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A two-layer model of laser interaction with skin: A photothermal effect analysis
Abstract
In order to understand the photothermal effect mechanism of laser interaction with skin, we employed a two-layer model to describe the heat generation, transportation, and dispersion in the procedure of laser interaction with skin. A skin temperature distribution corresponding to the laser interaction direction is calculated to describe the time of skin gasification and the possible thermal injury. The magnitude of time is ms. This basic process provides a possible quantitative recognition of the applications of laser in clinical skin care.
... Several mathematical models have been developed for predicting thermal response when tissue is irradiated by lasers [1][2][3]. During the last decade, histological examinations of cartilage and skull tissue using mid-infrared lasers were studied for potential surgical applications [4,5]. ...
... where ' ' r q is the heat flow per unit time and per unit area for the heat loss by radiation, ε is the material emissivity and σ is the Stefan-Boltzmann constant, which is equal to 5.67 x 10 −8 W/ (m 2 .K 4 ). Note from Eq. (2), that the radiation loss depends on the fourth power of the temperature, which means that this mode of heat transfer is very important as temperature increases. Some experimental data will be fitted to an expression based on this mode of heat transfer later in the experimental section and it will be shown that this mode of heat transfer is more important as bones start cooling. ...
The rate of cooling of domesticated pig bones is investigated within the temperature range of 20°C-320°C. Within the afore-mentioned temperature range, it was found that different behaviors in the rate of cooling were taking place. For bones reaching a temperature within the lower temperature range of 20°C-50°C, it was found that the rate of cooling is mostly governed by the empirical Newton’s law of cooling. It is also shown that a transition is taking place somewhere within 50°C-100°C, where both the heat conduction equation and Newton’s law apply. As bones can be raised at a fairly high temperature before burning, it was found that the rate of cooling within the range 125°C-320°C is mostly behaving according to the heat conduction equation and Stefan-Boltzmann radiation law. A pulsed CO2 laser was used to heat the bones up to a given temperature and the change of temperature as a function of time was recorded by non-contact infrared thermometer during the cooling period.
... It should be noted that although we have considered a four-layer structure in TMM to compute the light intensity distribution inside skin tissue, however, we have considered a one-layer structure for thermal analysis of the issue. By doing this, we neglect to spend massive calculations that is out of the aim of this work (interesting reader would be inferred to [26][27][28][29][30]. By exerting the value of Q r into Equation (18) and applying boundary conditions with respect to the adiabatic heating condition, the temperature profile in the skin tissue has been calculated at any depth with time variation. ...
This work is concerned with the investigation of the photo-thermal response of the human skin tissue. A four-layer, one-dimensional air–skin tissue structure has been considered. The considered structure is irradiated by the different light intensities and wavelengths. Light propagation in the skin tissue has been modeled by using a full electromagnetic method. To solve electromagnetic equations numerically, Transfer Matrix Method (TMM) has been used. Inhomogeneity of the multilayer skin tissue structure has been considered by the wavelength dependence of the Complex Optical Refractive Index (CORI). In order to determine the temporal and spatial temperature distribution inside the skin tissue, the TMM has been coupled with Pennes’ bio-heat transfer equation. The equation has been solved semi-analytically with the adiabatic condition by using the Green’s function method. Based on the preliminary findings of this study, the effect of the light intensity and wavelength on the resultant temperature distribution inside the skin tissue structure has been reported. Obtained results could be helpful for designing and improving the practical protocols through the skin photo-thermal therapy process.
... When the skin temperature exceeds 100 °C, the tissue will be gasified and carbonised. Rapid gasification can aid tissue cutting [8][9][10][11][12][13] . Generally, when the skin temperature exceeds its threshold, the thermally unstable molecular crosslinks in skin collagen gradually break down, resulting in a macro-scale thermal shrinkage, a process known as thermal denaturation [14] . ...
This study aimed to find a parameter that can accurately characterise the thermal damage of skin tissues during laser welding. Combining this parameter with an experiment, we proposed a method to quantify this parameter. A model between laser processing parameters and thermal damage accumulation parameter of skin tissue in vitro was established based on full-factor experiments. The statistical analysis software Minitab was used to design the full-factor experiment. The laser power, spot moving speed, and pulse frequency were taken as independent variables, and the thermal damage parameter of the skin tissue in vitro were taken as dependent variables. A direct correlation model between the thermal damage parameter and the three influencing factors was established, and the laser processing parameters were optimised and verified by experiments. For a laser power of 2.5 W, a spot moving speed of 150 mm s⁻¹, and a pulse frequency of 150 Hz, no irreversible thermal damage occurred in the skin tissue in vitro, and the tensile strength met the requirements. The significant factors influencing the thermal damage of skin tissue in vitro were obtained by conducting a variance analysis, and the related mechanism was analysed.
