The authors reported an original 1.9 micrometer diode laser assisted microvascular anastomosis (LAMA) in human. This technique has been applied in 12 patients during reconstructive surgery for digital replantations (n equals 2), for digital revascularizations (n equals 3) and for free flap transfers (n equals 7). Fourteen end-to-end anastomoses (10 arteries, 4 veins) were performed. LAMA were always performed on vessel which did not impede the chance of success of the surgical procedure in case of thrombosis. LAMA was performed with a 1.9 micrometer diode laser after placement of 2 equidistant stitches. The didoes spot was obtained by means of an optic fiber transmitted to the vessel wall via a pencil size hand piece. The used parameters were as followed: spot size equals 400 micrometer, power equals 70 to 220 mW, time equals 0.7 to 2 seconds, mean fluence equals 115 J/cm². The mechanism involved is a thermal effect on the collagen of the adventitia and media leading to a phenomena which the authors have termed 'heliofusion.' This preliminary trial has permitted to define the modalities of its use in human. The technique is simple, rapid and easily learned. The equipment is not cumbersome, sterilizable and very ergonomic. LAMA does not replace sutures but is complementary, thanks to a reduction in the number of stitches used and to an access to surgical areas which are not easily accessible. This study must be completed by a larger scale study to confirm this technique and its reliability. Others uses could performed on different tissues such as biliary and urinary track, specially under laparoscopic conditions.

A series of 830 nm diode laser assisted longitudinal aortorrhophy with a condition of 400 to 500 J/mm² for one cm length of anastomosis versus conventional manual anastomoses were performed in 90 Wistar rats. With comparing with normal media process, a histologic examination of aneurysm formation was conducted. The results show that there are two important factors to cause aneurysm formation after laser assisted anastomosis: (1) vessel wall is damaged by laser heating; (2) proliferation of collagen fiber at adventitia is absent when media reconstruction.

In 50 rats, a full thickness abdominal skin incision was made with a CO₂ laser (LI) (Kaplan PenduLaser 115) and in another group, an incision was made by a scalpel (SI). These groups were divided into 2 subgroups: a low power CO₂ laser skin welding of 500 mw was used in LI and SI subgroups and, in the other, the wound was closed with nylon sutures. On the fourth postoperative day in the LI welding group, a complete dehiscence wound was formed in one animal and in the SI welding group, a partial dehiscence. After one week, a thin line of young (fresh) scar was observed with complete healing after 2 weeks. Histology revealed after four days, a deep cleft in the LI and SI welding groups and a superficial ulcer after one week and after two weeks, all groups showed complete healing. Histochemical studies (Picrosirius red stain) revealed in the first two weeks, greenish-yellow polarized colors and red-orange colors of a matured collagen in the well-formed scar. It appears that there was no delay in the healing process between the LI and SI groups and between welding and suture healing.

Our ongoing studies out to one year postoperatively of healing in sheep menisci and condylar articular cartilage prompt the study of photochemical bonding of other connective tissues with our 1,8-naphthalimide dyes. Here we report our ex-vivo experiments in photochemical bonding of the dermis of rabbit dorsal skin with 1,8-naphthalimide dyes, in which effects of varying the externally applied pressure during the welding and of the addition of an albumin-collagen protein filler are studied.

Laser-induced corneal welding has been tested in vivo to perform experimental trials of penetrating keratoplasty (corneal transplantation). Twenty rabbits of 2500 - 3500 g in weight were selected to undergo laser-assisted corneal transplantation and then subjected to follow up on 2 - 15 postoperative days. Good sealing along the entire cut length was obtained soon after the surgical operation. Histology examinations evidenced that the laser-assisted surgical procedure provides a satisfactory healing process of corneal tissue in times that are substantially shorter than those of the conventional procedure.

Our purpose is to develop an optical technique for in-vivo and non-invasive diagnosis using backscattered light measurements. We have already demonstrated that optical coefficients of turbid media ((mu) _a, (mu) _s) can be derived from time and space-resolved reflectance in the case of semi-infinite geometry. This procedure was then applied to the investigation of multi-layered media: the upper layer was an aqueous solution of calibrated latex microspheres in water and the lower layer of the sample was a solid phantom. Two different types of phantoms were used. In the first set of experiments, we used an absorbing medium for under layer. In the second case, the lower layer was an absorbing and scattering phantom. Comparison with Monte-Carlo simulations were achieved for the resolution of the inverse problem.

