The effects of thermal coagulation on the macroscopic optical transport parameters that govern the distribution of light in tissues were studied. The optical absorption coefficients, pa, and the reduced scattering coefficients, j.ts (1-g), were deduced from measurements of total transmission and total reflectance of HeNe laser radiation ( = 633 and 594 nm) directed to thin slices of dog myocardium heated in vitro. The first optical changes were detected at 45°and, at temperatures above 65°, there was a 2-fold increase in absorption and a 7-fold increase in scattering. Transmission electron microscopy of laser-induced thermally coagulated lesions in rat myocardium (cw argon ion, = 514 nm) revealed ultrastructural alterations that were considered responsible for the increased scattering based on Mie theory. These microscopic alterations included disruption of mitochondria to form aggregates of electron dense granules and granular transformation of thermally coagulated proteins of the sarcomeres and cytoplasm. Our preliminary analyses suggest that the mitochondrial granules and the protein granules contribute to the increased scattering oflight in thermally coagulated myocardium.

An Er:YSGG laser was used to evaluate cutting rate and residual thermal damage in skin and bone at pulse repetition rates of 2, 5, 10, 20, and 30 Hz. The pulse repetition rate did not affect the cutting rate or the residual damage in bone. In skin, etch depth per pulse was greatest at 2 Hz possibly because the beam profile is Gaussian only at this lowest fluence. The cutting rate in skin was similar at all other repetition rates. Residual damage in skin was greatest at high fluences (>25 J/cm2) and increased with pulse repetition rate at all fluences studied. The results are explained using a simple thermal model that is based upon the thermal cooling time of the zone of tissue damaged by a single laser pulse.

A high speed camera was used to record time-resolved the development and the shape of the holes drilled by Er:YAG laser pulses of high intensity. The experimental results suggest that above a critical radiant exposure the dynamics of the drilling mechanism changes and driven by a high vapor pressure gradient, hydrodynamic instabilities within the hole occur. They are evidenced by dramatic local variations of the channel diameter and by a decrease of the normalized recoil momentum. These instabilities influence strongly the extent, and velocity of the ejection. It is shown that, when cutting soft tissue, the extent of the zone of thermal damage is determined by the amount of hot liquefied material that remains in the hole and acts as a heat source.

A review of the effects of pressure pulses on materials is presented with an orientation toward laser-induced shock wave effects in biological tissue. The behavior is first discussed for small amplitudes, namely sound waves, since many important features in this region are also applicable at large amplitudes. The generation of pressure pulses by lasers is discussed along with amplitudes. The origin and characteristic properties of shock waves is discussed along with the different types of effects they can produce. The hydrodynamic code techniques required for shock wave calculations are discussed along with the necessary empirical data base and methods for generating that data base.

Measurements of thermoelastic and ablative stress transients generated by pulsed lasers in tissue samples have been made using fast time-response (nanosecond) piezoelectric film transducers. Studies of the ablation of cornea using excimer and CO2 lasers, and of vascular tissue using excimer, dye and solid-state lasers are described. It is shown that useful information on the pulsed laser interaction can be gained from these measurements.

A pump-probe technique was used to measure the shock wave velocity generated by laser-induced optical breakdown in water. The transient shock pressure field was mapped using the measured shock velocities, together with the equation of state of water and the jump equations. We find that the calculated shock pressures near the breakdown region are in the kbar range and scale as hr2. Depending upon the energy of the laser pump pulse, typically 3 to 15 mJ, the shock wave dissipates within 300-500 1um.

