Ablation of biological targets by highly-absorbed infrared laser pulses (duration 200 microsecond(s) ) is explained using a model that takes into account tissue liquefaction. The generation of a liquid layer in tissue leads to an enhancement of the drilling efficiency and to a reduction of the evaporation rate. Experimental drilling-depth and recoil-momentum data are explained by a combination of evaporation and ejection.

Due to their strong absorption in water IR-lasers are excellent sources for precision cutting with minimal thermal damage in various fields of medicine. To understand the laser tissue interaction process one has to take into account the liquefaction of target material at the region of radiation impact. The dynamics of the created liquid may cause unexpected and undesirable effects for surgical laser applications. We studied the thermal damage along the walls of incision craters in terms of the elastic material properties and the dynamics of the drilling process. We show that the extension of thermally altered tissue is strongly influenced by the amount of hot liquefied tissue material remaining in the crater. When drilling into mechanically homogeneous materials this amount is essentially determined by the laser intensity used. However, when drilling through a composite structure consisting of various tissue types with different material properties, this is no longer the case. Even at low intensities, the damage zone varies substantially between the different layers. In our investigations we compared histologically and ultrastructurally the instantaneously created damage in the connective tissue and the subjacent skeletal muscle of skin after laser cutting, with long-time heating injuries. This comparison allows a differentiation between thermal and mechanical damage and an estimation of the minimum temperature created in the crater during the laser impact. The light microscopical examinations shows that the thermal damage in the connective tissue is about three times smaller than in the subjacent muscle layer. Comparative studies made with a composite structure consisting of the tissue substitutes gelatin and agar reveal that the unexpectedly large damage in the skeletal muscle layer is a result of the abrupt change of the elastic properties at the material transition. This discontinuity changes the ejection dynamics leading to a confinement of hot liquefied tissue material. The trapped hot liquid acts as an efficient heat source observed in the large damage of the muscle layer.

Precise cutting of biological tissue is possible with the Er:YAG laser because of the strong absorption of radiation exhibited by water containing media at 2.94 micrometers wavelength. To achieve control over the thermal damage caused to the tissue and over the extent of the coagulation zone, a thorough knowledge of the local temperature distribution arising near the impact zone is necessary. Calculations are possible in some simple cases, whereas in others, where liquified tissue material acts as a secondary heat source long after the pulse, a time resolved direct measurement of the temperature distributions with microscopical spatial resolution would be desirable. We have developed a method for measuring two-dimensional temperature distributions in optically transparent media with a high time resolution (up to 4 ns) and with microscopical spatial resolution by imaging the temperature dependent fluorescence distribution of 2 micrometers thin films positioned inside the target. With this method we have measured the temperature distributions at different times after the impact of single pulses from an Er:YAG laser at various fluences in gelatin targets, which we use as model for biological tissue. The results are compared with the thermal damage inflicted in vitro to different types of animal tissue. A strong dependence of the temperature distributions and their dynamical behavior on pulse fluence and water content of the target is observed, in congruence with the coagulation zones observed biological tissue.

Er:YSGG laser ablation of fresh and oven-dried bone was quantified by measuring the mass lost during single pulse irradiation of tissue. The results indicate a statistically significant difference between the ablation of fresh and oven-dried bone. The difference is, however, not as great as would be expected from the difference in 2.78-micrometers absorption as measured using attenuated total reflectance.

The trend for laser-based medical therapies has been towards short- pulse, high-peak power lasers. A consequence of using these lasers is the production of pressure waves that may propagate deep into tissue. Our previous experiments have concentrated on describing these effects and examining methods for measuring laser-induced shock waves. These two approaches were combined to study the effects of well-defined shock waves on red blood cells in vitro. Red blood cells were exposed to shock waves in capillary tubes covered with either latex or polyimide. The latex was deformed by the expanding plasma bubble whereas the polyimide was not. In the latex experiments where the cells were exposed to the bubble expansion in addition to the shock waves, damage to the cells was much greater. When the cells were exposed only to the shock waves through the polyimide, much less damage was apparent. This is the first clear example of the separation of different mechanical effects on tissue damage. In addition, the shock wave damage is not as significant as the damage caused by the bubble expansion.

The objective of biomedical applications of lasers is frequently to remove tissue in a controlled manner. However, for ablation induced by thermal- or photo-decomposition, damage to surrounding tissue may be excessive in some instances. Tissue can also be ablated by a hydrodynamic process referred to as front surface spallation, in which a thin layer next to a free surface is heated to levels, below vaporization but, so rapidly that it cannot undergo thermal expansion during laser heating. This generates a stress pulse, which propagates away from the heated region, with an initial amplitude that can be calculated using the Gruneisen coefficient. As the pulse reflects from the free surface, a tensile tail can develop of sufficient amplitude, exceeding the material strength, that a layer will be spalled off, taking much of the laser-deposited energy with it. Because tissue is generally a low strength material, this process has the potential of producing controlled ablation with reduced damage to the remaining tissue. However, to achieve these conditions, the laser pulse length, absorption depth and fluence must be properly tailored. This paper presents hydrodynamic calculations and analytical modeling relating to both stress- and thermal-induced ablation as a function of laser and tissue properties to illustrate the potential benefits of stress induced ablation. Also, guidance is given for tailoring the exposure parameters to enhance front surface spallation.

