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Oxidative modification of the major cholesterol carrying lipoprotein, low-density lipoprotein (LDL), renders it more atherogenic as well as inducing unique fluorescence spectral characteristics that distinguish it from native (non-oxidized) LDL. This fluorescence feature has been identified using a microspectrofluorometry system capable of recording autofluorescence of individual cultured macrophages incubated with oxidized LDL. Differences in fluorescence spectra between individual control cells and oxidized LDL loaded cells could also be elicited using dye-enhanced fluorescence with neutral lipid probes such as nile red. Autofluorescence spectroscopy applied to the detection of intracellular oxidized LDL accumulation in circulating monocytes may be useful for identifying a novel risk factor in the assessment of atherosclerosis.

Laser opto-acoustic tomography based on time-resolved detection of laser-induced stress transients is tested in vitro (gel phantoms) and in vivo (mouse model of breast cancer) as a method for diagnostic screening of breast tumors. A small gel sphere with diameter of 2.5 mm and optical properties that resemble that of carcinoma was detected at a depth of 25 mm in a gel phantom with optical properties that simulate normal breast tissue. Substantially stronger signals were observed in tumors of two mice. The laser-induced stress profiles qualitatively different from profiles in normal mammary glands were observed. Combination of the two results encourages our group to plan first clinical trials on laser opto-acoustic mammography.

Purpose: Optoacoustic tomography (OAT) is inherently a three-dimensional (3D) inverse problem. However, most studies of OAT image reconstruction still employ two-dimensional (2D) imaging models. One important reason is because 3D image reconstruction is computationally burdensome. The aim of this work is to accelerate existing image reconstruction algorithms for 3D OAT by use of parallel programming techniques. Methods: Parallelization strategies are proposed to accelerate a filtered backprojection (FBP) algorithm and two different pairs of projection/backprojection operations that correspond to two different numerical imaging models. The algorithms are designed to fully exploit the parallel computing power of graphic processing units (GPUs). In order to evaluate the parallelization strategies for the projection/backprojection pairs, an iterative image reconstruction algorithm is implemented. Computer-simulation and experimental studies are conducted to investigate the computational efficiency and numerical accuracy of the developed algorithms. Results: The GPU implementations improve the computational efficiency by factors of 1, 000, 125, and 250 for the FBP algorithm and the two pairs of projection/backprojection operators, respectively. Accurate images are reconstructed by use of the FBP and iterative image reconstruction algorithms from both computer-simulated and experimental data. Conclusions: Parallelization strategies for 3D OAT image reconstruction are proposed for the first time. These GPU-based implementations significantly reduce the computational time for 3D image reconstruction, complementing our earlier work on 3D OAT iterative image reconstruction.

