This PDF file contains the front matter associated with SPIE Proceedings Volume 6437, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

The Twente Photoacoustic Mammoscope (PAM) is based on generating laser-induced ultrasound from absorbing structures in the breast. The heart of the instrument is a flat PVDF based detector matrix comprising 590 active elements. The exciting source is an Nd:YAG laser operating at 1064 nm with 5 ns pulses. The instrument is built around a hospital bed. A study protocol was designed to explore the feasibility of using the photoacoustic technique as embodied in PAM to detect cancer in the breasts of patients with suspect/symptomatic breasts. The protocol was approved by a Medical Ethics testing committee and the instrument approved for laser and electrical safety. The protocol was executed at the Medisch Spectrum Twente by using the mammoscope to obtain photoacoustic region-of-interest (ROI) images of the suspect/symptomatic breasts. We report on one case and compare the photoacoustic images obtained with x-ray mammograms and ultrasound images.

We have designed, fabricated and tested a new laser optoacoustic imaging system (LOIS-64/16) for quantitative optoacoustic tomography of breast cancer. The system was designed to create a single slice of an optoacoustic image of the breast with 64 ultrawide band acoustic transducers. Other 16 transducers on the back of the acoustic probe were used to reconstruct the light distribution inside the breast. The system resolution was at least 0.5 mm for high-aspect-ratio objects. Maximum system sensitivity was 4.8 mV/Pa and the RMS noise of 3.1 mV, which allowed imaging of small (less than 1 cm) tumors at depths over 3 cm. The directivity of the optoacoustic transducers used in LOIS-64/16 assured that the signal detection was better than 70% of the maximum for about 75% of the imaging slice and reduced quickly for signals coming from out of the imaging slice. Implemented signal processing allowed significant reduction of the low-frequency acoustic noise and localizing the small OA signals. The system was able to differentiate phantoms mimicking tumors and malformations visualized in clinics based on the contrast and morphology of their images obtained at 1064 nm and 757 nm.

Continuous monitoring of cerebral blood oxygenation is critically important for successful treatment of patients with severe traumatic brain injury. At present, the techniques for monitoring blood oxygenation are invasive. Recently we proposed noninvasive monitoring of cerebral blood oxygenation by using optoacoustic probing of blood circulating in the internal jugular vein (IJV). A major source of error in the optoacoustic measurement with a single-element optoacoustic probe is the spatial misalignment between the probe and the IJV. We built a LabView®-based scanning system that automatically moves our optoacoustic probe across the IJV while continuously taking measurements. Automatic signal processing determines the signal with the best probe-vessel alignment which then is used for further processing. The scanning system was tested in phantoms using solutions with different absorption coefficients and with blood with various levels of blood oxygenation. Amplitudes and profiles of the optoacoustic signals recorded from the phantoms closely followed the blood oxygenation changes in accordance with blood optical properties. These data indicate that the scanning system is capable of improving the accuracy of non-invasive monitoring of blood oxygenation by minimizing errors associated with lateral misalignment of the probe with respect to blood vessels.

Continuous monitoring of cerebral blood oxygenation is critically important for treatment of patients with life-threatening conditions like severe brain injury or during cardiac surgery. We designed and built a novel multiwavelength optoacoustic system for noninvasive, continuous, and accurate monitoring of cerebral blood oxygenation. We use an Optical Parametric Oscillator as a light source. We successfully tested the system in vitro as well as in vivo in large animals (sheep) through thick tissues overlying blood vessels which drain venous blood out of the brain (e.g., superior sagittal sinus or jugular vein). Here we present the results of clinical tests of the system for continuous noninvasive cerebral blood oxygenation monitoring in the internal jugular vein of healthy volunteers. We applied our custom-built optoacoustic probe (which incorporated a wide-band acoustic transducer and an optical fiber) to the neck area overlying the internal jugular vein. We performed measurements with volunteers at 18 wavelengths in the near-infrared spectral range. Despite a thick layer of overlying connective tissue and low energy used in the experiments, we recorded signals with high signal-to-noise ratios for all volunteers. We found that the temporal (independent of signal amplitude) parameters of recorded profiles for different levels of blood oxygenation correlated well with the spectrum of effective attenuation coefficients of blood.

This research reports progress in our earlier investigation of detecting specific drug diffusion into eye tissue using photoacoustic spectroscopy (PAS). A key improvement to the technique is using short pulse tunable laser source to stimulate the photoacoustic effect in tissue. An optical parametric oscillator (OPO) laser system was used as a pumping source to generate ultrasonic photoacoustic signals and employed to scan through different wavelengths with 0.1nm wavelength resolution to determine spectra of different drug solutions in an ocular phantom. The short pulse duration (5-10ns) of the OPO laser has significantly increased the photoacoustic efficiency conversion, and the ability to tune its output from 210nm to1800nm has provided a wide selection range that is useful for optimizing spectroscopic studies. PAS spectra of different solutions of molecules, such as Trypan Blue, Rose Bengal, Indocyanine Green (ICG), and Amphotericin B (AB), at concentrations as low as 1 &mgr;g/ml, were constructed and compared to their actual optical absorption spectra. Ultrasonic hydrophone and photothermal deflection technique (PhDT), a noncontact optical method, were both used to record the photoacoustic signals, and compared in terms of sensitivity and applicability to record signals from the ocular tissue-bearing phantom. The results show good agreement between the optical and photoacoustic spectra, which supports moving to an in vivo application of recording the PAS responses from the eye. Future work will be directed at adapting this method for in vivo measurements, as well as improve the data acquisition system for faster PAS signal analysis.

In many clinical and research applications including cancer diagnosis, tumor response to therapy, reconstructive surgery, monitoring of transplanted tissues and organs, and quantitative evaluation of angiogenesis, sequential and quantitative assessment of microcirculation in tissue is required. In this paper we present an imaging technique capable of spatial and temporal measurements of blood perfusion through microcirculation. To demonstrate the developed imaging technique, studies were conducted using phantoms with modeled small blood vessels of various diameters positioned at different depths. A change in the magnitude of the photoacoustic signal was observed during vessel constriction and subsequent displacement of optically absorbing liquid present in the vessels. The results of the study suggest that photoacoustic, ultrasound and strain imaging could be used to sequentially monitor and qualitatively assess blood perfusion through microcirculation.

The prognostic value of exposing malignant melanoma poses an urgent need for an efficient, accurate screening method for metastatic cells. We propose a system for the detection of metastatic tumor cells based upon the thermo-elastic properties of melanoma. The method employs photoacoustic excitation coupled with a detection system capable of exposing cells within the circulatory system in vitro. Initial trials provided a threshold on the order of ten individual cells. Results imply the potential to assay simple blood draws for the presence of cancerous melanoma providing an unprecedented method for routine cancer screening.

Early and accurate determination of burn depth is crucial to monitoring the burn wound and aiding in the precise excision of necrotic tissue. A simplified model of a partial-thickness burn wound can be described as a layer of necrotic dermal tissue, containing thermally coagulated blood, atop a layer of inflamed dermal tissue characterized by the presence of viable (non-coagulated) blood. Using photoacoustic methods it is possible to discriminate between coagulated and non-coagulated blood and, therefore, discriminate between the two layer types. However, the effectiveness of such a photoacoustic method is limited by the thickness of the upper coagulated layer. Sufficient laser energy must be deposited into both layers to create detectable and characteristic signals from those layers. To determine the maximum thickness of the coagulated layer, at which the underlying non-coagulated layer was still able to generate detectable acoustic waves, we performed a Monte Carlo simulation on a human burn wound model with varying depths of the coagulated layer. The depths of the coagulated layer ranged from 100 to 1,100 &mgr;m, in 100 &mgr;m increments. Our analysis concluded that burn depth measurements can be achieved up to a burn depth of 900 &mgr;m with an incident radiant exposure of 0.255 J/cm² at 543nm and 0.1275 J/cm² at 633 nm.

Existing noninvasive imaging modalities fail to provide high-resolution depth-resolved imaging of skin burns in large depths. Hence, the measurement of burn depth, especially for partial-thickness burn, remains inaccurate. We used photoacoustic microscopy to measure the depth of acute thermal burns by imaging the microvasculature damage. In this work, partial-thickness burns were induced in vivo on pig skin. Limited by the flexibility of the photoacoustic scanning system, photoacoustic images of the burns were acquired after skin excision. Experimental results show that burn depth increases with longer heating duration. The maximum imaged burn depth measures ∼1.7 mm with a depth resolution of 15 &mgr;m.