Argon-fluoride excimer laser ablation of stratum comeum causes deeper tissue damage than expected for thermal or photochemical mechanisms, suggesting thatphotoacoustic waves have arole in tissue damage. Laserirradiation (193 nm, 14 ns pulses, 1-2 Hz) attworadiantexposures, 60 and 160 mJ/cm2perpulse was usedto ablate the stratumcomeumofskin. Light and electron microscopy ofimmediate biopsies demonstrated damage to fibroblasts as deep as 88 and 220 jun, respectively, below the ablation site. Ablation throughwaterwas usedtoinertially confine the ablation zone. Partial ablationofs.c. through airproducedno damage, whereas partial ablation through water damaged skin to amean depth of 1 14.5 8.8( Full thickness ablation of s.c. through air and water produced damage zones measuring 192.2 16.2 and 293.0 71.6 rim, respectively (p <0.05). The increased depth ofdamage in the presence ofinertial confinementprovided by the layer of water strongly supports a photoacoustic mechanism ofdamage. The depths ofdamage for thelarge spot, line, and small spots were 43 1 164 urn, 269 96xni, andno damage. The spot size dependence ofthedepthofdamage is consistentwiththe geometric attenuation one would expect to be present from a pressure wave related phenomena. Sequential biopsies were taken over a 7 day period for light and transmission electron microscopy. At 24 hours, there was necrosis of the epidermis and papillary dermis subjacent to the ablation site, with neutrophils surrounding and demarcating the affected area. The necrotic zone sloughedby48 hours. Thereepithelializationwas completeby7 days. The sequenceofrepairis similartoknife wound healing which we have previously studied, and is analogous to other wound healing processes. We have used an experimental model of ArF excimer laser ablation of stratum corneum to investigate laser-induced photoacoustic damage. The evidence for the injury being due to pressure transients is indirectbutcompelling. Whether these pressuretransients are acoustic transients orshockwaves has notbeendetermined, although itis ourprejudicethatshockwaves are the predominant force under these conditions. It is important to consider the possible effects of pressure transients in evaluating laser-tissue interactions, particularly when using short pulse, high peak power lasers.

In order to elucidate the mechanisms of photosensitized injury to mitochondria, two photosensitizers have been compared. Both doxycycline (DOTC) and rhodamine-l23 (R123) localize preferentially within the mitochondria of MGH-IJ1 bladder carcinoma cells j1 vitro, and both sensitize phototoxic injury that is selective for mitochondria. Mitochondria of cells pretreated with DOTC and irradiated with UVA (1 J/cm2, 320-400 rim) undergo massive swelling that begins by 10 mm after irradiation, is maximal by 1 h, and is partially repaired by 4 h; damage caused by exposure to a higher UVA dose (6 J/cm2), however, is not repaired. In contrast, cells pretreated with R123 and irradiated with an argon-ion laser (10 J/cm2, 514.5 rim) undergo a different type of mitochondrial injury, characterized by the delayed (4 h) onset of moderate mitochondrial swelling and striking mitochondrial distortion and fragmentation, which is not repaired by 48 h after irradiation. These differences indicate that the reactions underlying cellular phototoxicity can be distinguished even on an ultrastructural level. Probably both the primary photochemistry and the submitochondrial targets of these reactions differ with the two photosensitizers.

Transcutaneous oxygen electrodes are used to non-invasively measure tissue oxygen tension during photodynamic therapy (PDT). Measurements are performed on VX-2 skin carcinomas in rabbit ears. The degree of tumor oxygen tension reduction is proportional to the applied light dose. In the absence of irradiation, oxygen tension returns to pre-irradiation levels until a "damage threshold" has been reached. For 50mW/cm2 irradiations of Photofrmn II (at 630 nm) and tetraphenylporphine tetrasulfonate (at 657 tim), the cumulative dose required to irreversibly deplete tumor iranscutaneous oxygen was approximately 300 kJ/M2 and 600 kJ/M2, respectively.

A striking consequence of 248-nm excimer laser irradiation of skin is a stable vivid blue fluorescence easily visualized under UVA illumination. Its spectral properties (excitation maximum at "335 nm; emission maximum at ''43O nm) are somewhat similar to those of the fluorescent pigments associated with aging of tissue and peroxidation of lipids, and the fluorescent pigments formed during exposure to high temperatures. This study explores the spectral properties, magnitude, dose response, and laser pulse intensity dependence of this phenomenon in both isolated stratum corneum and epidermal cell suspensions from human skin.

An electronic optical imaging system consisting of a computer-controlled CCD array with 576 x 384 detection elements and 14 bits of digitization is presented. The system is used to obtain internal and external light intensity distributions for diffusing optical fiber tips used with photodynamic therapy and laser angioplasty. Significant intensity distribution variations were observed between fiber tips from 5 different sources. The effect of launch numerical aperture on the measured light distributions is also presented.