Our studies are directed towards the development of a pulsed laser based optoacoustic technique to visualize absorbed light distribution in irradiated tissues. Optoacoustic technique utilizes the time-resolved detection of laser-induced stress transients to visualize absorbed laser fluence distribution in opaque and heterogeneous tissues. The acoustic signal induced under confined stress conditions of irradiation by an Nd:YAG laser pulse displays Z-axial light distribution and may be used for imaging the tissue layers where a temperature-rise of about 1 degree(s)C is achieved. Scanning of the acoustic transducer along the tissue surface line by line until entire surface of interest is tested, permits reconstruction of a 3D optoacoustic image of the irradiated tissue. Z-axial resolution of optoacoustic imaging is defined as a product of the temporal resolution of piezoelectric transducer and the speed of sound in tissues. Lateral (radial) resolution of optoacoustic images is a funtion of piezoelectric detector diameter, the diameter of laser-induced acoustic wave, and a depth of optoacoustic probing (acoustic diffraction factor). The role of various parameters, such as tissue optical properties, tissue thickness, laser beam diameter, and diameter of piezoelectric element, were evaluated for imaging resolution in lateral direction. The results of our studies demonstrated that a broad- band acoustic transducer may become a useful tool for in vivo diagnostic imaging and feed- back information during clinical laser procedures. First in vivo measurements were performed and found to be in agreement with tissue histology. This study is a very first step in the basic design of optoacoustic tomographic technology for light absorbing tissues.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Insufficient light absorption in hard dental tissues makes laser ablation in near UV, visible or near IR spectral ranges very inefficient to be employed for tooth cavity preparations. We used deposition of a liquid absorber, indocyanine green (ICG) dye, to overcome this problem. Experiments employed Alexandrite laser anticipating that future near IR diode laser technology will replace existing medical lasers. Ablation kinetics and mechanisms for both free-running and Q-switched modes of Alexandrite laser were studied with the aim to determine optimal parameters of laser irradiation and optimal volume of the dye. Four experimental parameters were monitored during each ablation event: (1) incident laser fluence, (2) temporal profile of the laser pulse, (3) temporal profile and magnitude of laser-induced stress transients, (4) temporal profile and spectrum of plasma emission. We also examined kinetics of plume by probing ablation products with CW He-Ne laser beam. Results depicted ablation process as a complex multistage phenomenon. Two distinct stages associated with the tooth ablation are revealed in the free-running mode: (1) ablation of a dye droplet from a tooth surface by the first laser micropulse of a 250-microsecond(s) macropulse, (2) plasma mediated ablation of a melted layer of enamel produced by thermal explosion of the dye. Plasma jet formation was delayed 10-100 microsecond(s) against the beginning of free-running pulse. Ablation stages and their efficiency are defined by laser irradiation parameters, dye concentration and its total volume. In contrast, Q-switched (nanosecond) laser ablation occurs as a one stage process, and, therefore, less efficient. In addition, Q-switched mode irradiation induces shock waves amplitudes that are about an order of magnitude higher compared with that induced by the free-running irradiation. Experimental comparison of Q-switched and free-running modes of irradiation is evident in favor of free-running mode that produces a nice smooth crater without noticeable thermomechanical damage to surrounding tissues.

Plasma mediated ablation of collagen gels and porcine cornea was studied at various laser pulse durations in the range from 350 fs to 1 ns at 1053 nm wavelength. A time resolved stress detection technique was employed to measure transient stress profiles and amplitudes. Optical microscopy was used to characterize ablation craters qualitatively, while a wide band acoustic transducer helped to quantify tissue mechanical response and the ablation threshold. The ablation threshold was measured as a function of laser pulse duration and linear absorption coefficient. For nanosecond pulses the ablation threshold was found to have a strong dependence on the linear absorption coefficient of the material. As the pulse length decreased into the subpicosecond regime the ablation threshold became insensitive to the linear absorption coefficient. The ablation efficiency was found to be insensitive to both the laser pulse duration and the linear absorption coefficient. High quality ablation craters with no thermal or mechanical damage to surrounding material were obtained with 350 fs laser pulses. The mechanism of optical breakdown at the tissue surface was theoretically investigated. In the nanosecond regime, optical breakdown proceeds as an electron collisional avalanche ionization initiated by thermal seed electrons. These seed electrons are created by heating of the tissue by linear absorption. In the ultrashort pulse range, optical breakdown is initiated by the multiphoton ionization of the irradiated medium (6 photons in case of tissue irradiated at 1053 nm wavelength), and becomes less sensitive to the linear absorption coefficient. The energy deposition profile is insensitive to both the laser pulse duration and the linear absorption coefficient.

Plasma mediated ablation of collagen gels and porcine cornea was studied at various laser pulse durations in the range from 350 fs to 1 ns at 1053 nm wavelength. A time resolved stress detection technique was employed to measure transient stress profiles and amplitudes. Optical microscopy was used to characterize ablation craters qualitatively, while a wide band acoustic transducer helped to quantify tissue mechanical response and the ablation threshold. The ablation threshold was measured as a function of laser pulse duration and linear absorption coefficient. For nanosecond pulses the ablation threshold was found to have a strong dependence on the linear absorption coefficient of the material. As the pulse length decreased into the subpicosecond regime the ablation threshold became insensitive to the linear absorption coefficient. High quality ablation craters with no thermal or mechanical damage to surrounding material were obtained with 350 fs laser pulses. The mechanism of optical breakdown at the tissue surface was theoretically investigated. In the nanosecond regime, optical breakdown proceeds as an electron collisional avalanche ionization initiated by thermal seed electrons. These seed electrons are created by heating of the tissue by linear absorption. In the ultrashort pulse range, optical breakdown is initiated by the multiphoton ionization of the irradiated medium (6 photons in case of tissue irradiated at 1053 nm wavelength), and becomes less sensitive to the linear absorption coefficient. The energy deposition profile is insensitive to both the laser pulse duration and the linear absorption coefficient.

