In order to reach a higher level of accuracy in simulation of port wine stain treatment, we propose to discard the typical layered geometry and cylindrical blood vessel assumptions made in optical models and use imaging techniques to define actual tissue geometry. Two main additions to the typical 3D, weighted photon, variable step size Monte Carlo routine were necessary to achieve this goal. First, optical low coherence reflectometry (OLCR) images of rat skin were used to specify a 3D material array, with each entry assigned a label to represent the type of tissue in that particular voxel. Second, the Monte Carlo algorithm was altered so that when a photon crosses into a new voxel, the remaining path length is recalculated using the new optical properties, as specified by the material array. The model has shown good agreement with data from the literature. Monte Carlo simulations using OLCR images of asymmetrically curved blood vessels show various effects such as shading, scattering-induced peaks at vessel surfaces, and directionality-induced gradients in energy deposition. In conclusion, this augmentation of the Monte Carlo method can accurately simulate light transport for a wide variety of nonhomogeneous tissue geometries.

IR tomography (IRT) is a non-contact technique to determine the size and position of sub-surface and position of sub- surface chromophores, using a recorded time sequence of IR emission images following pulsed laser irradiation. A potential application for IRT is the laser treatment of port wine stains (PWS), where the clinician needs diagnostic information to select optical irradiation parameters. In this paper, we report recent progress using IRT to determine diagnostic information regarding blood vessels comprising a PWS. In our simulations, a 3D skin model containing an epidermis and blood vessels is used, from which a time sequence of IR emission images is calculated. Size and position of sub-surface chromophores computed from the time sequence of IR emission images by application of an inversion algorithm are very close to actual values. For in vivo studies, the chick chorioallantoic membrane is an ideal PWS mode. Size and position of a sub-surface blood vessel are correctly predicted by application of an inversion algorithm, given a recorded time sequence of IR emission images.

The present study was undertaken to evaluate the feasibility of thermal damage assessment of blood vessels by using laser-induced release of liposome-encapsulated dye. Experiments were performed in a hamster skin flap model. Laser irradiation was achieved with a 300micrometers fiber connected to a 805nm diode laser after potentiation using a specific indocyanine green (ICG) formulation. Liposomes- encapsulated carboxyfluorescein were prepared by the sonication procedure. Carboxyfluorescein was loaded at high concentration in order to quench its fluorescence. The measurements were performed after i.v. injection of DSPC liposomes and lasted 40 minutes. Fluorescence emission was measured with an ultra high sensitivity intensified camera. Three different shapes of fluorescent spots were identified depending on target and energy deposition in tissue: (i) intravascular fluorescence, (ii) transient low fluorescence circular spot and (iii) persistent high intense fluorescence spot. These images are correlated with histological data. The advantages of this liposome-dye system are (1) direct measurements can be obtained, (2) several repeated readings can be made from one injection, (3) continuous monitoring of the fluorescence can be made, (4) temperature-sensitive range can be adapted using different liposomes compositions, (5) circulation times of several hours can be achieved using DSPC liposomes (6) the tissue microcirculation and the vessel macrocirculation can be investigated simultaneously, therefore changes in response to a treatment regimen and/or ICG formulations can be detected. One main constraint exists: the fluorescent dye encapsulated into the liposomes has to be carefully chosen in order to avoid any direct absorption by the dye itself. In conclusion, one of the most significant applications of this experimental technique is the evaluation of various degrees of tissue thermal damage. It could be possible to consider the application of this technique in ophthalmology and dermatology and possibly for the evaluation of burn injury.

Linear birefringence is a property of collagenous tissue that results from both its composition and structure. Previous investigations have shown that birefringence provides an indication of structural changes in collagen during slow heating. We now report the birefringent response of both mature and young rat tail tendon to laser-heating. The results indicate that denaturation of collagen from mature rats induced by a 200-microsecond(s) -long Ho:YAG laser pulse may not be described accurately by kinetic parameters. Several second-long pulses of CO₂ laser pulse may not be described from young rats fit an Arrhenius model with E_a equals 12.1 kcal/mol and A equals e^18.03 s^-1. Typically, for slow-heating of collagen, E_a equals kcal/mol and A equals e¹²⁰ s^-1. Thus, it seems likely that the temperature and energy needed to initiate collagen denaturation is lower in young collagen, possibly due to its decreased hydroxyproline content and consequent decreased thermal stability.

In the present paper basing ont he free boundary model proposed previously we analyze the characteristic properties of local thermal coagulation depending on the applicator form. This model assumes that direct absorption of laser light in a small region causes the temperature to attain sufficiently high values leading to the immediate tissue coagulation. Heat diffusion into the surrounding live tissue gives rise to further thermal coagulation and the subsequent growth of the necrosis domain. Keeping in mind the possible forms of applicators we study the necrosis growth considering the heat generation rate of the cylindrical and spherical symmetry. In particular, it is shown that for the 3D case the necrosis growth exhibits saturation when thermal coagulation is limited by heat diffusion. For the 2D case heat diffusion provides continuous growth of the necrosis domain during the whole time of thermotherapy treatment. It turns out that the time dependence of the temperature in the region where thermal coagulation is under way is practically insensitive to particular details of the growth conditions.