The laser has found numerous applications in medicine, beginning with uses in ophthalmology in the 1960's. Today, lasers are used in tissue cutting, blood coagulation, photo-dynamic cancer therapy, arterial plaque removal, dental drilling, etc. In this paper, we examine those areas of laser medicine in which plasmas (ionized gases) are produced. In fact, the presence of a plasma is essential for the application at hand to succeed. We consider examples of the plasmas produced in ophthalmology (e.g. lens membrane destruction following cataract surgery), in urology and gastroenterology (e.g. kidney and gall stone ablation and fragmentation) and in cardiology and vascular surgery (e.g. laser ablation and removal of fibro-fatty and calcified arterial plaque). Experimental data are presented along with some results from computer simulations of the phenomena. Comments on future directions in these areas are included.

Based upon a consideration of the morphology of calcified tissue, a two component picture of ablation is postulated in which the soft connective tissue matrix is vaporized and entrains and removes the hard calcium salts. The dependence of the ablation process on laser irradiance, fluence and wavelength is discussed, including estimates of optimal ranges of those parameters for practical applications.

The paper contains the reu1ts of calculations and experimantal reseach of processed of ablation of soft fatty and bony biotissues by pulsed C132 - laser beam. Specific mass energies for bony and fatty biotissue ablation were defined for pulse- widths, causing minimum damage of adjacent nonradiated biotissue regions. Authors determined the dependence of energy of ablation for various biotissue types on the absorbed laser energy density and found optimal laser pulse parameters for bony C dental ) biotissue drilling.

Repetitively pulsed (rp) hydrogen fluoride (HF) chemical laser interactions with human cardiovascular tissue (normal aorta) have been studied to understand tissue ablation phenomenology, effects, and mechanisms under well characterized laser irradiation conditions. RP HF chemical laser experiments have been performed at two wavelengths (A = 2.78 ,m and 2.91 pm) over a fluence range of 0.5 to 1 1 J/cm2 to determine ablation efficiencies and effective enthalpies of ablation (Q) as a function of wavelength and fluence. The experimental results are analyzed to consider the physical and chemical processes associated with thermochemical ablation of human cardiovascular tissue by pulsed infrared lasers.

Laser angioplasty continues to attract interest as a potential method for treating atherosclerotic arterial disease. Current efforts are aimed at finding the most effective combination of laser and delivery system. High energy pulsed ultraviolet or infrared lasers demonstrate good photoablative properties but there remain practical difficulties with the optical fibre delivery. Continuous wave lasers are widely used in conjunction with "hot-tip" fibres for thermal ablation but their direct (optical) ablation efficiency is low, causing significant surrounding thermal damage in soft tissue. While considerable attention has been directed previously at the ablative effects for different laser wavelengths, little systematic study has been made of the efficacy for different temporal rates of energy deposition. We have compared the efficacy for tissue ablation in cadaveric human aorta of three different laser systems with similar wavelengths in the visible (green) but different temporal rates of energy deposition. The laser sources were the continuous wave argon ion laser (514.5 nm), the high pulse energy, frequency doubled Nd:YAG laser (532 nm) and the copper vapour laser. The copper vapour laser is a high repetition rate, high average power, pulsed laser emitting in the green (511 nm) and yellow (578 nm) which has temporal characteristics intermediate between those of the Nd:YAG laser and the argon ion laser, and has the potential to be effective both for direct optical ablation and hot-tip thermal ablation.

Experiments in the action of Nd-YAG laser on gelatin with introduced dye (china ink) are presented. They can be treated as a simulation of interaction of laser light of various wavelengths with organic matter.A fiber 600,iiin in diameter or lens focusing system are used to deliver thelaser light to the tissue.

Solid state lasers based on the trivalent thulium ion which operate within the strong water absorption band in the 2zin region are discussed. This water band has maximum absorption in the range 1.92- 1.94pm. The relative merits of thulium lasers in the crystalline hosts YAG, YSGG, and YLF which operate in this wavelength region are discussed.