Hyperlipidemic states are associated with focal accumulations in arterial walls of oxidatively modified low density lipoprotein (LDL) and monocyte-derived liquid-laden macrophages, biochemical and cellular hallmarks of atheromatous lesions. Mechanisms underlying the generation and cellular uptake of oxidized LDL are still incompletely understood. We have used laser-induced fluorescence spectroscopy to monitor the formation, intracellular accumulation, and tissue distribution of oxidized LDL. Oxidatively modified LDL excited by a XeCl excimer laser (308 nm) exhibits unique spectral characteristics that distinguish it from native (non-oxidized) LDL. The same spectral characteristics were demonstrated in lipid-rich atheromatous lesions, macrophages after incubation with oxidized LDL, and peripheral blood monocytes from hyperlipidemic, but not normolipidemic subjects. Detection of oxidized LDL in peripheral blood monocytes and arterial tissue may provide information on the atherogenic activity of hyperlipidemic states and serve as a novel risk factor for the assessment of atherosclerosis.

Pulsed laser ablation of biological tissues under confined stress conditions (PACS) is demonstrated to be precise tissue removal with minimal thermal and mechanical damage to adjacent tissue layers. PACS was described based on results obtained with four complementary techniques: (1) laser-flash photography, (2) time-resolved stress detection (TRSD), (3) laser Michelson interferometry, and (4) pendulum recoil measurements. Comparison of ablation process was made between aqueous solutions and collagen gels. Experiments demonstrated two distinct ablation stages with different ablative forces that drive material ejection at temperatures substantially below 100 degree(s)C. The first stage is associated with the ejection of thin subsurface layer of the irradiated volume. The material ejection from a superficial layer (d << 1/(mu) eff) is caused by a rapid growth of cavitation bubbles produced by laser-induced thermoelastic expansion in irradiated volume. The second, delayed stage of material removal occurs due to the collapse of coalesced cavitation bubbles in the depth (d

A new method to determine optical properties of biological tissue, time-resolved stress detection (TRSD), was tested on albino rat skin, heated in vitro. The TRSD method is based on the detection of pressure waves that are generated by short laser pulses. The pressure waves carry information about the optical properties (absorption and effective attenuation coefficients) of the medium. The evaluation was done in a systematic way by comparing the results of the TRSD method with results obtained using an integrating sphere. Measurements for both methods were performed on the same skin samples to minimize biological variations. The TRSD method proved to be reasonably successful to determine optical properties of tissue for the used wavelength of 355 nm: (1) The effective attenuation coefficients of both methods agreed very well; (2) The TRSD method gave absorption coefficients of a factor 2 + 0.5 times higher than the integrating sphere method; and (3) The reduced scattering coefficients were sensitive to error.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Fluorescence spectroscopy experiments were focused on circulating monocytes and endothelial cells, two cell types that play a central role in the development and progression of atheromatous lesions. Our aim was to gain new insights into the cellular physiology of atherosclerosis and allow the development of diagnostic tests for the evaluation of atherosclerosis risk.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Research is being performed at several locations in consideration of learning the value of the front surface spallation process for tissue ablation. The experimental results are illustrating complicated features associated with the microscopic behavior of materials during the spallation process. This paper summarizes the current understanding of the spallation process in various materials and presents an interpretation of recent laser-tissue interaction experiments.

A new technique based on time-resolved detection of laser-induced stress transients is proposed to visualize the distribution of absorbed laser fluence in turbid and layered biological tissues.

