Infrared tomography is a non contact method to determine the size and position of port wine stain blood vessels from analysis of recorded infrared emission images immediately following pulsed laser irradiation. To develop a diagnostic tool for clinical use, infrared emission images from laser irradiated tissue have been simulated on a computer. Analysis of the simulated infrared emission images shows that size and position of the laser heated vessels can be predicted.

Achievements and prospects of PPTR-method for monitoring/control of laser treatment in surgery purposes are presented. Connection of output laser parameters and ones of irradiated zone in tissue with parameters of measured thermal emission from irradiated tissue are analyzed. There are presented the results of investigations of the high power laser-tissue interaction and of the processes of tissue destruction stimulated by middle-infrared-range pulsed lasers radiation in different sorts of tissues carried out by PPTR-technique. PPTR and Laser Mass-Spectrometry are united in study of evolution of tissue destruction under action of power CO₂ laser. The evolution of thermal signal and, correspondingly, temperature in tissue under single laser pulse treatment with rise of energy density are obtained. Method of the temperature stroboscopy which allows to see very fast tissue heating stimulated by pulsed laser treatment more elaborate are presented.

We report the results of a study aimed at comparing Opto-Thermal Transient Emission Radiometry (OTTER) with established techniques of assessing skin condition, namely evaporimetry (TEWL), skin dielectric constant measurement, ATR-FTIR and clinical assessment. Comparisons were made during a week-long study of the effects of intensive washing on the volar forearms of 14 subjects. The study also provided a comparison of skin condition after washing with two different cleansers, a mild isethionate betaine cleansing bar and a soap bar. The subject-averaged results from OTTER and TEWL were found to correlate with the clinical assessments, namely that intensive washing with the soap bar produces greater skin damage than with the isethionate betaine bar. Skin dielectric constant measurements were found to be sensitive to changes of skin condition other than hydration, as evidenced by a daily oscillation that dominate the results. The ATR-FTIR measurements proved difficult to evaluate, because of interfering calcium deposits from the soap bar. On the practical side, OTTER and skin dielectric constant measurements were found to be quicker and more convenient to use than TEWL and ATR-FTIR.

Opto-Thermal Transient Emission Radiometry has been shown to be able to measure stratum corneum hydration in-vivo. We now present a study that shows how the technique can be extended to give information about the distribution of water within the stratum corneum. This depth profile measurement capability is based on a new theoretical expression for the opto- thermal signal that takes a gradient of water concentration explicitly into account. The technique is illustrated with measurements on untreated skin and skin treated with petroleum jelly and DMSO.

Holmium:YAG laser thermokeratoplasty corrects hyperopia (farsightedness) by producing small areas of corneal collagen shrinkage that cause the central cornea to bulge outward, increasing optical power. Collagen shrinkage is probably caused by laser-heated corneal water, but details of the shrinkage mechanism are not known. We investigated the shrinkage mechanism by measuring changes in corneal ultrastructure, surface shrinkage, water content, and strength following Ho:YAG laser exposures. Morphological changes in collagen were documented by measurements from electron micrographs. Corneal adhesive strength was determined by measuring tearing force in a plane parallel to the corneal surface. Laser-induced water loss was measured by weighing corneal samples before and after exposure. Corneal surface shrinkage was assessed by photographing the movement of particles on the cornea. Lasered collagen fibrils increased in diameter, lost their orderly arrangement, and appeared `frayed.' The corneal surface contracted toward lasered areas with a maximal shift of approximately 190 micrometers , more than could be explained by a model based on collagen fibril changes. Water loss plays a minor role in corneal shrinkage since corneal samples lost about only about 1.4% of their weight after massive laser exposure. Despite marked changes in collagen structure, corneal adhesive force was unchanged.

Recent scientific studies evaluating laser energy for tissue welding and thermokeratoplasty have demonstrated that the application of laser energy at non-ablative levels can alter collagen's structural and biochemical properties. A recent pilot study has demonstrated that the non-ablative application of holmium: yttrium-aluminum-garnet (Ho:YAG) laser energy to the joint capsule of patients with glenohumeral instability shrank the joint capsule, stabilizing the shoulder in the majority of the patients treated. Based on the collective findings of these studies, we hypothesized that thermal modification of dense collagenous tissues such as joint capsule, ligament, and tendon can be achieved by applying non-ablative laser energy. The purpose of this study was to evaluate the effect of laser energy at non-ablative levels on joint capsular mechanical, biochemical, histological, and ultrastructural properties in an in vitro rabbit model.

This study extends our previous studies of the photochemical cross-linking of collagen and bonding of collagenous sheets of dura mater using the hydrophobic 1,8-naphthalimide N,N'- bis-(2-({hexylamino)-5-bromo-1H-benz [de]isoquinolin-1,3(2H)-dion-6- yl]amino}ethyl)hexanediamide. We have obtained hydrophilic forms of the bifunctional molecule introducing a spacer and ligands containing alternating carbon-oxygen bonds (polyethers) wherein the oxygen moieties form hydrogen bonds. Additional hydrophilicity is attained by incorporating of an amino group (positive charge) at the end of each ligand. Ongoing studies with these forms of the bifunctional 1,8 naphthalimides have demonstrated welding of meniscal cartilage, articular cartilage, and cornea. These results suggest that the hydrophilic form of the dyes is able to penetrate readily the anionically charged proteoglycan matrix of these tissues and cross-link collagen molecules and possibly the protein cores of the proteoglycans. Gel electrophoretic studies have been performed to assess the photochemical cross-linking of these connective tissue proteins with these new forms of the paththalimide dyes.

In laserthermokeratoplasty (LTK) thermal denaturation and shrinkage of corneal collagen is used to correct hyperopia and astigmatism. In order to optimize dosimetry, the temperature at which maximal shrinkage of collagen fibrils occurs is of major interest. Since the exposure time in clinical LTK-treatment is limited to a few seconds, the kinetics of collagen denaturation as a rate process has to be considered, thus the time of exposure is of critical importance for threshold and shrinkage temperatures. We investigated the time-temperature correlation for corneal collagen denaturation within different time domains by turbidimetry of scattered HeNe laser probe light using a temperature controlled water bath and pulsed IR laser irradiation. In the temperature range of 60 degree(s)C to 95 degree(s)C we found an exponential relation between the denaturation time and temperature. For the typical LTK-treatment time of 2 s, a temperature of 95 degree(s)C is needed to induce thermal damage. Use of pulsed Holmium laser radiation gave significant scattering of HeNe laser probe light at calculated temperatures of around 100 degree(s)DC. Rate parameters according to the formalism of Arrhenius were fitted to these results. Force measurements showed the simultaneous onset of light scattering and collagen shrinkage.