For the therapeutic application of laser light it is necessary to minimize defects in the non-irradiated tissue. These defects depend on the primary mechanism of interaction which is determined by the duration of laser action. In the case of continuous wave laserlight a tissue layer surrounding the irradiated volume is thermally affected. On using laser pulsed with a certain energy this layer becomes smaller with decreasing pulse duration. With the pulses of a Q-switched laser tissue cutting will be obtained by the laser-induced breakdown (LIB). Thereby shockwaves are emitted which stress the tissue mechanically. Even in this case thermal lesions can be found. To be able to distinguish between thermal and mechanical effects by histological examination, experiments were performed with ns- and microsecond(s) -laserpulses under the same conditions. A Nd:YAG-laser at 1064 nm was used either Q-switched (pulse duration: 8 ns) or flashlamp-pulsed (100 microsecond(s) ) with a pulse repetition rate of 10 Hz. The beam was focused through air below the tissue surface (focal length in air: 80 mm). The beam geometry in the focal region was identical for both cases. The position of the focal plane relative to the surface was exactly controlled, as it influences extension and kind of the defect. To produce evaluable defects in the microsecond(s) experiments 200 laserpulses with an energy of 340 mJ per pulse had to be applied. The unfixed striated muscle samples of Sprague Dawley rats were immediately dissected prior to laser exposure. For the microsecond(s) experiments the defect region could be divided into 4 zones surrounding a crater, which was found at a focal plane position 2 mm below the surface. Zone 1 shows vacuoles and intensive staining. In zone 2 the myofibrils were displaced and torn apart. Zone 3 represents a sharply bordered intensively stained region. In zone 4 muscle cells are contracted. The zones are all of thermal origin, which could be derived from experiments, wherein an electrically heated wire was fixed inside the samples. In the ns experiments in general larger craters were found. Even a single laser pulse already produces a crater which did not happen in the microsecond(s) experiments. After the application of 5 to 10 pulses only some vacuoles could be found outside the crater. Increasing the number of pulse to 200 the picture is similar to that produced with microsecond(s) pulses. These results show that a few ns pulses suffice to form a crater. Additional ns-pulses lead to heat accumulation and produce thermal lesions like those of the microsecond(s) case and mechanical changes produced by shockwaves may be concealed.

Photographic studies of pulsed infrared laser irradiation of a pure water target illustrate two aspects of mass removal: (1) surface evaporation, and (2) explosive vaporization. A pulsed Erbium:YAG laser provided radiation at a 2.9-micrometers wavelength for delivery to the target site and triggered a second visible laser (nitrogen/dye laser) for illumination of the target site for photography. A variable time delay between the Er:YAG and dye lasers allowed selection of the time of the photograph (>=1 microsecond(s) ). The photographs distinguish between (1) rapid surface evaporation when the energy deposition achieves high temperatures but does not supply the full enthalpy of vaporization, and (2) explosive vaporization of water when the entire enthalpy of vaporization has been provided by the laser pulse.

Chemiluminescence (CL) and optical upconversion (OU) are used to measure photosensitizer singlet oxygen production indirectly. Both methods produce light that is blue-shifted relative to the normal photosensitizing wavelength. The emission intensity is proportional to the concentration of singlet oxygen generated and is the result of either decomposition (CL) or energy transfer (OU) processes occurring inside a thin polymer film. Techniques for fabrication and characterization of CL and OU polymer films are discussed.

Irradiation using short pulse, high peak irradiance sources can lead to novel photochemical reactions due to multiphoton absorption and consequent population of upper excited states. A brief review of biological reactions emanating from multiphoton photochemistry is presented.

Photodynamic therapy has been shown to directly kill cancer cells in vitro. PDT can also kill cancer tumors in vivo, although in such a case the tumorcidal effect may be a result of several mechanisms rather than direct cancer cell kill alone. For example, PDT usually causes an inhibition in blood flow in the treatment volume and it is conceivable that such an effect might compromise the tumor. In investigating the behavior and mechanisms of PDT in vitro and in vivo, we performed PDT on a rat mammary adenocarcinoma (MTF7) both in tissue culture and in an animal model. The animal experiments were done using a rat dorsal skin flap window chamber in which a solid tumor was grown. The window chamber model allowed us to observe, over a period of weeks, the growth of the tumor and the effect of PDT on the tumor and vasculature. We utilized two different photosensitizing dyes in this study- Photofrin and chloroaluminum sulfonated phthalocyanine. In order to compare the tumorcidal efficacy of the photosensitizers in vivo and in vitro, we quantitated the photosensitizer present in the cancer cells by measuring their fluorescence.

The dynamics of the ablation process was investigated in bone using two pulses separated by a variable time delay. In one experiment two sub- threshold 7.5 nsec duration pulses at (lambda) equals 355 nm with pulse separation ranging between 1 ns and 100 msec were used to ablate bone. Crater depths remained approximately constant for pulse separations up to approximately equals 100 nsec, then decreased monotonically in time to zero at 10 msec pulse separation. In another experiment the second pulse was replaced by a 7.5 nsec duration pulse at (lambda) equals 532 nm. The combination of sub-threshold pulses at two different wavelengths also ablated bone with a cutting quality matching that of the more strongly absorbed (lambda) equals 355 nm wavelength. Crater depths from dual wavelength ablation increased with increasing fluence in either contributing pulse. Practical consequences of these experiments are discussed.