In this study, high resolution reflection-mode (backward-mode) photoacoustic microscopy (PAM) is used to noninvasively image progressive extravasation and accumulation of nanoshells within a solid tumor in vivo. This study takes advantage of the strong near-infrared absorption of nanoshells, a novel type of optically tunable gold nanoparticles that tend to extravasate from leaky tumor vasculatures (i.e., passive targeting) via the "enhanced permeability and retention" effect due to their nanoscale size. Tumors were grown in immunocompetent BALB/c mice by subcutaneous inoculation of CT26.wt murine colon carcinoma cells. PEGylated nanoshells with a peak optical absorption at ~800 nm were intravenously administered. Pre-scans prior to nanoshell injection were taken using a 584-nm laser source to highlight blood content and an 800-nm laser source to mark the background limit for nanoshell accumulation. After injection, the three-dimensional nanoshell distribution inside the tumor was monitored by PAM for 7 hours. Experimental results show that nanoshell accumulation is heterogeneous in tumors: more concentrated within the tumor cortex and largely absent from the tumor core. This correlates with others' observation that drug delivery within tumor cores is ineffective because of both high interstitial pressure and tendency to necrosis of tumor cores. Since nanoshells have been recently applied to thermal therapy for subcutaneous tumors, we anticipate that PAM will be important to this therapeutic technique.

Laser Activated Nano-Thermolysis was recently proposed for selective damage of individual target (cancer) cells by pulsed laser induced microbubbles around superheated clusters of optically absorbing nanoparticles (NP). One of the clinical applications of this technology is the elimination of residual tumor cells from human blood and bone marrow. Clinical standards for the safety and efficacy of such procedure require the development and verification of highly selective and controllable mechanisms of cell killing. Our previous experiments showed that laser-induced microbubble is the main damaging factor in the case cell irradiation by short laser pulses above the threshold. Our current aim was to study the cell damage mechanisms and analyze selectivity and efficacy of cell damage as a function of NP parameters, NP-cell interaction conditions, and conditions of bubble generation around NP and NP clusters in cells. Generation of laser-induced bubbles around gold NP with diameters 10-250 nm was studied in Acute Myeloblast Leukemia (AML) cultures, normal stem and model K562 human cells. Short laser pulses (10 ns, 532 nm) were applied to those cells in vitro and the processes in cells were investigated with photothermal, fluorescent and atomic force microscopies and also with fluorescence flow cytometry. We have found that the best selectivity of cell damage is achieved by (1) forming large clusters of optically absorbing NP in target cells and (2) irradiating the cells with single laser pulses with the lowest fluence that can generate microbubble only around large clusters but not around single NP. Laser microbubbles with the lifetime from 20 ns to 2000 ns generated in individual cells caused damage and lysis of the cellular membrane and consequently cell death. Laser microbubbles did not damage normal cells around the damaged target (tumor) cell. Laser irradiation with equal fluence did not cause any damage of cells without accumulated NP clusters.

We investigate the feasibility of using iron oxide nanoparticles as a contrast agent for radiofrequency (RF) induced thermoacoustic tomography. Aqueous colloids of iron oxide (Fe3O4) nanoparticles have been synthesized and characterized. The synthesis method yielded citrate-stabilized, spherical particles with a diameter of approximately 10 nm. The complex permittivity of the colloids was measured with a coaxial probe and vector network analyzer, and the microwave absorption properties were calculated by using a relationship between the complex permittivity and absorption coefficients. Using our pulsed thermoacoustic imaging system at 3 GHz, the time-resolved thermoacoustic responses of those colloids were measured and compared to that of deionized water. Finally, two-dimensional thermoacoustic images were acquired from iron oxide colloids in a tissue phantom. The iron oxide colloids produced an enhancement in RF absorption of up to three times that of deionized water at 3 GHz. The enhancement increased rapidly with decreasing frequency of the RF excitation source. A corresponding increase in time-resolved thermoacoustic signal of more than two times was demonstrated. Our results indicate that iron oxide nanoparticles have the potential to produce enhanced thermoacoustic signals and to provide molecular imaging with functionalized contrast agents for thermoacoustic tomography.

Recent advances in fabrication techniques have accelerated development of optical generation and detection of ultrasound, a promising technology to construct high-frequency arrays for high resolution ultrasound imaging. A two-dimensional (2-D) gold nanostructure has been fabricated to optically generate high frequency ultrasound. The structure consists of 2-D arrangements of gold nanoparticles, sandwiched between a transparent substrate and a 4.5 &mgr;m thick PDMS layer. A pulsed laser beam is focused onto the optically absorbing gold nanostructure, and consequently, a localized volume is heated, and thermal expansion launches an acoustic wave into the overlying layer. The high optical extinction ratio of the gold nanostructure provides a convenient method to construct an integrated transmit/receive optoacoustic array. A thin polymer Fabry-Perot etalon is used for optoacoustic detection. The etalon is an active optical resonator, where the relatively low elasticity of the polymer and the high quality factor of the resonator combine to provide high ultrasound sensitivity. An integrated device combining the gold nanostructure and the etalon has been fabricated. Preliminary results demonstrate its promise as an all-optical ultrasound transducer.

The development of gold nanoparticles for molecular optoacoustic imaging is a very promising area of research and development. Enhancement of optoacoustic imaging for molecular detection of tumors requires the engineering of nanoparticles with geometrical and molecular features that can enhance selective targeting of malignant cells while optimizing the sensitivity of optoacoustic detection. In this article, cylindrical gold nanoparticles (i.e. gold nanorods) were fabricated with a plasmon resonance frequency in the near infra-red region of the spectrum, where deep irradiation of tissue is possible using an Alexandrite laser. Gold nanorods (Au-NRs) were functionalized by covalent attachment of Poly(ethylene glycol) to enhance their biocompatibility. These particles were further functionalized with the aim of targeting breast cancer cells using monoclonal antibodies that binds to Her2/neu receptors, which are over expressed on the surface of breast cancer cells. A custom Laser Optoacoustic Imaging System (LOIS) was designed and employed to image nanoparticle-targeted cancer cells in a phantom and PEGylated Au-NRs that were injected subcutaneously into a nude mouse. The results of our experiments show that functionalized Au-NRs with a plasmon resonance frequency at near infra-red region of the spectrum can be detected and imaged in vivo using laser optoacoustic imaging system.

We have studied the potential of gold nanorods to target cancer cells and provide contrast for photoacoustic imaging. The elongated "rod" shape of these nanoparticles provides a mechanism to tune their plasmon peak absorption wavelength. The absorption peak is shifted to longer wavelengths by increasing the aspect ratio of the rods. Particles 15 nm in diameter and 45 nm long were prepared using a seed mediated growth method. Their plasmon absorption peak was designed to be at 800 nm for increased penetration depth into biological tissue. They were conjugated with a specific antibody to target prostate cancer cells. We have applied photoacoustics to image a prostate cell culture targeted by conjugated gold particles. Images confirm the efficiency of conjugated particle binding to the targeted cell membranes. Photoacoustic detection of a single cell layer is demonstrated. To evaluate the applicability of the technique to clinical prostate cancer detection, we have imaged phantom objects mimicking a real tissue with small (2 mm size) inclusions of nanoparticle gel solution. Our photoacoustic imaging setup is based on a modified commercial ultrasonic scanner which makes it attractive for fast implementation in cancer diagnosis in clinical application. In addition, the setup allows for dual mode operation where a photoacoustic image is superimposed on a conventional B-mode ultrasound image. Dual mode operation is demonstrated by imaging a mouse with gold nanorod gel solution implanted in its hind limb.

In photoacoustic imaging, the difference of optical absorption determines the contrast between two media. In this study, a contrast enhancement method based on choosing various frequency subbands for photoacoustic imaging is proposed. Typically, a laser beam irradiates a medium of interest, and the optical energy decays with different rates as the optical absorption changes. The decay profiles result in acoustic pressure waveforms to propagate with various frequency components, which cause the acoustic frequency variation. The frequency band for a medium with high absorption is found significantly up-shifted from that for a medium with one order lower absorption. Accordingly, besides the amplitude difference due to the absorption, the contrast between two media with varied absorption can be further enhanced by choosing a high frequency band of the receiving signals for imaging. This method was demonstrated by simulations and experiments. The simulation, which is based on the Beer-Lambert law, verified the appearance of frequency variation due to the disparity of absorption coefficients. The experiments were performed by using agar phantom with various concentrations of graphite to create optical absorptions with more than tens times difference. For absorbers with absorption coefficients from 2.5 cm^-1 to 100 cm^-1, the peak frequencies and the -6 dB bandwidths of the PA signals increase from 1.17 to 3.83 MHz and from 2.17 to 7.58 MHz, respectively. The subband image at band 7-14 MHz shows 13-25 dB intensity difference between two agars with respective absorption of 41.75 cm^-1 and 5.01 cm^-1, while the difference is 9-15 dB at band 0-7 MHz, thus demonstrating that the contrast can be enhanced between two media using the selective subband imaging. The potential of improving the contrast between biological tissues and contrast agent with a significant higher absorption is revealed.

