A thermal model to predict the effects of laser parameters on the zone of thermal injury produced by laser ablation of biological tissue is presented. A dimensionless parameter based on the ablation velocity and the optical and thermal properties of the target is key in determining the resulting zone of thermal injury. The zone of thermal injury is minimized when this parameter, known as the Peclet number (Pe), is much larger than one. This occurs because the rapid movement of the ablation front prevents the diffusion of energy beyond the laser that absorbs the laser radiation. For Pe less than one, the slow movement of the ablation front allows for diffusion of energy away from the region of energy deposition and leads to larger zones of thermal injury. The model predictions are compared with data available in the literature. Deviations between the model predictions and published data are discussed and potential effects of pyrolysis, temporally varying pulse shapes and pulse repetition rates are explored.

Holmium ((lambda) equals 2.09 micrometers ) and excimer ((lambda) equals 308 nm) lasers are used for ablation of tissue. In a previous study it was demonstrated that both excimer and holmium laser pulses produce fast expanding and collapsing vapor bubbles. To investigate whether the excimer induced bubble is caused by vaporization of water, the threshold fluence for bubble formation at a bare fiber tip in water was compared between the excimer laser (pulse length 115 ns) and the Q-switched and free-running holmium lasers (pulse length 1 microsecond(s) to 250 microsecond(s) , respectively). To induce bubble formation by excimer laser light in water, the absorber oxybuprocaine-hydrochloride (OBP-HCl) was added to the water. Fast flash photography was used to measure the threshold fluence as a function of the water temperature (6 - 90 degree(s)C) at environmental pressure. The ultraviolet excimer laser light is strongly absorbed by blood. Therefore, to document the implications of bubble formation at fluences above the tissue ablation threshold, excimer laser pulses were delivered in vitro in hemoglobin solution and in vivo in the femoral artery of the rabbit. We conclude that the principal content of the fast bubble induced by a 308 nm excimer laser pulse is water vapor. Therefore, delivery of excimer laser pulses in a water or blood environment will cause fast expanding water vapor bubbles, which may induce mechanical damage to adjacent tissue.

How micro is laser microdissection? This study compared the spatial resolution of laser microdissection achieved by two laser systems: the ArF excimer laser which is strongly absorbed by tissue protein, and the Er:YAG laser which is strongly absorbed by tissue water. Both lasers penetrate tissue only a couple microns and are suitable for laser microdissection, and in this report the lasers ablated the outer dead-cell layer of the skin called the stratum corneum. The study involved dorsal skin sites on 8 rats for the ArF excimer and 10 rats for the Er:YAG. Ag/AgCl-gel electrodes were used to measure the passive DC resistance (R) and the active DC voltage (V) of skin sites which had received increasing numbers of ablative laser pulses (9 mJ/pulse, Er:YAG; 48 mJ/pulse, ArF excimer). About 8 pulses were required before a sudden drop in Ra and a sudden rise in V was observed. The R dropped from 4 +/- 0.2 (18) Mohm down to 1.5 +/- 0.2 (18) Mohm; mean +/- standard deviation (number of skin sites). The V was initially -56 +/- 5 mV, then dropped to -3 +/- 0.4 mV after laser ablation penetrated and destroyed the battery. The Er:YAG laser required 8.3 +/- 1.5 pulses to achieve 50% of the full change in R and V; the ArF excimer laser required 76 +/- 2 pulses. The changes in R and V per depth of tissue ablated were identical for the two lasers, despite their distinct differences in absorbing chromophore and efficiency of ablation.

The application of pulsed photothermal radiometry (PPTR) diagnostics to characterize port wine stain (PWS) lesions is discussed. Influence of epidermal absorbance and PWS depth on the PPTR signal is analyzed using an in-vitro model of PWS skin consisting of multilayered collagen films. An instrument is constructed and used for patient and animal model PPTR measurements. When an infrared fiber is incorporated, the instrumentation facilitates convenient skin-site accessibility. Modulation of the PPTR signal and homodyne detection substantially improve the system signal-to-noise ratio over previous methods. Given the measured PPTR signal, an algorithm computes the initial temperature distribution in the PWS skin immediately after the laser pulse. The use and limitations of the algorithm are presented and discussed.

Laser-induced fluorescence has been used to measure tissue levels of chloroaluminum phthalocyanine tetrasulfonate versus time in order to determine its pharmacokinetics. A hamster cheek pouch carcinoma model was used in vivo. The data have been modeled using a four compartment pharmacokinetic model, yielding rate constants which describe the transport. A minimum of 13 rate constants was needed to achieve acceptable fits to the tumor and normal tissue as well as plasma data. The model gives insight into the role of binding and unbinding processes that are not otherwise evident.