The optical properties represented by the absorption coefficient ((mu) a ) and reduced scattering coefficient [(mu) s (1-g) or (mu) ' s ] at (lambda) equals 355 nm of thermally altered albino rat skin were measured in vitro by two methods: (1) a time-resolved stress detection (TRSD) technique which directly measured the effective attenuation coefficient ((mu) eff ) and the absorption coefficient ((mu) a ), and (2) the well-known integrating sphere technique which measured total transmittance (T t ) nd total diffuse reflectance (R d ). The skin pieces were wrapped in water-tight packets and heated for 20 minutes in a calibrated water bath (temperature range: 20 degree(s) - 90 degree(s)C) and the same skin samples were used for both measurement methods. The experimental data were analyzed to specify the absorption and the scattering properties. The results, which were in general agreement for both methods, indicated that denaturation of the rat skin caused a decrease in scattering due to melting of the collagen fibers. The decrease began at 55 degree(s)C and plateaued at 65 degree(s) - 70 degree(s)C and was essentially unchanged at higher temperatures. Absorption was not significantly affected by denaturation except for a transient rise at 50 degree(s) - 60 degree(s)C.

The threshold radiant exposure [Hth (J/cm2)] at the retina which produces a minimal visible lesion (MVL) has been investigated as a function of the laser pulse duration (tp). By considering the optical absorption coefficient of the melanosomal interior, (mu) a.melanosome, one can calculate the threshold deposited energy, Qth equals (mu) a.melanosomeHth (J/cm3), for the MVL. The tp-dependence of Qth is adequately explained for tp > 16 microsecond(s) by the thermal relaxation of heated melanosomes in the retinal pigmented epithelium (RPE). However, at very short pulses (< 100 ps), there is an apparent on the order of 10-fold drop in the Qth which is possibly due to the onset of a photomechanical mechanism of damage. Thermoelastic expansion of the laser-heated melanin granules (approximately 10 nm in size) within the 1.5-micrometers melanosome is induced by laser pulses less than 50 ps in duration. This expansion occurs faster than the induced pressure can dissipate from the granules at the speed of sound. The stress relaxation time of a 10-nm melanin granule is about 7 ps. As the accumulated pressure attempts to propagate out of the granule as a pressure wave, the pressure wave suffers reflectance at the granule surface boundary due to the difference in acoustic impedances of the granule and surrounding intramelanosomal matrix. About 12% of the acoustic energy is estimated to be reflected back into the granule as a negative (tensile) pressure wave. This negative stress is hypothesized to elicit cavitation within the melanin granule. This mechanism of intragranule cavitation is a working hypothesis for the mechanism of the MVL in the sub- 50-ps regime. An experimental test of feasibility was conducted using a Q-switched laser and a liver/saline interface. A negative reflectance of about -22% was demonstrated at the liver/saline interface, indicating the ease with which negative stress can be generated at interfaces with mismatched acoustic impedances.

The optoacoustic probe for prostate cancer detection was developed and tested. The 10‐ns pulses of the YAG:Nd laser were delivered by an optical fiber with a turning mirror at its tip. A fiber tip was placed above an ultrasonic array which was employed for the detection of acoustic transients excited inside prostate tissue. The increased infrared light absorption inside prostate tumors resulted in acoustic pulses with enhanced peak pressure providing 200%–300% optoacoustic contrast. The transducer array and the optical fiber were wrapped inside a 20‐mm diameter thin cylindrical shell filled with ultrasonic gel transparent for infrared radiation. Each acoustic transducer was made of 0.05‐mm thick PVDF film with dimensions of 1 mm×12 mm. The frequency bandwidth of transducer array provided 0.3‐mm axial in‐depth resolution. The lateral resolution is defined by the array length and was estimated as 0.8‐mm for 32‐element array with 1‐mm gap between transducers. Transducer sensitivity of 0.05 mV/Pa allowed the ...

