A double-ring photoacoustic sensor to image and monitor blood content in tissue has been developed. This sensor has a very small opening angle. Using this sensor we are able to image artifical blood vessels, as well as vessels in a rabbit ear. Furthermore, the feasibility of in vivo imaging is demonstrated with a photoacoustic reconstruction of the joining of two palmar veins a few centimeter proximal to the wrist in a human arm.

We previously reported that for rat burn models, deep dermal burns and deep burns can be well differentiated by measuring the propagation time of the photoacoustic signals originated from the blood in the healthy skin tissue under the damaged tissue layer. However, the diagnosis was based on point measurement in the wound, and therefore site-dependent information on the injuries was not obtained; such information is very important for diagnosis of extended burns. In the present study, we scanned a photoacoustic detector on the wound and constructed two-dimensional (2-D) images of the blood-originated photoacoustic signals for superficial dermal burns (SDB), deep dermal burns (DDB), deep burns (DB), and healthy skins (control) in rats. For each burn model, site-dependent variation of the signal was observed; the variation probably reflects the distribution of blood vessels in the skin tissue. In spite of the variation, clear differentiation was obtained between SDB, DDB, and DB from the 2D images. The images were constructed as a function of post burn time. Temporal signal variation will be also presented.

Most dermatologic laser procedures must consider epidermal melanin, as it is a broadband optical absorber which affects subsurface fluence, effectively limiting the amount of light reaching the dermis and targeted chromophores. An accurate method for quantifying epidermal melanin content would aid clinicians in determining proper light dosage for therapeutic laser procedures. While epidermal melanin content has been quantified non-invasively using optical methods, there is currently no way to determine the melanin distribution in the epidermis. We have developed a photoacoustic probe that uses a Q-switched, frequency doubled Nd:YAG laser operating at 532nm to generate acoustic pulses in skin in vivo. The probe contained a piezoelectric element that detected photoacoustic waves which were then analyzed for epidermal melanin content, using a photoacoustic melanin index (PAMI). We tested 15 human subjects with skin types I--VI using the photoacoustic probe. We also present photoacoustic data for a human subject with vitiligo. Photoacoustic measurement showed melanin in the vitiligo subject was almost completely absent.

Time-resolved optoacoustic (OA) method was employed to measure changes in glucose concentration in the whole and diluted blood. An increase of the glucose level in tissue results in a corresponding decrease of optical scattering. Relative changes in tissue optical scattering can be obtained by measuring the effective optical attenuation coefficient, μ_eff by exponential fitting of the time-resolved optoacoustic profiles. Glucose effects in blood have been investigated using the forward mode of OA detection performed in the visible (at the wavelength, λ=532 nm) and near infrared (λ=1064 nm) spectral ranges. In our previous set of experiments, the OA studies performed in model media in vitro and biological tissue (sclera) in vivo demonstrated gradual reduction of optical scattering with the increase in glucose level. The present study has supported our previous observations. However, one novel effect was observed comprised of a transient increase in μ_eff during the first 5-10 minutes after injection of glucose. This phenomenon may be explained by changes in erythrocytes shape and size as a result of their adaptation to hyperglycemic conditions. Our observation was supported by light microscopy images of red blood cells under normal and hyperglycemic conditions. With glucose concentration changing rapidly (osmotic shock), any small reduction in µeff due to the glucose-induced decrease of relative refraction index of blood, can be compensated or even overwhelmed by the increase in µeff due to erythrocyte shrinkage and/or spherulation. Further cellular adaptation to glucose make erythrocytes return to their normal shape of biconcave disks about 7-μm in diameter. The kinetics of the effective optical attenuation was studies in response to glucose injection in order to better understand the mechanisms of erythrocyte adaptation to osmotic shock and to determine the time course of RBCs adaptation to various glucose concentrations. Finally, Mannitol as alternative osmolyte, which cannot penetrate through the RBC membrane, was used in the study. The effect of Mannitol on optical properties of blood was found to be even more pronounced compared to the effect of glucose. In this study, blood was chosen as an experimental medium with perspective of using the optoacoustic monitoring of glucose concentration either inside veins or in tissues that are well supplied with blood. The results of this study help in designing an optoacoustic measurement protocol for monitoring blood glucose in diabetic patients.