Photoacoustic imaging provides optical contrast with good penetration and high spatial resolution, making it an attractive tool for noninvasive neural applications. We chose a commercial dye (NK2761) commonly used for optical imaging of membrane potential to enhance photoacoustic images of the live lobster nerve cord. The abdominal segment of the nerve cord was excised, stained and positioned in a custom neural recording system, enabling electrical stimulation and recording of compound action potentials. Photoacoustic and pulse echo images were also collected using a commercial ultrasound scanner and a 10-MHz linear probe. A wavelength-tunable pulsed laser source (Surelite^TM, 5 ns, ~15 mJ, 30 mJ/cm²) operating at 20 Hz produced photoacoustic waves. Longitudinal photoacoustic scans of a 25-mm segment of the excised nerve cord, including ganglionic and axonal processes, were collected and displayed every 7 seconds. Without the contrast agent, an average of 10 scans produced a peak photoacoustic signal 6 dB over background noise. An additional 29 dB was obtained after the nerve was submerged in the dye for 20 minutes. The gain decreased to 23 dB and 14 dB at 810 nm and 910 nm, respectively - consistent with the dye's optical absorbance measured using a portable spectrometer. The contrast-enhanced photoacoustic signal had a broad spectrum peaking at 4 MHz, and, after high pass filtering, images approached 200-&mgr;m spatial resolution. The hybrid imaging system, which provided several hours of electrical stimulation and recording, represents a robust testbed to develop novel photoacoustic contrast for neural applications.

We are working to develop a molecular imaging agent that will allow for in vivo imaging of proteases by use of optoacoustic tomography. Proteases are protein-cleaving proteins known to be overactive in a number of pathologies, including cancers and vascular disease. Protease-sensitive "smart probes" have previously been developed in the context of pure optical imaging. These involve pairs of mutually quenching fluorophores attached to a backbone by protease-cleavable peptide side chains; cleaving of the side chains liberates the fluorophores and leads to increase in fluorescence. Optoacoustic imaging is sensitive not to fluorescence but to optical absorption and so a smart imaging probe for protease imaging would need to shift its absorption peak upon cleavage. Naturally, the absorption peaks of the cleaved (and, ideally, uncleaved) molecules should be in the near infrared for maximum tissue penetration. We have designed a molecule that should achieve these specifications. It comprises two active sites, derivatives of natural photosynthetic bacteriochlorophylls that absorb in the near IR, conjugated to a lysine backbone by peptide spacers specific to the protease being imaged. When these bacteriochlorophylls dimerize and stack in the uncleaved molecule, their absorption peak shifts about 20-30 nm. When they are cleaved from the molecule the absorption peak shifts back to that of bacteriochlorophyll monomers. We have performed a preliminary synthesis of the molecule and confirmed by use of a spectrometer that the pairing of the bacteriochlorophylls leads to the expected absorption shift.

Protein nanospheres capable of frequency controlled oscillation in response to laser stimulation are presented as contrast agents for photoacoustic imaging. Incident laser energy absorbed by dye-labeled protein nanospheres causes thermoelastically generated sound production. Plotted A-line graphs reveal a distinctive morphology and greater than 2 orders of magnitude increase in signal amplitude subsequent to converting labeled proteins into nanospheres. Evidence of nonlinearity and enhancement of ultrasound backscatter indicate a potential use in contrast-enhanced harmonic imaging. Photoacoustic and ultrasound imaging of protein nanospheres in phantom vessels show enhanced contrast at low concentration and clear delineation of the phantom vessel wall.

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 can be used to determine the signal from arbitrary objects. 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.

A method for phoatoacoustic tomography (PAT) is presented that uses line integrals over the acoustic wave field from a photoacoustic source for the reconstruction of a three-dimensional image. The line integrals are acquired with an optical line sensor based on a Mach-Zehnder interferometer. Image reconstruction is a two-step process. In the first step data from a scan of the line outside the object is used to reconstruct a linear projection of the source distribution. In the second step the inverse linear Radon transform is applied to multiple projections taken at different directions. This study focuses on the optimization of the first step using a frequency-domain algorithm and input data from a scan of the line detector in an L-shaped curve around the object. Simulations and a phantom experiment demonstrate that equally high resolution in all directions in the projection plane can be achieved with this method.

We present a camera-based optical detection scheme designed to detect the transient motion created by the acoustic radiation force in elastic media. An optically diffusive tissue mimicking phantom was illuminated with coherent laser light, and a high speed camera (2 kHz frame rate) was used to acquire and cross-correlate consecutive speckle patterns. Time-resolved transient decorrelations of the optical speckle were measured as the results of localised motion induced in the medium by the radiation force and subsequent propagating shear waves. As opposed to classical acousto-optic techniques which are sensitive to vibrations induced by compressional waves at ultrasonic frequencies, the proposed technique is sensitive only to the low frequency transient motion induced in the medium by the radiation force. It therefore provides a way to assess both optical and shear mechanical properties.

We have built a photoacoustic microscope (PAM) using an amplitude-modulated continuous-wave (CW) laser source, which is an inexpensive 120-mW laser diode. By using a bowl-shaped piezoelectric transducer, whose numerical aperture is 0.85 and resonance frequency is 2.45 MHz, we have experimentally demonstrated a lateral resolution of 600 &mgr;m, an axial resolution of 700 &mgr;m, and a signal-to-noise ratio (SNR) as high as 43 dB. Although the SNR in the CW PAM system is an order of magnitude worse than that of the pulsed-laser-based PAM system, CW PAM shows potential for biomedical applications as it uses durable and inexpensive semiconductor lasers, which will significantly reduce operation costs of the PAM systems.

We proposed and have been developing real-time, noninvasive monitoring of blood oxygenation, total hemoglobin concentration, and thermotherapy including hyperthermia, coagulation, and cryotherapy. In this paper we propose to use the optoacoustic technique for monitoring of nanoparticle-mediated photothermal therapy (NPT) of tumors. NPT is based on heating exogenous strongly-absorbing nanoparticles selectively delivered in tumors. Real-time monitoring of NPT is necessary for precise tumor therapy with minimal damage to normal tissues. In this study we injected PEGylated and non-PEGylated carbon nanoparticles in nude mice bearing human tumors (5-15 mm) and irradiated the tumors for 10 minutes with nanosecond Nd:YAG laser pulses which produced both thermal damage to the tumors and optoacoustic signals for monitoring NPT in real time. Irradiation of tumors was performed during or after (3 or 24 hours) nanoparticle injection. Amplitude and temporal parameters of optoacoustic signals (measured with a custom-made wide-band optoacoustic probe) correlated well with nanoparticle injection, temperature rise in tumors, and tumor coagulation. Substantial thermal damage in large areas of the tumors was produced when optimal irradiation parameters were used. Monte Carlo modeling of light distribution in tumors and optoacoustic theory were applied to study kinetics of nanoparticle concentration in the tumors. Our results demonstrated that the optoacoustic technique can be used for real-time monitoring of NTP and provide precise tumor therapy with minimal damage to normal tissues.

The effects of an inappropriately chosen speed-of-sound in photoacoustic imaging reconstructions are to cause blurring of images and impairment of contrast. Here we outline a new methodology to measure the speed-of-sound in a photoacoustic imager with little or no additional cost and without the need to perform extra measurements. The method uses a strong absorber of light which is placed in the path of the light illuminating the sample. This acts as a source of ultrasound whose interaction with the sample can be measured at the far-end of the sample using the same ultrasound detector used for photoacoustics. This yields time-of-arrival measurements of the ultrasound transient at the multi-element detector. Such measurements are made at various angles around the sample in a computerized tomography approach. Reconstruction of the time-of-arrival or speed-of-sound tomogram of the object can be made by implementing a fan-beam projection reconstruction algorithm. We present the concept and validate the method on a speed-of-sound phantom.