To study the onset of tissue ablation by cw laser irradiation, aortic and myocardial bovine tissue samples were exposed in air to a 1 mm diameter laser beam of either 15 W Nd-YAG or 3.5 W Argon. Exposure times varied from 0.1 to 30 s. The surface of the tissue was filmed with a resolution ofO.O1 mm and recorded on video with time coding. A photodiode positioned underneath the tissue samples measured forward transmitted light. After exposure the tissue was processed for histology. The observed phenomena were divided in three phases: Phase A: The tissue surface slightly discolored while the transmission signal remained constant or slightly diminished. Phase B: Simultaneous with a 'pop' sound, the surface rose and the transmission dropped abruptly to about 50 % and remained at this level for several seconds during vaporization of tissue water. Histology showed ruptured layers and multiple vacuoles beneath the surface. Phase C: In the middle of the beam a spot ofcarbonized tissue was formed which grew concentrically from the center while a crater was formed. No drop in the transmission was observed when carbonization started. On the crater bottom carbonization and vaporization followed in rapid succession while light transmission increased. Histology showed along the crater edge a 20 jim thick layer of carbonization and a 200 pm thick layer of vacuoles due to tissue water boiling whereas on the crater bottom these layers were only 10 and 20 jim thick respectively. Thus, the small zone of vacuoles at the crater bottom suggest that a large temperature gradient existed at the ablation front. The 'popcorn' phenomenon is attributed to the explosive formation ofvapor bubbles underneath the tissue surface caused by the anatomically or heat-induced layered structure of the tissue resulting in enhanced reflection and scattering of the laser light due to multiple tissue-vapor transitions.

Retinal lesions produced with a pulsed laser beam of 1-20 kHz frequency and 10-100% duty cycle were compared with lesions produced with a continuous wave (cw) laser of the same peak power and total energy. Photocoagulation was applied to the retina of three black pigmented rabbits using krypton red laser (647.1 nm) equipped with an acousto-optical modulator to convert cw laser emission to a pulsating beam. An optical fiber fed the laser beam into an optical system delivering a collimated beam of predetermined divergence; the animal's eye focused this beam to a 50-pm spot on the retina. Peak power was kept constant at 0.2 W, and energy was kept constant at 20 mJ. After 7 months the animals were sacrificed and retinal tissue examined by light microscopy. The central section of each lesion was identified and photographed. For lesions with the same energy per pulse and the same pulse duration, the most influential factor, in the frequency range of 1-20 kHz, appeared to be the duty cycle: the smaller the duty cycle, the smaller the lesion, and vice versa. In other words, the shorter the time interval between consecutive pulses, the larger were the pulsed laser lesions.

We have previously demonstrated the detection of reversible and irreversible changes on MR images oflaser energy deposition and tissue heating and cooling1. It is possible to monitor and control energy deposition during interstitial laser therapy. This presentation describes some first steps toward optimizing the power and total energy deposited in various tissues in vivo, by analyzing the irreversible tissue changes and their spatial distribution as revealed by spin echo imaging. We used various power settings of an Nd.YAG laser delivered by a fiber optic inserted into several tissues (brain, muscle, liver) of anesthetized rats and rabbits. MR imaging was performed at 1.9 T. Photothermally-produced lesions were seen on both T1- and Ta-weighted images. The overall size of the lesions correlated with the magnitude of the energy applied. The MR image appearance depended not only on the laser energy but also on the way it was delivered, on the type of tissue, and the MR pulse sequence applied. While Ti-weighted images adequately demonstrated an area of tissue destruction, T2- weighted images showed a more heterogeneous and more extensive lesion which could be better correlated with the complex histological representation of these lesions. Typically, when rabbit brain, liver, and muscle had been exposed to laser power of 2.5 Watts for a range of 55 to 120 seconds, depending on the tissue, a central area of signal void was surrounded by an inner hypointensity and an outer hyperintensity on T2-weighted images. The 3D extent of the lesions was well demonstrated on multislice images, providing correlation of the affected volumes seen on MRI with volumes seen in histological or histochemical preparations. We are developing an analytical model of laser heating and its effect on MR images to assess whether heating during imaging will produce unacceptable artifacts during surgery. The effect of heating is modeled as a change in magnetization during image acquisition. The region in which the change occurs is blurred by the Fourier transform of the change in magnetization as a function of time. Thus, blurring is minimized when changes occur slowly, compared to image acquisition times. We conclude that MRI can demonstrate the 3D extent of the lesions induced by lasers and can be used to investigate and optimize the control of induced tissue change within the affected volume.