In order to study light-induced reactions during PDT, the fluorescence response of the photosensitizer meso-tetra(4-sulfonatophenyl)porphyrin (TPPS₄) was observed in different cell systems and correlated with the sensitivity to photodynamic induced destructions. RR 1022 epithelial cells from the rat were grown on microscopic slides at a high and low cell density. Using video microscopy in combination with microspectrofluorometry we observed a different fluorescence behavior for high and low cell conditions during light exposure. A fluorescence relocalization from the cytoplasm to the nucleus and an intensity increase-- correlated with the formation of a new molecular species--could be detected only for low cell density. Moreover, cell cultures at a high density showed to be less sensitive to photodynamic destructions. In addition to cell culture-experiments, we observed the light-induced reactions of TPPS₄ accumulated in multicellular tumor spheroids. For these measurements laser scanning microscopy was used. Fluorescence relocalization and intensity increase could be detected only for the peripheric parts of the spheroids. The different fluorescence response seems to reflect different metabolic and physiologic states of the cells.

The validity of a threshold model in photodynamic therapy (PDT) has been demonstrated in various normal animal tissues with porphyrin and phthalocyanine photosensitizers. This model states that, given a specific tissue and photosensitizer (PS), necrosis will result if the number of photons absorbed by the PS per unit volume of tissue exceeds a threshold value. The purpose of this study was to determine the threshold value for normal brain tissue and for intracranially implanted VX2 tumors using a rabbit model. Additionally, the dependence of the threshold value on other factors currently not included in the threshold model, such as oxygenation of the target tissue, was investigated. Calculation of the threshold requires knowledge of three parameters: (1) the radius of necrosis (for interstitial irradiation), (2) the light fluence at the necrotic boundary and (3) the photosensitizer concentration in the tissue. The animals were sensitized using Photofrin^TM at various time-delays between injection of the sensitizer and illumination. The output from an argon laser was coupled into an optical fiber, which was implanted in the brain under stereotactic guidance. The animals were sacrificed 24 to 48 hours after PDT and the radius of necrosis was determined by serial microscopy. These data were used to calculate the photodynamic thresholds for normal brain and tumor tissue.

The observation of fluorescence in a tissue offers a method of real-time monitoring of chemical reactions when one of the reagents or products in fluorescent. This paper discusses how tissue optics affects one's observation of fluorescence in a turbid medium such as tissue. An experimental example is also shown: the photobleaching of a photosensitizer molecule (protoporphyrin IX) within rat tumors. A simple algorithm (Gardner et al., 1993) is given which enables the corrections for tissue optics.

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

The mechanisms responsible for the generation of stresses by pulsed-laser energy deposition in solids are elucidated with special attention given to laser-tissue interactions. These mechanisms include thermal expansion, subsurface cavity formation, ablative recoil and plasma formation and expansion. Scaling laws are presented for the magnitude of the stresses generated by each of these processes. The effect of laser parameters and material properties on the magnitude and temporal behavior of the stress transients is considered. The use of these scaling laws in conjunction with measurement of stress transients induced by pulsed laser sources may be a powerful tool in determining the physical processes which control the response of materials to pulsed energy deposition. In addition, the controlled generation and accurate measurement of acoustic transients may have important diagnostic and therapeutic applications.

In surgical applications of lasers, it is often necessary to know when a laser has penetrated a bone. There are many instances when it is critical to avoid damaging tissue beneath the bone. We are developing a system to monitor the ablation of bone. We have found a method to detect when the bone has been penetrated by measuring the photo acoustic signal generated by a pulsed laser. Using a transducer on samples of temporal bone and several model substances, we can see a decrease in the power spectrum near 350 kHz as softer materials is ablated. The current results are from a carbon dioxide laser operating in the super pulse mode. We are developing the technique for use with the Vanderbilt Free Electron Laser as part of our computer assisted surgery techniques program.

A new approach to drug delivery has been developed at the Wellman Laboratories of Photomedicine that is analogous to photodynamic therapy except that it utilizes high pressure impulse waves to increase the effectiveness of a variety of drugs rather than light activated drugs. This therapeutic modality offers a generic technology that can be used in a variety of conditions including infections, abscesses, and cancer.

We are exploring new applications of the technology of energy deposition and dynamic response. Early studies involved analytical solutions of the coupled thermal and elastic response of materials to pulsed energy deposition. Experiments designed to test the theory led to determinations of thermal pressure coefficients for a variety of materials and an understanding of the effects of the time dependence of the energy source on dynamic response. Subsequent experiments at higher deposited energies required analysis by an energy deposition-wave propagation code to explain the observed elastic-plastic behavior. Instrumentation included laser interferometry and holographic interferometry for multi- dimensional response. A possible application of this technology to Biomedical Science is a technique to measure ion transport in biological material. It requires a combination of holographic interferometry and spectroscopy, namely, Resonant Holographic Interferometry Spectroscopy (RHIS). The technique involves the absorption and refraction of light near absorption lines. Stress waves arising from the absorbed light can be assessed with the energy deposition-wave propagation code. Such calculations will require the inclusion of appropriate biomaterial properties.