We investigated the optical properties (mu) _a, (mu) _s, and g of human blood under flow conditions using integrating sphere measurements and inverse Monte-Carlo-simulations. The experiments were conducted at 633 nm with regard to the influence of the most important physiological and biochemical blood parameters. In addition, a spectrum of all three parameters was measured in the wavelength range 400 to 2500 nm for oxygenated and deoxygenated blood.

By using an isotropic fiber-optic probe together with an accurate mechanical positioning system, the fluence rate distributions (2-D, 3-D) were measured in normal and squama- carcinomatous human lung tissues in vitro for incident irradiance of 22 mW/cm^{2$ and wavelength 413.1 nm of a Kr(superscript +} laser for beam diameters of 1 mm and 4.5 mm respectively. Monte Carlo simulations of light distribution in the tissue were performed and compared with the experimental results. The optical properties of human lung tissues at 413.1 nm were estimated.

Measurement of photoacoustic transients excited by absorption of short (ns-) laser pulses yields information about the optical properties and the layer structure of tissue. The method is based on the generation of thermoplastic stress in the irradiated tissue volume, which acts as the source of the photoacoustic wave. Interpretation of the experimental results is strongly facilitated if the influence of acoustic diffraction at the boundaries of the spatially limited source is minimized. In our study this was achieved by using a special optical technique for the recording of stress waves. By combining this detector with an optical parametric oscillator (OPO) and a fiberoptic delivery system, we could obtain wavelength dependent values of the effective attenuation coefficient of tissue samples. First measurements in living tissue have shown the capability to resolve the layer structure of human skin. As it was also demonstrated on a simple two-layered phantom, wavelength tuning offers the possibility to achieve maximum contrast in the photoacoustic distinction between layers with different optical properties.

These studies are directed towards an on-line monitoring of tissue optical properties during medical laser diagnostics and therapy. The method of detection is based upon photoacoustic measurements. The detection of the laser induced acoustic transients was realized by means of piezoelectric transducers. Results concerning the optical properties of layered tissue- like samples are presented for two experimental configurations, i.e. transmission mode and reflection mode. The corresponding transients are calculated analytically and account for different influences which contribute to the signal, such as the laser parameters. Especially the effect of optoacoustic diffraction was investigated experimentally and verified theoretically. Its influence could be eliminated by applying fitting functions to the detected transients which take into account the distorting effect of diffraction.

The correlation between Doppler spectra due the emission of tissue destruction products at laser ablation and tissue types was established. The possibility of biotissue type relative determination at the passage of CO₂ laser beam through various sequences of tissue layers is demonstrated.

Pulsed photothermal radiometry (PPTR) is a contact free nondestructive method of material inspection. It also is a promising tool for in vivo tissue spectroscopy and on-line dosimetry. For this approach, one has to consider light scattering and thus a complicated signal interpretation. Different models of light propagation in scattering media are compared, such as Monte Carlo simulation (MCS) and various approaches of diffusion theory. Simulated PPTR signals with these calculations are compared. The validity of the different models in order to describe PPTR measurements correctly is discussed.

A non-invasive method to measure the optical properties of a turbid medium such as biological tissue is described. To determine the absorption and reduced scattering coefficient, the sample is illuminated with spatially intensity modulated light and the backscattered light is analyzed. The spatial modulation of the backscattered light and the diffuse reflectivity of the sample are used to deduce its optical coefficients from a table generated by Monte Carlo simulations. The sensitivity of the method is analyzed. Error influences on the method are investigated. The optical coefficients of human skin in vivo and of human bladder ex vivo are determined at 400, 500, 633, 700 nm and 633 nm respectively.

