In order to assess the potential clinical utility of using thermoacoustic computer tomography (TCT) to image the breast, we conducted a retrospective pilot study of 78 patients. We recruited patients in three age groups (<40,40-50,>50 years). The study population was further segregated into normal and suspicious based on the results of the previous x-ray mammography and ultrasound. Image quality was evaluated qualitatively by consensus of two trained mammographers using a 4-point scale. The appearance of normal anatomy, cysts, benign disease and cancer was noted. Patients were also asked to rate the comfort of the TCT exam and to indicate a personal preference for x-ray mammography or TCT. Analysis of the data indicated that TCT image quality was dependent upon both patient age and breast density, improving with both increasing breast density and decreasing patient age. Fibrocystic disease was well seen, cysts appearing as areas of low RF absorption. Fibroadenomas did not demonstrate contrast enhancement with the exception of one patient with associated atypical hyperplasia. Cancer displayed higher RF absorption than surrounding tissues in 4/7 patients in whom cancer was confirmed, including one patient with a 7-mm ductal carcinoma in situ (DCIS).

A clinical prototype of the laser optoacoustic imaging system (LOIS) was employed for breast cancer detection and localization in patients with confirmed breast cancer and scheduled for radical mastectomy. The prototype LOIS used a single optical fiber for delivery of laser pulses, an arc shaped 32-element PVDF transducer array for ultrawide-band piezoelectric detection of optoacoustic signals and a single-channel data acquisition card for signal processing. The resonance ultrasound frequency of the 110 micrometers PVDF film was outside detectable range of ultrasound. Spatial resolution of the transducer array was slightly better than 1mm in radial direction and slightly worse than 1 mm in lateral direction. The system was optimized for contrast and sensitivity. Data acquisition, signal conditioning and image processing were significantly improved and optimized resulting in reduced image frame rate of 2 seconds employing 700 MHz Aphlon processor. The computer code for digital signal processing employed band-pass hyper-Gaussian filtering and denoising. An automatic recognition of the optoacoustic signal detected from the irradiated surface was implemented in order to visualize the breast surface and improve the accuracy of tumor localization. Radial back- projection algorithm was employed adopting combination of integration along spherical wavefronts and integration along planar wavefronts (as in Radon transform) for image reconstruction. The system performance was evaluated initially in breast tissue-like phantoms with embedded blood vessels. Clinical studies in breast cancer patients scheduled for surgical mastectomy were performed and compared with x-ray radiography, ultrasound and pathology reports.

An endoscopic photoacoustic probe is designed and tested for use in PDT treatment of esophageal cancer. The probe, measuring less than 2.5 mm in diameter, was designed to fit within the lumen of an endoscope that will be inserted into an esophagus after PDT. PDT treatment results in a blanched, necrotic layer of cancerous tissue over a healthy, deeper layer of perfused tissue. The photoacoustic probe was designed to use acoustic propagation time to determine the thickness of the blanched surface of the esophagus, which corresponds to treatment depth. A side-firing 600 micrometers fiber delivered 532 nm laser light to induce acoustic waves in the perfused layer of the esophagus beneath the blanched (treated) layer. A PVDF transducer detected the induced acoustic waves and transmitted the signal to an oscilloscope. The probe was tested on clear and turbid tissue phantom layers over an optically absorbing dye solution.

To localize and monitor the blood content in tissue we developed a very sensitive photo-acoustical detector. PVDF has been used as piezo-electric material. In this detector also fibers for the illumination of the sample are integrated. Resolution is about 20 (m in depth and about 50-100 m laterally). We use 532 nm light. We will show how photoacoustics can be used for measuring the thickness of tissue above bone. We will also report measurements on tissue phantoms: e.g. a vessel delta from the epigastric artery branching of a Wistar rat, filled with an artificial blood-resembling absorber. The measurements have been carried out on phantoms containing vessels at several depths. Signal processing was enhanced by Fourier processing of the data.