A 3D photoacoustic imaging instrument for characterising small animal models of human disease processes has been developed. The system comprises an OPO excitation source and a backward-mode planar ultrasound imaging head based upon a Fabry Perot polymer film sensing interferometer (FPI). The mirrors of the latter are transparent between 590 - 1200nm but highly reflective between 1500-1600nm. This enables nanosecond excitation laser pulses in the former wavelength range, where biological tissues are relatively transparent, to be transmitted through the sensor head into the tissue. The resulting photoacoustic signals arrive at the sensor where they modulate the optical thickness of the FPI and therefore its reflectivity. By scanning a CW focused interrogating laser beam at 1550nm across the surface of the sensor, the spatial-temporal distribution of the photoacoustic signals can therefore be mapped in 2D enabling a 3D photoacoustic image to be reconstructed. To demonstrate the application of the system to imaging small animals such as mice, 3D images of the vascular anatomy of the mouse brain and the microvasculature in the skin around the abdomen were obtained non invasively. It is considered that this system provides a practical alternative to photoacoustic scanners based upon piezoelectric detectors for high resolution non invasive small animal imaging.

Photoacoustic imaging (PAI) has been shown to have higher spatial resolution and optical contrast than conventional ultrasound imaging due to the high contrast absorption of blood on green and near infrared (NIR) light. We conducted in-vivo PAI of tumor blood vessels - monitoring 3-24 days development of tumor after the implant of the tumor cells on the back of nude mice. We observed that the PAI signals increased with the blood vessel developments and the growth of the tumor. This study shows that PAI is suitable for monitoring of the development of blood vessels near the superficial tumors. These results demonstrate the usefulness of the PAI for tumor angiogenesis monitoring in small animals.

We have developed and tested a photoacoustic imaging system based on a 128 element curved-phased ultrasonic array, which spans a quarter of a complete circle with a radius of curvature equal to 25mm. The center frequency of the array is 5 MHz with 60% bandwidth. The physical dimensions of the elements are 10x0.3mm (elevation x azimuth) with an elevation focus of 19mm. Earlier we reported acoustic measurements of the axial and lateral resolutions of the system that were limited by the impulse response of the narrowband source used in the test. In this paper we discuss photoacoustic characterization of the system including resolution and sensitivity. The array forms the building block for a 512-element ring designed for complete tomographic imaging of small animals. Imaging results of phantoms will be compared with simulations.

Exact photoacoustic tomography requires scanning over a 4&pgr; solid angle in 3D. The ultrasound detection window, however, is often limited, which makes a full scan impossible. For example, when a boundary lies closely to an object, the scanning region can cover only less than 4&pgr; in 3D. Because of incomplete information, the resolution, SNR, and fidelity of the resulting image deteriorate. Boundaries, however, can be used to our advantage; we proposed post-processing algorithms in image reconstruction to make partially scanned data complete. Here, we show the efficacy of the post-processing algorithms with both numerical and experimental results. Indeed, the algorithms can improve the resolution, SNR, and fidelity.

Severe energy dissipation and waveform aberration in ultrasonic wave propagation due to the human skull remain major challenges to achieving good focus in high intensity focused ultrasound (HIFU) brain therapy and high-resolution thermoacoustic tomography (TAT) of the human brain. With the inclusion of the skull insertion loss, we numerically simulated the ultrasonic wave propagation using the pseudospectral time domain (PSTD) algorithm for TAT setup. We then applied the redatuming scheme through downward and upward continuation originated in seismic signal processing to eliminate the diffraction caused by the irregularity of the skull and to recover the insertion loss due mainly to the diploes layer of the skull. This approach, after further validation, is aimed to recover wave energy dissipation and waveform aberration in ultrasonic measurements applied to both trans-skull imaging (ultrasound propagates outward) and therapy (ultrasound propagates inward).

In many iterative algorithms for photoacoustic tomography (PAT), images are reconstructed from an integrated data function g(r&vec;, t) rather than from the measured acoustic pressure data function p(r&vec;, t). The integrated data function is related to the object by a spherical Radon transform, which can be inverted by use of standard reconstruction algorithms. In this work, we investigate a different reconstruction approach that utilizes the measured pressure data function p(r&vec;, t) to directly invert the PAT imaging model. We reveal that these two reconstruction approaches, which are preconditioned versions of each other, have distinct statistical and numerical properties. Each is demonstrated to have characteristics that are advantageous to certain types of data inconsistencies. Numerical results are presented to corroborate our analysis.

Purpose: The purpose of this study is to evaluate the influence of photon propagation on the NIR spectral features associated with photoacoustic imaging. Introduction: Photoacoustic CT spectroscopy (PCT-S) has the potential to identify molecular properties of tumors while overcoming the limited depth resolution associated with optical imaging modalities (e.g., OCT and DOT). Photoacoustics is based on the fact that biological tissue generates high-frequency acoustic signals due to volume of expansion when irradiated by pulsed light. The amplitude of the acoustic signal is proportional to the optical absorption properties of tissue, which varies with wavelength depending on the molecular makeup of the tissue. To obtain quantifiable information necessitate modeling and correcting for photon and acoustic propagation in tumors. Material and Methods: A Monte Carlo (MC) algorithm based on MCML (Monte Carlo for Multi-Layered edia) has been developed to simulate photon propagation within objects comprised of a series of complex 3D surfaces (Mcml3D). This code has been used to simulate and correct for the optical attenuation of photons in blood, and for subcutaneous tumors with homogenous and radially heterogeneous vascular distributions. Results: The NIR spectra for oxygenated and deoxygenated blood as determined from Monte Carlo simulated photoacoustic data matched measured data, and improving oxygen saturation calculations. Subcutaneous tumors with a homogeneous and radially heterogeneous distribution of blood revealed large variations in photon absorption as a function of the scanner projection angle. For select voxels near the periphery of the tumor, this angular profile between the two different tumors appeared similar. Conclusions: A Monte Carlo code has been successfully developed and used to correct for photon propagation effects in blood phantoms and restoring the integrity of the NIR spectra associated with oxygenated and deoxygenated blood. This code can be used to simulate the influence of intra-tumor heterogeneity on the molecular identification via NIR spectroscopy.

In photoacoustics, characteristics of the absorbed optical energy are a major determining factor of the characteristics of the photoacoustic signal. In other words, objects with different absorption properties generate photoacoustic signals with different frequency contents. Therefore, effects of absorption properties on photoacoustic spectral characteristics need to be fully understood in order to devise optimal setup for photoacoustic signal detection. The main purpose of this paper is therefore to numerically investigate such effects using a finite-difference time-domain (FDTD) approach. Photoacoustic signal generation can be described by the thermal conduction equation, the continuity and the Navier-Stokes equations, and the state equations in the system. To reduce computational and storage requirements, an axis-symmetrical cylindrical coordinate system with the z-axis parallel to the laser irradiation direction is adopted in our study. Moreover, the MacCormack scheme, which is fourth-order accurate in space and second-order accurate in time, and the first-order Mur absorbing boundary conditions are introduced to implement the FDTD code. The absorption coefficient range from 1cm^-1 to 100 cm^-1 and the signals are detected in forward modes. Results show that the peak frequency of the signal with absorbing coefficient 1cm^-1, 10cm^-1, 20cm^-1, 30cm^-1, 50cm^-1, and 100 cm^-1 is 2.4MHz, 2.4MHz, 4.2MHz, 4.8MHz, 6MHz, and 7.8MHz when detected forwardly. Note that although the peak acoustic frequency increases with the absorption coefficient, but the linear relationship between the two parameters does not hold. The results also imply that photoacoustic image contrast can be improved by properly selecting the receiver signal bandwidth. Assuming two objects with absorption coefficient of 10cm^-1 and 100cm^-1, respectively, it is estimated that a 11.34dB contrast improvement is achievable in the forward mode, while a 8.13dB improvement is possible in the backward mode.