It has been established that the infrared (Ho:YAG at 2.09 micrometers ) ablation process involves direct heating of tissue water followed by subsurface pressure build up that ultimately leads to a violent explosion. Recently, we presented evidence that the same mechanism plays a role in ultraviolet (XeCl at 308 nm) ablation. It is expected that this process is dependent upon the mechanical strength of the irradiated tissue. A qualitative study was done to demonstrate the effect of the tissue mechanical properties on the pulsed laser ablation process and resulting mechanical damage to tissue.

In studies of pulsed laser-tissue interactions it is often necessary to vary the wavelength of the incident radiation over a wide range and at the same time keep the other parameters such as laser pulse duration constant. Dye lasers will allow continuous tuning over limited ranges only, while the use of different types of lasers may lead to orders of magnitude changes in pulse duration. Optical Parametric Oscillators (OPO) can simultaneously satisfy all these demands. In the current experiments an OPO based on the nonlinear crystal (Beta) - bariumborate (BBO) was pumped by the third harmonic of a nd:YAG laser at 355 nm Tuning was possible from 400 nm to 3000 nm with a 3 ns wide output pulse. Maximum available energy was approximately 25 mJ. For initial study with this OPO system we investigated the wavelength dependency of shock wave formation after pulsed irradiation on a urinary stone in distilled water. The generated shock waves were detected using a so called Schlieren technique. Measurements were taken in the range from 460 nm to 960 nm. Results indicate a gradual increase in threshold energy for shock wave formation with increasing wavelength with no strong peaks in the spectra.

The goal of this study is to measure the average impulse induced pressure in porcine aorta by one laser pulse for both a free running Tm:YAG (2.014 microns) laser and a free running Ho:YAG (2.1 microns) laser. The pulsed Tm:YAG and Ho:YAG lasers operated at a repetition rate of 2 Hz and a FWHM pulse width of 150 microseconds. The laser energy was delivered to the tissue via a 600 micron core diameter low-OH silica fiber. The impulse pressure induced in the aortic wall was evaluated using tissue samples (n equals 50) mounted on a pendulum to measure the transferred momentum from the laser pulse to the tissue. The impulse induced pressure was determined using a contact mode, where the fiber tip and tissue surface are within 0.5 mm of each other. The impulse induced pressure was studied over a fluence range of 35 - 350 J/cm², fifteen measurements were taken at each fluence level. The results showed that the average impulse induced pressure of the Ho:YAG pulsed laser in the fiber contact mode was 2 - 6 times greater than in the non-contact mode, where the energy at the fiber tip was imaged onto the tissue surface. The results also showed that both the Ho:YAG and the Tm:YAG average impulse induced pressure, in the contact mode, increased linearly at lower fluences, reaching a peak at approximately 13 - 14 atm.

To evaluate whether the size of the tissue necrosis achieved by interstitial laser photocoagulation (ILP) is dependent on the fiber core diameter, the fiber cladding, and the laser wavelength used. Methods: Forty six rats were anaesthetized and their livers exposed at laparotomy. A single bare fiber was inserted interstitially into the liver which was then irradiated at 2 W for 100 s or 500 s. Four fibers were used: 0.2 mm and 0.4 mm all silica fibers had their buffer coats/jackets stripped off the distal 3 mm, leaving the cladding intact; 0.2 mm and 0.4 mm plastic clad fibers had their buffer coats/jackets and cladding stripped off the distal 3 mm. A 1064 nm Nd:YAG and an 805 nm diode laser were used. One day after ILP the rats were sacrificed and measurements made of the size of necrosis and of any charring. Results: 1. There was no difference in the size of necrosis from a 0.2 mm or 0.4 mm fiber. 2. Larger areas of necrosis and greater charring were produced with the plastic clad fibers. 3. The 805 nm diode laser gave the largest necrotic size and the greatest charring for a given energy. 4. There was a positive correlation between the degree of charring and the size of the necrosis.

Interstitial laser photocoagulation (ILP) was performed ex vivo between tissue slabs by delivering continuous-wave laser energy from an optical fiber (400 micrometers core, plane-cut, 5 mm cladding stripped) either directly, or by depositing the energy into a 2.4 mm diameter steel sphere mounted on the fiber tip. The dependence of the size and nature of the resulting thermal coagulation lesions on the following variables were assessed: (1) energy source: Nd:YAG - 532 nm, 1064 nm +/- steel sphere, (2) tissue type: porcine muscle (light), bovine muscle (dark), (3) delivered power: P equals 1.5 - 3.0 W (porcine), 1.0 - 2.5 W (bovine), (4) exposure duration: T equals 300 - 700 s. The resulting cross-sectional ILP lesions are summarized as follows: 532 nm: elongated; central charring in bovine and porcine at all powers. 1064 nm: circular; central charring only in bovine for P >= 2.0 W, T >= 500 s, sphere: circular; central charring in bovine for P >= 1.5 W and porcine for P >= 2.0 W. These experiments confirm a recent report suggesting ILP lesion size decreases as optical penetration increases. The results indicate that ILP lesions of clinically useful size (diameter >= 8 mm) must necessarily involve central charring in heavily pigmented tissues, but ILP lesions greater than 10 mm diameter can be made without charring in lightly pigmented tissues by delivering 3.0 W of 1064 nm laser energy for 700 s.