Isolated rat sciatic nerve was exposed to 10.6 and 0.622 un laser radiation in a plexiglass chamber perfused with Krebs solution. A LS88OE Surgical Laser System supplied the laser beam which was aimed at the nerve laying on a zinc selenide window mounted into the chamber bottom. The nerve was Irradiated with CO2 laser CW radiation at 40, 70, 100, and 150 W/cm2 for 1, 3, 5 and 10 seconds and with pulse radiation (pulse duration 20 ms, frequency 10 Hz, 150 W/cm2) for 1 minute. The compound action potential (CAP) was recorded at the end of exposure periods and compared to that recorded right before irradiation. Two effects of the CO laser radiation on the CAP were seen to depend on the intensity and exposure time. One manifested itself in narrowing of the CAP, shifting it to the shorter latencies and a slight change in amplitude; it was completely reversible upon cessation of irradiation. The other effect was irreversible in that the reduced amplitude was not fully restored after irradiation. The irreversible conduction block had a temperature threshold around 42-45°C. The exposure to HeNe laser radiation at 0.6101 W/cm2 was conducted in the dark for 10 sec. 1, 5, 10, and 20 mm. Under no cOnditions of HeNe laser irradiation was an effect observed on the parameters of the CAP.

The radiation spectrum of CO 1aser has a favourable absorption value for laser surgical effect on bloodful biotissues. This property provides more effective large vessels coagulation than C02 laser radiation. A lower level of CO-laser radiation power, required for surgical intervention, provides more safety and better clinical result.

The present study attempted to assess the in vivo effects of Nd-YAG laser irradiation in different gaseous environments on liver and brain. Such an investigation is critical for determining the extent of injury under such conditions for improving further clinical applications. We intended to define the influence on laser-tissue interaction of Room Air, 30% Oxygen, Helium, and Nitrogen. The anesthetized rats were placed in a special chamber and kept breathtng via a tracheostomy tube to the outside, and craniotomy or laparotomy was performed. Nd-YAG laser fiber was directed with a fixed distance at the exposed brain/liver. The staining drug for brain study was 2,3,5 triphenyltetrazolium chloride, which was injected into the aorta before sacrificing the animals. The 44 rats studied were divided into: liver and brain groups. The resulting lesions were photographed macroscopically. In the liver group, statistical analysis showed that laser-liver tissue interaction in helium and nitrogen created a well defined and less hemorrhagic lesions. Macroscopically, in the brain group, we found that the target zones were well delineated with Nitrogen concentration. Moreover, we observed smaller lesions and more sharply defined areas with Helium concentration. In Room Air and Oxygen concentrations, more carbonized and bloodish lesions were found. Laser-tissue interaction in Helium and Nitrogen environments produces more sharply defined lesions with less involvement of the sorrounding tissue, less hemorrhagic lesions to the target, and reduce smoke production. This effect may be of benefit in clinical application of Nd YAG laser, where a more specific target-laser interaction could be achieved avoiding undesired complications due to penetration on the surrounding healthy tissue.

An open-loop temperature control was introduced to control evolution of the maximum temperature on the tissue surface to be within upper and lower limits. For this purpose, the temperature evolutions of sample shots were analyzed and optimal sequences of laser pulses were computed. The 1.06 tm pulsed Nd:YAG laser was used and the thermal camera measured temperature. Experiment on animals in vivo and in vitro was performed to test the technique. Upper and lower temperature limits during laser irradiation were set below 100 °C since thermal coagulation was ofprimary concern. Usually, difference between the upper and lower limits was set to 1 5°C during experiment. However, this difference depended on the laser specifications such as power, pulse width, and repetition rates, as well as on tissue properties. Coagulation studies showed a clear relation of temperature versus cogulation depth. Therefore, the heating temperature and the duration time can be used as primary parameters instead of laser power and exposure time or energy.