This lecture will discuss the current status and prospectives of optoacoustic imaging and advantages of combined optoacoustic‐ultrasonic imaging systems and their biomedical applications. Optoacoustic system provides optical contrast in tissue while mapping tissue structures with ultrasonic resolution. The main advantage of imaging using optical contrast is the possibility to map distribution of blood concentration and its oxygen saturation (functional imaging) in the vasculature or other tissue of interest (such as malignant tumors). High optical contrast of blood (specifically hypoxic blood) makes visualization of the tumor angiogenesis possible, thereby providing functional information for differentiation of malignant and benign tumors. The idea that drives present developments in the field of optoacoustic imaging is that coregistration of optoacoustic and ultrasonic images would be a useful enhancement of almost every application of medical ultrasound.

A technique is described for the measurement of optical properties in clear and turbid media based on time-resolved detection of acoustic transients. Thermal expansion of the irradiated volume of a sample heated by short laser pulses causes a pressure-rise that is proportionally to the laser fluence and the absorption coefficient in the sample. The exponential profile of the acoustic signal formed by the initial stress distribution corresponds to z-axial light distribution in the irradiated volume. Therefore, the absorption and scattering properties of tissue can be determined from the profile and amplitude of the acoustic signals induced by the laser pulses. Stress waves generated in phantom aqueous medium and biological tissues by laser pulses were detected by a broad-band lithium niobate acoustic transducer. The results indicate that absorption coefficients in soft biological tissues in the near infrared spectral range are significantly (5 - 10 times) lower as compared to previously reported from integrating sphere measurements.

Tissue optics affect the observation of tissue fluorescence. This paper outlines how tissue optical properties affect the observation of fluorescence from human aorta.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Short-pulse laser ablation of soft biological tissues has a number of advantages as compared with long pulse (CW) regime that include: minor thermal damage to layers adjacent to the irradiated volume, higher efficiency and spectral selectivity. Mechanism of ablation upon confined stress conditions was determined. It is essential for realization of full potential precision of pulsed laser ablation applied in medical treatments. The following questions were answered: (1) ablation process is not driven by photodissociation of biopolymers, why ablation occurs at temperatures below 100 °C, (2) the major fraction of absorbed laser energy undergoes tissue heating, why energy' densely required for tissue removal is ten times less as compared with enthalpy of vaporization, (3) why the crater depth per pulse can never reach the penetration depth of light. The thermomechanical model of pulsed ablation is developed. Ablation of water containing media is found to include the following stages: instant heating of irradiated volume, sharp increase of pressure, formation of negative stress transient, cavitation by tensile wave, intense ejection of an irradiated substance by growing cavitation bubbles, collapse of cavitation bubbles with formation of a shock wave, spallation of tissue by this shock wave. Important role of tissue optical heterogeneity is revealed and studied.

We present our findings from a real-time laser optoacoustic imaging system (LOIS). The system utilizes a Q-switched Nd:YAG laser; a standard 128-channel ultrasonic linear array probe; custom electronics and custom software to collect, process, and display optoacoustic (OA) images at 10 Hz. We propose that this system be used during preoperative mapping of forearm vessels for hemodialysis treatment. To demonstrate the real-time imaging capabilities of the system, we show OA images of forearm vessels in a volunteer and compare our results to ultrasound images of the same region. Our OA images show blood vessels in high contrast. Manipulations with the probe enable us to locate and track arteries and veins of a forearm in real time. We also demonstrate the ability to combine a series of OA image slices into a volume for spatial representation of the vascular network. Finally, we use frame-by-frame analysis of the recorded OA video to measure dynamic changes of the crossection of the ulnar artery.