Interstitial laser photocoagulation is a new technique of tumor ablation using low power (2 W) laser light over a long time (500 s) via thin (0.2 mm) optical fibers. We have treated 26 patients with 70 liver metastases measuring 1 to 15 cm (median 2.5 cm). There were 1 to 8 treatment sessions per patient (median 3). Each tumor was treated via 1 to 4 optical fibers. The median energy used was 16000 J (range 3000 to 34000 J). Treatment effects were monitored in real-time with ultrasound, and the extent of tumor necrosis evaluated 1 to 3 days later using dynamic enhanced CT which showed laser-induced necrosis as well-defined new areas of non-enhancement. Greater than 50% necrosis of tumor volume was achieved in 86% (60 out of 70) of the tumors treated, and 100% necrosis in 53% (37 out of 70). Metastases under 4 cm were treated more effectively and required fewer treatment sessions than those over 4 cm. In eleven patients there was evidence of disease progression (follow-up 14 months or longer) and in 15 patients there has been overall tumor reduction (follow-up less than 1 year, median 4 months). Conclusion: With further development, ILP may offer a practical and minimally invasive alternative to major surgery for eradicating small, deep seated tumors, and debulking larger ones.

In this study a model was developed to quantify levels of laser-cavitron energy deposition and to correlate this with MRI and histological damage. The Ho:YAG laser (2.1 micron wavelength) and cavitron were used on separate matching tissues, with exposure to succeedingly higher energy densities by an accurate and reproducible method of dosimetry. Post laser/cavitron treatments were then evaluated using the Signa 1.5 T MRI (GE Medical Systems). In this study, fast gradient echo (rapid SPGR) pulse sequence was used to visualize and quantify signal changes. Thereafter, biopsies were taken for standard H&E preparation and quantification of laser or cavitron damage. FSE and Turbo Flash images taken demonstrated a linear correlation between energy deposition in the tissue and signal intensity changes for the laser or signal void for the cavitron. These signal changes correlated well with the histological measurements of the same tissues. This initial study demonstrates the potential clinical feasibility of these two energy delivery systems on various tissues.

Imaging plays a crucial role in the treatment of liver tumors by interstitial laser photocoagulation (ILP). Ultrasound allows location of the tumors and enables guided placement of thin hollow needles (through which the optical fibers are passed) into the appropriate part of the tumor. Heating of the tumor during ILP is seen as an enlarging echogenic zone around the fiber tips. However, the margins of the echogenic zone are often ill-defined and irregular and ultrasound cannot clearly differentiate treated from untreated tumor on follow-up scans. CT (pre-contrast, dynamic, and delayed) is used to define the number and sizes of metastases prior to ILP. 24 hrs after ILP dynamic enhanced CT clearly shows the laser-induced necrosis as a well-defined non-enhancing area, although real-time CT monitoring of ILP shows very little change around the fiber tip. MRI (standard spin-echo sequences) has been used to evaluate lesions post-ILP. On T1-weighted images the lesions appear heterogenous with areas of high and low signal intensity. With these current sequences the lesion-to-liver contrast is not as good as with dynamic enhanced CT. Conclusion: Ultrasound plays a useful role in treatment delivery. At present the post-ILP evaluation is best performed using CT. MRI has the potential for real-time monitoring of ILP using temperature sensitive sequences.

Preliminary studies of various laboratories indicate that the scattering coefficient and scattering anisotropy of tissues can change with the temperature, especially in the temperature range where coagulation processes take place. The objective of this work was to correlate light scattering changes with ultrastructural alterations in tissue as a function of temperature. Initial results for the dependence of tissue optical properties on the thermal heating of a variety of different tissues and corresponding ultrastructural changes are presented.

Thermal coagulation of albino rat skin heated in vitro results in prominent changes of light scattering but relatively little in light absorption based on measurements using an integrating sphere spectrometer. The reduced scattering coefficients, (mu) _s(1-g), gradually increase as temperatures increase from room temperature to 55 degree(s)C then rapidly decrease to plateau after 70 degree(s)C is reached. The differences among the (mu) _s(1-g) values for the different wavelengths were greater at the lower temperatures than at higher temperatures. The absorption coefficient, (mu) _a, changed very little over the test temperature range (room temperature to 90 degree(s)C) and then only at higher temperatures and for longer wavelengths. The optical property changes were associated with thermally induced light microscopic and ultrastructural changes in the dermal collagen, a major tissue component of skin.

