The Mark III free-electron laser (FEL) is an rf linac-driven infrared laser continuously tunable from 2 to 9 microns which delivers diffraction-limited optical pulses with durations of several picoseconds and peak output powers of several megaWatts. By driving the laser with electron pulses which exhibit a linear energy chirp over time, one can create broadband optical pulses which exhibit a linear frequency chirp and are susceptible to pulse compression in an external dispersive delay line. Numerical simulations of the Mark III FEL at 3.35 μm using electron pulses with only a 2% energy chirp over 4 ps have demonstrated subsequent optical pulse compression from 3.4 ps to 230 fs, and compressed peak output powers of 30 MW; such pulses can be focused to intensities greater than 10¹³ W/cm². We review the extension of the technique to other wavelengths, and describe the design of prism compressors in the wavelength ranges of 2.5-4.1 μm and 7.5-9.1 μm which have been procured for applications in a user-oriented program at Duke University.

Free-electron lasers (FELs) provide tunable, pulsed radiation in the infrared. Using the FEL as a pump beam, we are investigating the mechanisms for energy transfer between localized vibrational modes and between vibrational modes and lattice or phonon modes. Either a laser-Raman system or a Fourier transform infrared (FTIR) spectrometer will serve as the probe beam, with the attribute of placing the burden of detection on two conventional spectroscopic techniques that circumvent the limited response of infrared detectors. More specifically, the Raman effect inelastically shifts an exciting laser line, typically a visible frequency, by the energy of the vibrational mode; however, the shifted Raman lines also lie in the visible, allowing for detection with highly efficient visible detectors. With regards to FTIR spectroscopy, the multiplex advantage yields a distinct benefit for infrared detector response. Our group is investigating intramolecular and intermolecular energy transfer processes in both biopolymers and more traditional materials. For example, alkali halides contain a number of defect types that effectively transfer energy in an intermolecular process. Similarly, the functioning of biopolymers depends on efficient intramolecular energy transfer. Understanding these mechanisms will enhance our ability to modify biopolymers and materials with applications to biology, medecine, and materials science.

Controlled exposure of photoactive compounds to light prior to their use in biological targets results in the formation of heretofore unknown photoproducts. This process of photoproduct generation, termed "preactivation," renders the photactive compound capable of systemic use without further dependence on light. Preactivation of mercyanin 540 (MC540) and several other photoactive compounds is achievable by exposure to CW and pulse laser radiation. The singlet oxygen generated at excited states attacks the dye molucule itself, resulting in the formation of biologically active photoproducts. For preactivated MC540 (photoproducts of MC540) generated by exposure to argon laser light (514 nm) and light from free-electron laser, we have demonstrated its effectiveness in selective killing of certain types of cultured tumor cells as well as human immunodeficiency virus type 1 (HIV-1) with very low, if any, damage to normal cells and tisues. For example, approximately 90% of the Burkitt's lymphoma Daudi cells and HL-60 leukemic cells are killed by preactivated MC540 at a concentration of 120 μg/ml. A two-hour treatment of cultured cells with buthionine sulfoxamine followed by the treatement with preactivated MC540 reults in 99.99% inhibition of clonogenic tumor stem cell growth. We also have demonstrated that preactivated MC540 is very effective in killing cell-free and cell-associated HIV-1. It also is very effective in killing HIV-1 and simian immunodeficiency virus (SIV) in virus-infected blood in vitro as determined by reverse transcriptase, P₂₄, P₁₇, core antigen expression and synctium formation. Treatment of HIV-1 with preactivated MC540 renders the treated HIV-1 incapable of binding to CD₄ target molecules on T cells as determined by immunofluorescence and radioimmunoprecipitation assays. In vivo toxicology studies show that preactivated MC540 is very well tolerated and does not produce any signs of adverse reaction at the therapeutic doses, as determined by physical examination, serum eletrolyte measurements, blood chemistry, and histopathology. Photoproducts generated via the process of preactivation are currently under investigation for optimum activity and yield using high-peak, power-pulse lasers.

In this paper, we briefly discuss both the design of the optical system for combination of the two laser beams from the perspective of the experimental design and the study of the effects of combined beams on vascular tissues.

A pulsed spiking and nonspiking Cr; thulium (Tm):YAG flash lamp pumped laser operating at 1.96 and 2.01 μm was investigated in vitro in the clinically relevant power range for its basic laser-tissue interaction with soft, cartilaginous, and bone tissues. Some explanations of the differences and possible medical applications are discussed.

