In vitro ablation experiments of calf brain tissue using a picosecond Nd:YLF laser system are reported. The laser lesions were studied with special regard to the ablation rate and to possible injuries of tissue adjacent to the excisions. Histological examinations prove the excellent quality of the lesions, showing no signs of thermal damages or carbonization. Furthermore, the influence of higher photon energies on the ablation threshold was studied.

In this work two aspects of the ablation of brain by Erbium laser have been mainly addressed: the time evolution of the phenomenon and the damages, both thermal and mechanical, produced in the tissues. The time resolved images acquired during the laser interaction revealed that deep lacerations develop in the tissue due to a mechanical stress. The damages have been evaluated by studying the changes in the autofluorescence emission properties and the reduction in enzymatic activities (NADH Oxidase and ATPase). The results obtained in this study indicate that the thermal alterations resulting from the exposure to Erbium laser are limited, whereas the mechanical damages can be very pronounced.

In this paper the physics processes proceeded in the tissue under laser radiation action are described. From heat and mass transfer analysis the tissue destruction model is received as a many boundaries Stefan problem with sudden change of thermophysical parameters on phase boundaries. A numerical solution of the 1-D problem is presented for model testing. It permits us to define a size of carbon zone and a boundary movement velocity. The resulting analysis shows that the model satisfies the real process of the tissue destruction.

An alexandrite laser has been used in the fixed-Q and Q-switched modes, at the fundamental and frequency doubled wavelengths on a selection of hard and soft tissue. In an investigation into the potential use of the laser for the removal of deep lying lesions such as cutaneous vascular lesions and tatoos, studies have been carried out to characterize the depth and extent of the laser/tissue interaction in samples of tissue which greatly absorb the 750 nm radiation. The interaction of the laser radiation with extracted teeth was investigated looking at healthy enamel and dentine, and caries. Surface profile measurements of the enamel and dentine before and after irradiation show little physical effect of the laser irradiation, whereas caries appear to be ablated.

This study evaluated the effects of the FiberTome^TM (10, 20, 30 W), conventional Nd:YAG (10, 20, 30 W), and KTP (5, 10, 15 W) surgical laser systems, with respect to pigmented and nonpigmented tissues. The cutting width, cutting depth, and thermal damages were measured on the skin and liver of farmer pigs. The results show that there was no significant difference in the cutting width and depth, for nonpigmented versus pigmented tissues. KTP showed the most significant thermal damage reduction in pigmented tissue; 318 micrometers in nonpigmented vs 94 micrometers in pigmented tissue.

We investigated the effect of varying fluence and pulse width on the ablation rate and consequent thermal damage of the Ho:YAG (2.130 micrometers ) laser. The rate of ablation on fresh bovine knee joint tissues, fibrous cartilage, hyaline cartilage, and bone in saline was determined after varying the fluence (160 - 640 J/cm²) and pulse width (150, 250, 450 microsecond(s) ec, FWHM) at a repetition rate of 2 Hz. A 400/440 micrometers fiber was used. The ablation rate increased linearly with the fluence. In fibrocartilage, different pulse durations generated significant changes in the ablation rates, but showed minor effects on hyaline cartilage and bone. The heat of ablation for all three tissue types decreased after lengthening the pulse.

The purpose of this study was to evaluate the cutting abilities and thermal effects of different fiber optic delivery systems on the lung, after varying the power levels and system modes. The Nd:YAG (1.064 micrometers ) laser was tested using 600 micrometers bare, and sculptured (600/300 micrometers cone and 1000 micrometers chamfer) fiber tips in the continuous wave (cw) and pulsed (P) modes. A standard porcine model (n equals 18) was used for this study. We did not observe any ablation in the noncontact mode with the 1000 micrometers chamfer (P and cw) and the 600 micrometers bare fiber, in pulsed mode. We observed tissue ablation in the contact mode for all fibers. In contact mode, there was no significant difference in the cutting width between the 600/300 micrometers cone, and 1000 micrometers chamfer. In the range of the parameters tested we observed no carbonization. There is an advantage using Nd:YAG laser and flexible fibers in minimal invasive thoracic surgery.