In optics of biotissues it is often required to determine the radiance distribution function of a finite beam propagating through a turbid medium. One particular instance of this problem is the determination of the signal registered by a detector with a limited field of view, placed on the axis of beam propagation. Since most biological tissues exhibit a high anisotropy factor of scattering, the problem can be approached by applying the small-angle approximation of the radiative transfer theory. We have developed a technique for solving the small-angle problem in case of the Henyey-Greenstein phase function. The technique accounts for an arbitrary spatial and angular profile of the incident beam. The results of the numerical tests have shown a good agreement with the predictions of forward Monte Carlo simulations for a variety of optical properties typical for biological tissues.

Single pulses in the near-infrared (1060 and 1064 nanometers) were used to measure ophthalmoscopically minimum visible lesion (MVL) thresholds in the rhesus monkey eyes for pulsewidths of 7 nanoseconds (ns), 20 picoseconds (ps), and 150 femtoseconds (fs). MVL thresholds for 1 hour reading and 24 hour reading are reported as the 50% probability for damage (ED₅₀) together with their fiducial limits. These measured thresholds are compared with previously reported thresholds for near-IR and visible wavelengths for the complete range of pulsewidths (ns, ps, and fs). Threshold doses were lower at the 24 hour reading than at the 1 hour reading and both ED₅₀ for the fs pulsewidths were less than 25% of those for ns pulsewidths. MVL thresholds ranged from 19 (mu) J at 7 ns down to 1 (mu) J at 150 fs. Thresholds measured for the nanosecond and picosecond pulsewidths using infrared laser pulses were an order of magnitude larger than for the visible wavelengths at similar pulsewidths while the 150 fs threshold was only about double the value for the 580 nm visible wavelength at 90 fs.

Since it is difficult to measure the temperature in the small focused area of the laser-irradiated zona pellucida, we have developed a model to characterize the temperature field associated with this process which uses optical properties of water and measured beam profiles. In this presentation, we use this model to examine the temperature field for laser irradiation conditions which result in a given hole size.

Zona Pellucida (ZP), the outer protective layer of mammalian eggs, is composed of glycoproteins that can be dissolved by temperature elevation. The kinetics of this thermo-dissolution process can play an important role in understanding the mechanism of photothermolysis by a laser. While a temperature of around 70 degrees Celsius has been previously reported as the critical dissolution temperature for the ZP, we found in this study that dissolution temperature has a wider range and depends on duration of exposure which suggests a kinetic rate process. In this paper we report the results of our measurements at different prescribed temperature elevations and subsequent calculations of the kinetics constants using an Arrhenius model.

New method for investigations of a biological tissue was developed. Created method is based on images processing. Imaging of hidden objects in a scattering and absorbing medium is considered in the framework of eight-flux model for transport equation. For investigation of scattering and fluorescent signal laser-fiber spectrum-analyzer 'LESA-6' ('BioSpec,' Russia) was applied. Some experiment were carried out using CCD-camera. We present a reconstruction scheme allowing to solve the inverse problem for eight-flux approximation. The algorithm utilizes angular property of the linear scattering operator. The algorithm solves the inverse problem using set of optical images. For large number of phantoms hidden in a strongly scattering and absorbing medium we have got optical density reconstruction with satisfactory quality.

The aim of our study was to compare biolgic effects such as tissue damage and regeneration in rat skeletal muscle after CO₂ laser application with pulses of constant energy density, but with varying power and exposure time. We used three different laser conditions: 20 W by 0.5 s, 10 W by 1.0 s and 5.0 W by 2.0 s, all having the same energy density (10 joules in a constant area). The animals were sacrificed immediately after laser application, and after 1, 4, 7, 15 and 21 days. Tissue damage produced by the CO₂ laser with high power provokes more profound craters with less tissue damage at the margins. As shown by lesser tissue carbonization, fewer foreign body giant cell granulomas and less tissue fibrosis and, on the other hand, an accelerated regeneration of muscle tissue.