Threshold radiant exposures for pulsed holmium laser ablation of tissue were measured using a high speed framing camera. The effects of laser irradiance and beam size on threshold radiant exposure were investigated. Threshold radiant exposure decreased with increasing laser irradiance over the range 10⁵-10⁷ W/cm². An increase in laser beam diameter reduced the laser irradiance required to achieve ablation over the range of spot sizes examined (243-970 μm). These results suggest that both the rate of energy deposition and laser beam geometry affect the events preceding the onset of ablation and should be considered when modeling or quantifying laser ablation of tissue.

The Er:YAG laser ablation of hard dental tissues and artificial dental stones was studied. The hole depth after ablation was measured as a function of pulse energy and pulse number. The dental gypsum products allowed systematic study of how pulse energy, pulse number, and water content affected ablation. The studies on human teeth compared teeth stored in saline vs teeth stored in formalin.

A new cell surgery technique has been developed to produce well-defined alterations in cells and tissues without detectable heating and/or other structural damage in the surroundings. The technique involves the use of a 193 nm argon fluoride excimer laser which is guided through a glass pipette filled with a positive air pressure. To demonstrate the method holes were drilled in the zona pellucida of mouse oocytes. The diameter of the drilled hole was determined by the pipette tip size, and its depth by an energy emitted per pulse and number of pulses. Scanning electron microscopy of the drilled mouse oocytes showed uniform, round, well circumscribed holes with sharp edges. Oocytes that had their zona pellucida drilled with this new method fertilized in vitro and developed to the blastocyst stage in a rate similar to that of control group. These results demonstrate the non-perturbing nature of this cold laser microsurgical procedure. In addition to the extension of our results for clinical in vitro fertilization purposes, such as enhancement of fertilization and embryo biopsy, there are wide ranging possible uses of our method in fundamental and applied investigations that require sub-micron accuracy in cellular alteration.

Several medical and surgical fields are concerned by thermal actions of lasers. However, quantification of laser thermal action on biological tissue is not currently performed at the time of treatment. We propose to use a self-tuning control system, in order to improve the safety and efficiency of medical lasers. This study aims to develop a control strategy for real-time control of coagulation using a millisecond pulsed Nd:YAG laser. Temperature evolution was used as a relevant parameter since it is a key factor for thermal damage. A control law was stated using the dynamic programming algorithm which is an optimization technique based on the Bellman's optimization principle. Because the control law was depending on tissue parameters, a parametric adaptive algorithm, based on the least square method, was developed to estimate in real-time the tissue parameters altered by thermal action. Finally, the structure of the self-tuning algorithm was elaborated by linking the dynamic programming algorithm and the parametric adaptive algorithm. The self-tuning control was tested using computer simulation. The results shows that sequences of pulses are producing well-controlled thermal action and that unexpected perturbations are taken into account.

We considered the biotissue boring (or cutting) by transparent probes conditioned by direct absorbing of mid-infrared radiation into tissues. A strong radiation absorption by tissue water presents the possibility to control coagulation depths on the cut's walls by choice pulse durations of radiation. Preliminary results have been obtained with mechanically modulated current-wave CO₂ and CO-lasers. Experiments showed that combining the contact transparent probe technique and the pulsed strongly absorbed near-infrared radiations increases the cutting efficiency, the decrease of average power needed for transport through fibers, and the increased security of laser thermocontacted surgery.

Vascular lesions such as port-wine stains and telangiectases are sometimes treated with carbon-dioxide lasers, argon lasers or argon-pumped dye lasers; however these lasers are non- specific in their thermal effect on tissues and as a result often cause significant scarring. Recently, evidence has accumulated that the flashlamp-pumped dye (585 nm) and copper- vapor (578 nm) lasers, which produce pulsed light that is efficiently absorbed by hemoglobin, are more selective in coagulating abnormal vascular tissue and as a result give a superior clinical result. It is not yet clear what the most important physical and biological mechanisms are during the light-tissue interaction mediated by these two lasers. The post-treatment sequence of events is different for tissue irradiated by each laser; most significantly, the flashlamp-pumped dye laser causes significant transient purpura, whereas the copper vapor laser causes blanching and eschar formation. The clinical outcome, that is regression of the lesion, is equally successful with either laser although some evidence has accumulated showing that the flashlamp-pumped dye laser is best suited to the treatment of small vessel disease while the copper-vapor laser is better for the treatment of large vessel disease. In this paper, we will discuss our observations of the treatment of vascular lesions on humans with the copper-vapor and flashlamp-pumped dye lasers using empirically derived efficacious treatment parameters. Mathematical models of light and heat propagation and in vivo experiments involving mice ears and rat skin flaps will be used to elucidate what we feel are the important underlying mechanisms of this vascular lesion laser therapy.