The aim of this work is the experimental and theoretical investigation of the influence of variable laser parameters (wavelength, fluence, pulse repetition rate) and the optical and thermophysical properties of bone tissue (absorption coefficient, tissue inhomogeneity) on the ablation thresholds and the extent of thermally influenced zones. Ablation and perforation experiments were realized by using (1) a semiconductively preionized TEA CO2 laser (10.6 micrometers ) and (2) a sliding discharge TEA HF laser (2.9 micrometers ).

The channel propagation in liquid water caused by Er³⁺:YAG laser radiation at a wavelength of 2.94 micrometers is investigated experimentally and compared to a theoretical model. The propagation velocity of the phase front is measured with the beam delivered to the open water surface as well as with a fiber submersed in the water. For laser intensities up to 4 MW/cm² the results agree with model calculation. We suggest that the hydrodynamic behavior of water is a key feature for the propagation mechanism.

Laser ablation of corneal tissue by mid-infrared laser radiation was studied in dependence of the laser wavelength and the pulse duration. The thermally and mechanically laser induced damage was determined by light microscopy (LM) and scanning electron microscopy (SEM). Fresh porcine eyes were irradiated with three different lasers: Er:YAG ((lambda) equals 2.94 micrometers ), Er:YSGG ((lambda) equals 2.79 micrometers ) and Ho:YAG ((lambda) equals 2.1 micrometers ). The experiments were performed with both, free running (pulse duration (tau) equals 200 microsecond(s) ) and Q-switched pulsed ((tau) equals 100 ns) lasers. The extent of thermally damaged tissue was found to be the same for Er:YAG and Er:YSGG lasers: 8 - 45 micrometers and 2 - 10 micrometers , with the long and the Q-switched pulses, respectively. The Ho:YAG laser induced coagulation zones were 200 - 600 micrometers thick in the free running and 40 - 80 micrometers in the Q-switched mode. The ablation efficiency of Er-lasers was between 4 and 13 times higher than that of the Ho:YAG.

The thermal response of cortical bone targets to fiber delivered pulses from free running Er:YAG and Ho:Tm:YAG lasers was investigated experimentally and theoretically and compared to histological and morphological findings. The temperatures found at the ablation sites were found to be markedly lower for the strongly absorbed 2.94 micrometers Er:YAG than for the 2.1 micrometers Ho:Tm:YAG pulses. Furthermore, the temporal behavior of the temperature underneath the ablation crater when using 2.1 micrometers pulses suggests that the absorption characteristics of the bone targets are irreversibly changed by the thermal action of the ablating pulse which leads to a significantly different ablation behavior of the first pulse compared to the following pulses. In contrast, at the Er:YAG wavelength the thermal response of the bone targets and the ablation behavior of the pulses were found to remain unchanged after multiple pulses.

The surgical properties of pulsed 0.53 m Nd:YAG laser have been investigated. The tissues of genitals have been treated with 0.53 mm laser beam and investigated microscopically, injury depth to width ratio was established. The spectrophotometry of the same tissues was performed and optical densities were compared. The advantages of 0.53 micrometers , 1.06 micrometers , 1.32 micrometers Nd:YAG lasers used for gynecological operations have been discussed.