A photoacoustic imaging system has been evaluated by mapping the temporal distribution of photoacoustic signals generated in a tissue phantom. The phantom comprised an Intralipid scattering solution (μ_s'=1mm^-1 and μ_a=0.01mm^-1) containing two regions of enhanced absorption; a 1.5mm thick layer (μ_a=1mm^-1) and a 75μm layer (μ_a=40mm^-1). These were located 1cm beneath the surface of the Intralipid which was irradiated with 7ns Q switched laser pulses of fluence 0.05 J/cm². A Fabry Perot polymer film ultrasound sensing interferometer was used for the detection of the photoacoustic signals. A 1D ultrasound array was simulated by illuminating the sensing interferometer with a large diameter laser beam and line scanning a photodiode across the reflected output beam. The detection sensitivity of the system was 3kPA over a 25MHz measurement bandwidth, the 3dB acoustic bandwidth was 17.5MHz, the sensitive azimuthal 'array' aperture and element size were 12mm and 0.8mm respectively. Greyscale images of the time-resolved photoacoustic signals enabled both absorbers to be clearly identified with an axial spatial resolution of less than 100micrometers . It is envisaged that this approach could form the basis of a practical photoacoustic tissue imaging system.

Photothermal method was applied to improve sensing and imaging capabilities of a light microscope in cell studies. We describe the methods, technical details and testing results of cytometric application of Laser Photothermal Phase Microscope (LPPM). The merits of the proposed approach include living single cell monitoring capability, quantitative measurement of cell functional features through the use of cell natural chromophores as the sensors. Such intracellular sensors are activated by the laser pulse and transform an absorbed energy into the heat. The latter causes thermal and mechanical loads to a cell and its components. The second stage of the process includes the reaction of the cell as integral system or of its components to such loads. This reaction is caused by the changes of cell functional and structural state and includes alterations of cell optical properties. Both processes are monitored for a single cell non-invasively with probe laser beam. Pulsed phase contrast dual beam illumination scheme with acquisition of several laser images at different stages of cell-laser interaction was introduced. An acquired cell image is considered as spatially and temporally resolved cell response to non-specific load that is induced in a cell with a pump laser. This method eliminates any cell staining and allows to monitor cell viability and cell reaction to the environmental factors. Also LPPM offers further improvement of spatial and temporal resolution of optical microscope: with pulsed probe laser monitoring we can detect components with the size down to 50 nm and temporal resolution of 10 ns. In our set up the cell is pumped by pulsed laser at 532 nm, 10 ns , 0.01-0.4 mJ. The source of probe beam is a pulsed dye laser (630 nm, 10 nJ, 10 ns) which forms cell phase image. The results obtained with living cells such as drug impact control, single cell dosimetry, immune action of light on a cell demonstrate basic features of LPPM as the tool for the study of the spectral and spatial low-absorption distribution in living cells in addition to techniques of active thermal probing of sub-micron, low-absorbing samples.

In this work, we show the capability of laser optoacoustics to localize the position of the ciliary body on enucleated porcine and rabbit eyes. Our findings correspond well with histological sections of the measured area. Different wavelengths for an optoacoustical detection system in combination with laser cyclophotocoagulation have been compared taking grayscale images of the region of interest of rabbit and porcine eyes for various wavelengths in the NIR spectral range. Additionally, the changes in the optical properties of the tissue induced by coagulation with a diode laser were observed. First online measurements of the changes due to coagulation show that the method of laser optoacoustics is suitable for an online therapy control system.