The minimum invasive endoscopic treatment for early gastric cancer has been popular in Japan. The endoscopic mucosal resection and laser coagulation by Nd:YAG laser irradiation has been the popular treatment method in this field. However, the submucosal cancer has not been successfully treated by these methods. To treat the submucosal cancer endoscopically, we developed a new coagulation therapy using hot steam generated by Nd:YAG laser. The steam of which temperature was over 10 deg. in Celsius was generated by the laser power of 30 W with 5 ml/min. of saline. The steam was emitted to canine gastric wall under laparotomy or endoscopy for 50 s respectively. Follow up endoscopy was performed on 3, 7, 14, 28 days after the treatment. Histological examination was studied on 7, 28 days, and just after the emission. In the acute observation, the submucosal layer was totally coagulated. On the 7th day, ulceration with white coat was seen. The mucosal defect, submucosal coagulation, and marked edema without muscle degeneration were found by the histological study. On the 14th day, the ulcer advanced in the scar stage. On the 28th day, it completely healed into white scar with mucosal regeneration and mucosal muscle thickening. We could obtain reproducible coagulation up to deep submucosal layer with large area in a short operation time. Moreover there were no degeneration of proper muscle. This treatment effectiveness could be easily controlled by the steam temperature and emission duration. We think that this method can be applied to early gastric cancer including the submucosal cancer, in particular poor risk case for operation. Further study should be done to apply this method to clinical therapy.

The accurate prediction of the temperature and damage distribution, respectively, is a prerequisite for the successful application of the laser-induced interstitial thermotherapy of brain tumors. For the precise calculation of the resulting damage, the exact structure of the tissue in the region of interest has to be taken into account. We used the MR images of the respective brain tumor as the starting point for the computation of the light, the heat, and the consecutive damage distribution within the individual tissue structures selected for treatment. The simulation technique is based on the combination of the Monte Carlo method to calculate the light distribution and a finite difference alternative directions implicit scheme to solve the heat transfer equation. The model accounts for the specific geometry of the applicator, the changes of the optical and thermal tissue properties during the heating process, and the blood perfusion. The boundary conditions are determined by the thermal exchange between the tissue, the applicator, and the ambient medium. The predictions of the model were compared with the MR image of the resulting laser lesion induced in a patient. The relative error in the determination of the size of the damage area was found to be within 10 per cent in the direction of the applicator axis and within 20 per cent in the radial direction.

We propose a new mathematical model for the growth of a necrosis domain due to thermal coagulation. We assume that laser light is locally delivered to a small internal region where laser energy absorption causes the temperature to attain high values leading to immediate tissue coagulation. Heat diffusion into the surrounding tissue causes its further thermal coagulation, giving rise to the growth of the necrosis domain. The coagulation is treated in terms of irreversible phase transition, i.e. the coagulation is assumed to occur after the tissue temperature exceeds a certain threshold value T_cg. Two alternative models, the developed one and the model previously used are compared. The effects of the blood flow nonuniformity, the tissue response to temperature variations, and the delay of this response are analyzed in detail.

Laser tumor tissue photothermal interaction was studied using an 808 nm diode laser and indocyanine green as the photosensitizer. This in vitro study employed laser power in the range of 3 to 5 watts and an aqueous ICG solution that was administered to murine mammary tumor tissue by intralesional injection. Histology revealed a highly selective photothermal tumor tissue destruction at the center of the ICG injection, while the tumor cells not in the ICG area were spared. Also studied was the retention of photosensitizer in tissue following different methods of administration. The absorption spectra of tissue in the range of 400 to 900 nm were obtained at different time intervals after ICG injections for liver and kidney tissue, as well as tumor tissue. Our results showed, shortly after intracardiac injection, a rapid increase of ICG concentration in liver and virtually no accumulation of ICG in the subcutaneous tumor tissue. In contrast, the intratumoral-injected ICG remained in the tumor with sufficient concentration for a duration up to 48 hours, particularly in the case of well- circumscribed tumors. The combination of the in situ ICG administration and the 808-nm diode laser provided selective and controllable cancer tissue destruction when appropriate laser powers and dosage of ICG were employed.

A CW Tm:YAG laser with a tuning range from 1.86 to 2.08 micrometers was used to create thermal lesions on chicken breast, calf liver, and porcine cornea. All tissue types were exposed to a constant irradiance (450 mW, 0.92 mm spot diameter, 0.5 and 1.0 second exposures) at five different wavelengths about the 2 micrometers water absorption peak. We estimated tissue absorption coefficients from the thermal history recorded by a 8 - 12 micrometers thermal camera. Our data confirmed that the absorption of all tissue types tested followed the water absorption curve. The effect of these differences in absorption upon the coagulation lesion size was evaluated by histologic measurements.

The application of lasers to achieve localized thermal tissue damage is a common technique in minimally invasive surgery. Currently, there is no control during these treatments. In glaucoma therapy the laser energy applied and the beam direction are estimated prior to treatment, according to clinical experience and anatomic norm values. This lack of on-line control may limit success and lead to side effects. Precision and efficiency of treatment could be improved markedly by analysis of spatially resolved, temperature-dependent data obtained by Ultrasound Reflectometry. Thermally induced changes, as well as their localization were detected qualitatively in B-scan. Quantification was achieved by integration of high frequency RF-signals with the following resolution: spatial 50 micrometers , temporal 200 microsecond(s) , temperature 0.5 degree(s). The presented method is suitable for a non-invasive on-line therapy control.