Interstitial laser photocoagulation (ILP) is a new minimally invasive technique which can produce localized necrosis in the center of solid organs. Breast cancer can now be treated safely in selected patients by lumpectomy followed by radiotherapy. Taking the concept of conservative treatment a step further would it be possible to destroy breast cancers in situ leaving the area to heal by resorption and fibrosis? Thirty seven patients with early breast cancer scheduled for surgery underwent ILP in the interval between diagnosis and surgery. The technique was performed under local anaesthesia, and ultrasound (US) used to place an optical fiber into the tumor. Treatment was curtailed in 4 patients because of pain. Thermal necrosis varied from 2 - 25 mm as measured microscopically but in 4 patients no necrosis was evident. No complications arose as a result of treatment. When charring was present in the excised tumor a larger diameter of necrosis was evident than when charring was absent (mean 15 mm vs 5 mm) and a study in 10 patients using a pre-charred fiber produced a more predictable and larger diameter of necrosis (mean 17 mm). Ultrasound did not accurately predict the extent of laser induced necrosis (although it was better at predicting tumor size).

Preliminary investigations of the interaction between a continuous wave diode laser and biological tissue are presented. The laser used has a wavelength of 980 nm and a maximum output power of 10 W. The tests were carried out on porcine liver samples. The interaction was seen to develop over three different stages during laser irradiation. The initial stage of tissue coagulation was followed by a distinct tissue surface eruption, followed by the ablation of a crater into the tissue. Measurements of the time evolution of these stages are presented over a range of incident spot diameters, and related to theoretical expectations. Measurements taken from the resultant crater are presented, including the extent of residual tissue damage around the crater, over a range of applied laser exposure times.

Arthroscopy of TMJ has become a clinically important and more and more accepted method for diagnosis and treatment of TMJ alteration. This minimal invasive method is clearly limited by the anatomical dimensions of the TMJ. A 308 nm excimer laserlight has already found clinical applications in angioplasty, ophthalmology, and dentistry. The aim of the presented study was to find out if it is possible to ablate TMJ related structures under arthroscopic conditions. It also aims to evaluate the energy-threshold for ablation and the maximal possible rate of ablation. Contrary to other laser systems it offers a unique combination of minimal tissue alteration, precise tissue ablation guidability through optical fibers, and a good transmission through water.

UV-laser irradiation has similar injuring effect when applied to arterioles and venules of the same diameter and may be used in the study of thrombogenic and thromboresistant properties of microvasculature. The thrombogenic potential of arterioles is greater than the same of venules but this difference under studied pathological processes tends to decrease (hypertension, hypoxia, tumor growth).

Researchers in biomedical optics use either the photon diffusion model or the Monte Carlo simulation to approach the `forward problem' of image reconstruction of the optical diffusion tomography for turbid media. Solving the photon transport equation is an alternate method to solve the `forward problem,' and might be more accurate because light propagation in turbid media is supposed to be better depicted by the photon transport equation than the other two methods. A solution to the time-dependent integro-differential equation has been found, using a hybrid (finite-difference and analytic) method. When the spatial and directional distributions of the initial light beam are given, analytical solutions to the photon transport equation are obtained for following discrete instances. This new approach has potential application to the time-resolved optical diffusion tomography as a more accurate solution to the `forward problem.'

The spatially resolved transmission through porcine liver is measured with a CCD-Camera. A solution of the diffusion equation is derived to investigate whether the optical coefficients can be deduced from this experimental geometry. The inverse problem is solved with a nonlinear regression program using Monte Carlo simulations as solutions of the forward problem. The experiment is tested with latex-spheres, which are described by Mie-Theory.

Time-resolved transillumination experiments on turbid media have been performed using a streak camera with 6 ps time resolution and a mode-locked Ti:sapphire laser with a pulse width of 160 fs. One dimensional and two dimensional images have been recorded. The resolution of the images is related to the optical properties of the media as obtained from experimental data taken with the double-integrating-sphere method, and from fitting the time- resolved pulse profiles to a diffusion model.

The absence of satisfactory criteria for discrete model parameters choice during computer modeling of thermal processes of laser-biotissue interaction may be the premier sign for the accuracy of the numerical results obtained. The approach realizing the new concept of direct automatical adaptive grid construction is suggested. The intellectual program provides high calculation accuracy and is simple in practical usage so that a physician receives the ability to prescribe treatment without any assistance of a specialist in mathematical modeling.