Fiberoptic evanescent wave Fourier transform infrared (FEW- FTIR) spectroscopy using fiberoptic sensors operated in the attenuated total reflection (ATR) regime in the middle infrared (IR) region of the spectrum (850 - 1850 cm^-1) has recently found application in the diagnostics of tissues. The method is suitable for noninvasive and rapid (seconds) direct measurements of the spectra of normal and pathological tissues in vitro, ex vivo and in vivo. The aim of our studies is the express testing of various tumor tissues at the early stages of their development. The method is expected to be further developed for endoscopic and biopsy applications. We measured in vivo the skin normal and malignant tissues on surface (directly on patients) in various cases of basaloma, melanoma and nevus. The experiments were performed in the operating room for measurements of skin in the depth (under/in the layers of epidermis), human breast, stomach, lung, kidney tissues. The breast and skin tissues at different stages of tumor or cancer were distinguished very clearly in spectra of amide, side cyclic and noncyclic hydrogen bonded fragments of amino acid residuals, phosphate groups and sugars. Computer monitoring is being developed for diagnostics.

An innovative spectroscopic diagnostic method has been developed of investigation of different regions of normal human skin tissue. This new method is a combination of Fourier transform infrared fiberoptic evanescent wave (FTIR-FEW) spectroscopy and fiber optic technique for the middle infrared (MIR) wavelength range (3 to 20 micrometer). The fiberoptical sensors we have used are characterized by low optical losses and high flexibility for remote analysis. Our new fiber optical accessory can be utilized with a diversity of standard commercial Fourier transform spectrometers. FTIR-FEW, using nontoxic unclad fibers in the attenuated total reflection (ATR) regime, is suitable for noninvasive, fast, sensitive investigations of normal skin in vivo for various medical diagnostics applications including studies of acupuncture points. Our method allows for direct interaction of the skin tissue with the fiber. Here we present the first data on infrared spectra of skin tissue in vivo for several acupuncture points in the range of 1300 to 1800 cm^-1 and 2600 to 4000 cm^-1.

The effects of distant light diode irradiation with various spectrums of the trunk vessels on reactivity of microvessels in the small intestine mesentery treated with threshold doses of norepinephrine (NoE) are compared. The character of changes in reactivity of microvessels to NoE was found to depend on the wave length and irradiation dose. Ultraviolet irradiation (470 nm, 0.03 J/sm²) was noticed to increase reactivity of the vessels to NoE (vasoconstriction increase). In green light irradiation (540 nm, 0.3 J/sm² sm²) no changes in reactivity were observed. Red light irradiation (670 nm, 2.0 J/sm²), infrared particular (980 nm, 1.0 J/sm²), lowered reactivity to NoE. Thus, noninvasive light-diode irradiation of the blood results in different systemic changes of endothelial dependent reactivity of microcirculation due to specify of photochemical processes involved.

When heated the living tissue exhibits random nonuniformities in the temperature that are due to the discreetness of vessel arrangement. Because of strong temperature dependence of the thermal coagulation rate these nonuniformities should substantially affect the necrosis growth induced by local heating. In the present work we study their effect on the form of a necrosis domain when its growth is limited by heat diffusion into the surrounding tissue. In particular, we analyze the mean amplitude and the correlation length of the interface perturbations depending on the main characteristics of the random temperature nonuniformities.

The purpose of this work was to gain better overall picture to the thermal process involved in ablation of biological media performed by means of both continuous waves (cw) and pulsed laser irradiations. The theoretical analysis is based upon a computerized laser evaporative model. This scheme has been applied in order to clarify the following situations: (1) Ablation of tissue assimilated as water with finite absorption coefficient. (2) Tissue ablation by cw argon laser using gel (tissue-like phantom material) as a model system for tissue. (3) Skin ablation performed by pulsed Er:YAG laser radiation. In each case the role played by optical absorption in the dynamics of the ablation process is depicted. Typically, it appears that the position of the ablation front (or crater depth) and the transient ablation velocity are both influenced by tissue absorption while the steady-state stays independent on this parameter and evolves linearly with power density. Additionally, the ablation temperature beyond the moving front can reach a maximum higher than ablation threshold temperature. The peak temperature and its location are mediated by tissue type. Calculations show that for small absorption coefficients higher temperatures are reached at deeper levels. In contrast, at fixed absorption coefficient, the growing of the power density increases the peak temperature but reduces the penetration depth of the heated volume. The whole of computed data confirms that thermal laser ablation of tissue can be described as an explosive event and that a decrease of water content in the target alters the penetration depth which control the ablation rate.