The effects of wavefront distortions induced by acoustic heterogeneities in breast thermoacoustic tomography (TAT) are studied. First, amplitude distortions are shown to be insignificant for different scales of acoustic heterogeneities. Next, the effects of phase distortions (errors in time-of-flight) in our numerical studies are investigated, and the spreads of point sources and boundaries caused by the phase distortions are studied. After that, a demonstration showing that the blurring of images can be compensated for by using the distribution of acoustic velocity in the tissues in the reconstructions is presented. Last, the differences in the effects of acoustic heterogeneity and the generation of speckles in breast TAT and breast ultrasound imaging are discussed.

Photoacoustic tomography, also called opto-acoustic tomography when laser excitation is used, is a novel medical imaging modality that combines the merits of both light and ultrasound. Here, we present our study of laser-induced photoacoustic tomography of organs of small animals. Pulses of 6.5 ns in width from an Nd:YAG laser at 532 nm or 1064 nm are employed to generate the distribution of thermoelastic expansion in the sample. A wide-band ultrasonic transducer that is non-focused in the imaging plane scans around the sample to realize a full-view detection of the imaged cross-section. A modified back-projection algorithm is applied to reconstruct the distribution of optical absorption inside the biological sample. Using optical energy depositions that fall below safe levels, tissue structures in ex-vivo rat kidneys and in-situ mouse brains covered by the skin and skull are imaged successfully with the high intrinsic optical contrast and the high spatial resolution of ultrasound.

The laser optoacoustic imaging system (LOIS) for prostate cancer detection, localization and characterization with 32-element transducer array was developed and tested. Each transducer was made of 50-μm thick PVDF film with dimensions of 1mm x 10 mm. The thickness of the PVDF film allowed 100-μm axial in-depth resolution. Cylindrical shape of the 5-cm long transducer array provided an improved lateral resolution of 0.35 mm. Original design of low noise preamplifiers and wide band amplifiers was employed. The system was optimized for contrast and sensitivity. An automatic recognition of the opto-acoustic signal detected from the irradiated surface was implemented in order to visualize the prostate surface and improve the accuracy of tumor localization. Radial back-projection algorithm adopting Radon transform was used for image reconstruction. The advantages and limitations various contrast enhancing filters applied to the whole image were studied and discussed. Images were acquired in real time with the rate of one frame per second. The system performance was evaluated initially via acquisition of two-dimensional optoacoustic images of small blood volumes in prostate tissue phantoms. Clinical ex-vivo studies of prostatectomy specimen were also performed and compared with transrectal ultrasound.

The further development of a time-resolved photothermal (PT) method for sensing nano-scale absorbing targets is presented. This method is based on irradiation of nano-targets with a short laser pulse and time-resolved PT visualization of laser-induced thermal effects around targets. It is accomplished with a second probe beam that senses the target with an adjustable time delay after the pump laser pulse. The practical capability of this new approach is demonstrated for visualization of nano-scale gold particles (2-250 nm), liposomes (30-90 nm), Neutral Red (NR) particles (30-500 nm), and polystyrene beads as a calibration tool, with a tunable parametric pulse laser-OPO (420-570 nm, 0.10300 μJ, 8-ns width) as a pump laser and Raman-Shifter (639 nm, 13 ns width, 10 nJ) as a probe laser with a tunable delay of the probe pulse relative to thepump pulse in the range of 0-5000 ns. The further development of our PT technique is discussed with a focus on visualization of nano-scale absorbing zones during laser-cell interaction.

Precise knowledge of the acoustic field in biological tissue after pulsed laser irradiation is necessary in order to perform medical imaging. Often, the photoacoustic pulse is delivered as a circular, stress confined laser spot incident upon a planar surface, such as skin. The resulting photoacoustic wave propagates in the tissue and may be detected by a detector or array of detectors. The detected signals can then be used to reconstruct the optical properties and, perhaps, the structure of the tissue. In this paper, we study the beam profile of a circular laser spot incident upon tissue phantoms and propose a paradigm for further investigations, comparing the photoacoustic source as a baffled acoustic piston. Two phantom geometries were studied, a hemisperical acrylamide gel and a dyed water solution. An acoustic transducer with an active area of 200 μm was scanned about the circumference of the hemisphere. The laser spot was incident on the face. For the dye solution, the transducer was submerged and rotated about the laser spot on the water surface. The beam profile of the gel showed some diffractive lobes while the profile from the dyed solution did not, though the main lobe was more robust.