The five year survival rate of deep-seated malignant brain tumors after surgery/radiotherapy is virtually 100 percent mortality. Special problems include: (1) Lesions often present late. (2) Position: lesion overlies vital structures, so complete surgical/radiotherapy lesion destruction can damage vital brain-stem functions. (3) Difficulty in differentiating normal brain form malignant lesions. This study aimed to use the unique properties of the laser: (a) to minimize damage during surgical removal of deep-seated brain lesions by operating via fine optic fibers; and (b) to employ the propensity of certain lasers for absorption of dyes and absorption and induction of fluorescence in some brain substances, to differentiate borders of malignant and normal brain, for more complete tumor removal. In the method a fine laser endoscopic technique was devised for removal of brain lesions. The results of this technique, were found to minimize and accurately predict the extent of thermal damage and shock waves to within 1-2mm of the surgical laser beam. Thereby it eliminated the 'popcorn' effect.

This paper considers some issues pertinent to laser welding of elastin-based biomaterials to tissues using a pulsed diode laser (lO-is pulse) and indocyanine green (ICG) as an absorbing chromophore to localize laser heating to the "weld surface", the elastin/tissue interface where welding occurs. Experiments involved laser welding of elastin heterographs to the intimal surface of the carotid artery (in vitro, porcine) as a '-4x5 spotweld, then determining the breaking strength when the two tissues were pulled in a direction parallel to the plane of the spot weld while submerged in water. The questions answered are: . WHAT IS THE PEAK TEMPERATURE REQUIRED FOR WELDING ELASTIN HETEROGRAPH TO THE INTIMAL SURFACE OF CAROTID ARTERY? ANSWER: 3OO °C threshold, -6OO °C for maximum strength. This estimate is based on optical measurements of dye accumulation in stain layer and measurements of thickness of stain layer via fluorescence microscope examination. . WHATIS THE DEPENDENCE OF WELD STRENGTH ON THE LASER EXPOSURE? ANSWER: Breaking force (g) = Max*(1 exp(-(E Eth)/U67)), where Max is the maximum strength achievable by laser welding, expressed as the breaking force in g when elastin heterograph and tissue are pulled. E is the laser pulse energy. Eth is the apparent threshold laser pulse energy that will break the weld. Uth is the laser energy above threshold which achieves 67% of Max. Max was about 15 g for the -2O --2weld area of our experiments. Eth was 0.8 J. U67 was 1 .44 J. . DOES WELD STRENGTH DEPEND ON HYDRATION CONDITIONS? ANSWER: Not on the amount of excess unbound water. There was no significant difference in weld strength between welding dripping wet tissues vs well blotted tissues. S WHAT DIFFERENCE IS THERE BETWEEN IRRADIATING THE WELD SURFACE THROUGH THE BIOMATERIAL VS THROUGH THE TISSUE, WHEN THE BIOMATERIAL IS PARTIALLY STAINED WITH ICG? ANSWER: There is a difference if the stain layer is heavily stained. Irradiation through the tissue allows direct irradiation on the weld surface which achieves the highest peak temperatures for the least laser pulse energy. Irradiation through the elastin heterograph causes direct irradiation of the rear surface of the stain layer, within the biomaterial and away from the weld surface, and thermal diffusion must bring the heat to the weld surface. This difference occurs only when the absorption by the stain layer is sufficiently high that little laser energy directly reaches the weld surface.

In this study, the Z-scan technique was used to examine the thermal lens effect in turbid media containing micro-spheres suspended in water. The scattering coefficients of the turbid media were varied from 10 to 100 cm^-1. The experimental results suggested that the nonlinear changes in the refractive index of the scattering medium induced by high power laser heating could significantly impact the propagation of laser beam through the scattering medium. Presence of thermally-induced nonlinear changes in refractive index of tissue during high power laser irradiation may significantly alter the distribution of laser light in tissue during laser therapy.

Optimal laser light delivery into turbid biological tissue was studied using MOnte Carlo simulations. The goal was to efficiently deliver maximum amount of optical power into buried tumors being treated while avoiding damage to normal tissue caused by strong optical power deposition underneath the tissue surface illuminated by the laser beam. The buried tumors were considered to have much higher absorption than the surrounding normal tissue via selective uptake of absorption-enhancement dye by the tumor. The power delivering efficiency to buried tumors was investigated for various diameters of the laser beam. An optimal beam diameter was estimated to achieve the maximum produce of the power coupling efficiency and the power delivered to the buried tumor. The distribution of power deposition was simulated for single beam delivery and multiple beam delivery as well. The simulated results showed that with an appropriate dye enhancement and an optimal laser delivery configuration, a high selectivity for laser treatment of tumor could be achieved.

In vivo LIF spectroscopy and imaging, using 442 nm exc. light, delineates normal form adenomatous colonic tissue. This study investigates the microscopic origin of the observed tissue autofluorescence differences using confocal fluorescence microscopy, histological analysis, confocal microspectrofluorimetry and Monte Carlo simulation. A multilayered tissue model is developed for normal, flat and polypoid adenomas, based on: (1) tissue architecture, (2) optical properties, (3) fluorescence distribution, and (4) illumination/detection geometries. Major differences were found in fluorescence intensity and spectral lineshape between normal and adenomatous colon samples. Fluorescence spectral composition of normal and adenomatous colon autofluorescence is tissue depth-dependent for normal, premalignant, and malignant tissues. Mathematical modeling suggests that LIF measurements are primarily sensitive to changes in tissue morphology specific to pathological progression, including mucosal thickening, alteration in mucosal tissue constituents, redistribution of submucosal collagen and significantly increased blood volume.