The possible application of the vital dye Congo Red to enhance the selective ablation of plaque was investigated. Fresh healthy human aorta samples and samples with varying degrees of atherosclerotic disease were incubated for 3 minutes in a 0.25 mg/ml solution of Congo Red in saline, washed and then irradiated either in air or under saline using an argon laser ((lambda) equals 488 and 514.5 nm, spotsize equals 1.1 mm). The experiments were repeated with undyed healthy and diseased tissue samples. The effect of Congo Red staining on ablation was evaluated by comparing the minimum irradiance and the average amount of time needed to create ablation onset, which was defined as a 'pop' sound followed by carbonization of tissue in air. Ablation thresholds in air for dyed normal tissue, fatty and fibrous plaque were lowered by 42, 60 and 66% respectively. The average time to start ablation dropped from 40 to 2 s, 13 to 1 s and 20 to 14 s respectively. When tissue samples were submerged under saline, Congo Red paradoxically reduced the difference between the ablation threshold of healthy tissue and fatty threshold. During the initial irradiation a concentration of dye around the irradiation spot was observed. This unusual finding may be due to the transport of dye during irradiation. This may explain the observed effect that tissue adjacent to the initial irradiation site had a lowered ablation threshold. By examining the complex mechanisms involved in dye enhanced ablation in may be possible to select a combination of dye and irradiation parameters to achieve selective ablation of plaque.

135 transmission spectra were obtained from 26 autopsy specimens of healthy as well as lipid atheromatous human abdominal aorta. The tissue samples were irradiated using a Nd:YAG laser (1064 nm, continuous wave, 60 W, 1 s) and analyzed by microspectrophotometry (spectral range 250 nm to 800 nm, cryosections 25 micrometers thick, area measured 16 micrometers in diameter). Spectra were recorded from native tissue and from the coagulation zone adjacent to the ablation area. The optical density of the coagulation zone was significantly increased compared to the untreated areas within all tissue samples over the entire spectral range investigated. In the ultraviolet increases were 2 to 6 fold for intima, 1.2 to 3 fold for media, 2 to 4 fold for adventitia and 1.2 to 2.5 fold for lipid atheromatous plaque respectively. In the visible spectral range increases were 7 to 12 fold for intima, 2 to 5 fold for media, 5 to 9 fold for adventitia and up to 3 fold for lipid atheromatous plaque respectively (data p < 0.001). In addition, differences in the optical density between lipid atheromatous plaque and normal vessel wall decreased after laser irradiation. This indicated that due to the changes of optical properties during laser irradiation data obtained from transmission spectra of the native vessel wall may not be sufficient to predict laser-tissue interaction at least with respect to continuous wave lasers.

The variations of reflection and transmission coefficients, as well as of the fluorescence response of the arterial wall are studied as a function of temperature. In a first experiment, reflection coefficient, transmission coefficient, and temperature, are measured during laser irradiation of the tissue. The fluorescence spectrum is recorded in a second experiment, together with temperature, during heating of the arterial wall in a vapor bath. The temperature dependence of the optical parameters and of the fluorescence response are then deduced. For the reflection and transmission coefficients, a direct correlation of their variations with temperature is measured. Extrema are observed in the temperature region at about 50 degree(s)C. A global decrease of the fluorescence intensity is characteristic of a temperature increase. These variations can be used as thermal markers, which in turn could give a criterion to control heat deposition in the tissue during laser anastomose.

The reveal and results of the investigations of nonlinear scattering and wideband radiation from different tissues (epidermis, muscle tissues, pulp of the apples, potatoes and others) under their illumination by microsecond pulses of Nd-laser radiations ore reported.

The ablative removal of bone tissue and the accompanying acoustic wave have been studied in a liquid environment using an ultraviolet excimer laser (Argon Fluoride and Krypton Fluoride) and a mid-infrared Erbium Yttrium Aluminum Garnet (Er:YAG) laser

The interaction of 15 ns pulses from an ArF excimer laser with hard dental tissue was investigated for the purpose of obtaining practical information on the ablation process. Dark field fast photography utilizing an auxiliary, 15 ns Nd:Yag laser 'probe', was used to study the ablation plume dynamics as a function of time, luminescence were studied at different fluence levels and prr. Dentin ablation was found to be about four times as efficient as ablation of enamel in the higher fluence levels tested (> 10 J/cm²) and about twice as efficient as the ablation in the lower fluence regime (approximately equals 1 J/cm²). The dentin etch depth per pulse was found to increase exponentially with fluence (at least up to the tested level of 11 J/cm²), while in enamel the etch depth per pulse appears to increase logarithmically with fluence. Dentin ablation yields a larger, more dense plume which can be ejected (depending on the fluence level) to a height of several millimeters above the surface with observed ejection velocity in access of 1200 m/s. The dentin plume consisted of a relatively uniform particle size distribution (0.1 micrometers to 10 micrometers in diameter). Enamel ablation, on the other hand, yields a smaller observed ejection velocities (about 800 m/s), and a much smaller plume of fine particles (about 0.1 micrometers in diameter) and gases, confined to within 0.5 mm of the surface. In addition, an even smaller amount of highly non-uniform debris, (from ten to several hundred micrometers in size) is observed to be ejected to higher levels, and reach roughly half the height of the corresponding dentin plume for similar fluence levels. Although both dentin and enamel yield lower ablation efficiencies at 1 Hz, no significant difference is detected between the ablation efficiency at 5 Hz and ablation 10 Hz prr. Both materials remained within 20 degree(s)C of room temperature even at fluences as high as 20 J/cm² and prr as high as 10 Hz for enamel and 20 Hz for dentin. Although both materials attained temperature higher than 100 degree(s)C at prr greater than 50 Hz, enamel tends to become much warmer than dentin at this higher prr regime, and its surface temperature is more sensitive to fluence level. Both enamel and dentin ablation with fluence levels higher than approximately equals 1 J/cm² and at prr lower than 50 Hz, yielded smooth, thermal-damage-free surfaces. At sufficiently low fluence (< 1 J/cm²) only partial ablation is observed while rough and irregular surfaces are left behind. Spectral luminescence signatures generated by the ablation were found to be similar in both dentin and enamel, and to consist, initially, of broad plasma continuum, and later, of well defined Calcium atomic and ionic transition peaks.