Experiments in dog and rat liver compared the 805-nm wavelength of the diode laser and the 1064-nm wavelength of the Nd:YAG laser. (1) The major optical differences are in the absorption. The optical properties of dog liver were determined by integrating sphere experiments. The scattering coefficients were similar at both wavelengths, but the absorption was 3.5-fold greater for the diode laser wavelength. Consequently, the diode laser penetrated less deeply and heated the liver surface more strongly than the Nd:YAG laser. (2) Blood is a major component of liver absorption. When blood accumulated in the lower region of a rat liver held sideways, surface heating (measured by infrared camera) by the diode laser increased due to increased absorption by the pooled blood. (3) Tissue optics and irradiance geometry together affect the zone of thermal coagulation caused by each laser. The sizes of coagulation lesions in rat liver in vivo indicated larger zones of coagulation with the Nd:YAG laser. Our working hypothesis is that the diode laser caused greater surface heating and water evaporation which means less energy remained in the tissue. Enlarging the spot size of irradiance should alleviate such surface overheating and evaporation losses and maximize the zone of coagulation. The similarities between the two lasers are more striking than the differences.

Small bits of rabbit hepatic tissue were irradiated with CW Nd:YAG laser at powers of 5 - 45 W and auto-grafted subcutaneously together with unaffected (control), boiled or frozen-thawed samples of the same weight. Unlike control and cryo-processed grafts, which showed moderate local inflammation and swift resolution of the transplants with phagocytes, high power-lased samples provoked long-term purulent reaction around the graft without appreciable phagocytic scavenging of the debris by phagocytic cells. At lower powers and after boiling of the samples the scavenging was less inhibited and the resolution of the debris took shorter durations. In the biochemical series high resistance of rat liver homogenates to pancreatic proteinases was demonstrated in high power Nd:YAG vs. XeCl excimer, mixed Nd:YAG + XeCl excimer lased as well as US-processed samples after their incubation in proteolytic media. The results obtained suggest that the resistance of necrotic debris to phagocytic scavenging in the wound may be an important parameter responsible for local recovery, and that parameter is strongly dependent on the damaging factor, either laser or an alternative surgical tool, applied.

A treatment for eleven eyes on posterior capsular opacifications after artificial intraocular lens (IOL) implantations with a Nd:YAG laser is reported, and a secure range of laser energy under which IOLs are not injured in the operations is given in this paper.

Most clinical procedures using lasers are based on thermal laser-tissue interactions resulting in tissue damage. A mathematical model allowing temperature and damage prediction would be a useful tool to choose laser parameters able to produce the damage required. This study aimed at the development of a model called HELIOS. The capability of the model to predict thermal coagulation was evaluated by comparison with experimental in-vivo results. Conversion of laser light in tissue was studied using a beam broadening model, which is typically a modification of Beer-Lambert's law and takes into account scattering of light in tissue. HELIOS was validated making experimental studied in-vivo on liver (homogeneous tissue) of rats using a CW Nd:YAG laser, a CO₂ laser, and an Argon laser. For a given set of laser parameters, temperature measurements were performed using an infrared camera (Agema, Sweden). The histological examination was carried out on samples to quantify the depth of coagulation necrosis. Experimental data obtained in-vivo were compared with those calculated previously using HELIOS and similar sets of parameters. The difference between the predicted temperature evolution on tissue surface and that measured by the infrared camera was less than 5 degree(s)C in all cases. The difference between the predicted coagulation necrosis depth and the corresponding experimental one was less than 10%. In conclusion, HELIOS allows good prediction of tissue temperature and coagulation necrosis. This model appears sufficiently precise for clinical applications and so should be well adapted to control thermal action of lasers in biological tissues.