We measured angular distribution of the light scattering from live mouse embryo with 632.8nm in wavelength to evaluate the embryo viability. We aim to measure the mitochondrial density in human embryo which have relation to the embryo viability. We have constructed the light scattering measurement system to detect the mitochondrial density non-invasively. We have employed two optical fibers for the illumination and sensing to change the angle between these fibers. There were two dips on the scattering angular distribution from the embryo. These dips existed on 30 and 85 deg. We calculated the scattering angular pattern by Mie theory to fit the measured scattering estimated scattering size and density. The best fitting was obtained when the particle size and density were 0.9 micrometers and 10¹⁰ particles per ml, respectively. These values coincided with the approximated values of mitochondrial in the embryo. The measured light scattering may mainly originated from mitochondria in spite of the existence of the various scattering particles in the embryo. Since our simple scattering measurement may offer the mitochondrial density in the embryo, it might become the practical method of human embryo on in vitro fertilization-embryo transfer.

The paper presents the basics underlying development of an optical fiber spectrometer for measurement of skin bilirubin in new-born infants. It is possible to directly deduce the bilirubin levels in the skin using first principles of optical transport, rather than relying on a calibration study in a test population of neonatal subjects where optical measurements are correlated with laboratory serum levels of bilirubin. The optical method reported here measures cutaneous bilirubin despite variations in fibrillar development of the dermis, epidermal melanin content, or cutaneous blood content.

Purpose: to assess the early in vivo evolution of tissue response and wound healing from ultrashort pulsed laser retinal lesions by correlating the cross sectional morphology from sequential optical coherence tomography with histopathologic sectioning. Methods: single ultrashort laser pulses were placed in the Macacca mulatta retina and evaluated by cross-section optical coherence tomography (OCT). These images were compared at selected time-points with corresponding histological sections. Results: OCT was able to detect the acute tissue injury from laser delivery and the evolution of the healing response over 8 days after laser delivery. These OCT images correlated well with histopathologic findings. Conclusion: analysis of the extent of initial laser lesions and the type of healing response can be performed in serial sequence with OCT providing new insight into the healing response form laser injury. This information correlates well with microscopic data.

Minimum visible lesions (MVL) are reported for picosecond and nanosecond laser pulses at near-IR wavelengths in the primate eye, Macaca Mulatta. The 50 percent probability for damage (ED₅₀) dosages are reported for the 24 hour for both MVL and fluorescein angiography visible lesion thresholds at the 95 percent confidence level. The thresholds decreased by as much as 48 percent between the 1- hour reading and were lower in all cases at 24 hours. MVL- (ED₅₀) threshold doses were 19.1 uJ at 7 ns and 4.2 uJ and 4.6 uJ at 80 ps and 20 ps respectively. Our thresholds measured for the near-IR laser pulses were lower by a factor of 4 to 8 lower than previously reported values but almost an order in magnitude higher than visible MVL thresholds for similar pulsewidth in the visible wavelengths.

The melanin of the retinal pigment epithelial (RPE) cells is generally thought to have a photoprotective role in the eye, yet it is excited by light to a free radical which can react with cellular components. Soluble proteins extracted from the retina are photo-oxidized by the output of a Xenon arc lamp containing UVA and visible wavelengths. The oxidative damage in this model consists of carbonyl adducts to the peptides, and is proportional to the amount of UVA present. Melanosomes isolated from bovine RPE cells and added to the retinal protein extract partly protect the proteins from photo-oxidation resulting from this broadband exposure. However, if the proteins are instead exposed to the 488 and 514.5 nm outputs of an Argon continuous wave laser, the amount of protein oxidation is markedly increased when melanosomes are present. This observation suggests that the melanin free radical is optimally excited by wavelengths in the blue-green region of the visible spectrum, and in fact the action spectrum for the photo-oxidation of NADPH by laser-excited melanin peaks between 450 and 500 nm. The present data do not distinguish between two alternative hypotheses, i.e. that the apparent action spectrum peak is due to (1) a chromophore different from the one determining the overall optical absorption of melanin, or (2) the lower efficiency of UVA photons in activating melanosomes because of their strong absorption at the solution surface. Nevertheless these data implicate melanin in the so-called 'blue light' retinal hazard.

We have made an indirect in-vivo determination of spot size focusing in the rhesus monkey model. Measurement of the laser induced breakdown threshold both in-vitro and in-vivo allow correlation and assignment of a spot size after focusing through the living eye. We discuss and analyze the results and show how trends in minimum visible lesion data should be assessed in light of chromatic aberration. National laser safety standards are based on minimal visual lesion (MVL) threshold studies in different animal models. The energy required for a retinal lesion depends upon may parameters including wavelength and retinal spot size. We attempt to explain trends in reported MVL threshold studies using a model of the eye which allows calculation of changes in retinal spot size due to chromatic aberration.

We present dispersion curves for aqueous media measured with a white-light interferometric technique. Data is presented for wavelength ranges in the visible and near IR. Materials investigated include high purity water and primate vitreous. Results are examined with regard to retinal damage mechanisms associated with ultrashort laser pulse exposures. In this pulse duration regime the dispersive nature of the medium can play a critical role in predicting damage thresholds. These values are also critical to the development of nonlinear propagation models. We compare our results to other measurement techniques which determined index of refraction at a series of single wavelength points. Results indicate that he dispersive trends in the materials can be quickly and accurately mapped over a wide spectral range using this technique.