Heat and stress generation in water are investigated after irradiation with single, short pulses (5 ns duration) from an optical parametric oscillator (OPO), using piezoelectric stress detection, an optical pump-probe technique and laser-flash photography. The wavelength dependence of these effects is studied in the near infrared by choosing some wavelengths at the absorption peak between 1900 nm and 2050 nm from the wide tuning range of the OPO (400 nm - 2500 nm). Time-resolved temperature profiles show the relatively slow decay of the heat generated by absorption of a pulse, leading to the possibility of heat accumulation by successive pulses. Relaxation of the thermoelastic stress near a free surface causes cavitation and liquid ejection. This effect can be intensified by vaporization. The observation that the ablative effects of heat and stress are maximized at the absorption peak demonstrates the applicability of the OPO for optimized tissue ablation.

Mechanical effects induced by laser pulses with nanosecond to microsecond duration are studied on an optical fiber as a model target. The strength of the mechanical effects is documented by monitoring the stresses induced on the target fiber and the pressure transients detected in its direct environment. Pressure transients and stresses are detected during the laser pulse and some hundreds of microseconds later at the collapse of the resulting cavitation bubble. For microsecond pulse duration, the largest induced stress is observed at the cavitation bubble collapse. For nanosecond duration already a very large stress is observed during the laser pulse, followed by a second large stress. A continuous transition between the two regimes is observed for intermediate pulse duration, as also confirmed by the pressure measurements.

The dynamics of the water absorption peak around 1.94 micrometers was examined. This peak is important for the absorption of holmium and thulium laser radiation. To examine the effect of temperature on the absorption coefficient, the transmission of light through water of 22, 49 and 70 degree(s)C was measured from 1850 -2150 nm with a spectrophotometer. It was found that the absorption peak at 1.94 micrometers (at 22 degree(s)C) shifts to shorter wavelengths with increasing temperatures, to 1.92 micrometers at 70 degree(s)C. It was also seen that the absorption peak increases slightly with increasing temperature. It is shown in this paper that the dynamics of the absorption coefficient has a significant influence on the expected zone of damage and ablation parameters in the 2 micrometers wavelength range.

The coupling of proteins to a chromophore allows us, in principle, to achieve a high temperature for a short time in a highly confined microvolume. On the one hand this can be used for the study of very rapid thermal denaturation, which may happen in microseconds. On the other hand it is possible to produce very precise damage to special cellular and subcellular structures by heating a microscopic volume. Preliminary optoacoustic experiments were conducted to explore the possibility of detecting denaturation of proteins in such conjugates by their volume change, which is associated with the change of the tertiary structure. As expected, the melanin granules proved to be efficient photon-to-heat converters. Nevertheless, the volume change due to protein denaturation may be too small compared to the thermal expansion of water to allow quantitative evaluations.

To study possible preferential light penetration along the axis of white matter tracts in bovine brain, red laser light was interstitially delivered and detected by isotropic fiber tips, both parallel and perpendicular to the nerve fibers. A significant difference of 0.11 mm_-1 was found between the mean effective attenuation coefficients of both directions (p < 0.02).

Individual blood vessels in the chick chorioallantoic membrane (CAM) were selectively coagulated through photothermolysis, using pulsed laser irradiation at 585 nm. Pulse durations were chosen to be 0.45 ms and 10 ms, which correspond to the thermal relaxation times in blood vessels of 30 micrometers and 150 micrometers diameter, respectively. The dose vs diameter (D vs d) relationship for coagulation was calculated for the two pulse shapes. The energy deposited in a cylindrical absorber of diameter d by an optical field, incident perpendicular to the vessel, was expressed analytically and compared with the energy required to coagulate a blood vessel of the same lumen diameter. When thermal diffusion is incorporated into the model, our findings can be accounted for quantitatively. This information will be of use for improving the laser treatment of port wine stains and other vasculopathies.