Profiles of light, temperature, and thermal damage distributions in tissue based on measured optical properties are examined theoretically for high power diode laser (810 nm) and Nd:YAG laser 1064 nm). Generally higher absorption and effectively lower optical penetration has been experimentally observed at the wavelength of diode laser as compared to that of Nd:YAG laser. Results of this study indicate that similar thermal damage volumes are expected to be obtained by the two lasers, in general. However, for same irradiation conditions a larger volume of damage and more charring near the surface is predicted when using the diode laser on prostate tissue, and similarly for myocardial tissue. Role of blood presence throughout tissue, in terms of its optical interaction, as well as the role of a small layer of blood between the laser and the tissue is also investigated for both wavelengths.

The Monte Carlo method of statistical simulations was used to evaluate the effects of anisotropic scattering and refractive index mismatch at the boundaries of the medium on light and temperature distribution in the multilayered biotissues during laser irradiation. The influence of optical parameters of multilayered tissue on its stationary thermal response was examined by using an adaptive version of finite element solution of the heat conduction equation. Calculations were performed for two models of human skin and adjacent tissues for visible (five layer model) and UV (three layer model) ranges of wavelengths. The topic of this paper is especially pertinent to laser percutaneous irradiation of blood and laser PUVA therapy.

Application of the nonlinear thermal ablation model to the chemical HF laser corneal ablation data gives the dropping of the effective absorption coefficients by two orders of magnitude relative to the native tissue. This effect is believed to be due to the hydrogen bond weakening at subthreshold fluences and to the homogeneous broadening removal by ablation products' expansion at higher fluences.

This paper discusses optimum regimes of laser destruction of tissue, while maintaining optimum action with feedback.

The present study uses photodynamic therapy (PDT) for the treatment of IH in the rat carotid artery model. PDT is a relatively new cancer treatment technique which utilizes light-activated photosensitizers (PS) to produce injury to targeted cells. PS have no cytotoxic effects unless they are activated by the appropriate wavelength of light. Upon absorption of a photon, the activated PS can either by directly cytotoxic or give rise to cytotoxic oxygen species, commonly singlet oxygen, via energy transfer to the ground state molecular oxygen. Singlet oxygen then becomes the mediator of cellular injury by affecting cellular membranes and subcellular organelles.

Both PtCl₄ (Rh-123)₂ and PtCl₄ (Nile Blue)₂ interact positively in trimodality therapy including each drug, x-ray, and light (400 - 800 nm) in killing human mammary carcinoma cells (MCF-7) in vitro. The combination treatment results in more than additive killing as assessed by product of the surviving fractions and a significant reduction of the shoulder of the x-ray survival curve. Both of the drugs participate in photodynamic therapy (PDT) in drug and light dose dependent manner. The primary subcellular targets for the neutral platinum complexes were indicated to be the nuclear DNA as opposed to the mitochondria for Rh-123 or the lysosomes for Nile Blue. Because the nuclear DNA is also the primary target for ionizing radiation, drug plus light plus x-ray might cause supraadditive killing. Both PtCl₄ (Rh-123)₂ and PtCl₄ (Nile Blue)₂ have been reported to be relatively non-toxic in vivo compared to the anionic or cationic compounds currently being used in photodynamic therapy. Based on these results PtCl₄ (Rh-123)₂ and PtCl₄ (Nile Blue)₂ have the potential for use in photodynamic therapy and in trimodality therapy.

Two sets of experiments dealing with the upper excited states of DNA are presented here. In one investigation, one-photon excitation of DNA with low intensity UV laser radiation (199.8, 217.8, and 239.5 nm) is exploited to assess damage done upon excitation to singlet states higher than the first excited singlet state. In another, the sequential excitation of DNA with 266 nm laser radiation and 532 nm photons was explored using ps laser radiation.

Frequency domain photon migration (FDPM) non-invasively investigates the optical properties of multiple-scattering media such as tissue. In this work, FDPM is used to examine the optical properties of an EMT-6 murine tumor model in Balb/c mice. In-vivo measurements are recorded before and after intravenous administration of the photosensitizer, chloroaluminum tetra-sulfonated phthalocyanine (CASPc). Optical properties are used to: (1) discriminate between tumor and healthy tissue and (2) monitor the rate of tumor drug uptake. Our data show substantial frequency-dependent contrast between tumor and normal tissue. In addition, drug pharmacokinetics are effectively recorded in situ. Although FDPM can quantitatively measure absorption and scattering coefficients in infinite homogeneous media, exact values of these optical properties are difficult to extract in vivo due to boundary condition uncertainties.