We have done a study addressing the effects of blood of irradiation with different vascular- lesion lasers. Irradiation of dilute whole blood with light (577 - 585 nm) produced by an argon-dye laser, copper-vapor laser, flashlamp-pumped dye laser or Q-switched frequency- doubled Nd:YAG dye laser causes the release of hemoglobin from the red blood cells. The magnitude of this release is shown to be proportional to the magnitude of thermal insult. With this experimental technique, we compare the efficacy of each laser for producing thermal damage, using clinically realistic irradiation parameters. We have determined that an equivalent energy fluence from these lasers produced comparable amounts of thermal damage in whole blood, except in the experiments where the flashlamp-pumped dye laser was used. The flashlamp-pumped dye laser produced much greater damage than the other lasers at equivalent irradiance levels. This was felt to be due to the flashlamp-pumped dye laser's relatively large spot size. Measurable differences in thermal damage elicited by irradiation of red blood cells with the argon-dye laser and copper-vapor laser emphasized the fact that the latter is a pulsed laser, while former is a continuous-wave laser. All of the experimental results show that laser induced thermal damage is a function of not only the energy fluence, but also of the energy fluence rate and geometry of the irradiation.

It has been suggested that laser welds of collagenous tissues form by interdigitation and chemical bonding of thermally 'unraveled' collagen fibrils. We investigated this proposal by attempting to weld highly collagenous, avascular corneal tissue with an infrared (IR) diode laser as follows. First, the temperature at which corneal collagen shrinks and collagen fibrils 'split' into subfibrillary components was determined. Second, since use of a near-IR laser wavelength necessitated addition of an absorbing dye (indocyanine green (ICG) to the cornea, we measured absorption spectra of ICG-treated tissue to ensure that peak ICG absorbance did not change markedly when ICG was present in the cornea. Third, using gel electrophoresis of thermally altered corneal collagen, we searched for covalently crosslinked compounds predicted by the proposed welding mechanism. Finally, we attempted to weld partial thickness corneal incisions infused with ICG. Principal experimental findings were as follows: (1) Human corneal (type I) collagen splits into subfibrillary components at approximately 63 degree(s)C, the same temperature that produces collagen shrinkage. (2) Peak ICG absorption does not change significantly in corneal stroma or with laser heating. (3) No evidence was found for the formation of novel compounds or the loss of proteins as a result of tissue heating. All tissue treated with ICG, however, exhibited a novel 244 kD protein band indicating chemical activity between collagen and corneal stromal components. (4) Laser welding corneal incisions was unsuccessful possibly due to shrinkage of the sides of the incision, lack of incision compression during heating, or a less than optimal combination of ICG concentration and radiant exposure. In summary, these experiments demonstrate the biochemical and morphological complexity of ICG-enhanced IR laser-tissue welding and the need for further investigation of laser welding mechanisms.

Autofluorescence is a known property of skin, however, its temporal dynamics are not well known. In this study, we measured the autofluorescence changes in vivo skin during continuous exposure to 442 nm He-Cd laser light. The measurements were performed using a computerized autofluorescence and diffuse reflectance spectroanalyzer system developed in our laboratory. A novel calibration procedure was used to reduce the autofluorescence signal fluctuations over the exposure time of 11 minutes. Integral intensities were calculated for various wavelength regions from the spectra and plotted as a function of time. Non-linear regression fitting of the time function revealed a double exponential decay process [I(t) equals a exp(-t/(tau) ₁) + b exp(-t/(tau) ₂) + c] in which (tau) ₁ and (tau) ₂ differed by an order of magnitude. Autofluorescence imaging demonstrated that a complete recovery of the exposed areas requires approximately 6 days. The variations of parameters a, b, c, (tau) ₁, (tau) ₂ with exposure intensity were measured. A hypothesis for the physical meanings of the double exponential decay process is proposed.

The optical absorption and transport (or reduced) scattering coefficient of tissue in vivo can be deduced from in situ measurements of the light fluence rate at 2 or more points. This requires: (1) absolute measurement of the light fluence rate with small, interstitial light detectors; (2) knowledge of the irradiation and detection geometries and (3) a mathematical model relating the fluence-rate distribution to the absorption and scattering coefficients. The purpose of this study was to assess the accuracy of this technique using tissue-simulating phantoms with a wide range of known optical properties. Light fluence-rate measurements were made using either novel fluorescent-tip detectors with isotropic response of cut-end fibers of high or low numerical aperture. The light source was either a cut-end optical fiber or a fiber with a scattering tip which produced a nearly isotropic radiance distribution. Optical interaction coefficients of the phantom were derived from the fluence measurements using different solutions of the diffusion theory for infinite or semi-infinite media. Errors in the derived optical interaction coefficients, and their dependence on the optical interaction coefficients and on the source detector types are presented and discussed.