... This problem has hindered further development of PDT in widespread clinical applications. In addition, skin photosensitivity and photothermal injury are also common concerns from the patients receiving PDT treatment [43][44][45]. For the light penetration issue to be overcome, considerable research has focused on the development of photosensitizers in the near infrared (NIR) range [46]. ...
Indocyanine green (ICG) has been exploited as a photosensitizer for use in cancerous phototherapy including breast, brain, and skin tumors. Although ICG is of particular advantage for use in cancer phototherapy, it adversely tends to disintegrate in aqueous medium and such degradation can be markedly accelerated by light irradiation (photodegradation) and/or heating (thermal degradation). Furthermore, ICG after administered intravenously will be readily bound with blood proteins and hence leads to only 2 ± 4 min of plasmatic half-life. Among various pharmaceutical polymers, poly (lactic-co-glycolic acid) (PLGA) is the copolymer of poly (lactic acid) and poly (glycolic acid) and is one of the best defined biomaterials with FDA approval for drug encapsulation due to its biocompatibility, biodegradability, and controllability for drug release. This review shows that novel applications of PLGA polymers as nanocarrier for ICG to improve of physicochemical properties of ICG such as Half-life.
... It was found that when the heating spot became equal to the thickness of the cylinder, the numerical data agreed very well with the analytical results. A study on the photothermal mechanism of laser-skin interaction has been presented by Guan et al. (2011). Ozen et al. (2011) applied the TWMBT model and the Pennes equation to predict the burn injury of skin tissue exposed to microwaves. ...
The importance of microscale thermal energy transport has dramatically increased in the past several years and a number of significant investigations have been undertaken to better understand the fundamental phenomena that govern the behavior of energy transport at the microscale. While the original focus of this research was directed towards the thermal management of semiconductor devices and spacecraft thermal control, there has been an increased interest in the thermophysical phenomena occurring in biological systems, and bio-medical devices and applications.As a result a number of investigations have been reported, in which the thermal transport phenomena in microscale systems and passages are of significant importance.In this context, this following attempts to review the relevant literature on the theoretical and experimental investigations of microscale transport phenomena as specifically related to bioengineering and biomedical applications. Modeling methodologies, experimental studies, instrumentation techniques and research leading to the development of optimal bio-medical systems are reviewed and discussed.
... The initial temperature of the entire tissue is uniform, denoted by T 0 ¼ 37 C [29,[40][41][42]. Moreover, the temperature T s is evaluated between 45°C and 90°C in order to promote the denaturation of tissue and for preventing evaporation of the fluids [43,42]. On the other hand, the thickness of tissue is not defined but it is deep enough in such manner that the temperature at this coordinate is maintained at a reference temperature Tðx ¼ hÞ ¼ T 0 [40,44]. ...
... A study on the photothermal mechanism of laser-skin interaction was conducted by Guan et al. [12]. They applied a twolayer model to describe the heat conduction process in skin tissue. ...
This paper develops an analytical solution for a generalized dual phase lag (DPL) model based on the nonequilibrium heat transfer in biological tissues during laser irradiation achieved by performing volume average to the local instantaneous energy equation for blood and tissue. The obtained energy equation is solved by employing the separation of variables and Duhamel's integral method for both absorbing and scattering tissues. The generalized DPL model anticipates different results than that of the predicted by the classical DPL and Pennes bioheat transfer (parabolic) models. It is found that the generalized DPL model calculates lower temperature than that of obtained by the classical DPL model due to considering heat convection between blood vessels and tissue. The obtained analytical results are also compared with existing numerical and experimental data to verify the accuracy of the analytical solution. The results indicate that the generalized DPL model reduces to Pennes bioheat transfer model when both thermal lag times are zero. It is shown that effects of blood perfusion rate, coupling factor and the lag time τT on the temperature response of tissue are similar. Furthermore, the comparison between the analytical results and existed experimental data shows that the present mathematical model is an efficient tool for evaluation of bioheat transfer in biological tissues.
... Absorption coefficient of water from the ultraviolet to far-infrared region. [16]. In the case of other sources that are absorbed within the volume, the external heat source must be estimated as suggested by some authors [17][18][19]. ...
Temperature of water-based substances is investigated by aiming a pulsed CO 2 laser beam at the water–air surface. This method of controlling temperature is believed to be flexible in medical applications as it avoids the use of thermal devices, which are often cumbersome and generate rather larger temperature swing with time. The control of temperature in this laser method is modeled by the heat conduction equation. In this investigation, it is assumed that the energy delivered by the CO 2 laser is confined within a very thin surface layer of roughly 10 μm. It is shown that the temperature can be very well controlled by a CO 2 laser at a steady temperature, and we demonstrate that the method can be adapted to work in tandem with another laser beam.