Silent (LPa2 and LPa3) and spontaneously active (V3, V5, V17) neurons of subesophageal ganglia of Helix pomatia were irradiated via a 125-mm fiber probe with a 10-mW He-Ne laser (λ = 632.8 nm), and the rate of membrane depolarization, duration of latent period, and probability of spike activity were measured as the functions of light intensity. It was found that silent neurons can not be activated by He-Ne laser irradiation. When the spontaneously active neurons generating spikes every 7–10 min were irradiated in between their spontaneous spikes, the depolarization of membrane and generation of action potentials occurred as a function of light intensity, I. The probability of spike generation increased until the intensity reached 1 W/cm2, and when 1 = 4 W/cm2 was equal to 1. The depolarization of the membrane had a threshold at I = 0.1 W/cm2, then increased with increasing the intensity, and reached a plateau at I = 0.7 W/cm2 (depolarization rate 0.18 mV/s). Duration of the latent period decreased from 28 s to 17 s when the intensity was increased from 0.05 to 0.3 W/cm2. Further increase of the light intensity, from 0.3 to 1.5 W/cm2, caused a less pronounced change in the duration of the latent period (e.g., latent period equal to 11 s at I = 1.5 W/cm2). © 1992 Wiley-Liss, Inc.

We develop a system for three-dimensional whole-body optoacoustic tomography of small animals for applications in preclini- cal research. The tomographic images are obtained while the objects of study phantoms or mice are rotated within a sphere outlined by a concave arc-shaped array of 64 piezocomposite transducers. Two pulsed lasers operating in the near-IR spectral range 755 and 1064 nm with an average pulsed energy of about 100 mJ, a repeti- tion rate of 10 Hz, and a pulse duration of 15 to 75 ns are used as optical illumination sources. During the scan, the mouse is illumi- nated orthogonally to the array with two wide beams of light from a bifurcated fiber bundle. The system is capable of generating images of individual organs and blood vessels through the entire body of a mouse with spatial resolution of 0.5 mm. © 2009 Society of Photo-Optical

The effective management of cancer requires early yet reliable detection, localization, and diagnosis. Therefore, there is a definite and urgent clinical need for an imaging technique that is widely available, is simple to perform, is safe, and that can detect and adequately diagnose cancer. In this paper we present a hybrid imaging technology based on fusion of complementary imaging modalities ultrasound, optoacoustics, and elastography to take full advantage of the many synergistic features of these modalities and thus to significantly improve sensitivity and specificity of cancer imaging. To evaluate our approach, numerical and experimental studies were performed using heterogeneous phantoms where ultrasonic, optical, and viscoelastic properties of the materials were chosen to closely mimic soft tissue. The results of this study suggest that combined ultrasound‐based imaging is possible and can provide more accurate, reliable and earlier detection and diagnosis of tissue pathology. In addition, monitoring of cancer treatment and guidance of tissue biopsy are possible with combined imaging system. Practical and experimental aspects of combined imaging will be discussed with emphasis on data capture and signal/image processing algorithms. The paper will conclude with a discussion of the advantages, limitations, and potential clinical applications of the combined imaging technique.

We report on a technique utilizing time-resolved detection of laser-induced stress transients for the measurement of optical properties in turbid media specifically suitable for biological tissues. The method was tested initially in nonscattering absorbing media so that it could be compared with spectrophotometry. The basis of this method is provided by the conditions of temporal stress confinement in the irradiated volume where the pressure generated in tissues heated instantly by laser pulses is proportional to the absorbed laser energy density, and the exponential profile of the initial stress distribution in the irradiated volume corresponds to the z-axial distribution of the absorbed laser fluence. Planar thermoelastic waves can propagate in water-containing media with minimal distortion, and their axial profiles can be detected by an acoustic transducer with sufficient temporal resolution. The acoustic waves induced by 14-ns laser pulses in nonscattering media, turbid gels, and tissues were measured by a piezoelectric transducer with a 3-ns response time. Temporal profiles of stress transients yielded z-axial distributions of the absorbed laser energy in turbid and opaque media, provided that the speed of sound in these media was known. The absorption and effective scattering coefficients of beef liver, dog prostate, and human aortic atheroma at three wavelengths, 1064 nm (in near infrared), 532 nm (visible), and 355 nm (near UV), were deduced from laser-induced stress profiles with additional measurements of total diffuse reflectance.