Heat denaturation of egg white is usually followed by polymerization or gelatin of the denatured components, primarily albumin, and is associated with manifestation of a distinct increase in scattering or whitening of the egg white. In this study the effect pulsed laser coagulation of egg white was studied using a CTH:YAG laser delivered through a 600 micrometers diameter fiber into a cuvette filled with raw egg white. The dynamics of laser induced photothermal denaturation of egg white was observed by monitoring the increase of light scattering by time resolved video imaging. Two distinct laser induced processes were observed. At higher radiant exposure (> 30 - 40 J/cm²) the egg whites was rapidly heated above the water vapor transition temperature and a cavitation bubble was formed. Below threshold for bubble formation a bullet-like zone of whitened egg-white is formed at the top of the fiber.

To investigate the mechanism of light induced vasorelaxation, vascular tension change in normal buffer solution was compared with that in the buffer containing either the following drug: (1) guanylate cyclase inhibitor; (2) nitric oxide synthetase inhibitors and the optical isomers; and (3) vasodilating agent. The vasorelaxation caused by ultraviolet light irradiation was independent of the presence of an intact endothelium. It was inhibited by methyleneblue, but not influenced by either L-NMMA or D-NMMA. On the other hand, this vasorelaxation was enhanced by the agents containing nitro group (L-NAME, D-NAME) or sodium nitrite. These results show that the light activates guanylate cyclase, which results in the vasorelaxation. This activation was reinforced by the agents containing nitro group. We conclude that primary photochemical product, which is probably nitric oxide originated from photodissociation of nitro groups, may produce the vasorelaxation. The preliminary investigation suggests that ultraviolet irradiation may be benefit in the treatment for vasospasm.

A photochemical dosimetric system has been developed to record the spatially resolved light dose absorbed by a photosensitizer in a tissue-simulating medium. Relative measurements from these dosimeters match theoretical calculations of the number of photons absorbed per unit volume. These dosimeters have been used to compare the relative penetration of high intensity pulsed irradiation versus continuous wave for benzoporphyrin derivative monoacid ring `a' under typical tissue irradiation conditions. Calculations and experiments indicated an increase in relative light penetration for high intensity irradiation, due to transient changes in sample absorption. The relative deposited dose of different pulse intensities has been compared with the numerical simulation.

The oxygenation status of tissue is important in several medical applications. It has long been appreciated that hypoxic tumors are resistant to radiation therapy, while in photodynamic therapy (PDT), the dependence on oxygen is even more pronounced. Our laboratory studies several problems in PDT that originate with this ³O₂-dependence. Use of ³O₂-sensitive electrodes with small (<EQ 10 micrometers ) tip diameters enables accurate measurement of metabolic and photochemical ³O₂ consumption in photosensitized tumor spheroids in vitro. The temporal response of these devices (< 1 s) is sufficient to capture rapid depletion of ³O₂ during laser irradiation of individual spheroids. An important aspect of this research has been the development of appropriate mathematical models with which to interpret the microelectrode data and determine photophysical parameters. Recently, we have been establishing optical methods of measuring hemoglobin ³O₂-saturation in order to monitor oxygenation in rodent tumors during PDT. We have adopted a steady-state diffuse reflectance technique, in which white light is injected into the tumor via an optical fiber, and the spatially-resolved diffuse reflectance is monitored using appropriately spaced detection fibers. The signals are spectrally dispersed onto a CCD camera by a grating spectrograph, providing simultaneous acquisition of spatially-resolved diffuse reflectance data for a 160 nm range of wavelengths. Absorption spectra reconstructed using a diffusion theory approximation are then fit to linear combinations of oxy- and deoxyhemoglobin spectra to obtain the ³O₂-saturation.

Murine hybridomas were cultivated in tissue culture flasks. Dissolved oxygen tensions in the gas and liquid phases during cell growth were measured non-invasively by an optical oxygen sensor. Readings were made with caps both cracked open and completely closed. During cell growth, gas phase oxygen concentrations remained near atmospheric levels, while the oxygen tension at the bottom of the flasks eventually reached zero. These results suggest that the widespread practice of cracking open tissue culture flask caps during cell growth with a view to supplying adequate oxygen to cells is ineffective and unnecessary. The mass transfer characteristics of the tissue culture flask indicate the dominant resistance to oxygen mass transfer to the cells was the liquid media. The mass transfer rates through the liquid layer under standard laboratory conditions were found to be greater than those predicted by diffusion alone, suggesting microscale mixing. Volumetric and specific oxygen consumption rates were calculated from the sensor data, and were comparable to published values. A recently developed single fiber optic oxygen sensor is described. This new sensor will provide oxygen concentrations at various levels in the tissue culture flasks, allowing more accurate modeling of oxygen diffusion.

Lasers can be used to enhance the delivery of a number of molecules. The model that best fits our data is for the formation of aqueous pores. These pores are present for up to 80 seconds. Our experiments have shown that laser-induced stress transients can be utilized as a vector for intracellular delivery of molecules that may or may not normally cross the cell membrane. These two conditions have been tested with Photofrin and DNA. This technology may have applications in cell and molecular biology, cancer therapy, gene therapy, and others.

This study investigated the ability of laser-induced stress waves to enhance delivery of molecules into cells. Our experiment involved irradiating a thin layer of gel containing India ink at the tip of a capillary tube containing cells. The laser pulses (10 ns, 532 nm) deposited energy in the gel layer and generated intense stress waves which propagated into the capillary tube and affected the cells. The gel surface of irradiation was open to the air therefore the boundary condition was a free surface. The cell medium contained fluorescein attached to dextran of various molecular weights. After repeated laser pulses, the cells were collected on a glass slide and observed by fluorescence microscopy. Irradiated cells were observed to have taken up dye relative to unirradiated controls. The results indicate the ability of stress waves to enhance the uptake of large molecular weight reagents by cells.

We propose that displacement, velocity, and double-pulse holographic interferometry can be used to measure the dynamic response of tissue to pulsed lasers. Tissue material properties and dosimetry for pulsed laser interactions with tissue can then be addressed with these tools. The present use of fiber-optic coupled interferometric devices for biomedical applications suggests that these methods can be successfully adapted for the measurement of tissue material properties such as Gruneisen parameters, total energy absorption, energy deposition profiles, characteristics of multi-dimensional stress wave propagation, and heterogeneous effects. In this paper I will show (1) how Michelson interferometry was used to in stress confinement experiments with non-biological materials to make direct measurements of the Gruneisen parameters of a complex, composite material; (2) the advantages of velocity interferometry for determination of the energy deposition profile; and (3) the use of double-pulse holographic interferometry for multi-dimensional stress wave propagation and heterogeneous effects. We will propose a fiber-optic interferometer which is uniquely adapted for in-vivo biomedical applications.