A new method for surface processing of bioceramic materials like hydroxyapatite (OSPROVIT) has been investigated in this study. A proper surface preparation including retentive areas should lead to highly activated surfaces to adsorb biological active molecules for a good tissue/implant bonding. With the ArF-excimer laser ((lambda) equals 193 nm) it seems to be possible to get such surfaces by a process called photoablation. Mass spectroscopy and electron beam induced microanalysis of elements show that photoablation takes place without changes in the chemical structure. SEM pictures will point out the advantage of this laser type in the preparation for these biomaterials. Temperature measurements show that the thermal stress is acceptable and in a defined forming process the good workability is demonstrated.

Ablation of atheromatous plaques and normal aortic wall was undertaken using ball-tipped fibers, 9Fr and 7Fr multifiber catheters coupled to a pulsed dye laser (Model MDL-1P, Candela Laser Corp., Wayland MA operating at a wavelength of 504 nm. The delivery devices were placed in contact with, and perpendicular to, the endothelial surface of fresh aortic specimens. Catheters were weighted to produce identical pressures on the tissue (230 kPa). The depth and diameters of the craters were measured by microscopy to allow calculation of the volume of tissue removed. Laser energy produced craters in areas of normal aortic wall and in both fibrous and yellow plaque. For given energy levels the largest craters were formed in soft yellow plaque. A significantly smaller volume of tissue was ablated in fibrous plaques with the shallowest lesions in normal vessel wall. The most efficient ablation of fibrous plaque was obtained using the 7Fr multifiber catheter. Soft yellow plaque was encountered relatively infrequently in the specimens examined. These lesions were ablated more readily by the ball-tipped device than the multifiber catheters. Shattering of calcified plaque was produced at 100 mJ/pulse with the ball-tip and at 125 mJ/p with the multifiber catheters. Crater formation in calcified plaque was very variable and highly dependent on plaque color.

The holmium YAG and erbium YAG lasers operating at 2.1 micrometers and 2.9 micrometers respectively, are the subject of great interest for various medical applications. The interaction of both these pulsed lasers with biological tissue involves absorption of the radiation by water leading to rapid heating and ablation, however the different absorption coefficients at these two wavelengths give rise to different ablation efficiencies and haemostatic properties for the two lasers. It is this cut/seal ratio that determines for which medical applications each of these lasers is most suited. The lasers were used to produce incisions in various tissues by translating the tissue at fixed speed beneath a focused laser beam. The laser energy density was varied between 100 and 500 J/cm² and the lasers were operated at 2 Hz. After irradiation the tissues were fixed in formalin, processed routinely into paraffin wax, sectioned at 5 micrometers and stained with haemotoxylin and eosin. This allowed the dimensions of the incisions to be measured, as well as the depth of coagulative denatured tissue surrounding each incision. In this way the cut/seal ratio was determined for both the holmium YAG and erbium YAG laser in a range of hard and soft tissues. Results show that the latent heat of ablation for the holmium YAG laser interacting with soft tissue varies between 20-50 kJ/cm³, almost an order of magnitude larger than with the erbium YAG laser. Furthermore, the depth of coagulative necrosis with holmium YAG extends 100-400 micrometers , compared with 10-30 micrometers for erbium YAG. The two interactions clearly lead to vastly different results suggesting that the holmium YAG laser is suitable for producing lesions in highly vascular tissue where haemostasis is important, whereas the erbium YAG laser is better suited to avascular tissue requiring large depths of incision.

Continuous wave (cw) hydrogen fluoride (HF) chemical laser interactions with human corneal tissue have been studied in order to understand tissue heating phenomenology, effects, and mechanisms under well- characterized laser irradiation conditions. Time-resolved tissue front surface temperature measurements have been obtained during laser irradiation using an optical pyrometer system. Both front surface and in-depth temperature distributions have been calculated by 1D and 2D heat transfer models using realistic tissue optical and thermal properties data for comparison with experiment. The combined experimental/modeling results are useful for developing safe and effective laser microsurgical heating procedures.