Optoacoustic tomography is proposed and evaluated as a method for visualization and quantitative measurements of drugs, contrast agent and other substances penetration in skin and nails. Monitoring was made with the optoacoustic front surface transducer (OAFST) operating in backward mode. Denatured egg-white was used as a model for homogeneous porous tissue, and porcine skin in vitro was employed as a model for human skin. Experimental results demonstrated that transdermal penetration and penetration through egg- white have different mechanisms. Properties and behavior of percutaneous layers of porcine skin after application hydrophilic and lipophilic substances was different. An attempt to monitor drug delivery through a nail was made, however, no significant penetration of aqueous solutions in the nail was observed. The inner nail structure was visualized with optoacoustic tomography. We conclude that optoacoustic monitoring of tissue is feasible with spatial resolution limited only by duration of laser pulses. Axial in-depth resolution of 18 μm was achieved with 12-ns long laser pulses.

The selective damage of the retinal pigment epithelium (RPE) is a new treatment method for several retinal diseases. By applying a train of microsecond(s) laser pulses it is possible to selectively damage these cells and simultaneously spare the adjacent photoreceptor and neural tissue. Due to the ophthalmologic invisibility of the RPE cell damage we investigate an optoacoustic (OA) control system to monitor the RPE cell damage. Setup: The irradiation was performed with a frequency doubled Nd:YLF laser by applying a train of +s laser pulses. In vitro, the OA transients were received by an ultrasonic broadband transducer. During treatment an OA contact lens with embedded transducer was used. In vitro: Freshly enucleated porcine RPE samples with CalceinAM as life/death staining were used. Below RPE cell damage threshold a classic thermoelastic transient was found. Above cell damage threshold the OA transient differs form pulse to pulse. This can be explained by microbubble formation around the strong absorbing melanosomes inside the RPE cells. In vivo: We found the same pulse to pulse deviations of the OA transient above the fluoresceine angiographic detectable RPE damage threshold during treatment. This system give us a new approach to non-invasively monitor the selective RPE treatment.

The near-infrared photoacoustic technique is recognized as a potential method for the non-invasive determination of human glucose, because near-infrared light can incident a few millimeters into human tissue, where it produces an acoustic wave capable of carrying information about the composition of the tissue. This paper demonstrates a photoacoustic glucose measurement in a blood sample as a step toward a non-invasive measurement. The experimental apparatus consists of a near-infrared laser diode operating with 4 micro joules pulse energy at 905 nm, a roller pump connected to a silicon plastic tube and a cuvette for circulating the blood sample. In addition, the apparatus comprises a PZT piezoelectric transducer integrated with a battery-powered preamplifier to receive the photoacoustic signal. During the experiment, a glucose solution is mixed into a human blood sample to change its concentration. Although the absorption coefficient of glucose is much smaller than that of blood in the near-infrared region, the osmotic and hydrophilic properties of glucose decrease the reduced scattering coefficient of blood caused by the dissolved glucose surrounding the blood cells. This changes the distribution of the absorbed optical energy in blood, which, in turn, produces a change in the photoacoustic signal. Our experiment demonstrates that signal amplitudes in fresh and stored blood samples in crease about 7% and 10%, respectively, when the glucose concentration reaches the upper limit of the physiological region (500 mg/dl).

Microwave-induced thermoacoustic tomography was explored to image biological tissues. Short microwave pulses irradiated tissues to generate acoustic waves by thermoelastic expansion. The microwave-induced thermoacoustic waves were detected with a focused ultrasonic transducer to obtain two-dimensional tomographic images of biological tissues. The dependence of the axial and the lateral resolutions on the spectra of the signals was studied. A self-adaptive filter was applied to the temporal piezoelectric signals from the transducer to increase the weight of the high-frequency components, which improved the lateral resolution, and to broaden the spectrum of the signal, which enhanced the axial resolution.