We have newly synthesized a class of photochemical 1,8-naphthalimide dyes. Photochemical investigation strongly suggests that these dyes function as photoalkylation agents following activation to an intermediate state by visible light (circa 420 nm) excitation. The activated species reacts readily with nucleophilic amino acid residues, e.g., tryptophan, cysteine, and methionine. One dye, 1,14-bis-(N-hexyl-3'-bromo-1,8'-naphthalimid-4'-yl)-1,4,11,14- tetraazatetradecane-5,10-dione, which incorporates two reactive 1,8-naphthalimide groups at each end of an intervening structural bridge has been used to cross-link the protein monomers of F-actin, thus preventing its natural depolymerization at low salt concentrations, and to cross-link Apolipoprotein I of human high-density lipoprotein. These observations suggest continued study of these dyes as agents for protein cross-linking, tissue bonding, and inactivation of infectious agents.

Exposure of thymine and DNA to high-intensity 532 nm pulsed radiation from a Nd:YAG laser resulted in the cyclobutylpyrimidine dimers, which were measured by the method of high performance liquid chromatography. The in vitro transcription by RNA polymerase was markedly inhibited and could not be stimulated by spermine when the native double-strand DNA was replaced by irradiated DNA. It was shown that DNA damage was caused by 532 nm laser radiation and that the high-intensity visible radiation can initiate photochemistry in UV-absorbing biological molecules by two photon absorption. It is suggested that the use of very high-intensity laser radiation in medicine introduces the possibility that biomacromolecules may be damaged in cells as a result of two photon absorption.

The purpose of our study was to determine the efficiency of bone ablation and the thermal damage caused by a Nd:YAG pulsed laser beam having an experimental cooling irrigation system.

Pulsed laser ablation is a trade off between minimizing thermal damage (for relatively long pulses) and mechanical damage (for relatively short pulses) to tissue adjacent to the ablation crater. Often it is not known what the optimal laser parameters are for a specific application, since clinically used parameters have at least partially been dictated by physical limitations of the laser devices. We recently obtained a novel type of cryogenic continuous wave holmium:YAG laser ((lambda) equals 2.09 micrometers ) with a galvanometric drive outcouple mirror that acts as a Q-switch. This unique device provides pulse repetition rates from a few Hz up to kHz and the pulse length is variable from microsecond(s) to ms. The effect of pulse duration and repetition rate on the thermal response of chicken breast is documented using temperature measurements with a thermal camera. We varied the pulse width from 10 microsecond(s) to 5 ms and fond that these pulse durations can be considered impulses of thermalized optical energy. In this paper some theoretical considerations of the pulse length will be described that support the experimental data. It was also found that even at 1 pulse per second thermal superposition occurs, indicating a much longer thermal relaxation time than predicted by a simple time constant model.

Laser volatilization is a thermal process used for surgical incision and tumoral ablation. This experimental study was carried out with a view to quantify this thermal effect in terms of mass loss and temperature evolution. Three steps have been characterized during tissue removal: they are coagulation, water vaporization and combustion. This study has shown that each effect corresponds to a specific slope on mass loss and temperature evolution curves versus fluence. Those experiments allowed the identification of coefficients of volatilization.

The interaction of various pulsed lasers with meniscus and bone of freshly slaughtered bovines and pigs was examined. Our aim was to find lasers useful for accident surgical operations (e.g. bone or callus dystopy inside joints or nearby important vessels or nerves after fractures). Laser wavelengths of the UV- and infrared spectral range were investigated: XeCl- excimer lasers (wavelength 308 nm, pulse duration 28 ns, 60 ns, 300 ns) Nd:YAG (1.06 micrometers , 400 microsecond(s) ), Tm:YAG (2.01 micrometers , 400 microsecond(s) ), Ho:YAG (2.12 micrometers , 400 microsecond(s) ), CrEr:YSGG (2.79 micrometers , 400 microsecond(s) ), and Er:YAG (2.94 micrometers , 400 microsecond(s) ). The excimer laser radiation was guided by a tapered fused silica fiber, whereas for all other lasers the tissue samples were positioned in the focus of a lens with 100 mm focal length. Ablation rates were determined by perforating samples of defined thickness, and the effects of laser ablation on tissue were controlled macroscopically, by light microscopy and by scanning electron microscopy.