Laser-induced cavitation inside model objects--gels with various collagen concentrations (10 - 20%) was studied. Cavitation was produced by thermoelastic pressure waves generated under stress-confined conditions. Gel slabs were irradiated in air by nanosecond laser pulses from both sides. Under these conditions bipolar pressure pulses with compressional and tensile phases were generated on each side of the gel slabs. Due to interaction of these pressure pulses transient compression and tension with the amplitude two times higher are produced in the middle of the gel slabs. The tensile phase can induced cavitation. Two types of cavitation were found: transient (with bubble collapse) and permanent leading to formation of stable bubbles. To measure threshold of transient and permanent cavitation two methods were proposed. Transient cavitation was detected by scattering of a He-Ne probe beam from the cavitation bubbles and observed only for gels with low collagen concentration. Transient cavitation threshold for the gel with 10%-concentration amounts 28 bar. Transient cavitation lasts for approximately 3 microsecond(s) (approximately 1 microsecond(s) --growth for cavitation bubbles and approximately 2 microsecond(s) --collapse). Permanent cavitation threshold and total volume of permanent bubbles were studied by optical microscopy. Permanent cavitation can be divided into three types: cavitation with the following collapse accompanied by rupture and formation of small residual cavitation bubbles for gels with low (approximately 10%) collagen concentration; cavitation without collapse and with coalescence resulting in large bubble formation for gels with approximately 14% collagen concentration; cavitation without coalescence for gels with high approximately 20% collagen concentration.

Tissue ablation with ultrashort laser pulses offers several unique advantages. The nonlinear energy deposition is insensitive to tissue type allowing this tool to be used for soft and hard tissue ablation. The localized energy deposition leads to precise ablation depth and minimal collateral damage. In this paper we will report on our efforts to study and demonstrate tissue ablation using an ultrashort pulse laser. The ablation efficiency, and extent of collateral damage for 0.3 ps and 1000 ps duration laser pulses will be compared. Temperature measurements of the rear surface of a tooth section will also be presented.

Erbium lasers emitting at 2.94 micrometers and holmium lasers emitting at 2.1 micrometers are interesting tools for cutting, drilling, smoothing and welding of water containing tissues. The high absorption coefficient of water at these wavelengths leads to their good ablation efficiency with controlled thermally altered zones around the ablation sites. Combination of pulses with both wavelengths transmitted through one fiber were used to perform incisions in soft tissue and impacts in bone disks. Histological results and scanning electron microscope evaluations reveal the strong influence of the absorption coefficient on tissue effects, especially on the ablation efficiency and the zone of thermally damaged tissue. It is demonstrated that the combination of high ablation rates and deep coagulation zones can be achieved. The results indicate that this laser system can be considered as a first step towards a multi-functional medical instrument.

For intracorporeal CO₂ laser surgery, the laser beam is usually delivered through long focal length optics coupled to an operating microscope or an endoscope. While the target tissue is in focus for viewing, the power density in the spot of the beam can be affected by defocusing or irradiating tissue under a small angle of incidence. When the beam is used to drill a channel (e.g. transmyocardial revascularization, TMR), the power density along the beam will determine the shape of the channel. The area for effective ablation was studied for an ultra-pulsed CO₂ laser beam in combination with devices accommodating optics with focal lengths of 120 to 450 mm. The position of the ablation threshold along the waist of the beam in water and crater depths in (model) tissue were determined in relation to pulse energy (1 to 250 mJ) and angle of incidence. The crater formation during ablation of the model tissue and lateral thermal effects were recorded using fast photography and a thermal-imaging method based on Schlieren techniques. Using Gaussian beam theory, the ablation area in the beam of these optical systems was calculated. For the highest energies, the ablation area extended over a length up to 60 mm resulting in the formation of channels of similar length within several pulses. In the waist of the beam, the channels were only 100 - 300 micrometers with minimal thermal effects laterally. Away from the focus, more pulses were needed, larger diameter channels were formed and thermal effects became more pronounced. The theoretical predicted ablation area was in correspondence with of the measurements. For the beam delivery devices studied, tissue effects are along the `depth of focus' of viewing due to the relatively long `ablation waist' of the focused laser beam. However, for superficial applications, the depth of the narrow ablation craters is hard to appreciate and tissues in the depth can easily be perforated. Ablation is more controlled using larger spot sizes (> 0.5 mm) and delivering intermitted pulses with energies above ablation threshold.

Laser drilling of the zona pellucida of the mammalian egg can be achieved using a continuous wave 1.48 micrometers diode laser because of high absorption coefficient of water at this wavelength. Effect of increase in temperature of the medium on the holes produced by the laser is studied. A threshold temperature is identified and its relation to the radial temperature field in the laser irradiated zona is discussed.

The novel fiberoptic delivery system for the 193 nm excimer laser has been developed for vitreoretinal microsurgery. During the application of this laser in a liquid environment both the short-living cavitation bubbles and hydrogen gas-containing insoluble bubbles are produced. In present work we study the influence of these bubbles generated in free liquid on membranous tissue. Damage zones resulting from application of pulse trains at various repetition rates were investigated using vital stains which indicate the increase of cell membrane permeability. Cavitation bubbles were created by laser above the tissue in a highly absorbing liquid--Hartmann's solution with an addition of 7% albumin. These conditions simulate a situation in which a thin membrane separated from the underlying retina by layer of liquid is cut. After application of 50 pulses at 20 Hz at energy levels varying from 14 to 68 (mu) J per pulse we have detected cell damage at corresponding distances varying from 100 to 1200 microns. In Hartmann's solution (physiological medium), where the cavitation bubbles could not be formed at the same applied energies, the laser damage has been detected only at the distances varying from 150 to 200 microns. Penetration depth of the laser radiation in this solution is about 50 microns. The cells damage in this case probably has a photochemical nature. The difference in damage distance obtained at 1 and 20 Hz repetition rates can be explained by the influence of insoluble gas bubbles that grow at the tip exit and play a role of a transparent medium for the laser radiation. This effect probably determines the minimal distance at which the surgeon can apply the laser in standard physiological medium without being concerned with underlying cells damage. On the other hand, this phenomenon enable to destroy the upper level of cells in tissue without the deep penetrating mechanical influence associated with cavitation bubble-based tissue removal.

