonuniform ultrasound field and a heterogeneous distribution of optical parameters. We present an analytical solution based on the CDE for scattering of the temporal autocorrelation function from a cylinder of ultrasound in an optically scattering slab. The CDE is valid for moderate ultrasound pressures on a scale comparable with the optical transport mean free path, which must be greater than the ultrasound wavelength and smaller than or comparable to the sizes of both ultrasonic and optical inhomogeneities. These equations should be applicable to a wide spectrum of conditions for ultrasound-modulated optical tomography of soft biological tissues.

For diagnostic or therapeutic technologies using femtosecond laser-induced optical breakdown (LIOB) in turbid biological tissues, pulses of sufficient fluence must be delivered to the site of interest. As light attenuates and diffuses rapidly due to wavelength-dependent absorption and scattering, it is important to develop penetration optimization schemes. In this study, we use a high frequency (50MHz) ultrasonic technique to investigate the precision and penetration depth limitations of infrared femtosecond laser-induced photodisruption in excised pig skin. Optical parameters varied include laser fluence (energy density in J/cm²) and focusing numerical aperture. Our ultrasonic method uses sensitive detection of laser-induced bubbles to measure breakdown extent. Using a geometrically focused Nd:Glass laser (1053 nm, 800 fs) source, we show that acoustically detectable bubbles can be produced as deep as 900 um into excised porcine skin. As penetration exceeds several hundred microns, however, multiple bubbles stacked at different depths can be produced with a single laser excitation. Secondary bubble creation is more likely at supra-threshold fluences or with low NA (≤ 0.4) focusing, where optical self-focusing may occur near threshold fluences. However, as the numerical aperture is increased (> 0.4) for deeper focusing, aberrations can severely distort the beam, increasing the perceived LIOB-threshold with maximal penetrations of less than 500um. Using an index matching fluid (i.e. aqueous glycerol solutions) to help reduce scattering, we are able to improve penetration. However, multiple breakdown sites and the corresponding reduction in precision is still likely in skin even with glycerol treatment.

Photoacoustic tomography (PAT), which reconstructs the distribution of light-energy deposition in the tissue, is becoming an increasingly powerful imaging tool. For example, the technique has potential applications in the earlystage breast cancer sensing and the functional imaging of small animal brain. In PAT, the system signal-to-noise ratio (SNR) and the number of measurement positions (NMP) are the two main factors which affect the quality of final reconstructed image. Undoubtedly, the increase of SNR or the numbers of measurement positions will improves image quality. However, one has to pay a cost on the imaging speed for such improvement of image quality. In this paper, the factors influencing the imaging performance of PAT are investigated by means of computer simulations. The result shows that the increase of the number of averaging times in acquiring of acoustic signal and the number of measurement positions are efficient ways to improve image quality. However, there exists a turning point at which the further increase of NMP and averaging times makes the improvement of imaging performance negligible. Thus a tradeoff should be made to achieve the optimal reconstructed image according to the system SNR.

In this paper, we develop a temporal correlation transfer equation (CTE) for ultrasound-modulated multiply scattered light. The equation can be used to obtain the temporal frequency spectrum of the optical intensity produced by a nonuniform ultrasound field in optically scattering media. Derivation of the CTE is based on the ladder diagram approximation of the Bethe-Salpeter equation. We expect the CTE to be applicable to a wide spectrum of conditions in the ultrasound-modulated optical tomography of soft biological tissues.

Using a TEA CO₂ laser for explosive surface boiling of bulk water, oscillatory acoustic transients from steam bubbles were recorded using a contact photoacoustic technique. Multiple well-resolved, high-amplitude multi-MHz spectral features representing high-order combination acoustic oscillations of steam bubbles were revealed in spectra obtained by means of numerical Fast Fourier Transformation of these transients. A potential parametric generation mechanism for these high-order combination oscillation modes of steam bubbles is discussed.

The application of the frequency domain and steady-state diffusive optical spectroscopy (DOS) and steady-state near infrared spectroscopy (NIRS) to diagnosis of the human lung injury challenges many elements of these techniques. These include the DOS/NIRS instrument performance and accurate models of light transport in heterogeneous thorax tissue. The thorax tissue not only consists of different media (e.g. chest wall with ribs, lungs) but its optical properties also vary with time due to respiration and changes in thorax geometry with contusion (e.g. pneumothorax or hemothorax). This paper presents a finite volume solver developed to model photon migration in the diffusion approximation in heterogeneous complex 3D tissues. The code applies boundary conditions that account for Fresnel reflections. We propose an effective diffusion coefficient for the void volumes (pneumothorax) based on the assumption of the Lambertian diffusion of photons entering the pleural cavity and accounting for the local pleural cavity thickness. The code has been validated using the MCML Monte Carlo code as a benchmark. The code environment enables a semi-automatic preparation of 3D computational geometry from medical images and its rapid automatic meshing. We present the application of the code to analysis/optimization of the hybrid DOS/NIRS/ultrasound technique in which ultrasound provides data on the localization of thorax tissue boundaries. The code effectiveness (3D complex case computation takes 1 second) enables its use to quantitatively relate detected light signal to absorption and reduced scattering coefficients that are indicators of the pulmonary physiologic state (hemoglobin concentration and oxygenation).