The results of experimental studies of 2D temperature profiles reconstruction inside soft tissues are presented. The 2D images are obtained as the result of mathematical data processing in multichannel scanning acoustic thermotomograph (AT). Operation of this device is based on the receiving of weak acoustic emission produced by thermal motion of medium particles. The intensity of received signal is proportional to the acoustical brightness temperature of emitting object, i.e. to its temperature and sound absorption. Some evident applications of this method are related with early tumor detection and internal temperature measurement during hyperthermal treatment. This kind of passive scanning provides great safety of investigation combined with rather good spatial resolution due to short wavelength and high directivity of AT antenna. Our experiments demonstrated localization of overheated phantom objects inside tissue-like absorbing media with temperature contrast about 0.4 K and 2 mm resolution at 2D images with area size 210x40 mm. The advantages and possible applications of this kind of clinical investigations are illustrated by the results of in vivo experiments on the variation of acoustic brightness temperature of human limbs and liver during some physiological tests. This work was supported by RFBR (project #00-02-16600).

The acoustical thermovision (AT) method is based on the passive registration of thermal acoustical noise by ultrasonic antenna integrating signal over its receiving field. This method allows to find overheated object and measure its temperature contrast over surrounding medium. Various scanning and tomographic algorithms can be used to improve spatial resolution. One of the possible ways is to use focused antenna selecting signal from its focal area, but the localization quality strongly depends on the object transversal size. To reduce this bottleneck we can use the correlation processing from two focused antennae with combined focal areas. The spatial resolutions of AT with focused antenna and the correlation AT with two focused antennae were compared for various objects. The dependence of device output on the distance between antenna and object in both cases was calculated. The intensity maximum of emission received by the correlation AT is localized near antenna focus. Scanning this focal area across an object allows one to obtain spatial images. The 2D images of medium with inhomogeneous temperature distribution were calculated and the efficiency of correlation method was demonstrated. Thus correlation AT method provides new diagnostically significant information and allows us to approach to clinical application. This work was supported by RFBR (project #00-02-16600).

Mathematical model of image reconstruction for two-dimensional optoacoustic imaging system is described. It was assumed that receiving transducers are uniformly distributed along the perimeter of a 60 mm radius ring with 2.1 mm gaps between transducer centers and initial data were known with 0.1 mm increments. The algorithm of radial back projection with convolution was used for optoacoustic image reconstruction. The convolution was evaluated with modified Shepp-Logan (MSL) and rectangular (RECT) space spectrum windows. Linear interpolation was applied for calculation of the convolution at the intermediate space points. The following four criteria were employed for estimation of resulting image quality: noise level on entire tomogram, a jump transfer function, loss contrast function and the contrast-dimension reflation. Theoretical expressions for these parameters were derived and used for optimization of the proposed algorithm. Two-dimensional images of computer simulated spherical objects were reconstructed. It was shown that 0.1 mm spatial resolution could be obtained provided the signal-to-noise ratio equals approximately 3 at the tomogram. A very small (0.2 mm diameter) tumor and a small (2-mm diameter) tumor could be clearly revealed at the tomogram if their optical absorption contrast equals at least 2 and 0.1 respectively.

Optoacoustics is a method to gain information from inside a tissue. This is done by irradiating a tissue with a short light pulse, which generates a pressure distribution inside the tissue that mirrors the absorber distribution. The pressure distribution measured on the tissue-surface allows, by applying a back-projection method, to calculate a tomography image of the absorber distribution. This study presents a novel computational algorithm based on Fourier transform, which, at least in principle, yields an exact 3D reconstruction of the distribution of absorbed energy density inside turbid media. The reconstruction is based on 2D pressure distributions captured outside at different times. The FFT reconstruction algorithm is first tested in the back projection of simulated pressure transients of small model absorbers, and finally applied to reconstruct the distribution of artificial blood vessels in three dimensions.