Photothermal tomography (PTT) is applied to characterize diameter and depth of subcutaneous chromophores such as blood vessels that comprise port wine stain (PWS) birthmarks. PTT uses a fast infrared detector array to measure temperature rises in a PWS induced by pulsed laser radiation. A PTT record of PWS in response to pulsed laser exposure is composed of a sequence of infrared emission frames, each consisting of elevated temperature regions indicative of subcutaneous blood vessel heating. An analytic expression for recorded infrared emission frames is derived as a convolution integral of a PTT point spread function and the 3- D temperature distribution in the PWS immediately following laser exposure. Diameters of blood vessels comprising the PWS are best resolved in early infrared emission frames when radial heat diffusion is relatively small.

Light attenuation in a sample of human brain has been measured using a small (0.5 mm in diameter) fiber isotropic detector and similar to that an isotropic irradiator. Two lasers, He-Ne (633 nm) and GaAs (890 nm), were used. In vivo measurements were performed with rabbit brain. Penetration depth of 633 nm light was 1.3 mm and 2.4 mm respectively for human and rabbit brain. Propagation of 890 nm light was significantly higher, penetration depth was 2.8 mm for human tissue and 5.3 for rabbit.

This study compared the laser tissue effect of the Dornier FiberTome^TM system, the conventional Nd:YAG and the KTP laser on the stomach and liver. The cutting capabilities, thermal effects of the laser systems, as well as their dependence on power output and tissue type, were evaluated. A motorized fiber holder to maintain constant conditions (cutting speed fiber angle, and pressure) was developed. The results show that the cutting depth and cutting width are inversely proportional to the output level. The thermal damage remained constant as the output increased. Use of the FiberTome^TM system appears to be more advantageous than the conventional Nd:YAG laser in pigmented tissues, in the range of the parameters tested.

Since the start of Laser Medicine in the early 60's, there has been an increased interest in its application, throughout the world. Pelt's well4cnown expression when referring to laser: "a solution when looking for a problem" is as if he was revolted during research, as if he set to work feverishly in the search for practical application of characteristic laser effects. Due to laser characteristics, such as the brilliance, parallelism and monochromaticity, extensive practical medical expressions have found their place in therapy and diagnosis, which in the specialties of ophthalmology and neurosurgery, medical laser can be considered an essential instrument of great esteem.

June 1991 a new European initiative for the improvement of safety technology of medical laser application has been started. From the beginning different research groups in Germany, Italy, Spain, Portugal and Russia tried to raise the funds needed to carry out the proposed research activities which cover various fields of laser medicine. The approved research program of this European project focus particularly on all important fields in which research activities can improve the safety level of medical laser applications.

Various tissue samples have been irradiated with lasers generally used for surgical laser applications. Laser generated fumes were collected on charcoal tubes and chemical analysis of the pyrolytic products was performed by means of gaschromatographic and mass spectrometric (GC/MS) methods. First experimental results show clearly distinguishable features in the component spectra from various combinations of tissues and lasers or laser parameters, respectively. A method of standardization and calibration is presented and preliminary estimations of emission concentrations of particular substances are given.

This paper demonstrates the development of a minimized burn chamber for the in-vitro examination of laser plumes. This chamber resembles in its dimensions a human throat. The total volume is ca. 35 ml including the outlet to the collecting device. With this volume, a gas flow of 2 1/min results in a complete volume exchange in less than one second. First results of a gaschromatographic separation of the reaction products and their mass spectrometric analysis are presented.

Gas chromatography (GC), mass spectrometry (MS), GC/MS technique and standardized methods have been used to investigate harmful chemicals produced during laser treatment of tissue and to find relations between the emitted molecules and interaction processes. It has been found that the emission of carbon monoxide is strongly connected with carbonization. Hydrocyanic acid is well suited to get information on processes, characterizing laser tissue interaction, e.g., plasma behavior. From results obtained by variation of the reaction gas atmosphere (helium, nitrogen oxygen, air) it becomes clear that two processes must be taken into account, oxidation (combustion) and the preferred formation of compounds characterized by the cyano group. It is possible to reduce the amount of emitted hydrocyanic acid drastically working in an oxygen atmosphere. The typical substances arising during laser tissue interaction were compared with the emission obtained by laser treatment of fiber-reinforced plastics, pointing out that the high temperature region (plasma torch) is an important source of the emitted substances.