The use of 2.09 micrometers Ho-YSGG laser pulses for intra-vascular non- contact ablation of tissue has been investigated. Therefore the transmission and the temporal shape of the laser pulse transmitted through saline was measured. Also the interaction between the laser pulses (200 microsecond(s) FWHM) and saline was studied by time resolved flash photography. Finally, porcine aorta was ablated (in vitro) through either blood or saline. The lesions and adjacent tissue were examined histologically. The penetration depth (the depth for a decrease to 1/e of the transmitted energy) of the laser pulses in saline depended on the power density (0.01 to 12.4 J/mm²) and varied from 0.33 to 2.2 mm, respectively. The photography showed the development of a transparent water vapor cavity around the fiber tip (320 micrometers ) during the laser pulse. The maximum dimensions of the cavity varied as function of the intensity. Within the vapor cavity the laser pulse was undisturbed. Due to this 'Moses effect in the microsecond region' porcine aorta could be ablated through up to 3 mm of saline and blood. Especially after successive laser pulses, histology showed large fissures in adjacent tissue, presumably due to the expanding vapor cavity and the layered structure of the aorta. In conclusion, the formation of a vapor cavity during Holmium laser irradiation in physiological media enables non- contact tissue ablation and induces fissures into adjacent tissue, that may be undesirable.

On the basis of the experiments on the action of powerful pulse radiation at 2.088 micrometers wavelength on biological tissue the application of holmium laser in surgery is substantiated.

To optimize the laser ablation of soft wet tissues by Er:YAG laser radiation at 2.94 micrometers the dynamics of ablation is investigated by high-speed photography techniques. Gelatin as model material is used.

parameters of laser induced tissue ablation. Threshold fluence for ablation is related to some of the optical and thermal properties of the tissue. For this a necessary condition of ablation is imposed that the rate of deposition of laser energy should be greater than the rate of expending energy for vaporization and at threshold they should be equal. A simple expression for resulting depth of the ablation crater is derived assuming total conversion of the optical energy above threshold into kinetic energy of the ablation products. Evaporation that should follow the ablation in the superheated exponential tail in the tissue is also considered for contributing to the crater depth. The assumption of total conversion of optical energy above threshold into kinetic energy of the ablation products is also used to find the average velocity of the ablation products ejected from the tissue. The laser induced ablation is shown to produce a pressure wave which has two components. The first component originates because of the static pressure arising due to the conversion of condensed state of the tissue into a super heated fluid which remained confined to its original volume because of its kinetics and inertia. This component has been worked out using the ideal gas laws. The second component is the recoil pressure of the ejected ablation products which at any instant of time is governed by the instantaneous intensity of the laser beam and it is calculated using Rocket effect. The values of the aforesaid parameters calculated from this model are compared with our and others experimental results and are found to be in agreement with reasonable accuracy.

Threshold effects occurring in fragmentation of stones using a 504 nm microsecond pulsed dye laser were investigated in relation to the optical properties of the stone. Threshold measurements performed on 13 stones appeared to be roughly proportional to the diameter of the fiber. The optical penetration depth, measured opto acoustically yielded values between 0.7 and 3.2 micrometers . The threshold varied exponentially with the optical penetration depth.

The time-of-flight probing of supersonic motion of ablation products or preceding shock wave was used to extract the maximum values of initial pressure and temperature. Measurements were accomplished at three laser- tissue combinations - the TEA CO₂ laser plus artery wall, and the ArF excimer laser or the Q-switched Er:YSGG laser plus pig eye cornea and gave pressures from 1 to 40 MPa and temperatures from 450 to 700 K. These data favor the thermal ablation mechanism for all three cases.

A Q-switched Nd:YAG can be used to cause photodisruption of human crystalline lens. The nature of the acoustic transients that result from photodisruption of crystalline lens are presented in this paper, and a relationship between the laser energy and acoustic pressures is established.

The objectives of this research were to identify mechanisms responsible for the initiation of continuous wave (cw) laser ablation of tissue and investigate the role of pressure in the ablation process. Porcine aorta samples were irradiated in a chamber pressurized from 1 X 10^-4 to 12 atmospheres absolute pressure. Acrylic and Zn-Se windows in the experimental pressure chamber allowed video and infrared cameras to simultaneously record mechanical and thermal events associated with cw argon laser ablation of these samples. Video and thermal images of tissue slabs documented the explosive nature of cw laser ablation of soft biological media and revealed similar ablation threshold temperatures and ablation onset times under different environmental pressures; however, more violent initiation explosions with decreasing environmental pressures were observed. These results suggest that ablation initiates with thermal alterations in the mechanical strength of the tissue and proceeds with an explosion induced by the presence superheated liquid within the tissue.

A new inverted laser trapping microscope, combined with a linear diode array, has been developed to measure light scattering from a single cell over a large (up to approximately equals 45 degree(s)) angular range in real time. Diffraction profiles from 5 and 10 micrometers diameter polystyrene dielectric microspheres are in qualitative agreement with Lorenz-Mie calculations in terms of the separation and number of side-lobe features. Diffraction profiles from red blood cells (RBC), Chinese Hamster Ovary (CHO) cells, and liposomes exhibit features that can ultimately be correlated to their size and shape. Results obtained with this instrument can be used to determine the optical properties of the trapped cell, the location of the cell in the optical trap with respect to the laser focal point, and the forces acting on the cell.