Three-dimensional optoacoustic imaging uses detection of laser-generated thermoelastic waves with an ultrasound sensor array. Integrated acoustic signals (velocity potentials) are back projected into the source volume to give a map of absorbed laser energy. Since the number of array elements and the receiving solid angle are limited, radial back projection produces artifacts such as back projection arcs. To solve this problem, we developed in this study an iterative method for image reconstruction. A first image estimate was generated by simple radial back projection. A model for signal generation from a volume containing arbitrary optoacoustic sources was then used to calculate acoustic wave propagation from this estimate. Calculated signals at the array elements were compared with the measured signals and the difference was used to improve the image. In simulations and experiments we used the algorithm to obtain three-dimensional images of multiple optoacoustic sources. With signals from an array of 3 x 3 detector elements a significant improvement was observed after about 10 iterations compared to the simple radial back projection. Although computationally more intensive, iterative reconstruction can minimize the time and instrumentation for signal acquisition because a small number of array elements already gives a good quality optoacoustic image.

This paper is devoted to comparison new optoacoustic tomography with conventional breast tumors diagnostic techniques such as ultrasonography and X-ray radiography. Experiments were performed in phantoms simulating breast with tumors. The fundamental harmonic of Q-switched Nd:YAG laser (λ = 1064 nm) was used to generate optoacoustic pressure waves. Laser induced pressure waves were detected by a wide-band acoustic transducer. Digital oscilloscope controlled by PC was used to store and process optoacoustic signals. Gelatin phantoms with controlled optical parameters were prepared to simulate breast with tumors. Absorbing volumes colored with naphthol green and hemoglobin were embedded in the gelatin phantoms to model the breast tumors with increased optical absorption. Optoacoustic pressure waves form the phantoms were detected at different angles and 2D images were reconstructed. Comparison of optoacoustic images with images obtained with ultrasound and X-ray techniques proved that optoacoustic method has substantially higher contrast and resolution. Obtained results confirm that laser optoacoustic imaging technique can be an important tool for early breast cancer detection with tumors less than 5 mm in diameter.

Monitoring of laser energy absorbed inside tissue is very impotent for laser thermocoagulation of tumors, laser surgery etc. Experimental results have shown that analysis of optoacoustic signal magnitude induced by short laser pulse inside tissue can give quantitative information about laser fluence absorbed by the tissue. We have investigated some tissue phantoms with absorbing objects inside. The first harmonic (1064 nm) of Q-switched Nd:YAG-laser was used for generation of optoacoustic signals.

The results of theoretical and experimental investigations of pulsed optoacoustic (OA) method for tomography of biological objects in the frequency range 1-10 MHz at the depths of up to 5 centimeters are presented. Some key imaging problems - reducing of effect of surface OA pulse and light shock on the received signal, enhancing of the uniformity of light distribution inside the object keeping light irradiation at the reliable level - can be solved by choice of experiment configuration and spatial scanning. Taking into account distortions of OA signal due to frequency dependence of tissue properties and receiving/amplifying circuits the acoustical source position can be found using signal processing e.g. inverse filtering algorithm. The next step to the high-quality imaging is in use of reconstructive tomographic algorithms. The experimental setup was designed for OA tomographic investigation of phantoms (artificial objects and biotissue samples) as well as in vivo objects. The received signals were amplified, digitized and stored in PC. The experiments with the model objects were carried out to elaborate principles of multichannel receiving by weakly-directed probes and to study the methods for reconstruction of 2D tomograms. The results show that post-processing and choice of experiment geometry allow to improve significantly the quality of optoacoustic tomograms. This work was supported by RFBR (Project #00-02-16600).

The transient grating method acts as a monitor of the evolution of thermal energy following optical excitation of an absorbing molecule. The signals produced by a photodynamic therapy agent are shown to be strongly dependent on presence of oxygen in solution indicating transfer of energy from a triplet state of the dye to form excited ¹Δ_g oxygen. Analysis of the data shows that the efficiency of excited oxygen production can be determined by a recording of the diffracted light intensity versus time.

The results are presented which form the basis against which the technique of photoacoustic PA diagnostics of inhomogeneous liquid samples can be developed. Usually, a thin-layer small-volume inhomogeneous probe presents a considerable challenge for diagnostics. The paper shows that gas-microphone PA technique works well here, and moreover it presents an added bonus if the thickness of the layer is close to the dimensions of immersed inhomogeneities.