The optical properties diffuse reflection and transmission of normal porcine aorta and thermally coagulated aorta were measured using low power laser irradiation at 1060 nm, 633 nm, 514 nm, and 325 nrn (reflection measurements only). The results from three of the wavelengths (1060 nm, 633 nm, and 514 nm) were used with a Delta-Eddington diffusion approximation model12 to calculate absorption coefficients, j.ta, and effective scattering coefficients, j.ts, of the normal and thermally coagulated aorta at these wavelengths. The results show a strong wavelength dependency of irreversible changes in optical properties of thennally damaged aorta, with a trend of increasing changes in reflection approaching smaller wavelengths.

Theoretical modeling of the processes oflight and temperature distribution is rapidly developing both as a qualitative as well as a qtiantitative tool for the understanding of laser light interaction with tissue. Many applications oflasers in medicine involve volumetric coagulation of tissue. In these processes laser light incident on tissue is absorbed and scattered by tissue chromophores and subsequently converted to thermal energy resulting in a temperature rise in tissue. It is well known that elevation of temperature can cause tissue coagulation which is, in most tissues, accompanied by a change in optical properties of tissue (Gourgouliatos [1987], Spears et al. [1988]; Jacques and Gaeeni [1989]). The results by Jacques and Gaeeni [1989] provide a quantitative data for absorption and scattering coefficients of dog myocardium as a function of temperature at selected wavelengths. Parametric study of the effect of variation of optical properties on heat generation, temperature distribution, and ablation rate has been done by Motamedi et a! [1989] and Rastegar et al. [1989]. However, these and other analyses ( e.g. Armon and Laufer [1985], McKenzie [1986], Jacques and PraM [1987] )of temperature disiribution in tissue traditionally have not considered the role of dynamic variation of of the optical properties with temperature. This paper presents a method to analyze this effect and explores its potential impact on laser dosimetry prescription for laser applications involving coagulation of tissue.

Temporal and angular profiles of backscattered pulses are introduced as a viable, novel and noninvasive technique to probe biological and medical materials. The optical properties of these materia's are quantitatively measured in terms of two scattering length scales the transport mean free path and the absorption length of the light. We show that these two scattering lengths can be directly determined from an analysis of the temporal and/or the angular profile of the backscattered pulse intensity. Weak localization of the photon are observed in the tissues. Experimental results for transient light backscattering from the human eye, normal and cancer lung and breast tissues, and tooth; from chicken heart; and leaf are presented.

A single-beam gradient force optical trap was combined with a pulsed UV laser microbeam In order to perform laser Induced cell fusion. This combination offers the possibility to selectively fuse two single cells without critical chemical or electrical treatment. The optical trap was created by directing a Nd:YAG laser, at a wavelength of 1.06 jim, into a microscope and focusing the laser beam with a high numerical aperture objective. The UV laser microbeam, produced by a nitrogenpumped dye laser (366 rim), was collinear with the trapping beam. The maximum transverse force exerted on a moving cell (NS- 1) by the optical trap was determined by measuring the velocity at which the cell fell out of the trap. For a laser power of 55 mW and a cell diameter from 9 to 20 rim, the drag force was calculated using Stokes' law to be in the range from 1.42 0.22 x 106 to 1.13 0.25 x 10-6 dynes. For the fusions, the transverse force needed to capture two NS- 1 cells and to bring them into close contact, was > 200 mW. Once inside the trap, two cells could be fused with several pulses of the UV laser microbeam, attenuated to an energy of 1 &J/pulse In the object plane. This method of laser Induced cell fusion should provide Increased selectivity and efficiency in generating viable hybrid cells.

Tissue ablation using 193nm excimer lasers is being considered for a variety of surgical procedures, yet little is known regarding the potential mutagenic risk to human cells. The effects of sublethal doses of radiation on cellular DNA and gene expression have been examined in cultured human skin fibroblasts. Northern blot analysis of mRNA revealed an increase in the levels of the c-f. proto-oncogene, interstitial collagenase, and metallothionein transcripts after laser radiation at either 193nm or 248nm. Similar changes in gene expression have been previously observed in cells treated with different carcinogens, including classical UV light (254nm) and phorbol esters. In contrast to the conventional UV light or laser radiation at 248nm, the 193nm radiation did not cause significant pyrimidine dimer formation, as determined by measurements of unscheduled DNA synthesis. However, both 193nm and 248nm radiation induced micronuclei formation, indicative of chromosome breakage. These data indicate that exposure of actively replicating human skin cells to sublethal doses of 193nm laser radiation may result in molecular changes associated with carcinogenesis.