A novel photoacoustic breast phantom was developed using poly(vinyl alcohol) gel prepared by a simple technique that imparts optical scattering to the gel without the neccessity for scattering particles. Tumour simulating gel samples of suitable absorption coefficient were also prepared using a second technique, by adding appropriate quantities of dye at the time of formation; the samples were then cut into spheres. The optical absorption coefficient of the spheres was chosen as between 4 - 7 times that of breast tissue. A breast phantom with a thickness of 60 mm, embedded with such 'tumours' was developed for studying the applicability of photoacoustics in mammography. Time resolved photoacoustics, in a transmission mode, was used to image the inhomogeneities. Light excitation was from a liquid-light guide coupled Nd:YAG laser at 1064 nm, with a 5 ns pulse duration. The guide was mechanically scanned across the surface of the phantom, with the time-of-flight signals recorded using a PVDF based detector array. A modified delay and sum beamforming algorithm was used to reconstruct the photoacoustic sources. Results of these experiments are discussed.

A new method for reconstruction of optoacoustic images is proposed. The method of image reconstruction incorporates multiresolution wavelet filtering into spherical back-projection algorithm. According to our method, each optoacoustic signal detected with an array of ultrawide-band transducers is decomposed into a set of self-similar wavelets with different resolution (characteristic frequency) and then back-projected along the spherical traces for each resolution scale separately. The advantage of this approach is that one can reconstruct objects of a preferred size or a range of sizes. The sum of all images reconstructed with different resolutions yields an image that visualizes small and large objects at once. An approximate speed of the proposed algorithm is of the same order as algorithm, based on the Fast Fourier Transform (FFT). The accuracy of the proposed method is illustrated by images, which are reconstructed from simulated optoacoustic signals as well as signals measured with the Laser Optoacoustic Imaging System (LOIS) from a loop of blood vessel embedded in a gel phantom. The method can be used for contrast-enhanced optoacoustic imaging in the depth of tissue, i.e. for medical applications such as breast cancer or prostate cancer detection.

We have calculated analytically the temporal autocorrelation function of the electric field component of multiply scattered coherent light transmitted through an anisotropically scattering mediuma irradiated with a plane ultrasonic wave. The accuracy of the analytical solution is verified with an independent Monte Carlo simulation. The analytical model shows that an approximate similarity relation exists.

A classical medical ultrasound system was combined with a pulsed laser source to allow laser-induced ultrasound imaging (optoacoustics). Classical ultrasound is based on reflection and scattering of an incident acoustic pulse at internal tissue structures. Laser-induced ultrasound is generated in situ by heating optical absorbing structures, such as blood vessels, with a 5 ns laser pulse (few degrees or fraction of degree), which generates pressure transients. Laser-induced ultrasound probes optical properties and therefore provides much higher contrast and complementary information compared to classical ultrasound. An ultrasound array transducer in combination with a commercial medical imaging system was used to record acoustic transients of both methods. Veins and arteries in a human forearm were identified in vivo using classical color doppler and oxygenation dependent optical absorption at 660 nm and 1064 nm laser wavelength. Safety limits of both methods were explored. Laser-induced ultrasound seems well suited to improve classical ultrasound imaging of subcutaneous regions.

Acousto-photonic imaging (API) is a new approach in biomedical imaging that combines diffuse imaging by photon density waves (PDW) and light "tagging" inside the tissue by focussed ultrasound. This light "tagging" enables 3D optical imaging with mm resolution in tissue limited only by the geometrical extent of the ultrasound focus and the signal to noise ratio. We discuss some possible mechanisms of light "tagging" and its dependance of different parameters. We present several phantom measurements which investigate advantages and disadvantages of API against PDW. The main advantage of API is the possibility of real 3D imaging while its biggest disadvantage is the poor light intensity from deeper regions.