This study aimed to evaluate the feasibility of retinal thermal damage assessment in a rabbit eye model by using laser-induced release of liposome-encapsulated dye. After anethesia, thermosensitive liposomes (DSPC) loaded with 5,6- Carboxyfluorescein were injected intravenously to pigmented rabbits. Retinal photocoagulation were performed with a 810 nm diode laser (P equals 100 to 400 mW, (Phi) equals 500 micrometer, ls). Fluorescence measurements in the area of the laser exposures were then realized with a digitized angiograph. Histology was performed on retinal tissue and adjacent ares. Fluorescent spots were observed for fluences ranging from 50 plus or minus 10 J/cm² to 210 plus or minus 10 J/cm². The fluorescence intensity increased linearly with the fluence and reached a plateau at 150 plus or minus 10 J/cm². The fluorescence intensity was correlated to the maximum temperature at the center of the laser spot with a linear increase from 42 plus or minus 3 degrees Celsius to 65 plus or minus 3 degrees Celsius. The chorioretinal thermal damage quantified by histology was correlated to temperature. These results are in agreement with our two previous studies with DSPC liposomes for temperature measurements in a tissue model and then in a vascular model. In conclusion, this study demonstrates the possibility of a laser-induced release of liposome-encapsulated dye for a quantification of diode laser induced thermal damage in ophthalmology. Such a method could be useful for a real-time monitoring of laser photocoagulation for condition such as choroidal neovessels when a precise thermal damage is required near the foveolar area.

Laser ablation and associated bubble formation are known to damage biologic tissue. Imaging of tissue straining during ablation would help to understand and control laser-induced damages. We have used polariscopic imaging to monitor the strain induced by pulsed holmium laser irradiation. A photoelastic tissue phantom, poly(acrylamide) gel, whose viscoelastic properties could be controlled was used to mimic various tissues. The laser energy was delivered to the sample via an optical fiber placed either perpendicularly 1.7 mm from the surface or within the sample. Only compressive strain is observed when the bubble is formed within the phantom, whereas significant tensile strain is induced when the bubble is formed at or next to the surface.

Laser ablation of tissue can be accompanied by deleterious training. A polaroscopic technique was used to image induced strain. In a bovine cornea model, we demonstrate that strain induced by a cavitation bubble occurs over an area larger than the bubble. Further, although the bubble lifetime is only about 400 microseconds and the cornea tissue is strained greatly by the expanding and collapsing bubble, there is substantial strain for several milliseconds after the bubble collapse. The results indicate the utility of polaroscopic imaging as well as the magnitude and spatial extent of mechanical events at times long after the initiating mechanical insult.

Laser surgery of soft tissue can exploit the power of brief, intense pulses of light to cause localized disruption of tissue with minimal effect upon surrounding tissue. In particular, studies of Ho:YAG laser surgery have shown that the effects of cavitation upon tissues and bone depend upon the physical composition of structures in the vicinity of the surgical site. For photofragmentation of occluding structures within catheters and other implant devices, it is possible to exploit the particular geometry of the catheter to amplify the effects of photofragmentation beyond those seen in bulk tissue. A Ho:YAG laser was used to photofragment occlusive material (tissue and tissue analogs) contained in glass capillary tubing and catheter tubing of the kind used in ventricular shunt implants for the management of hydrocephalus. Occluded catheters obtained from patient explants were also employed. Selection of operational parameters used in photoablation and photofragmentation of soft tissue must consider the physical composition and geometry of the treatment site. In the present case, containment of the soft tissue within relatively inelastic catheters dramatically alters the extent of photofragmentation relative to bulk (unconstrained) material. Our results indicate that the disruptive effect of cavitation bubbles is increased in catheters, due to the rapid displacement of material by cavitation bubbles comparable in size to the inner diameter of the catheter. The cylindrical geometry of the catheter lumen may additionally influence the propagation of acoustic shock waves that result from the collapse of the condensing cavitation bubbles.