A method is proposed for mapping laser light distributions in gel phantoms by measurement of the distribution of a fluorescent compound that has been photolabeled to the gel by the laser irradiance. A preliminary study of photolabeling by an argon laser using tetrabrominated rhodamine (TBR) was conducted in collagen gel phantoms to illustrate the feasibility of the method. A discussion of the basic quantitative relationships for analysis of measurements is presented.

Optical properties of biological tissue can change as a result of thermal denaturation due to temperature rise; a familiar example is whitening observed in cooking egg-white. Changes in optical properties with temperature have been reported in the literature. Temperature rise due to laser irradiation is a function of the optical properties of tissue which themselves are a function of temperature of the tissue. This creates a coupling between light and temperature fields for biological tissue under laser irradiation. The effects of this coupling on the temperature response and light distribution may play an important role in dosimetry consideration for therapeutic as well as diagnostic application of lasers in medicine. In a previous study this problem was addressed in one dimension, for short irradiation exposures, using certain simplifying assumptions. The purpose of this research was to develop a mathematical model for dynamic optical changes with thermal denaturation and a computer program for simulation of these effects for a multi-dimensional geometry.

The ablation of cornea by pulsed 193-nm excimer laser radiation has been studied for individual laser pulses by real-time noncontact methods. The ablation event is consistent on a pulse-to-pulse basis, indicating no cumulative photochemical modification of the tissue. However, dynamic optical events occur within each laser pulse which may significantly impact the radiation transport process.

A Q-switched Er:YAG laser was used to ablate skin, liver, and agar gel. The materials were chosen for their varying degrees of stiffness. It was seen that the varying mechanical properties effected the dynamics of the ablation process. The lower stiffness materials demonstrated a lower velocity of the ablated material and a longer post-pulse ablation duration. The effects were mediated by high ablation pressures causing elastic expansion of the crater during and after the pulse.

Shock waves, whether generated by sparks or laser bursts, are capable of producing a wide range of biological effects. Thresholds for hemorrhage of kidney and lung, the killing of fruit fly larvae, developmental defects in embryos and changes in function of excitable tissues such as the heart fall in the range from 1 to 10 MPa. In endoscopic applications, the bubble that is created has the potential to act directly upon the surrounding tissues producing effects by mechanisms that are qualitatively different than the shock fields themselves.

When a transparent liquid or solid medium is present in front of an opaque target being irradiated by an intense laser beam, then the expansion of hot vapors generated (at the interface between the medium and the target) by the irradiated heating of the target is restrained by the medium. The tamping effect of the overlying liquid or solid can cause a much larger fraction of the deposited energy to go into kinetic energy, which leads to enhanced tissue disruption, compared to when a gas or vacuum is in front of the target. Condensable vapors and high thermal conductivity in the surrounding material facilitate rapid energy transport our of the vapor, which can cause a major reduction in the tamping enhancements. This contained-vaporization process is likely important in laser-medical applications such as, for example, laser angioplasty and laser lithotripsy. The work enhancement by the process is probably advantageous for lithotripsy in providing the necessary energy to break urinary stones; however, for angioplasty, the enhancement may provide little aid in removing plaque but may cause significant damage to arterial walls. If gas could be introduced into the artery preceding irradiation of the plaque, then the enhancements could be avoided. In summary, careful management of the tamping conditions during tissue irradiations in the clinical applications of lasers should lead to significant improvements in the overall desired outcome.

Shock waves generated by laser-induced plasmas were investigated using a pump-and-probe technique. The shock peak pressure was determined from the measured shock velocity, the jump conditions, and the equation of state for water. Two plasma geometries, spherical and cylindrical, were investigated. The symmetry of the shock wave, in the near field, reflects the symmetry of the plasma. The conservation of momentum was used to derive expressions for the shock velocity, particle velocity and pressure vs. the distance (r) from the center of the plasma. For a cylindrical shock wave the shock pressure is proportional to 1/r. We have, also, applied the Rankine-Hugoniot expression to calculate the energy deposited behind the shock front. We found that a substantial fraction of the laser energy is deposited in the medium in the form of internal energy following the passage of a shock wave.