Understanding of laser tissue ablation mechanisms may help to optimize laser ablation parameters in order to achieve minimal thermal and mechanical damage to surrounding tissues. One of the possibility to avoid unwanted thermal and mechanical effects is the use of cavitation-driven ablation. This regime provides ablation of aqueous media at temperatures substantially below 100 degree(s)C. Cavitation-induced ablation can be achieved at stress- confined irradiation of absorbing media with free surface. Under these irradiation conditions relatively powerful thermoelastic tensile pressure waves can be generated. The tensile waves can produce cavitation inside irradiated medium and cavitation-induced ablation. Temporal profile of the tensile wave depends on absorption coefficient of the medium while amplitude depends on both absorption coefficient and fluence of laser pulses. In our study effect of the amplitude and duration of the tensile waves on threshold of cavitation-induced ablation is investigated. Absorbing aqueous solutions of potassium chromate with various concentrations were irradiated by third harmonic of Q-switched Nd:YAG laser. On the basis of the experimental results, one can conclude that ablation threshold under this irradiation conditions is effected by both amplitude and temporal characteristics of the tensile pressure waves. Effect of duration of tensile pulse is caused by resonance and cooperative phenomena upon cavitation and coalescence of cavitation bubbles in the irradiated volume. Other effects (initial distribution of cavitation bubbles, etc.) are also discussed.

A computational model for the ablation of tooth enamel by ultra-short laser pulses is presented. The role of simulations using this model in designing and understanding laser drilling systems is discussed. Pulses of duration 300 fsec and intensity greater than 10¹² W/cm² are considered. Laser absorption proceeds via multi-photon initiated plasma mechanism. The hydrodynamic response is calculated with a finite difference method, using an equation of state constructed from thermodynamic functions including electronic, ion motion, and chemical binding terms. Results for the ablation efficiency are presented. An analytic model describing the ablation threshold and ablation depth is presented. Thermal coupling to the remaining tissue and long-time thermal conduction are calculated. Simulation results are compared to experimental measurements of the ablation efficiency. Desired improvements in the model are presented.

So far, laser angioplasty in minimally invasive cardiac surgery has been performed with the Excimer laser in the UV, and experimentally with IR lasers, at 2.1 micrometers (Holmium laser) and 2.94 micrometers (Erbium laser), using water as the prevailing absorber. Plaques are inhomogeneous, composed of various materials (fibrous tissue, cholesterol, hydroxyapatite calcifications) absorbing off the water absorption bands. The Free-Electron Laser FELIX (FOM, Nieuwegein/NL) was tuned to 9.5 micrometers wavelength (absorption maximum of hydroxyapatite and cholesterol) and to wavelengths with low absorption in plaque. Ablation was performed in vitro at an energy per pulse between 10 and 20 mJ and a laserpulse length (macropulse) of 4 micrometers . Ablation rates, ablation thresholds and collateral thermal damage were assessed by histologic micrometry for plaque and for normal vessel wall. The ablation rates were found to be higher in water containing plaque than in calcified plaque. With the high absorption of hydroxyapatite at 9.5 micrometers , the ablation thresholds for (calcified) plaque and vessel wall differ by 50% at least. Thus, accidental perforation is almost impossible, adding an element of selectivity and inherent safety not available so far. Collateral thermal damage at all applied wavelengths was of the same order as known from other IR sources (15 - 30 micrometers ).

This paper discusses a new approach for 3D optical imaging of tissue using a very low power, narrow-pulse, plane-wave illuminator, and a receiver which takes 2D images of the backscattered light in a number of widely spaced, narrow time-gates. An optical parameter of a single voxel is estimated using only the average backscattered energy in a single time-gate from a single detector. Thus a solution to the `inverse problem' becomes practical even for 3D images with spatial resolution on the order of a millimeter. The tissue parameters are computed one slab at a time using a `layer peeling' process. A multiple-pass imaging process using later time gates on later passes allows the image quality to be greatly improved. An analytical model has been developed for the accuracy with which the optical parameters of a given voxel can be computed as a function of voxel depth. This paper describes the computation of optimum gate parameters and optical tissue parameter estimation error versus tissue depth, image resolution, and receiver accuracy.

A new, simple and quick approach, oblique-incidence reflectometry, was used to measure the absorption and reduced scattering coefficients of a semi-infinite turbid medium. An obliquely incident light beam causes the center of the far diffuse reflectance to shift from the point of incidence, where the far diffuse reflectance refers to the diffuse reflectance that is several transport mean free paths away from the incident point. The amount of shift yields the diffusion constant by a simple formula, and the slope of the diffuse reflectance yields the attenuation coefficient. Only the relative profile of the diffuse reflectance is needed to deduce both optical parameters, which makes this method attractive in clinical settings because it does not require a stringent calibration for absolute quantity measurements. This method was tested theoretically by Monte Carlo simulations and experimentally by a reflectometer. Because this method can be used to measure optical properties of biological tissues quickly and requires on inexpensive equipment, it has potential clinical application to the diagnosis of disease or monitoring of treatments.

Laser optoacoustic imaging based on time-resolved detection of laser-induced thermoelastic pressure waves can be potentially used in medicine as a diagnostic tool (tomography) and as means to measure tissue optical properties in vivo. Information on tissue optical properties in vivo is important for laser dosimetry and tissue characterization. Analysis of the profile and amplitude of laser-induced transient stress can provide direct information about absorbed energy distribution in irradiated volume. Pressure wave profiles exactly correspond to temperature distributions generated in tissues under irradiation conditions of temporal stress confinement. In this study we experimentally measured pressure wave profile and amplitude upon irradiation of highly scattering gel phantoms and aqueous solutions colored with potassium chromate and made turbid with polystyrene spheres. A wide-band lithium niobate acoustic transducer was used to detect laser-induced pressure waves. The experimentally measured laser-induced stress profiles and the results of a Monte Carlo simulation were in excellent agreement. Computer code was developed to calculate separately absorption and scattering coefficients from transient stress profiles. Our study has demonstrated feasibility of time-resolved detection of laser-induced acoustic transients as a method for tissue optical properties measurements in vivo.