Self-focusing is a phenomena that is induced in certain materials when high irradiance laser light interacts with the material. High irradiances are most readily achieved with focused ultrashort laser pulses. Past theoretical calculations using the nonlinear wave equation have calculated the critical power for self-focusing by tightly focused beams in water at 580 nm to be 1 MW. The recent pulse propagation model by Feng et al. has been used to find the pulse duration where the self-focusing threshold can be most easily found. In addition, a first-order model of laser-induced breakdown developed by Kennedy has predicted that the threshold for breakdown at each pulse duration is independent of spot size. Thus self-focusing can be seen from a precise measurement of spot size and breakdown threshold. With several optical setups with different predicted spot sizes, we measured the spot size by knife- edge technique at energies far below the breakdown or self- focusing thresholds for a pulse duration of 2.4 ps, 800 fs, and 126 fs. We also measured the laser-induced breakdown threshold for each of these optical setups. The laser- induced breakdown irradiance threshold was constant for those spot sizes that were below the self-focusing threshold, as predicted by Kennedy's model. The measurements of self-focusing for ultrashort laser pulses in water and its implications on retinal damage will be discussed in this paper.

Our research investigates the mechanism of retinal injury from short laser pulses. We used an ex-vivo porcine model and time resolved strobe imaging to examine mechanical effects associated with sub-nanosecond absorption by the retinal pigment epithelium (RPE). We present a unique method of imaging high speed event sin the RPE with a resolution of approximately 1 micrometers . Microcavitation bubbles were first imaged around single melanosomes, after irradiation with a 40 ps, 532 nm pulse. The threshold for bubble formation was 50 mJ/cm². A few times threshold, stress waves could be observed around the melanosomes. At 0.68 J/cm² we measured shock waves with an average velocity of 2700 m/s, 1 ns after irradiation. After exposures above 50 mJ/cm², RPE cells were found to be non-viable. We compare these results with MVL data collected using live animal models.

Recent theoretical work has shown that damage to retinal pigment epithelium cells from nanosecond laser pulses is likely to occur by bubble formation at lower fluences than damage caused by thermal heating. The bubbles form around the strongly absorbing melanosomes and the bubbles remain within the cell in which they form. This suggests that damage due to bubbles will remain localized in cells containing the strong absorbing material, whereas thermal damage tends to affect surrounding cells. This same approach may be useful in treating pigmented skin which also contains strong absorbers. By killing cells with bubbles produced by nanosecond laser pulses, damage to surrounding healthy tissue may be minimized. Laser pulse lengths, fluence levels, and potential applications for treating melanoma and congenital nevi will be discussed.

A complex examination of results of UV laser action on normal and atherosclerotic wall has been undertaken. A considerable part of laser energy absorbed in tissue specimen was shown to be exhausted for photodissociation of lipid molecules. Moreover, substantial changes in molecular structure of amino acid and glucose have been demonstrated.

In experiments on white male rats short-term immobilization- sound stress was modelled. Decrease of glycogen content and myeloperoxidase activity, increase of lysosomal cationic proteins level and NBT-test parameters as well as fall of adrenaline, dopamine and 5-hydroxytryptamine amount in polymorphonuclear leukocytes were observed. Preliminary transcutaneous He-Ne laser irradiation modified metabolic reaction of leukocytes to stress and prevented stress- induced decrease of biogenic amines content in cells.

In experiments on white male rats stress was modeled by combined action of immobilization and sound stimulus during 2 h. It was established that stress induced storage of secretory material and accumulation of biogenic amines in mast cells. Preliminary transcutaneous He-Ne laser irradiation prevented stress-induced changes.

The number and correlation of skin stroma cells was studied on mice C57B1 with the subcutaneously transplanted melanoma B16 which was exposed to neodymium pulsed laser radiation. Within 1-5 days after the exposure the total number of the free skin stroma cells was found to increase in the periphery from the radiation epicenter and the number of lymphocytes, macrophages and leucocytes tended to grow. Lymphoid infiltration was also revealed in the preparations of the epithelized wound and cicatrix on the skin melanoma sites in the patients who had undergone pulsed laser radiation therapy.

Fluorescence intensity measurements from fluorophore molecules present in tissue can be affected by the intrinsic scattering and absorption of the tissue. Using detection of the fluorescence from small regions of tissue the fluorescent intensity signal is not significantly affected by the background absorption in the tissue. Confocal microscopic measurements have been examined here in an effort to develop a system in which the fluorescent intensity measured from tissue is linearly proportional to only the fluorophore concentration and the input light irradiance. Monte Carlo simulations confirm that by constraining the detected area to smaller than the average scattering length, that the fluorescence measurements are not significantly affected by intrinsic absorption. This type of system is useful for quantification of photosensitizer concentrations in tissue during photodynamic therapy or for pharmacokinetic measurements of uptake in tissue. Preliminary measurements confirm that this method should be equivalent to fluorescence-based tissue extraction methods of photosensitizer uptake in tissue.

The phenomenon of photobleaching of a photosensitizer during photodynamic therapy (PDT) is well known. For second generation photosensitizers it may be possible to exploit this effect to enhance the volume of damaged tissue and improve the efficacy of PDT. In addition, as a consequence of photobleaching, the fluorescence emitted by the photosensitizer will decrease during PDT. A diffusion theory model has been developed which simulates photobleaching of a photosensitizer during PDT and calculates the total fluorescence emission as a function of delivered light fluence. The fluorescence signal can be compared with tissue necrosis boundaries calculated for realistic treatment conditions using a threshold model previously described by us. The relationship between the fluorescence signal and the extent of tissue damage is discussed.