The ability of laser-induced plasmas to shield biological tissue from intense exposure to laser pulse energy has been in question for years. Current ocular surgical techniques rely on cone angles of incident pulses to protect delicate tissue anterior and posterior to the site of intraocular laser surgery. The so-called 'shielding' effect of the evolving plasma has been a proposed mechanism for providing additional protection to the retina. Concern is heightened when the procedures require work deeper int the vitreous cavity. For various pulse durations, we have examined the efficiency of the pulse energy to plasm conversion as well as the shielding effectiveness. Work has been done in both the nanosecond (ns) and picosecond (ps) time regimes, but little is known about breakdown and plasma behavior induced by femtosecond (fs) pulses. A simplistic model of the eye is developed using a quartz cell filled with ultrapure water. To focus the pulses in the cell and to induce optical breakdown, a 17 millimeter (mm) aspheric lens was selected to approximate the 17.1 mm reduced focal length of the human eye. Pulses of 10 ns at 1064 nanometer (nm) and 5 ps at 580 nm are used to compare with previous work. In the femtosecond time regime, 100 fs pulses at 580 nm are used with energies ranging from 0.5 - 120 microjoules ((mu) J). For all pulse widths, the ratio of the energy exiting the water cell to the input energy is recorded for cases with and without optical breakdown. Comparisons of shielding effectiveness are made between all three time regimes.

Application of the thermocamera is limited to imaging surface temperatures at an air interface. We describe a technique which enables color imaging of temperature gradients inside optically transparent media. In an optical setup, very small changes in optical density of the media, induced by flow or temperature gradients, are color coded. Depending on the geometry of the temperature distribution, colors in the image can be related to absolute temperatures. The calibration is performed by making use of calculated temperature distributions of specific geometry and by thermocouple measurements. This technique can be applied to study the thermal effects of cw and pulsed lasers during interaction with model and biological tissues. Using fast flash light photography or video imaging, temporal resolution in the microsecond region can be obtained. To study the feasibility of the technique, experiments were performed to image cw Nd:YAG and pulsed Holmium and Excimer laser light interaction with transparent gels and tissues submerged in saline. During the measurements, temperatures were also monitored using thermocouples on selected positions within the field of view. At present, it is still difficult to translate the color images directly to absolute temperature images. The real time color images obtained with this color schlieren technique, however, provide a good understanding of thermo dynamics and thermal relaxation during laser tissue interaction with cw and pulsed lasers.

The combination of rapid temperature decay along radial distance and rapid temperature decay with time suggests that effective laser-tissue interaction with limited thermal damage can be achieved using pulsed Ho:YAG laser. Our study confirms the physical hypothesis that short irradiation time with high pulse energy produces a rapid temperature rise.

High margin laser micromachining processes were developed lately in the aerospace industry for unique device manufacturing. These processes include precise laser ablation of cuts, grooves and holes in various media (silicon, sapphire, polyamides, etc.) with high aspect ratios (> 120:1) and dimensions close to 1 micrometer, while maintaining the integrity of neighboring integrated circuits. The relevance to biomedical therapeutic and surgical applications and to the interaction of lasers with soft and hard tissues is discussed.

In order to measure the dynamics of the ablation cavity which is generated in front of the fiber tip during Ho:YAG laser ablation in water containing material, we studied a fiber-optic laser probe method. The He-Ne laser beam as a probe laser simultaneously delivered through a silica glass fiber with the Ho:YAG laser beam as an ablation laser. The backscattering light of the probe laser from the fiber tip surroundings was measured at the laser input end of the fiber. We used water and agar as the water containing material. The measured backscattering light might be mainly taken place by the debris scattering in the ablation cavity. The transient intensity change of the measured backscattering light was predicted to offer useful information about the growth and extinction of the ablation cavity. From the experiment, the e-holding decay time of the backscattering probe light waveform indicated good agreement with calculated cavity collapse time in water using the Rayleigh equation. We also found that this e- holding decay time consisted with the theoretical cavity collapse time, which could be obtained by the viscoelastic model of agar. Therefore, we concluded that the e-holding decay time of the backscattering light waveform revealed the cavity collapse time of the ablation cavity, which is influenced by the material viscosity. We measured the e-holding decay time of vascular samples. Each tissue had individual e-holding decay time which might be explainable by the tissue viscosity.

Deep brain tissue lesions produced by XeCl excimer laser were studied in 14 rats. Fiberoptic was stereotactically implanted and the lesion was produced by one pulse. MRI and histological controls were performed at various dates following the lesion. The findings are relevant to other lesion procedures. Some XeCl laser properties seem useful in stereotactical procedures.

Isolated crayfish mechanoreceptor neuron was microirradiated into its different region with various lasers (mainly with helium-cadmium laser, 441.6 nm). Laser microbeam was about 5 - 9 micrometers , intensity varied from 10¹ to 1.5(DOT)10⁴ W/cm², and wavelength--from 454 to 633 nm. Polyphasic neuron response on laser microirradiation (impulse frequency acceleration, then inhibition, new acceleration, and sudden irreversible impulsation ceasing) was described and its dependence on intensity, wavelength and localization was investigated. Complex electrophysiological, ultrastructural and cytochemical study along with results of neuron response modification with supravital stains, bioenergetic inhibitors, antioxidants gave evidences that impulsation acceleration phases were caused by laser effect on neuronal membrane, but inhibition phase--on mitochondria. Main intracellular laser radiation photoreceptors were probably flavins.