The efficiency of RNA-protein crosslink and RNA chain break formation under nanosecond or picosecond UV-laser pulse irradiation of tobacco mosaic virus was determined. It was found that on high-intensity UV-laser irradiation the quantum yields of both reactions increase considerably as compared to the usual (low-intensity) UV-irradiation. The RNA-protein crosslink quantum yield was found to be 1.8 x 10(-5) and 1.2 x 10(-4) and that of RNA chain breaks 1.7 x 10(-4) and 8.9 x 10(-4) for nanosecond and picosecond irradiation, respectively.

The effect of two-photon absorption of powerful laser UV radiation by liquid water has been investigated. The two-photon absorption coefficients were measured at a number of wavelengths from 266 to 316 nm. Using a method of picosecond spectroscopy it is shown that the absorption of two UV quanta with λ = 266 nm by water results in its ionization with a quantum yield of 15% or in its dissociation with a quantum yield of 13%. The dependence of the quantum yield of ionization on the UV radiation wavelength has been obtained. The measured ionization limit of liquid water is 6.5 eV. The products of two-photon laser photolysis of water were studied. The reaction of water with the soluted oxygen resulting in superoxide ion (O 2 − ) formation is shown to be the main transformation of the hydrated electron e aq − . Other, less effective, channels of e aq − transformation are reactions resulting in the formation of the hydroxyl ion (OH − ). The measured lifetime of e aq − is0.3 μs. The relative contribution of e aq − , OH 2 − , O 2 − and H 3 O + to the photoconductivity of water under powerful UV laser radiatio been studied.

The effect of two-photon absorption of the powerful laser UV radiation by liquid water has been investigated. The two-photon absorption coefficients were measured at the number of wave-lengths from 266 nm to 316 nm. Using a method of picosecond spectroscopy it is shown that the absorption of two UV quanta with λ =266 nm by water molecule results in its ionization with quantum yield of 15%. The kinetics of transient absorption spectrum of water was measured at λ =532 nm.

Two-quantum photochemical processes, taking place under high-intensity UV laser irradiation of biomolecules in aqueous solution, have been studied with thymine, one of the DNA bases, as an example. It has been found that two-step high-energy excitation of thymine results not only in its ionization and relaxation but also in electronic energy transfer to the solvent, water. The probabilities of primary photoprocesses from the high-lying electronic vibrational states of thymine have been measured. It has been shown that the radicals of the solvent, water, formed by its sensitized photodecomposition and also by direct two-photon ionization and dissociation make the basic contribution to the formation of the final chemically stable products in the picosecond UV photolysis of thymine in aqueous solution.

The method of picosecond kinetic Spectroscopy has been used to measure some characteristics of excited singlet states of nucleic acid components in liquid aqueous solutions. It has been shown that the times τ 1 or non-radiative relaxation of the excited electronic stale S 1 to the ground state S 0 for uracil, uridine and thymine lie in the range 4 ± 2 ps, while for adenine and cytosine τ 1 XXX 4 ps.

We describe the synthesis and characterization of melanin-like nanoparticles (MNP) as novel contrast agents for optoacoustic tomography. Good dispersion stability of high concentration MNPs in different biological media was achieved with thiol-terminated methoxy-poly(ethylene glycol), which can be used for further functional conjugation. MNP-PEG were found biocompatible with human MCF-7 and 3T3 cells. Cell toxicity of MNPs was found lower than that of gold nanorods for concentrations that provide equal optical absorbance. Optoacoustic tomography images were obtained with Laser Optoacoustic Imaging System (LOIS-3D) from tubes filled with contrast agents and live mice. Imaging of tubes permitted verification of the system resolution

— The mechanism of photodecomposition of nucleic acid bases in a neutral aqueous solution upon two-step excitation of high-lying electronic states by a powerful laser UV radiation is discussed. Experimental dependences of photodecomposition efficiency versus UV radiation intensity are measured both under picosecond and nanosecond laser UV irradiations. By comparison of experimental dependences with a theoretical model, we obtain some characteristics of excited states, such as lifetime t1 of the first electronic excited state S1 intersystem crossing yield φ, photosensitivity from an intermediate excited state and others for all five nucleic acid bases.