Photoacoustic imaging is a promising non-invasive imaging technology due to its ability to combine the enhanced contrast of optical absorption with the spatial resolution of acoustic imaging. Co-registered three-dimensional (3-D) ultrasound and photoacoustic imaging takes advantage of both modalities to allow visualization of tissue structures within a volume using simultaneous structural and functional information. 1.75D acoustic arrays are well-suited for this application due to their ability to scan in 3-D volumes rapidly and accurately while maintaining a reasonable system complexity and cost. We have designed, fabricated, and tested a 1.75D 1280-ch ultrasound system for co-registered 3-D ultrasound and photoacoustic imaging. The system features a 1.75D 1280-channel ultrasound array with a center frequency of 5MHz and 80% bandwidth. The electronics includes 1280 high-voltage pulsers, 40 32-to-1 multiplexers, amplification circuitry, and a 40-channel data acquisition circuit. The system is able to drive the entire array simultaneously, and each array element independently, to scan a 3-D volume within +/- 40 degrees in azimuth direction and +/- 10 degrees in elevation respectively. System performance including axial and lateral resolution has been characterized and compared with simulations. Co-registered 3-D ultrasound and photoacoustic imaging has been successfully performed on phantoms with different geometries and contrast.

To perform ultrasound imaging using an array transducer, a focused ultrasound beam is transmitted in a particular direction within the tissue and the received backscattered ultrasound wave is then dynamically focused at every position along the beam. The ultrasound beam is scanned over the desired region to form an image. The photoacoustic imaging, however, is distinct from conventional ultrasound imaging. In photoacoustic imaging the acoustic transients are generated simultaneously in the entire volume of the irradiated tissue - no transmit focusing is possible due to light scattering in the tissue. The photoacoustic waves are then recorded on every element of the ultrasound transducer array at once and processed to form an image. Therefore, compared to ultrasound imaging, photoacoustic imaging can utilize dynamic receive focusing only. In this paper, we describe the image formation algorithms of the array-based photoacoustic and ultrasound imaging system and present methods to improve the quality of photoacoustic images. To evaluate the performance of photoacoustic imaging using an array transducer, numerical simulations and phantom experiments were performed. First, to evaluate spatial resolution, a point source was imaged using a combined ultrasound and photoacoustic imaging system. Next, image quality was assessed by imaging tissue imaging phantoms containing a circular inclusion. Finally, the photoacoustic and ultrasound images from the combined imaging system were analyzed.

In this paper, we present new Adaptive and Robust Techniques (ART) for microwave-based thermoacoustic tomography (TAT) and laser-based photo-acoustic tomography (PAT), and study their performances for breast cancer detection. TAT and PAT are emerging medical imaging techniques that combine the merits of high contrast due to electromagnetic or laser stimulation and high resolution offered by thermal acoustic imaging. The current image reconstruction methods used for TAT and PAT, such as the widely used Delay-and-Sum (DAS) approach, are data-independent and suffer from low resolution, high sidelobe levels, and poor interference rejection capabilities. The data-adaptive ART can have much better resolution and much better interference rejection capabilities than their data-independent counterparts. By allowing certain uncertainties, ART can be used to mitigate the amplitude and phase distortion problems encountered in TAT and PAT. Specifically, in the first step of ART, RCB is used for waveform estimation by treating the amplitude distortion with an uncertainty parameter. In the second step of ART, a simple yet effective peak searching method is used for phase distortion correction. Compared with other energy or amplitude based response intensity estimation methods, peak searching can be used to improve image quality with little additional computational costs. Moreover, since the acoustic pulse is usually bipolar: a positive peak, corresponding to the compression pulse, and a negative peak, corresponding to the rarefaction pulse, we can further enhance the image contrast in TAT or PAT by using the peak-to-peak difference as the response intensity for a focal point. The excellent performance of ART is demonstrated using both simulated and experimentally measured data.

The paper is dedicated to the analysis of applicability of passive acoustic brightness (PAB) method for noninvasive detection and localization of optical heterogeneities under laser impact. To estimate the opportunities of PAB method we used computer model of multilayered medium (similar to its optical and thermophysic properties to real human tissue). It was demonstrated that PAB method allows to achieve high-quality diagnostics results in case of optical heterogeneities located in depth range less than 20 millimeters. Topics of combining PAB method with Photo-Acoustic (PAT) method were also discussed in terms of sensitizing of both approaches.

We present a rapid, robust method of signal processing useful for optoacoustic monitoring of total hemoglobin concentration ([THb]) and oxygen saturation level in small blood vessels. Our method includes the wavelet-based regularization of the difference operator which is a typical discrete approximation of the derivative. The optimal degree of regularization is defined by the signal-to-noise ratio (SNR). We applied the proposed method to Monte Carlo-modeled signals from a cylinder simulating the human radial artery (diameter 1.6 mm, depth from skin 2 mm, and [THb] varied in a wide range from 4 - 16 g/dL). We obtained N-shaped signals and found that the maximum of the first derivative between the front and rear walls systematically correlates with the actual value of [THb]. We estimated the accuracy of [THb] reconstruction from the maximum of the first derivative as 0.32 ± 0.18 g/dL (mean value ± SD) at an SNR typical for our in vivo experiments at the wavelength of 1064 nm. We also demonstrated that the difference between the maxima of the first derivative of the signals obtained at 700 nm and 1000 nm depends on oxygen saturation level.

We devise and explore a ring-shaped acoustic detector associated with a virtual point detector concept for photoacoustic tomography. The center of the ring transducer scans a circle around the object to be imaged and then is treated as an omni-directional virtual point detector in photoacoustic image reconstruction. The virtual point detector introduces a space-invariant point spread function in photoacoustic image reconstruction and thus improves the tangential resolution, which is due to the finite aperture. Compared with a real point detector, the virtual point detector can provide similar spatial resolution but better SNR. Compared with a real finite-aperture detector, the virtual point detector can provide similar SNR but better spatial resolution. In addition, because of its virtual feature, the virtual point detector can be placed very close to and even inside of a tissue sample to locally scan a region of interest, which yields good SNR and spatial resolution.

Photoacoustic image reconstruction involves dozens or perhaps hundreds of point measurements, each of which contributes unique information about the subsurface absorbing structures under study. For backprojection imaging, two or more point measurements of photoacoustic waves induced by irradiating a sample with laser light are used to produce an image of the acoustic source. Each of these point measurements must undergo some signal processing, such as denoising and system deconvolution. In order to efficiently process the numerous signals acquired for photoacoustic imaging, we have developed an automated wavelet algorithm for processing signals generated in a burn injury phantom. We used the discrete wavelet transform to denoise photoacoustic signals generated in an optically turbid phantom containing whole blood. The denoising used universal level independent thresholding, as developed by Donoho and Johnstone. The entire signal processing technique was automated so that no user intervention was needed to reconstruct the images. The signals were backprojected using the automated wavelet processing software and showed reconstruction using denoised signals improved image quality by 21%, using a relative 2-norm difference scheme.

A finite element reconstruction algorithm for three-dimensional photoacoustic tomography is demonstrated to recover both the images of absorbed optical energy density and acoustic speed simultaneously. The recovered results show that the algorithm is able to reconstruct photoacoustic images quantitatively in terms of the location, size, optical and acoustic properties of the heterogeneous medium.

Filtered backprojection (FBP) reconstruction is the method of choice for diagnostic xray CT, despite the fact that backprojection is computationally costly. FBP image quality is superior over fast Fourier reconstruction techniques because interpolation errors are localized and the backprojector applies the Radon transform, annihilating all measurement errors orthogonal to its range. We discuss computational complexity, sampling rates, and quadrature techniques for FBP reconstruction of thermo/photo/optoacoustic data.

The reconstruction of images from projections, diffraction fields, or other similar measurements requires applying signal processing techniques within a physical context. Although modeling of the acquisition procedure can conveniently be carried out in the continuous domain, actual reconstruction from experimental measurements requires the derivation of discrete algorithms that are accurate, efficient, and robust. In recent years, wavelets and multiresolution approaches have been applied successfully for common image processing tasks bridging the gap between discrete and continuous representations. We show that it is possible to express many physical problems in a wavelet framework, thereby allowing the derivation of efficient algorithms that take advantage of wavelet properties, such as multiresolution structure, sparsity, and space-frequency decompositions. We review several examples of such algorithms with applications to X-ray tomography, digital holography, and confocal microscopy and discuss possible future extensions to other modalities.

Photoacoustic tomography (PAT) is an emerging imaging technique with great potential for a wide range of biomedical imaging applications. The reconstruction problem of PAT is an inverse source problem, in which the photoacoustic source of interest is induced by a probing optical wavefield. In this work, we revisit the PAT reconstruction problem from a Fourier perspective. By use of standard analytic techniques from inverse source theory, we derive a mathematical relationship between the pressure wavefield data function and its normal derivative measured on an arbitrary aperture that encloses the object and the three-dimensional Fourier transform of the optical absorption distribution evaluated on concentric spheres. We refer to this relationship as a "Fourier-shell identity", which is analogous to the well-known Fourier-slice theorem of X-ray tomography. Potential applications of the Fourier-shell identity are identified and discussed.