Photoacoustic imaging theory is presented based on fundamental physical processes involved in photoacoustic phenomena. Excessive pressure is used to characterize the photoacoustic signal. A solution to photoacoustic pressure derived for liquids is investigated under a rapid heating condition. Photoacoustic image reconstruction theory is proposed for short, square heating pulses. The limitation of the reconstruction theory is discussed. Relationship of the photoacoustic image reconstruction and the Radon transform is presented. The local radiation absorption energy density is determined by the reconstruction theory for three- and two- dimensional cases and for complete or partial data acquisition. The accurate solution is an infinite series. The general formulas of the zero- and first order approximations for three- and two-dimensional cases are provided. A computer simulation for a two-dimensional case exhibits good agreement between the first order approximation and true value.

We evaluated the particle density detectability and particle size detectivity of our fiber-optic light-scattering measurement system. In order to prevent the multiple pregnancy on current in vitro fertilization-embryo transfer, we have aimed to develop a new quantitative and non- invasive method to select a single viable human embryo. We employed the measurement of mitochondria localization in an embryo, which may have the correlation with development ability. We applied the angular distribution measurement of the light-scattering intensity from the embryo to obtain the information originated from the mitochondria. The latex spheres with a diameter of 1.0 micrometers were used to simulate the scattering intensity of the mitochondria. The measurement probes of our system consisted of two fibers for illumination and sensing. They were arranged at a right angle to a microscope optical axis to measure the angular distribution of the light-scattering intensity. We observed that the light-scattering intensity increased monotonically in the range from 10⁶ to 10¹⁰ particles per ml. Since the mitochondria density in a human embryo corresponded to 2.5 X 10⁷ per ml in the measurement chamber, we may measure the mitochondria density in the human embryo. The angular dependence of light-scattering intensity changed with the sphere diameters. This result showed the possibility of the selective measurement of the mitochondria density in the embryo in spite of the presence of the other cell organelle. We think that our light-scattering measurement system might be applicable to the evaluation method for the embryo viability.

A new optical probe which enables rapid and noninvasive measurement of the in vivo optical properties of tissues over a broad range of wavelengths is presented. The device was initially tested and calibrated in tissue-simulating phantoms whose optical properties were known to be similar to those of biological media. The technique was then applied in vivo to measure the optical properties of canine and human prostates.

For high-accuracy modeling of light propagation in biological tissue, the Monte Carlo method is currently the best tool in our arsenal. Its principal drawbacks include relatively slow convergence and, in the case of 3D problems, prohibitive computer memory requirements. Furthermore, in most problems, the number of iterations required to obtain satisfactory convergence varies substantially with distance from the light source; obtaining useful information at large distances from the light source requires that we generate more data than needed at smaller distances. We will present our work on an adaptive technique, which trades off spatial resolution in regions of low light intensity against the two aforementioned drawbacks to generate an `optimal' intensity map from the available data. With this technique, memory requirements scale with the degree of detail required, and not with the physical size or dimensionality of the problem. The reduced memory requirements make 3D problems tractable, and because the technique is adaptive, a generic approach is applicable to virtually any problem, without need to tailor the program to particular geometries.

Monte Carlo modeling of light propagation can accurately predict the distribution of light in scattering materials. A drawback of Monte Carlo methods is that they converge inversely with the square root of the number of iterations. Theoretical considerations suggest that convergence which scales inversely with the first power of the number of iterations is possible. We have previously shown that one can obtain at least a portion of that improvement by using van der Corput sequences in place of a conventional pseudo-random number generator. Here, we present our further analysis, and show that quasi-Monte Carlo methods do have limited applicability to light scattering problems. We also discuss potential improvements which may increase the applicability.

For erbium laser radiation a side-firing fiber delivery device was constructed for arthroscopic operations. The device basically consisted of a ZrF₄-fiber with a side-firing tip made of low OH-quartz. The feasibility of the underwater application and the dynamics of bubble formation at the submerged side-firing fiber tip were investigated by transmission experiments and time resolved flash lamp videography and compared to the bubble formation at the bare fiber tip. The drilling depth in water and the associated laser induced pressure waves were measured for both the side-firing and the bare fiber tip. The collapse pressure of the vapor bubbles generated at the side-firing tip turned out to be higher than those generated at the bare fiber tip. At a laser pulse energy of 200 mJ the collapse pressure at the side-firing tip and the bare fiber tip amounted to 47 bar and 14 bar, respectively. The maximum laser pulse energy reached at the distal fiber tip amounted to 200 mJ within a 400 microsecond(s) long pulse. These laser parameters are sufficient for medical applications like cutting biological tissue e.g. meniscus.

Short pulses of laser radiation are capable of producing strong stress transients in absorbing materials. If the stress waves contain negative components, material can be fractured or even ablated. This study investigates the propagation of thermoelastic stress waves and in particular the occurrence of tensile stresses due to acoustic diffraction in front of a submerged optical fiber. Pulses with a duration of 6 ns from an optical parametric oscillator (OPO) were transmitted through optical quartz fibers into aqueous dye solution or gelatin. The absorption of the samples could be widely varied by tuning the wavelength of the OPO. Stress wave propagation and tensile stress induced cavitation were observed by use of a time gated video camera. Depending on the absorption of the sample, tensile stress induced cavitation could be observed either outside (at high absorption) or inside the laser irradiated volume (at low absorption) in front a submerged fiber tip, the latter at temperatures calculated to be below the boiling point. The experimental findings together with theoretical calculations demonstrate that the limited size of the stress wave source, leading to diffraction of the wave is the reason for the occurrence of tensile stress waves. This was further supported by the results obtained from the comparison of cavitation bubble formation at a rigid boundary (fiber submerged) and at a free boundary (fiber outside the liquid). In medical applications where fiber-transmitted laser pulses are applied, tensile stress can weaken the mechanical strength of the tissue. This may facilitate the onset of tissue ablation but might also cause unwanted side effects, which can occur even far outside the optically heated zone.