The development of modern laser techniques provided wide range of variation of radiation parameters such as frequency, intensity, pulse duration, thus making it possible to carry out investigations on selective action on the substance. The term “selective action” in our understanding means that molecules or parts of molecules of the same type undergo considerable change (modification) under laser irradiation [1]. Such change can be caused by photoionization or photodissociation with subsequent chemical reactions.

There is a need to monitor tissue temperature and optical properties during thermotherapy in real time in order to control the boundaries of hyperthermia or coagulation of diseased tissues and minimize the damage to surrounding normal tissues. We propose to use optoacoustic technique for monitoring of tissue temperature. Efficiency of thermoacoustic excitation in water is dependent on temperature. Therefore, the optoacoustic technique may be utilized to monitor tissue temperature if the efficiency of thermoacoustic excitation in tissue is temperature-dependent. We performed experiments on real-time optoacoustic monitoring of temperature in freshly excised canine tissues (liver and myocardium) and aqueous solution during conductive heating. Laser-induced optoacoustic pressure signals were recorded from tissues and the aqueous solution during heating with the use of sensitive wide-band acoustic transducers. Fundamental harmonic of Q-switched Nd:YAG lasers were used for pressure wave generation. Amplitude of the optoacoustic pressure induced in tissues increased linearly with temperature. Good agreement was obtained between the experimental data and theory for the aqueous solution. We demonstrated that laser optoacoustic technique is capable of measuring 1 - 2 degrees Celsius temperature change in tissue and at the distance of up to several centimeters between the investigated tissue volume and the transducer.

Measurement of optical properties during coagulation can be used for monitoring of the extent of thermal injury in real time. Knowing optical properties of native and coagulated tissues one can optimize treatment parameters and provide precise coagulation of malignant and benign lesions. Attenuation coefficient and total diffuse reflectance of native and coagulated canine liver were measured with the use of optoacoustic and diffuse reflectance techniques. Absorption and reduced scattering coefficients were calculated using the obtained data. Total diffuse reflectance, absorption, reduced scattering, and attenuation coefficients of canine liver increase due to protein coagulation. The capability of optoacoustic technique to detect directly changes in attenuation coefficient can be used for monitoring of tissue coagulation in real time.

Understanding of laser tissue ablation mechanisms may help to optimize laser ablation parameters in order to achieve minimal thermal and mechanical damage to surrounding tissues. One of the possibility to avoid unwanted thermal and mechanical effects is the use of cavitation-driven ablation. This regime provides ablation of aqueous media at temperatures substantially below 100 degree(s)C. Cavitation-induced ablation can be achieved at stress- confined irradiation of absorbing media with free surface. Under these irradiation conditions relatively powerful thermoelastic tensile pressure waves can be generated. The tensile waves can produce cavitation inside irradiated medium and cavitation-induced ablation. Temporal profile of the tensile wave depends on absorption coefficient of the medium while amplitude depends on both absorption coefficient and fluence of laser pulses. In our study effect of the amplitude and duration of the tensile waves on threshold of cavitation-induced ablation is investigated. Absorbing aqueous solutions of potassium chromate with various concentrations were irradiated by third harmonic of Q-switched Nd:YAG laser. On the basis of the experimental results, one can conclude that ablation threshold under this irradiation conditions is effected by both amplitude and temporal characteristics of the tensile pressure waves. Effect of duration of tensile pulse is caused by resonance and cooperative phenomena upon cavitation and coalescence of cavitation bubbles in the irradiated volume. Other effects (initial distribution of cavitation bubbles, etc.) are also discussed.

The optoacoustic tomography (OAT) utilizes acoustic waves induced by short laser pulses in tumor with preferential light absorption. Waveform of the opto-acoustic signal detected by a wide-band transducer array provides information about shape of tumor and its specificity. The 32-element acoustic array was employed in clinical prototype of laser optoacoustic imaging system (LOIS). Piezopolymer PVDF film was used for the array transducer fabrication. Sensitivity of the array and its transient directivity pattern are discussed in this paper. The array performance was tested in experiments with phantoms.

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