So far most rigorous reconstruction algorithms for photoacoustic tomography (PAT), e.g., the modified back-projection algorithm, have been developed based on ideal point detectors. However, a flat unfocused transducer is commonly used in PAT, thus suffering from the finite aperture effect - tangential resolution deteriorates as the imaging point moves away from the scanning center. Based on a virtual-point-detector concept, we propose a PAT reconstruction with a focused transducer to improve the degraded tangential resolution. We treat the focal point of the focused transducer as a virtual-point detector, which means that delays applied in reconstruction are relative to the focal point. The geometric focus defines propagation path of photoacoustic signals. The simulation results show that compared with PAT with an unfocused transducer, PAT with a focused transducer having an f-number of 2.5 significantly improves tangential resolution by 29 microns up to 791 microns at the imaging positions of at least 4 mm away from the scanning center. The farther the imaging positions away from the scanning center, the larger the improvement. In the region of 4 mm away from the scanning center, PAT with a focused transducer slightly degrades the tangential resolution by up to 70 microns. The improvement in tangential resolution comes with a compromise of loss in radial resolution by 26 microns up to 79 microns depending on the distance from the scanning center. In terms of the significant improvement in tangential resolution, the loss in radial resolution is tolerable, especially for imaging of big objects, e.g., breast.

In principle, absorbed energy profiles can be exactly reconstructed from photoacoustic measurements on a closed surface. Clinical applications, however, involve compromises due to transducer focus, frequency characteristics, and incomplete measurement apertures. These tradeoffs introduce artifacts and errors in reconstructed absorption distributions that affect quantitative interpretations as well as qualitative contrast between features. The quantitative effects of target geometry, limited measurement surfaces, and bandpass transducer frequency response have been investigated using a ring transducer system designed for small animal imaging. The directionality of photoacoustic radiation is shown to increase with target aspect ratio, producing proportionate overestimates of absorption values for two-dimension apertures less than approximately 150 degrees. For all target geometries and orientations, mean absorption values approach the full view values for hemicircular measurement surfaces although the true spatial uniformity is recovered only with the complete surface. The bandpass transducer frequency spectrum produces a peaked amplitude response biased toward spatial features ranging from 1 to 8 times the system resolution. We discuss the implications of these results for design of clinical systems.

Photoacoustic imaging with planar sensor arrays suffers from a finite aperture or limited view problem due to the fact that the sensor array is finite in size, whereas exact reconstruction algorithms require that it occupies the entire infinite plane. Here, acoustic reflectors are proposed as a means of reflecting the acoustic waves, that would otherwise not be measured, back onto the sensor, and it is shown that an existing FFT-based image reconstruction algorithm can be used to reconstruct an image in this case without modification. A 2D simulation is used to show the improvement in the image quality when data from the reverberant field is included in the reconstruction. The improvement is explained by the matching of the periodicity implicitly assumed by the FFT algorithm and the periodicity in the virtual initial pressure distribution formed from the real initial pressure distribution and an infinite line of image sources generated by the acoustic reflectors.

Last researches on Osteoporosis disease show's that the correlation between BMD information that come from DXA, and the fractual risk are not enough correlated. For this reason, new techniques that could be more sensitive and give different information's on the bone are being looking for. The key for success is to find a technique that could be detected changes in the micro-structure inside the trabecular bone, and supply more information on this structure. We have shown that pulse ultrasound light tomography could work in well reflection configuration and on living tissue to measure locally the "reduce scattering coefficient" inside the body. This technique could work deep inside the body (few cm) and could supply very good resolution. In the beginning on this research we show clinical results that justified planning more accurate and larger clinical study. The system suffers from mechanical and body interface problems that only very skill and professional physician could get a good results. This system also suffers from low signal to noise ratio, and irrevocable data on the same people. In this lecture we will present an upgrade and improved system. The mechanical and body interface changes will present and discusses. A good repeated data will be presented on phantoms and on peoples. Finally, large clinical data that operated in clinical technician on 60 Ultra distal bone peoples will present in compare to DXA data that was measure on the same peoples on the same place.

We present derivation of the temporal correlation transfer equation (CTE) for multiply scattered light modulated by an ultrasound pulse. The equation can be used to obtain the time-varying specific intensity of light produced by a pulsed and nonuniform ultrasound field in optically scattering media that have a heterogeneous distribution of optical parameters. We also develop a Monte Carlo algorithm that can simulate the spatial distribution of the time-dependent power spectrum density of light modulated by a focused ultrasound pulse in optically scattering media with heterogeneous distributions of optically scattering and absorbing objects. Derivation is based on the ladder diagram approximation of the Bethe-Salpeter equation that assumes moderate ultrasound pressures. We expect these results to be applicable to a wide spectrum of conditions in the ultrasound-modulated optical tomography of soft biological tissues.

Ultrasound-Modulated Optical Tomography is a novel biophotonic imaging technique that provides optical contrast with ultrasonic spatial resolution. High temporal coherence laser light and focused ultrasound are transmitted into tissue. Light passing through the acoustic focal volume experiences modulation due to acoustically induced changes in optical index of refraction and optical scatterer displacement. A component of the modulated light may be detected using various detection schemes. One such scheme detects changes in the contrast of optical speckles using a CCD camera. We use statistical optics to derive expressions for the speckle statistics as a function of acoustic parameters. We demonstrate theoretically and experimentally that low acoustic frequencies induce much larger modulation compared with high frequency ultrasound. Theoretically computed values for the speckle contrast are compared with experimental values.

The in-line x-ray phase-contrast imaging method relies on changes in index of refraction within a body to produce image contrast. In soft tissue, index of refraction variations arise from density changes so that phase contrast imaging provides a map of density gradients within a body. An intense, short pulse laser beam that is differentially absorbed by an object within a body will produce a thermal wave with an associated density change that propagates outwardly from the interface between the object and the body. Experiments are described where a pulsed Nd:YLF laser is synchronized to an image intensifier to record the effects of the energy deposited by a pulsed laser.

We have made a comparison of various detection strategies for detection of acoustically modulated light in a scattering medium. Furthermore we have investigated the possibility to determine the local absorbance in a model system mimicking a blood vessel in tissue.

Because it has the advantages of optical contrast and ultrasonic resolution, the ultrasound-modulated optical tomography becomes a new and promising method for biomedical imaging. The propagation of the light modulated by ultrasound in the tissue plays an important role in this new technique. We already proved that the modulated depth of the modulated light (Identical Modulated Depth, ignoring the background diffused light) was dependent on optical and ultrasonic properties of tissue within the ultrasound zoom, and didn't change in the propagation in the recent research. However, the modulation depth detected at the surface in experiment (Real Modulated Depth) differs from the Identical Modulated Depth, which includes the background diffused light. In Diffuse theory and experiments it is shown that the Real Modulated Depth is dependent on the propagation process of the diffused light. The relations to the Real Modulated Depth contributed by the tissue thickness, optical properties, etc. are figured out in this paper. So the Real Modulated Depth detected in the experiments should be transformed into the Identical Modulated Depth (a dominant parameter to imaging) by a set of iterated algorithm decoding. All these should contribute to the practical applications of ultrasound-modulated optical tomography.

We present experimental evidence that the optical absorption coefficient images of heterogeneous media can be obtained with quantitative photoacoustic tomography (PAT). Photoacoustic images are obtained from a series of tissue-like phantom experiments using a finite element-based reconstruction algorithm coupled with a scanning photoacoustic imaging system. The experimental results show that optical absorption images can be quantitatively reconstructed when the photon diffusion model is coupled with the Helmholtz photoacoustic wave equation.

The aim in quantitative photoacoustic imaging (QPI) is to recover the spatial distributions of the optical absorption and scattering coefficients from an absorbed energy density distribution (a conventional photoacoustic image). This paper proposes a gradient-based minimisation approach and demonstrates that functional gradients may be calculated efficiently by using a finite element model of light transport based on the diffusion approximation, in conjunction with a related adjoint model. The gradients calculated using this adjoint method are tested against finite-difference estimates, and inversions for the absorption or scattering coefficient distributions (from simulated data) are shown for the case where the other coefficient is known a priori. Simultaneous estimation for both absorption and scattering is ill-posed, and so multiwavelength inversion, in which the specific absorption spectra of the constituent chromophores and the wavelength-dependence of the scatter are known, is proposed as a means of ameliorating the ill-posedness. The unknown parameters are now the spatial variation of the chromphore concentrations and scattering coefficient. It is shown that the functional gradients for both of these can be obtained straightforwardly from the gradients for absorption and scatter and require no significant additional calculations. A simulated example is given in which the distributions of two chromophores with different spectra are recovered from absorbed energy images obtained at multiple wavelengths, when the scattering is assumed known.