One concern during IR-laser ablation of tissue under water is the mechanical injury that may be induced in tissue due to rapid bubble expansion and collapse or due to strong laser-induced pressure waves. The objective of this study was to evaluate the feasibility of using a liquid which is transparent to the IR-region of the spectrum in order to minimize these undesired mechanical side-effects. As transmitting medium perfluorocarbon liquid was used. Free- running Er:YAG and Ho:YAG laser pulses were delivered into the liquid via a 400 micrometers fiber. Bubble formation during the ablation process was recorded with fast flash photography while pressure transients were measured with a needle hydrophone. The effect of the surrounding material (air, water, perfluorooctane) on the tissue response of chicken breast was evaluated in vitro using histology. It was observed that a large bubble (up to 6 mm in diameter) was formed under perfluorooctane driven by the ablation products. This bubble, however, does not generate a pressure wave when collapsing. Although perfluorooctane only shows a weak absorption for infrared radiation, laser-induced thermal lensing in the liquid strongly decreases the radiant exposure and therefore the ablation efficiency.

An analytical model is proposed for the heating of single spherical melanosome in eye biotissues under laser pulse action.

The problem of a purposeful change of the shape of cartilage is of great importance for otolaryngology, orthopaedics, and plastic surgery. In 1992 we have found a possibility of controlled shaping of cartilage under moderate laser heating. This paper presents new results in studies of that phenomenon. We have measured temperature and stress in a tissue undergoing to irradiation with a Holmium laser. Study of cartilage structure allowed us to find conditions for laser shaping without pronounced alterations in the structure of matrix.

Lacking established national laser safety standards for sub-nanosecond, single pulse laser systems operating in the visible to near infrared spectral regions has resulted in research efforts designed to define the risks associated with human ocular exposure. The bulk of this work has been focused on visible wavelength laser pulses and resulting retinal threshold damage. We report threshold measurements for Minimum Visible Lesions (MVL) at the retina for picosecond (ps) laser pulses in Macaca Mulatta eyes using near infrared wavelengths (80 ps and 1064 nm). The 50% probability for damage (ED₅₀) dosages are calculated for 1 hour and 24 hour post exposures using the SAS probit analysis. The MVL ED₅₀ threshold and the fiducial limits at the 95% confidence level were found to be 4.16 (3.00 - 5.77) microjoules ((mu) J). Fluorescein angiography (FA) was accomplished at both 1 hour and 24 hour post exposure, however the analysis for FA is currently underway and results will not be reported here.

Recent studies of retinal damage due to ultrashort laser pulses have shown interesting behavior. Laser induced retinal damage for ultrashort (i.e. less than 1 ns) laser pulses is produced at lower energies than in the nanosecond to microsecond laser pulse regime and the energy required for hemorrhagic lesions is much greater times greater for the nanosecond regime. We investigated the tissue effects exhibited in histopathology of retinal tissues exposed to ultrashort laser pulses.

Threshold measurements for laser-induced breakdown (LIB) and bubble generation for femtosecond laser pulsewidths have been made in vivo for rhesus monkey eyes. These LIB thresholds are compared with model-predicted thresholds for water and minimum visible lesion thresholds in Dutch Belted rabbit and rhesus monkey eyes. LIB thresholds in biological materials including vitreous, normal saline, tap water, and ultrapure water have been measured and reported using an artificial eye. We have recorded on video the first LIB causing bubble formation in any eye in vivo using albino rabbit eyes, pigmented rabbit eyes, and rhesus monkey eyes. External optics were used to focus the image within the vitreous and the bubbles generated were clearly formed anterior to the retina within the vitreous humor. The length of time that the bubbles are visible depends on the pulse energy delivered and may last for several seconds. However, for pulse energies near thresholds, the bubbles have a very short lifetime and may be seen on the video for only one frame. The plasma formation at the breakdown site acts as a limiting mechanism for energy transmission and may explain why high-energy femtosecond pulses at energies up to 100 microjoules sometimes do not cause severe damage to the retina. This fact may also explain why it is so difficult to product hemmorrhagic lesions in either the rabbit or primate eye with 100-femtosecond laser pulses.

As part of a research program to understand and model eye damage produced by exposure to subnanosecond laser pulses, an effort is currently being made to model and analyze ultrashort pulse propagation from the cornea to the retina. Both analytical models and numerical simulations are being used to analyze the effects of self-focusing, laser-induced breakdown (LIB), and plasma-pulse interaction. The modeling effort is coupled with experimental measurements of LIB thresholds and plasma shielding for visible, picosecond (psec) and femtosecond (fsec) pulses in water, which serves as a reasonable simulant for the vitreous humor of the eye. Comparison of LIB thresholds to the critical power for self-focusing indicates that self-focusing has little effect on LIB thresholds for long psec pulses. For short psec and fsec pulses, however, numerical simulations show that self-focusing is critical to LIB in water. These results indicate that self-focusing may play a role in fsec pulse ocular damage, by influencing whether LIB and plasma-pulse interaction occur at the retina, in the vitreous, or both. Both the location of the LIB event and the amount of plasma shielding can significantly effect the degree of damage.

Laser pulses of 60-ps and 80-ps at a wavelength of 532-nm and 1064-nm respectively were used to produce laser induced breakdown in triple-distilled water. The resulting luminescent flash from the plasma was captured with an imaging spectrograph coupled to a streak camera with a 5-ps time resolution. The wavelength range was 350 to 900-nm. We present the resulting experimental data which gives plasma duration and time-resolved spectral information. Plasma temperature is also computed from the data. All parameters are presented at a pulse energy of 1-mJ and are compared with time-integrated spectra at the same pulse duration and at 5 to 7-ns pulse duration in a similar energy range.