High quality (precise) laser ablative processing of materials, e.g. cutting of polymers or biotissues, is characterized by smooth bottom and edges of a crater and absence of droplets, expeffing from the irradiation zone. Until recently, the experimental realization of the above mentioned conditions has been achieved mostly with excimer lasers. The most popular 'chanism of precise organic material removal is photoablation, but it can be applied only to the case of UV radiation. Though recently, quite good quality tissue cuttin was obtained also by means of erbium (X =2.94 m) laser. In both cases we have laser radiation with relatively short pulse duration r 1O s and high absorption coefficients (a 1O"10 cm-i) in biotissues, but for excimer lasers radiation absorption takes place mostly in biomolecules while for erbium laser energy is deposited in water. That is why it could be quite natural to consider thermal mechanisms of precise tissue ablation. We believe that surface thermal stability under intense laser heating is a key factor which determines surface quality after material ablation. Really, numerous experiments with melting, ablation, laser induced chemical reaction on irradiated solid surfaces (metals, dielectrics, semiconductors) have shown that after sufficiently long heat treatment by CW radiation or by multiple pulse action various types of surface reliefs can be formed. In fact, one of the problems in laser materials processing is to find radiation parameters (wavelength, irradiation time or pulse number, intensity, spot size and beam polarization) when surface reliefs are not formed. Typically, under multiple pulsed action such reliefs show essential degree of order and. are called surface structures (see, e.g. [1]).

The model is elaborated and results of numerical computations of variation of radiation reflection factor for laminated eye biotissues during their laser coagulation are considered. Comparison of certain simulation results and experimental data is performed.

Energy absorption, heat transfer and thermodenaturation occurring during the interaction of laser radiation pulses with pigmented spherical and spheroidal granules in heterogeneous biotissues are investigated on the base of mathematical simulation. The possibility of selective interaction between short radiation pulses and pigmented biotissues is shown which results in the formation of thermodenaturation microregions inside and near the pigmented granules.

The goal of interstitial laser therapy is to destroy neoplastic tissue through localized heating. Temperature elevations to the necrotic coagulation threshold of about 60 degree(s)C cause the induction of irreversible, as well as, reversible alterations to tissues. While chronic effects prevail days following the treatment, the relevant issue from the perspective of magnetic resonance imaging-monitoring of laser therapy, is the ability of MRI in detecting these effects. In this study time-course MRI images of rabbit musculature, treated with 'temperature controlled' Nd:YAG laser irradiation were correlated with histological changes. The MRI appearance of the interstitial laser lesion can be divided into three categories of acute, sub- acute, and chronic. The dynamics of chronic lesion development, including variations in shape, size and composition of the lesion were successfully documented by T2-weighted spin echo MRI.

Finite-difference modeling offers a flexible approach toward modeling tissue ablation. The model presented here considers optical penetration of the laser, thermal diffusion, water diffusion, surface evaporation, carbonization, and subsurface explosive vaporization. An example simulation considers near-infrared diode laser heating and ablation of a low- absorption nonscattering tissue which has been superficially stained with an absorbing dye (indocyanine green, ICG). Computer simulation illustrates 5 distinct phases of the process: (1) initial heating due to ICG absorption, (2) evaporation with surface clamped at 100 degree(s)C which desiccates surface layer, (3) heating of surface after desiccation has slowed evaporation, (4) rapid heating after onset of carbonization due to combination of desiccation and heating, and (5) subsurface explosive vaporization which removes a superficial tissue layer and exposes a fresh surface which repeats the above cycle.

The effects of laser light on mineralized tissues can be studied by histological techniques such as SEM, TEM and light microscopy. Beside surface effects, laser induced tissue changes in the subsurface layers are important markers to control the side effects of laser treatment. To study these depth effects in the irradiated area cross sections of the lased bone or teeth are necessary. Mineralized tissues must usually be decalcified before cutting sections for histological analysis. Due to this process many laser induced alterations in mineralized tissues cannot be observed, especially in dental tissues. To avoid these disadvantages sawing and grinding techniques to cut undecalcified sections were developed.

Impulsive momentum is imparted to residual tissue during pulsed-laser ablation because the mass ablated is generally ejected with a sizable velocity. Accurate measurements of the impulse are possible, which can provide an important monitor of the ablation process. Simple models can be used to predict the impulse under a variety of conditions; in some cases, complex radiation-hydrodynamic code calculations are required. In this paper, this modeling is discussed along with the dependence of momentum on the pulsed heating and target conditions. Momentum measurement techniques are discussed briefly. The behavior is explained in terms of dimensionless parameters and the impulse coupling coefficient as a function of incident fluence, which has a well defined threshold as well as a maximum. Complications in the mixed liquid-vapor phase are also addressed.