Thermal mechanism is fundamental for the generation of ultrasonic waves in a course of absorption of laser radiation. Its efficiency is relatively low, but increases significantly with temperature (thermal non-linearity). If the phase transition of the irradiated medium occur, the efficiency may exceed its linear regime value several orders of magnitude. The idea of optoacoustic supercontrast for early cancer detection is based on this fact. We performed feasibility studies and describe requirements to and properties of the optoacoustic supercontrast agent based on nanoscopid particles. The results of the preliminary experiments with the metal and carbon nanoparticles as optoacoustic generation up to three orders of magnitude under irradiation conditions of laser optoacoustic imaging in the depth of human tissue.

Monte-Carlo modeling technique was used to simulate the ultrasound-modulated optical tomography. The difference between absorption and scattering objects was compared. Simulation result indicated that this technique is sensitive to object absorption property, while the scattering properties have less effect on the output AC/DC signal intensity. It was also demonstrated that inhomogeneity and the background optical properties of the scattering medium could change the AC/DC value. The signal-to-noise ratio problem in the experiment is carefully analyzed. The major noise source is the speckle noise caused by the small particle movement within the tissue sample. The decorrelation time of the speckle pattern was measured in the tissue sample. In order to reduce the speckle noise, the data acquisition time must be less than the speckle decorrelation time.

Acousto-optic imaging consists in tagging multi-scattered photon paths with a focused ultrasonic beam. This technique should give optical information on hidden structures in several centimeter thick scattering media, with a millimetric resolution. We have coupled our previous acousto-optic imaging setup with a suitably designed echograph. Thanks to a single 3 MHz multi-ring emitter, working either in pulsed or c.w. mode, we can get acoustic as well as acousto-optic responses of structures in biological tissues.

An analytic model of the ultrasonic modulation of multiply scattered coherent light in scattering media is provided. The model is based on two mechanisms: the ultrasonic modulation of the index of refraction, which causes a modulation of the optical path lengths between consecutive scattering events, and the ultrasonic modulation of the displacements of Rayleigh scatterers, which causes a modulation of optical path lengths upon each scattering event. Multiply scattered light accumulates modulated optical path lengths along its path. Consequently, the intensity of the speckles that are formed by the multiply scattered light is modulated. In water solutions, for example, the contribution from the index of refraction is slightly less than the contribution from displacement when the scattering mean free path is less than a critical fraction of the acoustic wavelength, and it becomes increasingly greater than the contribution from displacement beyond this critical point.

On the basis of the suggested model of coherent-optical interaction of laser radiation with ultrasonic wave, the influence of geometry of experiment on the amplitude and phase characteristics of ac photo-detector current at the ultrasound frequency was experimentally and theoretically analyzed. For intermediate scattering regime where the ballistic and low-step scattered light components are measurable their contributions to ac photocurrent were separated by experimental data processing. With a use of theoretically predicted and experimentally confirmed optimal conditions of the measurement, the images of the absorbing half-plane were reconstructed in weak scattering media with use of the distributions of the ac amplitude of the current at 3 MHz ultrasound frequency and dc current. The possibilities of the acousto-optical imaging of multiply scattering medium were discussed on the basis of the results obtained after comparison with traditional measurements of dc photocurrent distributions.

We have developed an optical cross sectional imaging method for scattering media with the aid of a pulse ultrasound wave. A converging pulse ultrasound wave and a He-Ne laser beam, which are set on the same axis and same direction, are irradiated simultaneously to a sample. As the pulse ultrasound wave travels into the sample, the instantaneous position of the wave changes the optical properties of a localized region of the sample and modulates the scattered intensity of the incident laser light. We detected the modulation of the scattered light that is transmitted through the sample to observe the absorptive features inside a thick sample. Depth resolving capability is achieved from the time-dependent measurement of the scattered light intensity. This system achieves 3D resolution determined by the focal spot of the focused ultrasound wave. The resolution of our system is measured at 1.5 mm in the axial direction and 0.3 mm in the transverse direction. We verify the system by observing an absorptive object, which is a silk thread stained with Victoria blue dye, that is embedded in silicone rubber. We also obtained a cross-sectional image of an absorbing object that is surrounded by a 10-mm thick highly scattering medium.