Photoacoustic methods can be used to make spatially resolved spectroscopic measurements of blood oxygenation when using a multiwavelength excitation source, such as an OPO system. Since these excitation sources are usually expensive and bulky, an alternative is to use laser diodes. A fibre coupled laser diode excitation system has been developed, providing two wavelengths, 850 and 905nm, each composed of 6 high peak power pulsed laser diodes. The system provided variable pulse durations (65-500ns) and repetition rates of up to 5KHz. The pulse energies delivered by the excitation system at 905nm and 850nm were measured to be 120&mgr;J and 80&mgr;J respectively for a 200ns pulse duration. To demonstrate the utility of the system, the excitation source was combined with an ultrasound detector to form a probe for in vivo single point measurements of superficial blood vessels. Changes in blood oxygenation and volume in the finger tip were monitored while making venous and arterial occlusions. To demonstrate the imaging capability of the excitation system, 2D photoacoustic images of a physiologically realistic phantom were obtained for a range of pulse durations using a cylindrical scanning system. The phantom was composed of cylindrical absorbing elements (&mgr;a=1mm^{-1}) of 2.7mm diameter, immersed in a 1% intralipid solution (&mgr;s=1mm^{-1}). This study demonstrated the potential use of laser diodes as an excitation source for photoacoustic imaging of superficial vascular structures.

The optical and acoustic properties of heterogeneous media are recovered simultaneously using finite element-based reconstruction algorithm coupled with a scanning photoacoustic imaging system. The results show that the images obtained are quantitative in terms of the shape, size, location and optical and acoustic properties of the heterogeneities examined.

In this paper, an efficient algorithm for quantitative reconstruction of optical absorption coefficient in backward mode photoacoustic imaging is presented. Compared to forward mode and sideward mode (tomographic) imaging, the setup of backward mode imaging generally has a wider range of clinical applications. However, due to the limited detection angles, quantitative image reconstruction has also been difficult. Previously, we proposed a method for reconstruction of optical energy deposition in backward mode imaging. This method is derived from the photoacoustic wave equations with line focusing, with which the focusing is utilized to reduce the reconstruction problem from three dimensions to one dimension. In this paper, we extend the previous reconstruction method for absorbed optical energy to a two-step procedure for optical absorption coefficient. By adding the second step, backward mode photoacoustic imaging becomes more quantitative as the image is directly related to the inherent properties of an image object. In the proposed method, comparison between the reconstructed and the predicted absorbed energy is then iteratively performed to find the optical absorption distribution. Numerical simulations are conducted to verify efficacy of this method. The errors in reconstructed optical absorption coefficient are generally within 10%. Phantom experiments are also performed. The results are presented with a discussion on effects of object position and geometry.

Purpose: The purpose of this study is to calibrate the PCT small animal scanner system with a blood phantom and to measure the blood hemoglobin concentration and oxygen saturation level in mouse tail vein and tumors. Methods and Materials: A blood phantom with variable blood flow and oxygen content was integrated into the PCT scanner with a circulation system. The circulation system consisted of a pump, an oxygen electrode detector and a tonometer. The SaO2 values were determined based on a linear combination model of oxy-hemoglobin and deoxy-hemoglobin absorption spectrum curves. Hemoglobin concentration (CHb) was determined by measuring the linear relationship for different blood dilutions. SaO2 and CHb as measured in vivo for the artery and vein in a mouse tail were also measured. Results: The PCT spectra of blood phantom samples were measured and compared with hemoglobin optical absorption spectra. The linear relationship between hemoglobin concentration and PCT intensities were observed by phantom study. The hemoglobin concentration of mouse is ~9.7g/dL. The saturation difference between arteries and veins in mouse tail is also measured by PCT scan. Conclusions: Both the phantom and living mouse tail vessel scans have shown that NIR PCT-spectroscopy can be used to measure the hemoglobin saturation level and hemoglobin concentration in small animal for future tumor hypoxia study.

There is a pressing need for noninvasive methods for continuous monitoring of total hemoglobin concentration ([THb]) in blood. We proposed to use an optoacoustic technique for noninvasive [THb] measurement by probing of arteries or veins. In our previous work we demonstrated that our optoacoustic system is capable of detecting signals from the radial artery with high resolution, contrast, and signal-to-noise ratio. In clinical studies, we confirmed the ability of our system to monitor [THb] changes continuously, noninvasively, and in real time. However, absolute measurements of [THb] with high accuracy must account for blood vessel diameter. We designed and built a new broadband optoacoustic transducer that detected both the anterior and posterior (back) walls of blood vessels with high resolution. The optoacoustic signals from our radial artery phantom (silicon tube with a diameter of 1.6 mm filled with arterial sheep blood and immersed in 0.625% Intralipid solution) contained a distinct peak from the back wall of the tube. The characteristic parameters derived from the signals (the amplitude and the derivative of the normalized signal near the back wall) proved to be linearly dependent on [THb] in the physiological range. We also tested our modified system in vivo in radial arteries of healthy volunteers. The posterior walls of the arteries were well resolved, permitting accurate measurement of vessel diameter. The characteristic parameters of the signals were compared to those of signals from blood obtained for close tube diameter and various THb concentrations. The [THb] values derived from the in vivo measured optoacoustic signals were close to invasively measured ones.

Multiwavelength photoacoustic imaging was used to make spatially resolved measurements of blood oxygen saturation (sO2) in vivo. 2D cross-sectional images of the initial absorbed optical energy distribution in the finger were acquired at near-infrared wavelengths using a photoacoustic imaging system. Using the structural information from these images, a 2D finite element forward model of the light transport was formulated to calculate the absorbed energy density in the tissue as a function of the concentrations of the tissue chromophores and scatters. Separate oxy- (HbO2) and deoxyhaemoglobin (HHb) concentrations were assigned to locations within the mesh that corresponded to the locations of blood vessels in the detected photoacoustic image. The surrounding tissue was regarded as a homogeneous medium with optical properties that were determined by the concentrations of water, lipids, optical scatters, and HbO2 and HHb in the capillary bed. The concentrations of the individual chromophores were the variable input parameters of the forward model. By varying their values in a minimisation procedure in order to fit the output of the model to the measured multiwavelength images, the HbO2 and HHb concentrations, and hence blood sO2, within the blood vessels were determined.

Previous studies on measurements of hemoglobin concentrations and oxygen saturations of human blood by photoacoustic spectroscopy assume that the exponential attenuation of the optical radiation in blood results in photoacoustic signals with exponential temporal profiles and absorption coefficients could be extracted by fitting exponential functions. In this paper, we demonstrate that the detected photoacoustic signals are the convolutions of the optical absorption distributions in samples, and the first derivative of the temporal profile of the excitation laser, and the impulse response of the ultrasound detector. The detected photoacoustic signals from absorbers with exponential optical absorption distributions do not keep exponential profiles. In addition, we present an improved approach to calculate the absorption coefficient by fitting the detected photoacoustic signal with a function that is the convolution of an exponential function, and the first derivative of the temporal profile of the laser, and the impulse response of the detector. This approach is validated by both numerical simulations and experimental results.

It is well documented that photoacoustic imaging has the capability to differentiate tissue based on the spectral characteristics of tissue in the optical regime. The imaging depth in tissue exceeds standard optical imaging techniques, and systems can be designed to achieve excellent spatial resolution. A natural extension of imaging the intrinsic optical contrast of tissue is to demonstrate the ability of photoacoustic imaging to detect contrast agents based on optically absorbing dyes that exhibit well defined absorption peaks in the infrared. The ultimate goal of this project is to implement molecular imaging, in which Herceptin^TM, a monoclonal antibody that is used as a therapeutic agent in breast cancer patients that over express the HER2 gene, is labeled with an IR absorbing dye, and the resulting in vivo bio-distribution is mapped using multi-spectral, infrared stimulation and subsequent photoacoustic detection. To lay the groundwork for this goal and establish system sensitivity, images were collected in tissue mimicking phantoms to determine maximum detection depth and minimum detectable concentration of Indocyanine Green (ICG), a common IR absorbing dye, for a single angle photoacoustic acquisition. A breast mimicking phantom was constructed and spectra were also collected for hemoglobin and methanol. An imaging schema was developed that made it possible to separate the ICG from the other tissue mimicking components in a multiple component phantom. We present the results of these experiments and define the path forward for the detection of dye labeled Herceptin^TM in cell cultures and mice models.