Recent experiments, using picosecond and subpicosecond laser pulses in the visible and near infrared spectral regions, demonstrated intensity dependent eye damage. As the pulse duration shortens new physical phenomena begin to play a role as evidenced by changes in the energy requirements needed to produce laser eye damage. We investigate some of the competitive interactions between the various dispersive and diffractive effects involved in laser propagation and determine the role of the competing physical parameters, such as the laser's temporal pulse width, intensity, spatial transverse beam size, wavelength and the material's dispersion and refractive index properties.

Shock waves and cavitation bubbles generated by optical breakdown may strongly influence the surgical effect of photodisruptive lasers. We have investigated the shock wave and cavitation bubble effects of femtosecond and picosecond laser pulses generated during photodisruption in corneal tissue and water. Laser pulses with 150 fs duration at approximately 620 nm wavelength have been focused into both corneal tissue and water to create optical breakdown. Pulses with durations of 20 ps have been applied for comparative studies. Time-resolved flash photography has been used to investigate the dynamics of the generated shock waves and cavitation bubbles. Femtosecond pulse engender rapidly decaying shock waves in both materials. The spatial range of shock waves induced by femtosecond laser pulses is considerably smaller than that of shock waves induced by picosecond optical breakdown. Cavitation bubbles excited by femtosecond pulses are observed to develop more rapidly and to reach smaller maximum diameter than those generated by longer pulses. In corneal tissue intrastromal cavitation bubbles generated by femtosecond pulses disappear within a few tens of seconds, notably faster than cavitation bubbles generated by picosecond pulses. The reduced shock wave and cavitation bubble effects of the femtosecond laser result in more localized tissue damage. Therefore, a more confined surgical effect should be expected from a femtosecond laser than that from picosecond (or nanosecond) lasers. This indicates a potential benefit from the application of femtosecond laser technology to intraocular microsurgery.

Laser-induced breakdown (LIB) has long been used in ophthalmic microsurgery as a mechanism for disruption of tissue. The goal of this surgery has been precise tissue cutting by plasma formation and a minimization of collateral damage due to shock wave and cavitation bubble formation. We investigate the strength of the shock wave emission, the size of the cavitation bubble, and the amount of plasma shielding to determine the efficacy of using femtosecond pulses in surgery to reduce collateral photoacoustic damage. A pump-probe technique is used to image the time-resolved evolution of the cavitation bubble produced by focused laser pulses with pulsewidths of 130 fs, 300 fs, 3 ps, and 60 ps. Simultaneously, a hydrophone is used to measure the pressure response generated by the initial plasma shock wave and subsequent shock waves generated by the collapse and rebound of the cavitation bubbles. In addition, transmission measurements are made which indicate the amount of energy shielded beyond the focus by the plasma. These measurements give a good indication of the degree to which collateral damage may be reduced as the pulsewidths is decreased from the picosecond to the femtosecond time regime.

For sub-nanosecond pulses incident on the retina, damage mechanisms arising from high pressure within the melanosome absorber become important. This paper investigates the formation of stress waves generated in tissue as a result of laser exposure. This will eventually lead to a determination of the manner in which thermomechanical waves cause damage in cellular material such as the retinal pigment epithelium. The first step in understanding how stress waves are generated within melanosomes requires determination of how energy `channels' into separate mechanisms. The present lack of an equation of state for the melanosome/cellular system limits our ability to determine how the absorbed energy is divided between thermal and mechanical processes. We will use the fundamental definition of the experimental Gruneisen parameter as an approach to get the necessary equation of state. This will lead to melanosome temperature and internal pressure at the end of an ultrashort pulse.

Melanin in vivo is usually packaged in melanosomes with protein coats that restrict direct interaction of the melanin with the surrounding medium. We found that disruption of the melanosomes by exposure to a pulsed laser increased the ability of the melanin radicals to oxidize NADPH in a photochemical reaction. Retinal pigment epithelial (RPE) melanosomes were prepared from fresh bovine eyes in 0.25 M sucrose. A reaction mixture of 7 mM NADPH, approximately 7500 RPE melanosomes, and 80 mM Tris buffer, pH 7.2, was prepared in a volume of 60 (mu) l. Of the two 25-(mu) l aliquots taken from this mixture, one was pre-exposed to the 2nd-harmonic output of a Q-switched Nd:YAG laser (532 nm, 1800 10-nsec pulses at 10 Hz), and then was exposed to an Argon ion continuous wave (CW) laser (488.1 and 514.5 nm) for five minutes. The other aliquot was exposed only to the Argon laser. The CW exposure excited the melanin radicals to a reactive state that oxidized NADPH, as assayed by the loss of absorbance at 340 nm. Native melanosomes oxidized less NADPH during Ar⁺ laser pumping than did melanosomes pre-exposed to the YAG laser. The YAG laser's stimulatory effect on melanosomes reactivity increased as the total energy it delivered rose above 3.5 J (0.14 J/cm²/pulse X 1800 pulses), up to a maximum NADPH oxidation at about 20 J (0.2 J/cm²/pulse X 1800 pulses, beam broadened at higher pulse energy). Electron microscopic analysis of the melanosomes confirmed the progressive physical disruption of melanosomes as the YAG pulse energy increased.

This study determine the threshold radiant exposure from a 10-ns pulsed laser (532 nm) which caused bovine melanosomes to present various observable endpoints of disruption. The endpoints tested were (1) a visible region of clearing in a uniform field of melanosomes under a glass cover slip, (2) an audible sound, and (3) the increase in melanin photoreactivity due to photodisruption of melanosomes. The thresholds were tested for different starting temperatures by pre-equilibrating the melanosomes in aqueous solution at various temperatures. Lower temperatures required larger exposures to attain a given endpoint. The data suggest that bovine RPE melanosomes are about 4-fold denser in melanin content than cutaneous melanosomes.

A technique for measurement of the index of refraction of ocular components using a dual- beam Michelson interferometer is described. Various coherent light sources with wavelengths between 400-nm and 1060-nm were used in the measurements. The accuracy of this technique allows certainty to one part in ten thousand. Data was collected on triple-distilled water. This method will be used to measure the refractive index of vitreous, aqueous, lens, and cornea in a future experiment. The data will be discussed in the context of linear dispersion effects in the propagation of sub-picosecond laser pulses in laser-ocular safety studies.