The evaporation of water by means of fiber guided pulsed mid- infrared laser radiation was investigated. The influence of the pulse duration and speckle formation on the ablation threshold was determined. Optical on-line monitoring of refractive index changes at the distal fiber tip has been used to determine the cavitation threshold. Temporal and spatial speckle pattern at the distal application fiber tip were measured by imaging the fiber tip to a reference plane. An IR- CCD camera and fast IR-photodiodes were used to monitor the intensity modulation. The measured cavitation thresholds show a strong dependence on the pulse energy and the pulse duration. Overheating of water at the evaporation threshold could be calculated for the Q-switched pulse mode. Regarding speckle, spatial peak intensities up to a factor of 1.5 and 2.6 over the average intensity were evaluated for the Q- switched and the free running mode, respectively. Speckle modulation plays an important role in the free running mode to achieve the boiling point of water with the threshold radiant exposure determined. Furthermore, transient intensity fluctuations were measured across the beam profile at the fiber tip.

The common method to remove the vitreous body of the eye (vitrectomy) is based on mechanical cutting and pulling of the tissue. Since this involves the risk of mechanical violation it can not be performed in close contact to the retina. This limitation should not exist for erbium laser radiation due to its short penetration depth. A flashlamp pumped, pulsed free- running erbium:YAG laser was used at a wavelength of 2.94 micrometer. Three different pulse length were used. The measurements were performed at freshly enucleated porcine vitreous bodies stored in cooled saline water. The ablation rate was taken by a high precision scale. The same experiments were performed with saline water replacing the vitreous body. Pulse energies under 25 mJ were ignored. The ablation rates did hardly differ here from the results measuring the evaporation of water. Energies above 125 mJ per pulse caused a visible movement of the surface. Therefore this energy range was not included in the experiment. The ablation rate was observed to increase linear with higher pulse energies and shorter pulse lengths. The ablation energy per mass decreases linear with increasing pulse energy for all pulse length. The results show that it is possible to ablate the vitreous body. It should be possible to perform the ablation process in less than half an hour.

Photoablative tissue processing at the wavelength (lambda) approximately equals 3 micrometer is of great interest in many medical applications but not yet really understood. A mathematical model of the photoablation process using a free- running infrared laser has been developed. It includes evaporation and thermoelastic pressure generation and was solved using the finite-element-method. Simulated thermoelastic pressure transients are in good agreement with the experiment. It has been shown, that the temperature dependence of the absorption and the volumetric expansion cannot be neglected. With higher laser intensities strong recoil pressure transients (greater than or equal to 100 bar) and strong thermoelastic pressure transients due to a partially evaporation are given. For this reason a new model including large tissue expansions, tissue overheating and recoil induced pressure transients has been developed and presented.

The stone fragmentation process induced during Ho:YAG laser lithotripsy was observed by time-resolved flash video imaging. Possible acoustic transient occurrence was simultaneously monitored with a PVDF needle hydrophone. We used artificial and cystine kidney stones. We observed that, although the fragmentation process is accompanied with the formation of a cavitation bubble, cavitation has a minimal incidence on stone fragmentation. Fragment ejection is mainly due to a direct laser stone heating and vaporization of stone organic constituents and interstitial water. The minimal effect of the cavitation bubble for fragmentation is confirmed by acoustic transients measurements, which reveal weak pressure transients. This is in contrast with the fragmentation mechanisms induced by laser of shorter pulse duration.

A method has been found to selectively ablate cholesterol esters accumulated in an arteriosclerotic region of a rabbit artery without damaging the blood vessel using a free-electron laser (FEL). A FEL is a pulsed laser source that generates a beam by amplifying the coherent radiation emitted by an electron beam that is traveling through a periodically alternating magnetic field at a relativistic velocity. The characteristics of the FEL include a broadly tunable wavelength and ultra-short pulse width. We have found that FEL irradiation of a rabbit arterial wall for 1 min with a power of 1.5 mW can ablate cholesterol esters without damaging the elastic fibers of the arterial wall. The FEL was tuned to 5.75 micrometer, which is a wavelength that is absorbed by cholesterol ester. This method may be used as a non-invasive surgical procedure for the treatment of arteriosclerotic arteries.