The UV corneal ablation model is presented wftLch is giving approximate analytical formula for the threshold fluence and the depth ablated per laser pulse. Only the thermal and optical tissue parameters are used in the formula. Model results are compared with the known experimental results.

A previously described numerical model which calculates heat transfer in biological tissue exposed to Co2 laser irradiation was adapted and applied to predict the amount of ablated tissue and peripheral damage under high power pulsed irradiation. The main object of this work is to search for the irradiation parameters which minimize damage. Simulations of pulsed irradiation were carried out employing rabbit brain tissue thermal constants and varying the following irradiation parameters: pulse duration, repetition rate of the pulse train and beam profile. A Gaussian profile with a beam radius of 1mm was employed over a series of l5OmJ single pulses differing in duration. An optimal value of lms for pulse length was obtained. No improvement was observed at lower durations. Next, the pulse length was kept constant and the effect of the repetition rate of a pulse train was investigated. The damaged volume was shown to increase with repetition rate, however, at very high repetition rates damage decreased to the minimal values observed for the low range of repetition rates. Similar procedures carried out using a rectangular beam profile showed that lateral damage reduced to widths of a few microns with pulse lengths shorter than ims. In addition, ablation thresholds and ablation rates were calculated for both profiles. The theoretical values obtained agree with previous experimental results.

The monitoring and control of the thennal dose in non-contact laser hyperthermia treatment of tumors requires a model and solution of the lasertissue interaction. Presented are two models for the thermal response of tissue during continuous wave, noncontact, laser hyperthermia treatment of tumors. The first model is where all the laser energy is absorbed at the surface of the tissue and the second model is where the laser energy penetrates into the tissue before absorption. Both models include the effect of blood perfusion in the solution for the temperature rise. It is shown that perfusion has a negligible affect on the temperature distribution for the surface absorption of laser energy. However, in the case of laser energy penetration, perfusion has a significant effect which increases with increasing optical penetration.

A model for calculating temperature fields and ablation processes in tissue was developed. The model assumes three different stages experienced by the tissue during a complete ablation process. The consecutive steps are: heating, boiling and ablation. The model has been examined by calculations and experimental measurements of temperature fields in various kinds of biological tissue, exposed to ablative or non-ablative laser beams in the range of low to moderate power densities. The model was able to determine temperature fields and thermal damage in tissue, as well as ablation parameters. This makes it a powerful tool for optimizing laser irradiation parameters for a variety of applications.

Although the CO2 laser radiation has a very short absorption length in living tissue, it can still cause extended damage around the incision. In order to reduce thermal damage in the tissue, one can optimize the irradiation parameters of the laser. Anoter method for reducing the thermal damage is to find a way to cool the tissue. We were able to show that the thermal damage surrounding a crater made by a pulsed CO2 laser may be reduced by irradiating the tissue under water. However, we first had to overcome the high absorption of the water layer. We used a pulsed laser beam of high energy and high repetition rate to create a stationary cavity in the liquid. Through this cavity the beam is transmitted to the material to be treated. Using a simplifid model, we found that the height of the cavity depends on the repetition rate and pulse energy of the beam but not on the pulse width. We also found that the temperature rise in the liquid surrounding the cavity increases with pulse width. In a series of experiments performed in bovine cornea, we found that the thermal damage surrounding the incision caused by a CO2 laser beam was significantly reduced when the tissue was irradiated under water, applying the cavity mode of beam transfer.

We report corneal curvature modifications for rabbit corneas which we achieved by controlled heating of the cornea with a continuous Co:MgF2 laser in the 2 micron region. Our experiments were designed to flatten the cornea, which is required for myopic correction. Large changes of 6-8 diopters in corneal curvature have been stable for the one year duration of the experiments. This is a new method for correcting corneal curvature which retains normal corneal function while not requiring invasive techniques such as cutting or ablation.