Theoretical modeling of the thermal and optical processes involved in the interaction of laser light with tissue has progressed both qualitatively and quantitatively as a tool for understanding laser coagulation and ablation. Optimal procedure and dosimetry prescription for laser irradiation of tissue in various applications requires a knowledge of the thermal damage process in tissue and its dynamic behavior. Previous work in the area of thermal damage modeling date back to the work of Henriques (1947) on pig skin. Agah et al. (1987) have evaluated damage coefficients for human vascular tissue. Interesting work on quantitative histology is in progress by Thomsen et al. (1989) in which thermal damage coefficients are correlated to birefringent microscopic observations. A study of the dynamics of thermal damage and ablation indicate the presence and variation of damage in different tissues even before the onset of the ablative process (Rastegar, et al. 1988). In the present study, using a kinetic rate model for thermal damage, the behavior of the damage model has been analyzed to determine sensitivities of tissue parameters, effects of temperature dependence, and possibility of predictions based on isotherm propagation.

An Nd:YAG laser hyperthermia system was used to induce hyperthermia in chemically-induced rat mammary adenocarcinomas. Excessive heating of the surface tissue limits the depth of heating during laser-induced hyperthermia. To determine whether surface cooling would allow heating of deeper tissues, treatment surfaces were cooled using two different techniques. (1) an IV drip in conjunction with oxygen flow directed toward the surface, and (2) moisture saturated oxygen flow from a nebulizer. The laser was interfaced to a computer and a thermometer that provided feedback to maintain the tumor temperature between 43.2- 43.5 degree(s)C. The thermocouple was placed in the base of the tumor and its temperature was used via the feedback system to control laser exposure. All tumors were 1.0 to 2.0 cm in diameter. While both cooling techniques lowered the surface temperature effectively, nebulizer technique was preferred due to better control of surface cooling and less fluid accumulation around the treatment area. Nd:YAG laser hyperthermia delivered in conjunction with surface cooling using the nebulizer technique produced efficient heating of rat mammary adenocarcinomas to an approximate depth of 15 mm without overheating the surface tissue.

The surface temperature behavior as a function of time of water and tissue phantoms irradiated with non-ablative CO₂ laser pulses (2 ms) is measured. The temperature decay is very slow and the temperature increase due to the pulse is still non-zero at 70 ms after the pulse is terminated. The slow decay results in accumulation of temperature after multiple pulses at 10 Hz. The thermal behavior suggests that to avoid temperature accumulation when applying (super) pulsed CO₂ irradiation, repetition rates must be smaller than 10 Hz.

The influence of laser-induced temperature increase in transcutaneous PDT was examined in this study. First the subcutaneous temperature and the relative light intensity between tumor and skin were measured as a function of the applied power density in a series of studies. In a second experiment the influence of temperature on the effect of photodynamic therapy was studied. Determination of temperature and of relative light intensity was performed on three groups of mice: one group of C3H mice with macroscopically strong pigmentation, a second group of the same species with weak pigmentation and a third group of extensively unpigmented, homozygote nude mice of the NMRI family were used. For the second series of experiments the SSK2 fibrosarcoma was used as a tumor model on the C3H mouse. The photosensitized tumors from three animal groups, each with 5 animals, were irradiated subcuratively. In Group 1 the tumor surface was cooled in order to prevent laser- induced temperature effects. In Group 2 and 3 no cooling was used. Evaluation of the therapeutic effect was performed in respect to the regrowth delay time. With the use of transcutaneous PDT it could be shown that the temperature and the relative light intensity between tumor and skin depended essentially on the concentration of pigmentation of the skin above the tumor. Dependent on pigmentation and cooling, temperatures of more than 42 degree(s)C were established with irradiation at power densities starting at about 300 mW/cm². In the second series of experiments a clear prolongation of the regrowth delay time, i.e. a better therapeutic effect, was achieved in uncooled irradiated tumors. Curative therapy was successful only in uncooled irradiated tumors. For this reason the synergistic influence of laser-induced hyperthermia on the therapeutical result of PDT could be shown on the tumor model used.

Pulsed photothermal radiometry (PPTR) and integrating sphere spectrophotometry analyzed by the theory of Kubelka and Munk (KM) were used to determine optical absorption coefficients of prosthetic grafts and sutures and arterial thrombus. The KM method, a purely optical theory and technique, resulted in higher absorption coefficients than those found using PPTR, a primarily thermal technique. This difference was statistically significant (t_.025) for the prosthetic materials. With the KM method, other properties such as scattering can also be quantified and the experiment can be performed over a range of wavelengths at one time. The PPTR technique is limited to a single wavelength but it has the advantage that most materials can be tested without any special preparation. In addition, with PPTR the measured quantity is the temporal temperature response of an object to a laser pulse, which is itself of interest. Clinically, the high absorption coefficient of thrombus as compared to that of the graft and sutures (t_.025 for the PPTR measurements) suggests that laser thrombectomy may be safe in polyethylene terephthalate (Dacron) grafts.