Neonatal hyperbilirubinemia affects more than half of the newborns and represents a potentially serious condition due to the toxicity of bilirubin to the central nervous system. A precise non-invasive technique for the monitoring of bilirubin concentration is desirable for the treatment of icteric babies. Transcutaneous bilirubinometry based on optical reflectance spectra is complicated by the superposition of the spectral absorption properties of melanin and haemoglobin with those of bilirubin. Diffusion theory forms a suitable model for the description of light propagation in tissue. In this treatment, an inverse diffusion approach is developed to measure bilirubin concentration in tissue by means of the reflectance spectrum. First results of its application to in vivo measurements are encouraging.

The growing number of laser applications in medicine and biology has determined a renewed interest on the study of the light transport in turbid media such as biological tissues. One of the most powerful methods used to describe this kind of process is given by the Monte Carlo techniques. We have developed a FORTRAN90 code, running on an Alpha Vax AXP DEC 2100 to simulate the transport of a photon beam with a Gaussian temporal and spatial profile through a multilayered sample. The code provides the sample transmittance and reflectance (both time and space resolved) that can be compared to the experimental data. Monte Carlo calculations have been performed to simulate time-resolved transillumination through water latex and intralipid water solutions with optical properties similar to those of biological tissues. The comparison of Monte Carlo results with experimental data and with analytical solutions to diffusion equation shows a good agreement, suggesting that Monte Carlo techniques are indeed a powerful tool for predictions on light transport in turbid media.

Formation of vapor bubbles is characteristic of many applications of short-pulse lasers in medicine. An understanding of the dynamics of vapor bubble generation is useful for developing and optimizing laser-based medical therapies. To this end, experiments in vapor bubble generation with laser light deposited in an aqueous dye solution near a fiber-optic tip have been performed. Numerical hydrodynamic simulations have been developed to understand and extrapolate results from these experiments. Comparison of two-dimensional simulations with the experiment shows excellent agreement in tracking the bubble evolution. Another regime of vapor bubble generation is short-pulse laser interactions with melanosomes. Strong shock generation and vapor bubble generation are common physical features of this interaction. A novel effect of discrete absorption by melanin granules within a melanosome is studied as a possible role in previously reported high Mach number shocks [Lin and Kelly, SPIE 2391, 294 (1995)].

The formation and the dynamics of cavitation bubbles were investigated when applying pulsed mid-IR laser radiation in water. HeNe laser light reflected at the distal application fiber end was measured in order to probe refractive index changes during ablation. A Cr:Tm:YAG laser, (lambda) equals 2.01 micrometer, was operated in the free running and the Q- switched mode. The pulses were transmitted through a 400 micrometer low-OH quartz fiber into a water filled cuvette. In the Q-switched mode the reflected HeNe-laser power was found to be nearly constant during the cavitation bubble lifetime, whereas in the free running mode, transient changes of the reflected power were measured. Initial bubble wall velocities of 20 m/s in the free running mode and about 330 m/s in the Q- switched mode could be estimated using pulse energies of 110 mJ and 18 mJ, respectively. The lifetime of the cavitation bubble in both cases was measured to be about 300 microseconds. Relative to the reflected power when the fiber tip was in air, it was significantly lower during the lifetime of the bubble. The results indicate that condensation or sublimation of steam takes place on the fiber tip owing to low vapor temperature within the bubble. Rapid cooling due to adiabatic steam expansion and the Joule-Thompson effect at the onset of bubble formation are the most likely explanations for this unexpected result. A simple gas kinetic model predicts temperature gradients in the order of 200 degrees Celsius within the first microseconds. In conclusion, the optical on- line monitoring described is an excellent tool to investigate the kinetics of ablation in any medium in vitro as well as in vivo.

To further evaluate the potential for intra-articular phototherapy, the optical properties of normal porcine ligament and cartilage were determined in vitro. The diffuser reflectance, Rd, was measured with an integrating sphere at 351, 365 nm and in the range 440 - 800 nm. The lateral spread of light introduced by a 400 mm optical fiber was measured and analyzed to yield the optical penetration depth, (delta) . The two measurements, Rd and (delta) yielded the absorption coefficient (mu) _a (cm^-1), and the reduced scattering (mu) _s' equals (mu) _s (1-g) (cm^-1) at 351, 365 nm and in the range 440 - 800 nm.