The first approximation in the evaluation of potential health hazards has been carried out for particles in laser smoke aerosols. The amount of emitted aerosol mass per second has been measured for typical lasers and application parameters. Combined with investigations of the flow distribution in operating rooms for two different types of ventilation, the estimated concentration values as a function of time during and after vaporization of tissue has been calculated and measured at certain points in the OP. Due to turbulence in the flow distribution high concentrations have been determined. Neglecting all potential toxic effects by the gaseous components or adsorbed vapors and liquids at the particle surface and regarding the aerosols as dust/air mixtures only the determined concentrations are of the same order of magnitude as typical threshold limit values for workplace pollution with inert dusts.

Information on particle size distribution and concentration of plume aerosols produced by laser treatment and electrosurgery is required to evaluate the potential health hazards of the surgical team and the patient during treatment of tissue. This paper describes the properties of aerosols in laser and electrosurgery plume. Experiments with different in-vitro conditions and laser parameters have been performed. The investigations were carried out by use of CO₂- and Nd:YAG-Laser with different power densities, beam diameters and tissue types. In addition in the preliminary experiments with 308 nm Excimer laser radiation quite different results have been obtained. The aerosols produced during laser treatment have been compared to those produced during electrosurgery.

Volatile organic compound (VOCs) emissions from laser surgery and electrosurgery were evaluated using a multisorbent sampler and capillary gas chromatograph system (GC). The experiments were carried out by vaporization of different tissues (liver, muscle, skin) under standardized conditions by use of CO₂ and Nd:YAG lasers. As comparison the VOCs produced with an electrocauter surgical unit were investigated. With the use of a described sample and analyzer train a mixture of about 150 VOCs has been obtained. Up to now 15 substances have been identified as non-aromatic aldehydes, nitrils and aromatic hydrocarbons. The studies focus on quantitative results for these identified components. By use of the Toluene production the dependence of the emitted concentration on parameters as wavelength, power density, total energy applied, and tissue type can be shown.

Medical laser treatment enlarged its application in recent years in an explosive way. By a given distance to the patient the laser surgeon can cut, coagulate, or evaporate human tissue in a very distinct manner. Due to the mainly thermal interaction of the laser light with the irradiated tissue it may be heated up to pyrolysis conditions. By pyrolysis of human tissue degradation products are generated, which may be harmful. Chemical substances and particles formed of tissue could be toxic, cancerogenic or irritant to skin and airways or after uptake. Special hazards of human laser plume in the health care environment may result from infectious viruses, bacterias, parasites, spread tumor cells and DNA fragments.

The collecting efficiency of surgical smoke evacuators depends strongly on the design of the suction nozzle. In order to study the flow conditions at the nozzle entrance we developed methods to visualize the flow in the surrounding area. The experimental set-up for visualization and recording of the streamlines is described. Flow patterns recorded on a video recorder allow us to estimate the efficiency of a particular nozzle design under various experimental conditions. The turn-over from laminar flow to turbulent flow due to the laser generated fume can be studied in detail.

Time resolved laser spectroscopic measurements have been used to investigate elementary processes responsible for the emitted substances during laser tissue interaction. The results confirm the data obtained with classical analytical methods. CN, OH, CH, and CH₂ radicals were detected during the laser treatment of tissue. It has been found that the original molecules of tissue were reduced into basic compounds from which the emission products were formed. The relative concentration of the radicals depends on the surrounding gas atmosphere (O₂, N₂, He). In the presence of oxygen oxidation processes arise in rivalry with the formation of CN radicals and by this lower the concentration of stable cyano compounds.