The Carbon Dioxide laser has been a proven medical tool for over ten years with established applications in Gynaecology, Dermatology, E.N.T. and Oral surgery. Over the years, major technological advances have been made leading to a number of products available on the market. These technological advances include: sealed tube technology, close 1oop cooling, superpulsed operation. However, despite several indications to the contrary, a flexible fibre delivery system for this wavelength has not emerged. This limits the application of the carbon dioxide laser in medicine to those where direct line of sight access to the treatment area is possible. This investigation of the interaction of CO laser light with tissue was motivated mainly by the possibility of fibre delivery of the 5 micron wavelength. If the CO laser has a similar tissue interaction to 2' then there could be a major market for such medical laser systems. The fibre delivery would open new potential treatments in fields such as angiopasty, laserthermia and other procedures possible only by fibre delivery.

Continuous-wave CO-laser seems to be useful in surgery of soft blood tissue because its spectrum contains the components with cutting and coagulating features. If CO-laser or another types of continuous wave lasers for surgering are employed the main cause of the health tissues damaging on the walls of cut of drilled hole, is heating walls by evaporation products. Tissue damaging by heated products are not excepting under the thermal process of tissue destruction principally. This is apparent with continuous water vaporjet induced by C-W laser beams particularly. It is shown that depth of cuts wall overheated by destruction products, may be decreased in some times by means of using the pulse repetition mode operation CO-laser and the mass removed rate decreased at the same time. This approach is appropriated in the case when thermal influence area decreasing is the main demand to the planed surgery intervention. Coagulative effect on the walls of laser wound may be provided by the radiation with relatively large penetrative length in tissue. In case of CO-laser its spectrum contains a sufficient part of such radiation for both continuous and pulsed modes.

A general mechanism is proposed, capable of accounting for the stimulating action of visible and infrared lasers on cell cultures, at low laser doses, and the damaging action at larger doses. Laser irradiation is assumed to accelerate the formation of a trans-membrane electrochemical proton gradient in mitochondria. This causes more Ca²⁺ to be released from the mitochondria to the cytoplasm by an 'antiport' process, using the proton-motive force (pmf). At low laser doses, the additional Ca²⁺ transported into the cytoplasm (among other factors controlled by the pmf) triggers mitosis and enhances cell proliferation. At higher laser doses, too much Ca²⁺ is released. This causes hyperactivity of Ca²⁺-ATPase and exhausts the ATP reserves of the cell. The nature of the photoacceptors and possible ways in which the visible and infrared laser energy is converted by the photoacceptors are discussed.

Surgical incision may have promotional effects on neoplastic lesions, possibly through release of tissue growth factors (e.g., EGF, FGF(beta) , IGF, TGF(alpha) ). The CO₂ laser may precipitate altered release of these factors. To test this, .5 cm laser, and scalpel incisions were made into fields treated by application of .5% DMBA in acetone, 3 times a week for 6 weeks (group 1) and 12 weeks (group 2). DMBA is a complete carcinogen (initiator and promoter). At 6 weeks, chemically, but not histologically, definable premalignant lesions are seen. Treatment for 12 weeks causes histologic neoplasia which can be graded with T-N-M classification. For both groups, the surgical sites were examined clinically and histologically 4 weeks post-op in a blind fashion. Standard criteria were utilized for defining neoplasia. For group 1, 3 out of 6 laser treated animals developed large exophytic squamous cell carcinomas, but no lesions developed in 12 contralateral, 3 control and 3 scalpel treated pouches. For group 2, 12 of 16 laser treated animals developed tumor with mean grade of 1.75 and mean size of 7.4 mm, 5 of 6 scalpel treated animals developed tumor with mean grade of 1.83 and mean size of 3.6 mm and 3 of 6 control animals developed tumor with mean grade of 1.00 and mean size of 1.5 mm. By the Student 't' test on the binomial distribution lasers cause significant promotion (p < .01). These results suggest that laser surgery may have earlier and more profound promotional effects than scalpel on initiated cells relative to tumor size in the vicinity of the wound site by increased release of growth factors.

Malignant breast tumors can be separated from benign and normal tissues using uv-fluorescence spectroscopic technique. Using the same method one can also distinguish cancerous tissues from noncancerous ones in case of cervix, uterus and ovary.

The separation of on-axis scattered and unscattered transmission through turbid media has been a difficult experimental task in recent years. This study suggests the use of a polarimeter to filter out the contribution of scattered light to the net on-axis transmission. Red blood cells (RBC) were used to produce the scattering effect. The scattering level was varied by: (1) altering the distance of the detector from the sample, (2) using erythrocytes from three different species, e.g., the dog, goat, and human, which are know to have different RBC sizes, and (3) allowing the RBCs from each species to shrink and swell osmotically. An He-Ne laser was used as the source of the radiation so that data were obtained at a wavelength in the spectral region used in oximetry and hemoglobinometry. In each case, the difference in the scattering cross sections obtained for each sample, with and without polarization filtering, gave us a measure of the filtered scattered light. The results obtained were in close agreement with the expected contribution of scattered radiation to the net axial transmission. This method may be used effectively for all studies involving measurements of on-axis transmission through turbid media, such as biological tissue.