Fluorescence spectroscopic detection and photodynamic therapy may provide an effective approach for early detection and treatment of oral cancer. Thus the development of a safe photosensitizer that could enhance the spectroscopic contrast between normal and neoplastic tissue, while allowing for selective photosensitization and treatment of pre-malignant and malignant lesions in the oral cavity, is highly desired. In this study, the pharmacokinetics and a safety of 5-aminolevulinic acid (ALA) that could induce an endogenous precursor of protoporphyrin IX and heme in the biosynthetic pathway was investigated. Two doses of ALA:25 and 75 mg/kg were administered intravenously to 4 and 3 dogs, respectively. A 'wash-out' period of 1 week between administration of each does was allowed to ensure against PpIX build-up. Using an optical multichannel analyzer, the fluorescence from the oral cavity was recorded at 3 sites: buccal mucosa, gums, and the tongue, and also from a remote site, the skin. A fiber optic probe was used to deliver excitation and collect the emitted fluorescence. Results showed that the ALA-induced fluorescence reached a peak at 2-4 hours, and returned to baseline in 24-31 hours. The dogs were stable during the course of the study, minimal vomiting was noted. In conclusion, the study showed that higher doses result in a higher peak at a later time.It was observed that different tissues have different pharmacokinetic response, the tongue and the gums have the highest peak fluorescence values, followed by the buccal mucosa and skin.

In this proceeding a summary is given of the slides presented at the meeting. For a detailed description of the research and clinical applications, references are included. An update of current research and clinical activities can be found on the web page of the medical laser center: www.cv.ruu.nl/LaserCenter. Links to other laser sites. At this site reprints can be requested.

Despite the superficial penetration of the light, the continuous wave (cw) CO₂ laser may induce a relatively large area of thermal damage in tissue next to the ablation crater. Ultra-pulsed laser systems, however, deliver pulse energies above the ablation threshold for tissue within a few hundred microsecond(s) , which instantly vaporize tissue. Since cw ablation models cannot be applied on pulsed ablation, a new model is introduced which is validated experimentally on phantom tissue. It discriminates between the cross section of the laser beam above the ablation threshold and the flanks of beam below threshold inducing thermal effects. Depending on pulse energy and beam diameter, the shape of the ablation crater in the tissue was calculated and validated experimentally in phantom tissue using a pulsed CO₂ laser and an Erbium laser. The diameter of the spot varied from 0.1 to 2.5 mm and the pulse energy from 25 to 250 mJ, 10-100 mJ. In the experimental crater depths up to 40 mm could be obtained in one pulse. The theoretical model agreed within 10 to 50 percent with the experimental data for spot sizes form 0.5 to 0.1 mm for the CO₂ laser. Although, the thermal effect of these pulses are minimal compared to cw lasers, the sub-threshold part of the laser beam can contribute to undesired thermal damage when repetitive pulses are applied within the thermal relaxation time. The model can serve as a good tool for predicting the depth of ablation for current clinical applications in dermatology, ENT and cardiac-surgery.

Laser tissue welding is a thermal process for binding tow tissues together. Optical and thermal models exist to calculate the temperatures of laser irradiated tissues. However, a rate process model is required to relate the time-temperature history to a weld strength. This paper proposes a first-order rate process model based on contraction during heating. The entropy and enthalpy associated with contraction of porcine intestine in a water bath was measured and used to calculate the fraction of altered molecules for both water bath and laser welding. Intestine was welded to intestine in a water bath at 60-80 degrees C for seven minutes. Pulsed laser welding used 10-30 pulses and an exogenous chromophore. The yield strengths of the welds were measured and found to roughly correlate with the fraction of altered molecules estimated for both the water bath and laser welds.

Interstitial laser coagulation (ILC) is a new method of producing localized tissue destruction, that may be used to eliminate soli tumors, such as liver metastases, pancreatic carcinomas, brain glioma and benign prostate hyperplasia. In ILC, Nd:YAG laser light is guided through flexible quartz fibers implanted directly into the tumor. Several experimental studies have shown the effectiveness of this therapy. Clinical application is feasible, however success in malignant tumors is limited by: (1) the restricted lesion size produced by a single optical fiber and (2) the lack of reliable on-line monitoring of the laser-induced effects. Research is therefore directed towards the development of multiple fiber application, guided by real time feedback of the laser-tissue interaction.

In the past years there has been a significant increase in the treatment of bladder outlet obstruction caused by benign prostatic hyperplasia. Transurethral electroresection of the abundant tissue (TURP) has since the early seventies been the golden standard. The main drawback of a TURP is the relative lack of hemostasis, due to a confined energy and heat distribution around the resection loop. As sufficient tissue needs to be removed to overcome the bladder outlet obstruction, the ideal treatment has to combine both ablative and hemostatic abilities. After 1992, endoscopic laser and 'non laser' treatment modalities have been introduced, that competed with TURP as to clinical outcome. These treatments have in common that a high amounts of energy is delivered to the prostate to remove tissue either indirectly by coagulation necrosis or directly by vaporization. Various in-vitro and clinical studies were performed using different energy sources, such as Nd:YAG and diode laser light in combination with a large variety of delivery devices. Also TURP was included in the evaluation. The in-vitro results provided understanding of the efficiency in energy delivery, the extent of heat induced in the prostatic tissue and possible side-effects, using thermal imaging techniques. Over the last five years clinical data have been collected for various techniques with a follow-up of two years showing the contact techniques to be superior over non-contact and comparable with the outcome of the 'standard' TURP.