A total of 364 optical source-detector pairs were uniformly deployed over a 9 x 9 cm² probe area initially, and then the total pairs were gradually reduced to 60 in experimental studies. For each source-detector configuration, 3D images of a 1 cm diameter absorber of different contrasts were reconstructed form the measurements made with a frequency domain system. The results have shown that more than 160 source-detector pairs are needed to reconstruct absorption coefficient within 60% of the true value and appropriate spatial and contrast resolution. However, the error in target depth estimated from 3D images was more than 1 cm in all source-detector configurations. With the a priori target depth information provided by ultrasound, the accuracy of the reconstructed absorption coefficient has been improved by 15% and 3% on average for high and low contrast cases, respectively. The speed of reconstruction has been improved by 10 times on average.

In this paper, we first report experiments on confocal tomography of sonoluminescence media with objects buried in both biological tissue and tissue-simulating media. A high-sensitive confocal scanning setup based on photon counting technique was developed. With the system, a carbon cube buried in tissue simulation medium was imaged with high contrast and a lateral resolution of about 100μm. For the tissue experiment a carbon stick buried in muscle tissue was optically sectioned with the sonoluminescence confocal scanning technique. The spatial resolution of the image at present is limited by the intensity of the sonoluminescence, and could be largely improved by increasing the intensity of emission by a sono-chemluminescence method. The optical tomography of sono-chemluminescence method has potential applications in clinical diagnosis.

Optoacoustic imaging is an imaging technique based on the time-resolved arrival of sound waves at an array of pressure transducers on the tissue surface, where the sound waves were generated by thermoelastic expansion of optically absorbing objects within the tissue when heated by a pulsed laser. This paper discusses the time-resolved pressures and velocity potentials create by pulsed laser irradiation of absorbing objects of varying shape, as well as the process of back-projection of such data to create an optoacoustic image of the distribution of absorbing objects.

We describe a new technique, Thermosonics, that can be used to detect cracks in teeth. This technique was initially invented and developed for finding cracks in industrial and aerospace applications. The thermosonics technique employs a single short pulse (typically tens of milliseconds) of ultrasound excitation combined with infrared imaging. Ultrasonic waves vibrate the target material. This vibration causes rubbing and clapping between faying surfaces of any cracks which are present, resulting in a temperature rise around the cracks. An infrared camera is used to image the temperature distribution during and after the ultrasound excitation. Thus, cracks in teeth can be detected. Although this technique is still under development, it shows promise for clinical use by dentists.

Probing photon density in diffusive media is very important in order to model and understand their propagation. It is possible to detect photons outside the medium, but their non-invasive detection inside it is still an unsolved problem. An elegant, semi-invasive approach to perform this task is to scan a small absorbing sphere inside the turbid medium and measure the light outside the sample when the sphere is present and when it is not. However this method requires the medium to be liquid and such a procedure cannot be performed in the case of biological tissues. Ultrasound tagging of light has been introduced initially for transillumination imaging in turbid media, and then extended to the case of reflection imaging. Here we present results showing that it is possible to map the photon density inside solid turbid media by locally tagging photons using an ultrasonic field. We experimentally retrieve the well-known banana-shaped photons distribution when the source and the detectors are in a back-scattering configuration, using a gel-based homogeneous phantom. We also present experiments where hemoglobin has been introduced inside the gel. By fitting the experimental results with the theoretical formula, we are able to quantitatively retrieve the amount of hemoglobin introduced inside the gel, not only from data obtained by scanning the ultrasound waist inside the phantom, the in put and output fibers staying fixed.