This paper presents a new technique for manipulating particles and cells using laser-induced pressure. This technique is based on irradiating absorbing mediums around the particles, creating thermal and pressure gradients. The authors discuss the role of laser-induced bubble formation in manipulating particles. The authors discovered competition from different forces, with each force acting on objects in a different manner - resultant near-field forces push objects away from the laser beam, while far-field forces pull objects toward the beam. During bubble collapse, dynamic suction has a pulling effect that moves the liquid containing the particle toward the beam. The authors use different pulse lasers - CO₂, Ho:YAG, Nd:YAG, and N₂ - to demonstrate the ability of this new technique to manipulate glass microspheres through different optical schemes, including ring geometry of the laser beam and delivery of laser radiation with fibers. The authors also discuss the role of thermal convection in the particle manipulation.

We have developed a low-coherence optical sensor for detection of laser-induced thermoelastic deformations in biological materials. The presented optical sensor utilizes a birefringent fiber-based dual channel low-coherence Michelson interferometer capable of differential phase measurements. We demonstrate that the low-coherence sensor can be used for spatially-resolved measurements of laser-induced thermoelastic deformations in biological materials with high axial resolution. Experimental studies were carried out using gelatin-based tissue phantoms.

Although tumors can show important contrast in their optical properties at an early stage of development, they are difficult to image optically due the diffusive nature of biological tissues. Such tumors can also be detected by "classical" ultrasound (US) imaging, but the acoustic constrast is often weak at early stages. Acousto-optical (AO) imaging combines light and ultrasound : light carries the desired information and ultrasound provides the spatial resolution. Based on a previous work made by the group of L.V. Wang, we present AO images obtained with chirped US. This modulation of the US frequency allows to encode a spatial region of the medium in the frequency spectrum of the AO signal. We can then obtain the optical contrast along the US path with improved resolution. The technique was apply to the imaging of buried objects in phantoms and to the vizualization of the "virtual source".

Theoretical and experimental investigations of point spread function of focused 32 elements array transducer for laser optoacoustic (OA) tomography have been carried out. The elements were imposed in the radial plane of a cylindrical surface. Specially developed software was used for numerical modeling the problem of OA tomography. Diode-pumped Q-switched Nd:YAG laser was employed for thermo-optical excitation of probe acoustic transients. The correspondence between numerically calculated and measured temporal profiles of acoustic transients detected by a single transducer over wide area around it focal zone was demonstrated. The maps of spatial sensitivity for transducers with aperture angles 30° and 60° were determined experimentally and theoretically. The possibility of localization of the array sensitivity area at the beam waist in the plane perpendicular to the imaging plane has been shown. Back projection algorithm was employed for image reconstruction based on experimentally obtained OA transients from point sources located at different distances from the array. The size of the images allowed to determine transverse and longitudinal resolutions in the imaging plane.

Optoacoustic tomography (OAT) is a medical imaging method for detection of cancerous tumors that uses laser pulses to produce transi ultrasonic waves with spatial profiles replicating distribution of absorbed optical energy. Unlike conventional ultrasonography that uses an external source of acoustic waves, OAT uses transient acoustic waves generated as result of thermal expansion of tissue preferentially heated with short laser pulses. Tissues with different optical properties have different optoacoustic profiles and this enables reconstruction of an acoustic image based on distribution of optical absorption. It is anticipated that the difference in optical absorption between very early tumors and normal tissues might be minimal, justifying application of a contrast agent. Gold Nanoparticles (NP) can be designed to strongly absorb desirable color of laser pulses and effectively produce acoustic waves. Therefore, gold NP can be potentially employed as an optoacoustic contrast agent. We studied sensitivity of optoacoustic imaging in phantoms resembling dimensions and properties of the breast with small objects loaded with gold NPs of various concentrations. Targeted selective loading of breast cancer cells in culture with 40-nm diameter NPs was experimentally demonstrated with electron microscopy and fluorescence labeling techniques. To achieve selective targeting, Herceptin, a monoclonal antibody raised against Her2 receptor was conjugated to NPs using streptavidin-biotin conjugation as a linker. Targeting experiments simultaneously demonstrated that Mab/NPs conjugates inhibit cell proliferation of Her/neu positive cells. These data present the first step in development of a new technology for highly selective cancer chemotherapy with capability to diagnose the presence of malignant tumors and monitor the effects of the treatment.