We present a study of the short-timescale fluid dynamic response of water to a fiber-delivered laser pulse of variable energy and spatial profile. The laser pulse was deposited on a stress confinement timescale. The spatial profile was determined by the fiber core radius, r, and the water absorption coefficient, (mu) _a. Considering 2D cylindrical symmetry, the combination of fiber radius and absorption coefficient parameters can be characterized as near planar, symmetric, and side-directed. The spatial profile study shows how the stress wave varies as a function of geometry. For example, relatively small absorption coefficients can result in side-propagating shear and tensile fields.

Laser injury by sub-nanosecond pulses in the eye and skin is related to strongly absorbing pigment particles such as melanin with dimension of order 10-15 nm. Single melanosomes, with size of approximately 1 micrometers and containing many such melanin particles, were isolated in water and irradiated with 100 psec pulses. Using time resolved imaging techniques, they observed the emission of a strong shock wave followed by rapid bubble expansion on a nanosecond timescale. The shock had a supersonic speed of approximately 2700 m/sec and an initial pressure of nearly 35 kbars. The shock wave can induce further tissue damage in addition to that produced by the bubble expansion and reduce the threshold for laser damage in the retina. In this work we simulate the system by using the hydrodynamic computer code LATIS with a realistic equation of state for water. We simulate both isolated melanin particles and whole melanosomes. Our melanosome model considers a spherical structure of order 1 micrometers in diameter with a uniform energy. This is consistent with the fact that the melanin particles are not stress confined while the melanosome is almost stress confined; thus, the pressure builds up uniformly in the melanosome. The details of the dynamics of the supersonic shock wave emission and rapid bubble evolution on both the melanin and melanosome scales are investigated. Comparison between modeling and experiments is presented. In order to achieve peak pressures and shock speeds comparable to the reported values, it is necessary to model the melanosome as having an absorption coefficient of approximately 6000 cm^-1. Another way to achieve agreement with experiment is if the superposition of shock waves from the many melanin particles inside the melanosome produces a stronger shock than calculated by assuming a smooth absorption, as in our melanosome model. A better experimental determination of the values of linear and non- linear absorption coefficients for a single melanosome is needed in order to decide between the two approaches.

Photophysical reactions have been the focus of laser-tissue interactions. However, these interactions usually have localized and short-term effect, hence only resulting in limited success against systemic lesions, especially against metastatic tumors. Could laser induce a photobiological reaction, specifically a long-term reaction in cancer treatment. Our experimental results on treatment of rat breast cancer indicated that a systemic and long term response against the tumors could be stimulated by a new laser-sensitizer-immunoadjuvant treatment. Long term impact of our method was observed; survival tumor rats and apparent ability against tumor re-challenge. The long term effect was also confirmed by our histochemical studies. Our results pointed to a humoral immune response. A new mechanism of laser-tissue interaction, namely laser-sensitizer- immunoadjuvant induced photobiological reaction, may prove to be crucial in laser cancer treatment.

We examined a participation of photochemical nitric oxide in the vasorelaxation induced by ultraviolet pulsed light. We measured a luminal diameter of rat femoral artery in vivo during a Krypton-fluoride (KrF) excimer laser irradiation. We also measured the vascular response when the artery was pretreated with sodium nitrite, superoxide dismutase or methylene blue. Histological changes in the vessels were examined by light microscopy. The vessel relaxed by KrF laser. The vasorelaxation was dependent on the repetition rate under the constant total energy. On the contrary, the vessel wall damage was inversely proportional to the repetition rate. Accordingly, the damage is most likely due to photoacoustic mechanism. The vasorelaxation was inhibited by methylene blue but enhanced by sodium nitrite or superoxide dismutase, which strongly suggests that the relaxation is closely related to nitric oxide. We conclude that the photochemical product of nitric oxide may be one of the possible mechanisms for the pulsed ultraviolet light induced vasorelaxation.

Free electron lasers (FELs) can be used to molecular operation such as the delivery of a number of molecules into cells. Cultured NIH3T3 cells are exposed to high-intensity short pulse FEL. The FEL is tuned to an absorption maximum wavelength, 6.1 micrometers , which was measured by microscopic FTIR. A fluorescence dye in the cell suspension is more absorbed into the cell with the FEL exposure due to the FEL- induced mechanical stress to the cell membrane. A quantitative fluorescence microscopy is used to determine the efficiency of delivery. The result showed that the fluorescence intensity of sample cells were higher than that of control cells, and there was significant difference between the control and the sample group. Blebbing and the colony formation of the cells were observed for cells with mechanical stress.

The paper presents theoretical and experimental results allowing to discuss and understand the mechanism of stress relaxation and reshaping of cartilage under laser radiation. A carbon dioxide and a Holmium laser was used for treatment of rabbits and human cartilage. We measured temperature, stress, amplitude of oscillation by free and forced vibration, internal friction, and light scattering in the course of laser irradiation. Using experimental data and theoretical modeling of heat and mass transfer in cartilaginous tissue we estimated the values of transformation heat, diffusion coefficients and energy activation for water movement.