Laser-flash photography was used to document the onset of cavitation bubbles induced in liquid media by Q-switched laser pulses. Such bubbles are hypothesized to indicate the 'breakage of water' by negative stress caused by the inverting reflectance of a laser-induced propagating stress wave at the air/liquid interface. Absorption of the pulsed laser by the liquid caused thermoelastic expansion which generated the initial stress distribution. These liquid phantom experiments model the situation for tissues where such 'breakage' of the tissue structure can contribute to the process of tissue removal by the phenomenon called 'spallation'. The threshold dosimetry for cavitation and explosive vaporization were determined. The threshold negative stress for onset of water cavitation under the dynamic stress conditions induced by approximately 140-ns laser pulses is approximately -8 bars.

We are testing the acoustic response of Ho:Tm:YAG and Tm:YAG laser pulses absorption in water and in tissue using a broadband ultrasound PVDF transducer. A clear acoustic signature of the cavity formation as well as its collapse is obtained. The cavity collapse echo carries higher amplitudes and can be eliminated or reduced in tissue by direct fiber-tissue contact. The laser pulse temporal profile--smooth or spiky--has a rather small acoustic impact. Smoother pulses generate somewhat lower acoustic amplitudes during the cavity formation phase.

Previous studies of mid-infrared ablation of tissue showed blast damage to adjacent tissue in the form of large fissures extending from the crater wall. We hypothesized that these fissures are due to the forceful expansion of a water vapor bubble within tissue. Time resolved flash photography documented an expanding vapor bubble within the tissue by the elevation of the tissue surface. The aim of this study was to determine the relation between the intensity of holmium:YSGG laser irradiation and bubble dimension in saline and in tissue. The influence of the spatial distribution of the intensity was determined by delivering 200 to 500 mJ laser pulses through a 320 micrometers and 600 micrometers diameter bare fiber, which was in light contact with porcine aorta. A holmium:YSGG laser pulse is a superposition (500 microsecond(s) long) of microsecond pulses. The first part of the laser pulse contains the most intense spikes. To determine the influence of the high peak power spikes on bubble formation, the first part of a 650 mJ laser pulse was absorbed. The modified laser pulse (400 mJ) was smoother and 75 microsecond(s) shorter than the original 400 mJ laser pulse. Increasing the fiber tip area by a factor of 3.5 (600 micrometers vs. 320 micrometers diameter) resulted in a decrease in the diameter of tissue elevation by only a factor of 1.25, indicating that the delivered energy determined the extent of tissue elevation. Elimination of the high peak power of the first part of the laser pulse did not decrease the dimension of the bubble in saline and of the tissue elevation. Therefore we anticipate that the extent of the blast damage in tissue cannot be reduced substantially by either reducing the fluence or by quenching high peak power spikes of the laser pulse.

This paper examines the thermomechanical effects of Er:YAG irradiation of tooth from a theoretical standpoint. Based on the high absorption coefficient of biological tissue at 2.94 micrometers , and the short interaction time between the photons at this wavelength and the tissue, effects of light scattering and heat conduction were neglected in calculating the temperature distributions in a tooth irradiated by Er:YAG laser. Radial and circumferential stresses developed as a result of temperature rise were then calculated on the surface of tooth. Radial stresses were always found to be compressive. Circumferential stresses, however, were compressive within a range inside the beam radius, and tensile outside the beam radius. Locations of the maximum tensile stress and maximum displacement were found to be outside and inside the beam radius, respectively. Maximum stresses and displacements decreased monotonically with increasing beam radius. Our results also revealed that ablation and thermal stresses beyond enamel strength could be reached within a single pulse.

The process of vapor cavity formation in light absorbing liquid under the action of free- running laser radiation is investigated and a mathematical model to simulate the process is presented. To minimize thermal and mechanical damage of tissue, it is proposed to choose an appropriate temporal profile and energy of laser pulses using results of the simulation.

Microplasma and shock waves produced by a Q-switched YAG pulse laser worked near the optical breakdown threshold of physiological saline are investigated by using optical shadowing exploring method. A series of optical shadowgraphs of microplasma and shock waves versus the incident laser energy and the delay time is obtained. The mechanical action of shock waves is discussed.