A common feature of the photosensitizers in current or proposed use for photodynamic therapy (PDT) is their lipophilicity which promotes their accumulation in cellular membranes. In spite of the absence of observable photosensitizers in the nucleus, photodynamic activation of photosensitizer-loaded cells produces substantial amounts of DNA damage. With either porphyrins or phthalocyanines as photosensitizers, the yield of DNA single-strand breaks plus alkali-labile sites (SSB) is less than that resulting from an equitoxic dose of ionizing radiation; however, these same photodynamic treatments produce high yields of DNA-protein crosslinks (DPC), which are not repaired during post-treatment incubation of the cells, in contrast to the DPC produced by ionizing radiation. Initial yields of DPC after photodynamic treatment of murine lymphoma L5178Y cells sensitized by chloroaluminum phthalocyanine (AlPcCl) are greater in the relatively PDT-sensitive strain LY-R as compared to the relatively PDT-resistant strain LY-S. Photodynamic treatment sensitized either by AlPcCl or by Photofrin II is mutagenic at the thymidine kinase (tk) locus in one or more sub-strains of LY-R and LY-S. With Photofrin II, the induction of mutations has been observed in the tk+/- heterozygous strains LY-R16 and LY-S1, but not in the hemizygous tk+/0 strain LY-R83. This pattern of strain-specific mutagenesis is found for other agents, such a ionizing radiation, which produce a high proportion of multi-locus lesions. With AlPcCl, mutation induction is found in strains LY-S1 and LY-SR1, but not in either of the sub-strains of LY-R. Treatment of strains LY-R and LY-S with identical doses of AlPcCl and red light results in degradation of the DNA, which occurs earlier and to a greater extent in the more PDT-sensitive strain. Examination of the size of the DNA during the period of degradation revealed a series of fragments with sizes which were multiples of approximately 190 bp. This suggests that PDT treatment of L5178Y cells induces the process known as apoptosis, or programmed cell death, in which endonucleolytic scission of the DNA occurs in the internucleosomal linker region. DNA degradation is also stimulated by treatment of murine L929 fibroblasts with phthalocyanine and light but not by gamma- irradiation alone. Combined treatment with a minimally lethal dose of PDT and a dose of gamma-radiation producing 90% cell death results in the induction of a PDT-type cell death in a substantial portion of the cells. When the level of programmed cell death is very high, the recovery of mutants may be compromised. Thus, PDT appears to produce extensive DNA damage, but events at other cellular locations may alter the expression of that damage.

A Lameta 22710 excimer laser operating at 70 mJ/mm2 per pulse, with pulse duration of 70 nsec, and pulse repetition rate of 10 Hz, equipped with a quartz filament as energy conductor was used to make incisions on rat liver. 2 to 5 sec after irradiation the specimens were fixed and further processed for electron microscopy and histochemical visualization of the endoplasmic reticulum (ER) marker enzyme glucose-6- phosphatase at the ultrastructural level. The additional series were: fixation before irradiation-(A); lasing with Nd:YAG laser (1064 nm, continuous wave mode, 40 J/mm²)-(B); incision with a white-hot steel needle-(C); and incision with an Esto-Rex ultrasound scalpel (66 kHz, 6 Wt, vibration amplitude of 15 micrometers )-(D). The results showed that unlike Series C and B, in which high temperature caused severe damage to all cellular organellae, the excimer action was much more specific. It caused vesiculation of ER without significant injuries to other cellular structures. The analogous effect was noted after US scalpel cutting, thereby allowing a conclusion that a kind of dynamic rather than thermal factor is responsible for the observed phenomenon of vesiculation. The time schedule of vesicle formation and molecular background of membrane transformation is considered in the light of the data of Series A and D, and also on the basis of available information of membrane behavior. Photoablative effect of pulsed excimer laser is thought to be based on chemical decomposition of organic molecules and their ejection from the tissue to the action of high energy photons. Pressure waves (either acoustic or shock) are presumably generated powerful enough to cause tissue and cell damage beyond the site of ablation. Some thermal and fluorescence events are also implicative in biological targets irradiated with excimer lasers. In our previous studies electron histochemistry was employed for the analysis of cellular alterations caused with a continuous wave mode-operating infrared (Nd:YAG) and a 308 nm ultraviolet (XeCl) laser in rate liver hepatocytes. A conclusion has been made on the predominantly nonthermal injuries produced by the excimer as opposed to clearly thermal damage by the Nd:YAG. Besides, it was suggested that a kind of dynamic effect should prevail in the excimer action. In the present study we continue our line of investigation by extending a spectrum of experiments designed for better understanding the biological action of the excimer laser.

We measured the photoacoustic absorption spectrum of rat tissues (skin, heart) and bovine tissues (liver, kidney, muscle) in the range 250-700 nm. The photoacoustic measurements are not significantly affected by scattering and provide directly the absorption spectrum of tissue. The measured values of the absorption coefficient of tissues are in agreement with the literature data from optical measurements, which however require the adoption of suitable models of light propagation in tissues.

Abstract not available.

Practical use of laser tecnique surgery demonstrated the true choice of lasers generating in the long wavelength spectrum. In this field the pigment content of biologic tissues is not noted at absorption spectrum. The latter is generally defined by the percentage of water in tissues. Among the known laser types the most proved and developed are the lasers generating at 1060 nm, 2090 nra, 2900 nii and 10600 nm. Ho-lasers of 2090 nm radia tion wavelength are perspective instruments for surgery purposes due to the high radiation absorption at biologic tissues: 30 cru This value is much higher than the absoptance at 1060 mit and a little bit lower at 1060 nm. An important feature that distinguishes Ho-laser among other types is the possibility of its use with fiber channel made of fused silica for abdominal operations.