The validity of an extended Rayleigh model for laser generated bubbles in soft tissue is examined. This model includes surface tension, viscosity, a realistic water equation of state, material strength and failure, stress wave emission and linear growth of interface instabilities. It is compared to detailed dynamic simulations using the computer program LATIS. These simulations include stress wave propagation, a realistic water equation of state, material strength and failure, and viscosity. The extended Rayleigh model and the detailed dynamic simulations are compared using a 1D spherical geometry with a bubble in the center and using a 2D cylindrical geometry of a laser fiber immersed in water with a bubble formed at the end of the fiber. Studies are done to test the validity of the material strength and failure, stress wave emission, and the interface instability terms in the extended Rayleigh model. The resulting bubble radii, material damage radii, the emitted stress wave energies, and the size of the interface distortions are compared. Conclusions are made on the validity of the extended Rayleigh model and on possible improvements to this model. The purpose of this study is to investigate the use of the extended Rayleigh model as a substitute for the detailed dynamic simulations when only limited information is needed. It is also meant to benchmark the detailed dynamic simulations when only limited information is needed. It is also meant to benchmark the detailed dynamic simulations and highlight the relevant physics. It is shown that the extended Rayleigh model executes over 300 times faster on a computer than the detailed dynamic simulations.

In numerical calculations of idealized bubble dynamics test problems, Los Alamos computational tools perform well. A realistic equation of state must be used and geometrical features must be carefully modeled to simulate experiments accurately. In this work, we compare numerical simulations taking these features into account with experiments performed at the Medizinisches Laserzentrum Luebeck. We compare the measured and calculated positions of the shock front and of the bubble wall as a function of time in the laser optical breakdown of water produced by 30-ps 1-mJ Nd:YAG laser pulses.

The formation and evolution of acoustic waves and vapor bubbles as a result of laser irradiation have received considerable attention, particularly with respect to angioplasty, thrombolysis, and ophthalmic laser applications. Pressure waves and bubbles have been implicated in undesirable tissue damage yet they can be beneficially utilized while limiting their negative impact. Either planar or spherical pressure waves can be produced through manipulation of irradiation parameters and geometry. An OPO laser emitting approximately 5 ns pulses of visible radiation was delivered through an optical fiber to a cuvette containing dye dissolved in either water or glycerin. Absorption was varied by altering the dye concentration and wavelength of the OPO laser and the spot size was varied by employing multiple sizes of optical fiber. A nitrogen-pumped dye laser with a pulse duration of approximately 5 ns was used as an illumination source. A Mach-Zehnder interferometer technique enabled visualization and quantification of the pressure waves; bubble evolution was monitored with shadowgrams. A comparison was made between experimental and theoretical results for water and glycerin.

Cavitation bubbles generated by fast overheating of water during pulsed laser applications in liquid medium have been shown to be a driving force of the soft tissue cutting. An alternative approach proposed in this paper is the generation of cavitation bubbles by fast overheating of liquid conductive medium by a short pulse of electric current. An electrical system based on a tapered microelectrode has been developed for generation of a high voltage sub-microsecond discharge in physiological medium. A highly localized ozone of power dissipation - about 20 micrometers in size - results in a low threshold energy of cavitation bubble generation - about 3 (mu) J. Efficiency of the pulse energy conversion to the bubble energy is about 12 percent, which is lower than the best results obtained with laser- based instrumentation. In spite of this, due to the low threshold energy, the cavitation bubbles that are required for effective cutting of soft tissue are generated at a lower energy than has been achieved with the laser-based instrumentation. The prospects and limitations of this newly developed technology are compared to the present applications of fiber-delivered pulsed lasers in microsurgery.

In various pulsed-laser medical applications, strong stress transients can be generated in advance of vapor bubble formation. To better understand the evolution of stress transients and subsequent formation of vapor bubbles, 2D simulations are presented in channel or cylindrical geometry with the LATIS computer code. Differences with 1D modeling are explored, and simulated experimental conditions for vapor bubble generation are presented and compared with data.

The computer code LATIS is used to simulate midplane and backplane spallation resulting from short pulsed laser absorption. A 1D planar geometry is simulated with an exponential laser absorption profile. The laser pulse length is assumed to be much shorter than the sound transit time across the laser absorption length. The boundary conditions are a fixed front plane and free backplane and a free front plane and a fixed midplane. The NBS/NRC equation of state for water is used with a self-consistent yet empirical material strength and failure model. The failure model includes the effects of void nucleation, growth and coalescence. Definite signatures of the nucleation and coalescence thresholds are found in the back surface motion for backplane spallation.

The formation of a stationary cavity by a sequence of CO₂ laser pulses was investigated in liquids of different viscosity and surface tension. Decreasing the surface tension of the liquid and increasing its viscosity increases the depth of the cavity and decreases the threshold energy needed for its formation. A theoretical model based on the Bernoulli-Stokes equation was developed. Good agreement between experimental and theoretical results was obtained. A method of reducing thermal damage, involving liquid layer effects during laser cutting of different materials, is proposed.