A deep reflection-mode photoacoustic (PA) imaging system was designed and implemented to visualize deep structures in biological tissues. To achieve good penetration depth, we chose near IR laser pulses at 804 nm wavelength for the generation of photoacoustic waves. To avoid overshadowing the deep PA signals by the surface PA signals, we employed dark-field illumination. To achieve good lateral resolution, we chose spherically focused high-numericalaperture ultrasonic transducers with 5 MHz or 10 MHz center frequencies. By using these transducers, we achieved 153 &mgr;m and 130 &mgr;m axial resolutions, respectively, at 19.5 mm depth in 10% porcine gelatin containing 1% intralipid. The system was applied to imaging internal organs of small animals. Compared with our previous high-frequency (50-MHz) photoacoustic microscope, we scaled up the imaging depth while maintaining the ratio of the imaging depth to axial resolution more than 100. In addition, we studied the scalability of the imaging depth and the resolution with ultrasound frequency.

In this paper, we report on sequential decreases in the amplitude of photoacoustic (PA) signals from nanosecond laser pulse irradiation of various samples. These samples include biological tissues, such as dental-enamel and chicken/turkey breast as well as some non-biological samples. Laser energy densities in the range of 80mJ/cm² to 300mJ/cm² were used in our experiments, typical of those used in PA imaging regimes. Induced temperature rises are modelled to show that the average temperature rise for each pulse in those biological tissues is less than one degree centigrade. Measurements reveal a rapid decay of photoacoustic signals within the first few laser pulses absorbed by the sample and this decay is irreversible in the short term. The phenomenon indicates that laser irradiation interacts with biological samples, causing long-term physical changes that can be attributed to a reduction of optical absorption within the samples.

Photoacoustic imaging is based on the generation of acoustic waves in a semitransparent sample after illumination with short pulses of light or radio waves. The goal is to recover the spatial distribution of absorbed energy density inside the sample from acoustic pressure signals measured outside the sample (photoacoustic inverse problem). We have proposed a numerical method to calculate directly the time reversed field by re-transmitting the measured pressure on the detection surface in reversed temporal order. This model-based time reversal method can solve the photoacoustic inverse problem exactly for an arbitrary closed detection surface. Recently we presented a set up which requires a single rotation axis and line detectors perpendicular to the rotation axis. Using a two-dimensional reconstruction method, such as time reversal in two dimensions, and applying the inverse two-dimensional radon transform afterwards gives an exact reconstruction of a three-dimensional sample with this set up. The resolution in photoacoustic imaging is limited by the acoustic bandwidth and therefore by acoustic attenuation, which can be substantial for high frequencies. This effect is usually ignored in reconstruction algorithms but has a strong impact on the resolution of small structures. It is demonstrated that the model based time reversal method allows to partly compensate this effect.

Governing equations for ultrasonic propagation in three spatial dimensions with attenuation obeying a frequency power law are derived. Quadratic attenuation corresponds to a partial differential equation of degree four whose operator factors into a product of two parabolic operators, and impulse response is related to the heat kernel. The solutions satisfy primitive causality, but not relativistic causality. For powers that are not even integers, the waves satisfy integral-differential equations.

To make a photoacoustic microscope (PAM) suitable for clinical applications, we have developed a portable PAM system with high frame rate. The probe for this portable PAM system is a small hand-held unit, which is connected with the rest of the system with an optical fiber and electrical cables. The probe can be maneuvered easily and pressed against the region of interest at various anatomical sites. By employing a tunable laser with a repetition rate up to 1 kHz, this portable PAM system has reduced its acquisition time to less than one second for each cross-sectional image. We have applied this system to animals as well as humans to acquire cross-sectional images with a maximum imaging depth of 5 mm, a lateral resolution of better than 100 &mgr;m, and an axial resolution of 35 &mgr;m.

We present a fiber-based optical detection system for high-frequency 3D ultrasound and photoacoustic imaging. Optically probing the surface of a thin polymer Fabry-Perot etalon defines the acoustic array geometry and element size. We have previously demonstrated wide bandwidth signal detection (>40 MHz) and element size on the order of 15 &mgr;m. By integrating an etalon into a photoacoustic imaging system, high-resolution 3D images were obtained. However, the previous system is limited for clinical applications because the etalon is rigidly attached to a free-space optical scanning system. To move etalon detector technology toward a practical clinical device, we designed a remote-probe system based on a fiber bundle. A fiber bundle, composed of 160,000 individual light guides of 8-&mgr;m diameter, delivers the optical probe to the etalon. Light coupled into a single guide creates an active element on the etalon surface. We successfully measured the ultrasound signals from 10 MHz and 50 MHz ultrasound transducers using a laser tunable around 1550 nm. With further progress on reducing the size of the etalon, it will be possible to build a practical device for in vivo high-frequency 3D ultrasound and photoacoustic imaging, especially for intravascular and endoscopic applications.

Diagnosis and treatment of atherosclerosis necessitates the detection and differentiation of rupture prone plaques. In principle, intravascular photoacoustic (IVPA) imaging has the ability to simultaneously visualize the structure and composition of atherosclerotic plaques by utilizing the difference in optical absorption. Extensive studies are required to validate the utility of IVPA imaging in detecting vulnerable plaques and address issues associated with the clinical implementation of the technique. In this work, we performed ex vivo imaging studies using a rabbit model of atherosclerosis. The intravascular photoacoustic (IVPA) and ultrasound (IVUS) images of the normal aorta and aorta with plaque were obtained and compared with histological slices of the tissue. The results indicate that IVPA imaging is capable of detecting plaques and showed potential in determining the composition. Furthermore, we initially addressed several aspects of clinical implementation of the IVPA imaging. Specifically, the configuration of combined IVPA and IVUS catheter was investigated and the effect of the optical absorption of the luminal blood on the IVPA image quality was evaluated. Overall, this study suggests that IVPA imaging can become a unique and important clinical tool.

With increasing demand for in-vivo observation of living cells, microscope techniques that do not need staining become more and more important. In this talk we present a combined multi-photon-acoustic microscope with the possibility to measure synchronously properties addressed by ultrasound and two-photon fluorescence. Ultrasound probes the local mechanical properties of a cell, while the high resolution image of the two-photon fluorescence delivers insight in cell morphology and activity. In the acoustic part of the microscope an ultrasound wave, with a frequency of GHz, is focused by an acoustic sapphire lens and detected by a piezo electric transducer assembled to the lens. The achieved lateral resolution is in the range of 1&mgr;m. Contrast in the images arises mainly from the local absorption of sound in the cells, related to properties, such as mass density, stiffness and viscose damping. Additionally acoustic microscopy can access the cell shape and the state of the cell membrane as it is a intrinsic volume scanning technique.The optical part bases on the emission of fluorescent biomolecules naturally present in cells (e.g. NAD(P)H, protophorphyrin IX, lipofuscin, melanin). The nonlinear effect of two-photon absorption provides a high lateral and axial resolution without the need of confocal detection. In addition, in the near-IR cell damages are drastically reduced in comparison to direct excitation in the visible or UV. Both methods can be considered as minimal invasive, as they relay on intrinsic contrast mechanisms and dispense with the need of staining. First results on living cells are presented and discussed.

A unique photoacoustic system was developed for neovascular imaging during tumor photodynamic therapy (PDT). In this system, a single pulse laser beam is used as the light source for both PDT treatment and for concurrently generating ultrasound signals for photoacoustic imaging. To demonstrate its feasibility, this system was used to observe vascular changes during PDT treatment of chicken chorioallantoic membrane (CAM) tumors. The photosensitizer used in this study was protoporphyrin IX (PpIX) and the laser wavelength was 532 nm. Damage of the vascular structures by PDT was imaged before, during and after treatment. Rapid, real-time determination of the size of targeted tumor blood vessels was achieved, using time difference of positive and negative ultrasound peaks during PDT treatment. The experimental results show that a pulse laser can be conveniently used to hybridize PDT treatment and photoacoustic imaging and that this integrated system is capable of quantitatively monitoring the structural change of blood vessels during PDT. This method could be potentially used to guide PDT and other phototherapies using vascular changes during treatment to optimize treatment protocols, by choosing appropriate types and doses of photosensitizers, and doses of light.