CO₂ lasers are among the most important lasers used for medical applications such as laser surgery. This laser is used mainly for tissue cutting and tissue removal, exploiting the high power of this laser systems and the high absorption of the biological tissue at this laser wavelength. Our research continues earlier studies conducted at the Tel Aviv University since 1987 that come out with a theoretical model used for simulating tissue irradiated with a CO₂ laser beam. When examining the previous studies on laser ablation we have found that the important mechanism of steam pressure ablation was neglected. Therefore in our work we added steam pressure ablation into the model and studied the effects of this new mechanism on the tissue irradiated with a CO₂ laser. The simulations results reveal that top-hat beam profile (uniformly distributed intensity) produces less thermal damage and its ablation efficiency is higher compared with standard Gaussian beam profile. We saw that the efficiency of the steam pressure ablation process is considerably higher than the purely thermal ablation process. We also noticed that the layers underneath the tissue surface are responsible for the pressure ablation.

Laser-assisted repair of nerves is often unsatisfactory and has a high failure rate. Two disadvantages of laser assisted procedures are low initial strength of the resulting anastomosis and thermal damage of tissue by laser heating. Temporary or permanent stay sutures are used and fluid solders have been proposed to increase the strength of the repair. These techniques, however, have their own disadvantages including foreign body reaction and difficulty of application. To address these problems solid protein solder strips have been developed for use in conjunction with a diode laser for nerve anastomosis. The protein helps to supplement the bond, especially in the acute healing phase up to five days post- operative. Indocyanine green dye is added to the protein solder to absorb a laser wavelength (approximately 800 nm) that is poorly absorbed by water and other bodily tissues. This reduces the collateral thermal damage typically associated with other laser techniques. An investigation of the feasibility of the laser-solder repair technique in terms of required laser irradiance, tensile strength of the repair, and solder and tissue temperature is reported here. The tensile strength of repaired nerves rose steadily with laser irradiance reaching a maximum of 105 plus or minus 10 N.cm^-2 at 12.7 W.cm^-2. When higher laser irradiances were used the tensile strength of the resulting bonds dropped. Histopathological analysis of the laser- soldered nerves, conducted immediately after surgery, showed the solder to have adhered well to the perineurial membrane, with minimal damage to the inner axons of the nerve. The maximum temperature reached at the solder surface and at the solder/nerve interface, measured using a non-contact fiber optic radiometer and thermocouple respectively, also rose steadily with laser irradiance. At 12.7 W.cm^-2, the temperatures reached at the surface and at the interface were 85 plus or minus 4 and 68 plus or minus 4 degrees Celsius respectively. This study demonstrates the feasibility of the laser-solder repair technique for nerve anastomosis resulting in improved tensile strength. The welding temperature required to achieve optimal tensile strength has been identified.

In the course of search for systems and media that could simulate optical characteristics of bio-objects we developed a method of preparation of tissue phantoms on a gel base. Examination of fluorescence spectra of agar and gelatin gel revealed that agar gel may be used in phantom preparation as a matrix free of reabsorption effect and with low intrinsic fluorescence. But in subsequent experiments we used 10% gelatin gel in order to reproduce native fluorescence of biotissue. Phantom samples were prepared as 2.2 or 3.2 mm thick 25 mm diameter gel 'tablets' with controlled content of blood and scatterer (polystyrene latex). In phantoms without scatterer we studied the dependence of the shape of fluorescence spectrum on blood content (excitation wavelength 350 - 380 nm, observation of emission spectra within 400 - 650 nm range). We obtained an agreement of fluorescence intensity and spectrum shape transformation, as the blood concentration in phantom increases, with the changes of in vivo tissue autofluorescence spectra (literature data) when going from normal tissue to the abnormal region. Using measurements of collimated transmittance we evaluated scattering coefficient for polystyrene latex embedded in 10% gelatin gel. When blood was added, the linear dependence of extinction in phantom on latex concentration was distorted within absorption band.