In our study we investigated the ablation characteristics of an aqueous dye solution with a defined absorption coefficient, irradiated by short (8 ns) and long (100 microsecond(s) ) pulses from a Nd:YAG laser (wavelength: 1064 nm). The experimental technique was schlieren photography with a second Nd:YAG laser at 532 nm as a light source and with a variable delay between the two laser pulses. With a special arrangement of the laser beams and the sample effects below and above the surface of the liquid could be simultaneously observed. We could distinguish three ablation mechanisms, depending on the pulse duration and the incident fluence. With short pulses and a fluence below the vaporization threshold the tensile pulse from the bipolar thermoelastic wave, propagating from the liquid-air interface into the sample, caused rupture and spallation of the liquid. At fluences generating a surface temperature in excess of 100 degree(s)C the short pulses caused explosive vaporization, characterized by shock wave emission both in air and in liquid. At the same fluence the long pulses caused slow vaporization, meaning that vapor and liquid ejection started during the laser pulse and was less violent than with the 8 ns pulses.

In this paper, 30 cases of extracapsular cataract extractions with a Nd:YAG laser are reported. Myosis phenomenon in laser operation is observed and its producing mechanism is discussed. Laser ophthalmology was the earliest field in laser application research, and pulse Nd:YAG laser ophthalmic instruments have been developed rapidly because of unique advantages in the anterior and posterior capsulotomy and trabecula shaping method. Thirty cases of extracapsular cataract extractions with the assistance of a Nd:YAG laser are reported in this paper. In laser operation, myosis phenomenon is observed, and its production mechanism is discussed.

We sought to compare the properties of cylindrical and ball tip fibers (in conjunction with a holmium:YAG laser) by examining both their beam characteristics and the characteristics of the channels formed when the fibers passed through myocardial tissue. We found that the cylindrical fiber tip was able to produce a wider, more uniform channel with less surrounding thermal damage than the ball tip fiber. We attribute these findings to the shorter focal length, smaller beam spot size, and greater beam divergence produced by the ball tip fiber.

We considered the boring soft biotissues with laser-energized fibers for the two-step photodynamic therapy of deep-seated tumors. The proposed method and the corresponding equipment included the dye laser pumped by copper vapor laser have been discussed. Experiments were performed with potato tissues fixing heat penetration depths. It was established that a quality of resulted bore walls (the small coagulation depth and the absence of carbon deposits) is determined by laser powers, boring speeds, and properties of tissues.

Local tissue optical parameters are of principal importance in determining the ideal dosage of laser energy and choosing the most suitable therapeutic wavelength for a specific organ. We have developed a non-invasive method for the determination of optical parameters of whole tissue in-situ. Backscattered laser light from a pencil beam incident on the tissue surface was measured using a CCD camera interfaced with a personal computer. Backscatter measurements were performed on in-vitro canine myocardium and on phantoms, with optical properties predetermined by the integrating sphere method. Backscattering radiance proved to be a function of the distance to the incident beam. Regression analysis of the backscatter showed that the backscatter can be expressed as an exponential function of the distance to the beam with a regression coefficient ((mu) _r) resembling the tissue diffusion coefficient ((mu) _eff), for tissue thickness much greater than optical free path. The CCD derived backscatter radiance exponent showed excellent agreement with the diffusion coefficient, with a correlation coefficient of 0.91. The backscattering technique might prove to be a simple, non-invasive, accurate new technique in the determination of in-vivo optical properties.

We present in this paper the effect of dehydration on the behavior of reflectance (R) and transmittance (T) on the arterial wall. The behavior of R and T at 633 nm as a function of tissue dehydration, measured on fresh arteries exposed to air, shows two domains of variation. For water loss up to a value of about 60% of the initial tissue weight, the tissue shrinks, but its reflectance and transmittance remain almost constant. In this region, the absorption and scattering coefficients, deduced from a diffusion model, increase with dehydration. For greater water loss, a significant increase of T and correspondent decrease of R are observed. This behavior is related to a tissue denaturation. This study shows that the dehydration is an important parameter in determining the correct dose of laser light that needs to be applied to obtain the desired effect, since it leads to variations of the tissue optical properties.

During laser treatment, coagulation affects the optical properties of the tissue. In particular, the formation of a white lesion increases the scattering coefficient significantly. This change in the optical properties in turn affects the laser light distribution in the tissue. For example, what is the effect of the white lesion formed during photocoagulation of the retina upon reflection and fluence rate? This problem was simulated on a model medium consisting of a thin absorbing black paint layer covered with a 1 cm thick layer of fresh egg white. The egg white layer was subdivided into coagulated (white) and uncoagulated (clear) layers. The optical properties of coagulated and uncoagulated egg white were determined. These values were used to model light distribution in the medium for varying thicknesses of the coagulated egg white layer using the one dimensional Adding Doubling method. Our results show that the fluence reaching the paint layer increase until the coagulated layer reaches 100 micrometers , after which it falls off exponentially. It was also found that the total reflected light increases almost linearly at first as the coagulated layer thickens, and then begins to level off to an R_(infinity ) at a coagulation thickness of 2 mm. Experimental measurements of reflection from a lesion with a CCD camera confirm the computed trends. These results provide a theoretical foundation for control of lesion thickness using reflectance images.