We have been evaluating the use of a pulsed Nd:YAG laser for ablating hard dental tissue. For this application we apply dye-drops of an IR absorptive fluid on the enamel, then irradiate with a laser pulse from the laser. By using ink- jet technology to deliver the dye-drops, we can attain micron- and millisecond-scale precision in drop delivery, with a 'burst' of drops preceding each laser pulse. To gain better understanding of the ablation process we have used a high- speed CCD camera system with 1 microsecond(s) exposure and 1 microsecond(s) inter-exposure-interval capability. Fast photography of the ablation process showed the following typical events. (i) The laser induced plasma plume erupts immediately after pulse onset, expands to maximum within 50 microsecond(s) , and lasts up to 200 microsecond(s) . (ii) Ejected particles flying away from the site of laser pulse/dye-drop impact are detected within 30 microsecond(s) of laser pulse onset, and continue up to 10 ms. These particles attain velocities up to 50 m/s with lower velocities from lower pulse power. (iii) The plasma plume has a peak height that increases with increasing laser fluence, ranging up to 10 mm for a fluence of 242 J/cm² on enamel. From this study, the dye-assisted ablation mechanisms are inferred to be plasma-mediated and explosion- mediated tissue removal.

A 805-nm diode laser ablation therapy with indocyanine green (ICG) was studied for the endoscopic treatment of early gastric cancer. The ICG solution with 1mg/ml was administrated to the submucosa of the resected porcine and anesthetized canine stomach for the purpose of enhancing the tissue absorption to the laser light. We established complete removal of the mucosa and the submucosa with laser power of 12 or 25 watts. The proper muscle was intact because the ICG solution prevents the laser light penetration to the proper muscle. An ablated depth could be easily recognized by observing the ablated surface color. Our results showed that the combination of the 805 nm diode laser irradiation and the submucosal injection of the ICG solution might provide selective and controllable endoscopic treatment for early gastric cancer.

Thirty patients with uterine leiomyomas which were treated by LITT were evaluated histologically, and biologically for oestrogen receptors and epidermal growth factor, at the time of treatment and often subsequently, up to three years later. Three lasers were evaluated: 5W were found to be necessary to produce a lesion 10mm in diameter when the Nd:YAG laser was employed for 500 seconds. The KTP laser alone, using the same number of Joules, produced a larger volume of vaporization and coagulation.However, the most striking effect was achieved with a combination of KTP and YAG. The diode laser alone produced a lesion equivalent to that of the Nd:YAG laser. However, when employed with a fiber splitter and four fibers, each delivering a power of 4W, the volume of necrosis around each fiber was doubled. Finally, the diode laser, when fitted with a diffuser fiber, produced a lesion three times larger than that of the Nd:YAG laser, and there was no char. Histochemical study of oestrogen receptors and immunohistochemical study of epidermal growth factor in the lesions produced by LITT have shown that they are destroyed.

We investigated plasma-mediated surface ablation in corneal tissue using picosecond and femtosecond laser pulses in order to achieve high precision, non-thermal tissue removal with a non-ultraviolet laser source. Experiments utilized three laser systems, a regeneratively amplified Ti:sapphire laser, a synchronously amplified dye laser, and a regeneratively amplified picosecond Nd:YLF laser. Tissue ablation was performed by tightly focusing the laser beam on the tissue surface. Ablation thresholds were determined by monitoring the plasma spark, as well as the tissue surface. Tissue ablations were then analyzed by standard histologic methods and scanning electron microscopy. We observed a decrease in the ablation fluence threshold as the pulse duration is shortened from 200 ps to approximately 140 fs, in agreement with our theoretical predictions. Using identical pulse energies, the femtosecond laser pulses ablated tissue at higher efficiencies than the picosecond laser, with an approximately two-fold improvement in the etch depth curve. Histologic analysis reveal minimal adjacent tissue damage at either pulse duration. Femtosecond laser pulses may offer advantages that make them ideal tools for high precision tissue ablation.

Time-resolved video photographs have been taken in order to investigate plasma induced surface ablation of soft tissue by IR picosecond laser pulses with energies of about 1 mJ. The emission and propagation of shock waves in the irradiated tissue as well as in the surrounding air environment was studied. The pressure amplitudes of the shock transients were determined from the measured shock velocities. A decay of the pressure amplitude below 100 MPa was observed within a distance of about 200 micrometers from the center of laser induced optical breakdown. The dynamics of the ablation crater and the ejection of the ablation fragments was studied on a larger time scale. The maximum expansion of the ablation crater was measured to be about 200 (Mu) m at temporal delays of 4-5 microsecond(s) after the impact of the laser pulse. Furthermore, generation and propagation of a surface deformation wave was observed. Thus, we present a detailed and consistent description of all phenomena occurring during plasma-mediated surface ablation of soft tissue.

We evaluated in vivo wound healing responses to plasma- mediated ablation in skin as a function of laser pulsewidth and energy. Experiments utilized a regeneratively amplified Ti:Sapphire laser operating at 800 nm with pulsewidths varied from 7 ns to 100 fs. Skin incisions were created in mice by tightly focusing the laser beam on the tissue surface. Incisions of equal depth were compared at time points ranging from 6 hours to 3 weeks using standard histologic methods. Incision depth was proportional to pulse energy at each pulsewidth. Fluence threshold dependence on laser pulsewidth agreed with those predicted by ex vivo testing. Histologic analysis revealed minimal adjacent tissue damage at pulsewidths less than a few picoseconds and energies near the fluence threshold. Longer pulsewidths and higher fluence levels were associated with more significant collateral effects. These in vivo results suggest collateral tissue damage and secondary effects may be minimized by controlling laser pulsewidth and energy.