Monte Carlo simulations provide accurate descriptions of light distribution in complex geometry, multiple layer tissues at the expense of long computation times. In addition, the accuracy of the light distribution quantity calculated is influenced by the resolution of the simulation if spatial convolution is used. A method of increasing the accuracy of the calculated fluence rate without increasing computation time is to consider the Beer-Lambert law source attenuation in the tissue analytically, separate from the Monte Carlo numeric calculation of the remaining multiple scattering process. A method of reducing the time of computation needed for a desired maximum statistical variance is to use a non-linear grid that provides high resolution in areas of interest and lower resolution far away from the source and boundaries. The theory of the source attenuation function is discussed. Results demonstrating the validity of the analytic function and the use of non-linear grids are provided and compared to standard Monte Carlo techniques.

We describe an improved measurement of tissue absorption of light by using a mirage effect due to the thermal gradient at the tissue surface. The laser beam enters a prism where internal reflection is highly dependent on the incident angle. The system is calibrated using an acoustic-optical cell. Finally, we discuss design of a compact system which can produce an electrical signal related directly with the sample optical radiation absorption.

The results of measurements of the optical parameters of the upper layers of human epidermis in UV wavelength range are presented. For making samples, the technique of skin surface strippings was used. For calculations of linear scattering and absorption coefficients, the Kubelka - Munk approximation was applied, taking into account the sample method. Use of skin surface strippings allows the transmission and diffuse reflection spectra of psoriatic epidermis.

The POLYCON system has been developed for simultaneous passage of different laser beams to a target. In this study, CW Nd:YAG (9 W/cm²) and XeCl excimer (30 mJ/pulse, 80 nsec, 10 Hz) laser beams were directed via individual fibers to a cone-shaped optical mixer continuously connected at the site of its outlet aperture with a common flexible cable destined for surgery. Contact mode cuts were made on rat liver and biopsy samples taken for electron microscopy. Injuries of mixed type have been found within the same cells, e.g., mitochondrial ballooning detected recently after Nd:YAG and vesiculation of the endoplasmic reticulum which is a feature characteristic of XeCl excimer laser. Taking into consideration satisfactory cutting properties, good hemostatic effect and favorable prognosis for recovery as evidenced by active phagocytosis along with proteolytic digestibility of debris in the wound, mixed beam lasing with the parameters tested can be approved as a perspective kind of surgery. In one study, a low potential for healing after CW mode Nd:YAG lasing has been revealed by specially devised tests. Contrary to that, the PW mode XeCl laser has been shown to provide better conditions for local recovery not incompatible with the impression of other workers. The excimers are also believed to be good cutters but they are mainly utilized in the experiments on tissues with low blood supply, due presumably to inherent poor hemostatic capacity of these tools. The lasers of that kind would undoubtedly be tried for tissues with high levels of blood supply provided hemostasis could be secured at the site of surgery by some means. In this paper, such a means is offered, and it is a mixed beam system in which XeCl excimer laser is used in the capacity of a cutter while Nd:YAG one is operating to stop bleeding with minimal side effects on healing in the post-operation period.

The current trend to using short-pulse, high-peak power lasers has led to better temporal and spatial confinement of effects; however, a consequence of these techniques is the production of pressure transients in tissue which can propagate for long distances. The terminology that is used in the literature has been confusing since there is no absolute physical criteria for the classification of some of the phenomena. It is suggested that investigators use some practical approaches for choosing the descriptions of their systems. The four commonly observed pressure effects include shock waves, acoustic waves, mechanical effects, and radiation pressure. The distinction is usually made between mechanical effects (ex: cavitation) and radiation pressure (ex: optical trapping). The distinction between shock and acoustic transients is frequently blurred. Shock waves with rise times in the order of picoseconds should be distinguished from acoustic waves with rise times in the order of nanoseconds because there are unique characteristics to this type of pressure phenomena.