... 23 The basic premise of MC simulation is that complex interactions of particles and biological matter can be treated as a stochastic process, with simulated random movement samples from probability density functions. Even though the MC simulation of two layers 15 shows a good result compared to two-layer DE, 24 it requires a great amount of computational time. 19 ...
Modeling behavior of broadband (30 to 1000 MHz) frequency modulated near-infrared (NIR) photons through a phantom is the basis for accurate extraction of optical absorption and scattering parameters of biological turbid media. Photon dynamics in a phantom are predicted using both analytical and numerical simulation and are related to the measured insertion loss (IL) and insertion phase (IP) for a given geometry based on phantom optical parameters. Accuracy of the extracted optical parameters using finite element method (FEM) simulation is compared to baseline analytical calculations from the diffusion equation (DE) for homogenous brain phantoms. NIR spectroscopy is performed using custom-designed, broadband, free-space optical transmitter (Tx) and receiver (Rx) modules that are developed for photon migration at wavelengths of 680, 780, and 820 nm. Differential detection between two optical Rx locations separated by 0.3 cm is employed to eliminate systemic artifacts associated with interfaces of the optical Tx and Rx with the phantoms. Optical parameter extraction is achieved for four solid phantom samples using the least-square-error method in MATLAB (for DE) and COMSOL (for FEM) simulation by fitting data to measured results over broadband and narrowband frequency modulation. Confidence in numerical modeling of the photonic behavior using FEM has been established here by comparing the transmission mode's experimental results with the predictions made by DE and FEM for known commercial solid brain phantoms.
... Molina et al. [11] presented an analytical solution for the hyperbolic heat conduction model in cylindrical coordinates for the following typical samples of heat-treated biological tissues: heating of the cornea for refractive surgery, cardiac ablation for eliminating arrhythmias, and hepatic ablation for destroying tumors. A study on the photothermal mechanism of laser-skin interaction was conducted by Guan et al. [12] . They applied a twolayer model to describe the heat conduction process in skin tissue. ...
The thermal wave and the Pennes bioheat transfer models are solved analytically by employing the Laplace transform method for small and large values of reflection power (albedo) during laser irradiation. Most of the previous studies have been based on the infinite heat diffusion velocity, but non-Fourier thermal behavior has been observed experimentally in biological tissue. At low initial albedo values, the temperature in the skin depth that directly results from conduction heat transfer process is caused by the lengthy thermal re-laxation time in skin tissue. This condition generates a big difference between the thermal wave and Pennes results at the beginning of the heating process. This difference increases under short-time heating condition and high heat flux. However, with high initial albedo, the temperature distribution in the skin depth becomes negligible because of the skin absorption of laser beams. The non-Fourier effect should be considered during laser heating with low albedo, because errors in the predicted temperature values may occur.
In order to further study the mechanism of heat conduction in bio-tissues, so as to provide a solid theoretical basis for the research of related fields, this paper summarizes in detail the theoretical models used by scholars from various countries to analyze the heat conduction process in the photothermal effect of laser-bio-tissues for many years. The rationality of the theoretical model largely determines the accuracy of the response process description, the development of various models is reviewed longitudinally and the critical nodes in the development process are introduced, and the comparative analysis of the respective advantages and limitations of different models are made horizontally, focuses on the latest theoretical breakthroughs and cutting-edge applications in this field while summarizing the methods. Finally, based on the analysis of a large amount of literature, the paper refines several main problems faced in the research in this field and makes some prospects for the development direction in the future.
Laser biological tissue welding is a potential alternative technology from traditional suturing skills. In this study, we explored the influence laws of laser scanning path on in-vitro skin tissue performance and designed four different scanning paths, including heartbeat, circle, peano20-2, and toothed paths. Then the influence laws of laser scanning paths on temperature distributions, mechanical properties and microstructure characteristics were discussed based on the simple factor design of experiments. The samples under the heartbeat path reached the highest peak temperature and the highest tensile strength, but possessed sparse microstructure. The peak temperature of the samples under the circle path was close to that of the heartbeat samples but the tensile strength was only 8.667 N/cm². The temperature of the toothed samples was much lower and obviously fluctuated. The toothed samples had very bad mechanical properties, but had uniform microstructure. The Peano20-2 samples achieved the best comprehensive microstructure characteristics and tensile strength was up to 19.939 N/cm² and suffered no color deepening or scars at the macroscale.
In order to obtain the effect of laser scanning mode on the fusion result in vitro biological tissue, the experiment was designed to explore the effect of laser scanning mode on the fusion morphology, tensile strength and stability of skin incision. The results showed that the use of segmented scanning welding method can significantly improve the incision's effective absorption of the laser energy and the stability of welding quality is higher. On this basis, the reliability test was carried out with the optimized process parameters, and the tensile strength of the incision was tested. The results showed that under the condition of the process, the incision can achieve full layer fusion and no defects such as burning and carbonization. Compared with the continuous scanning laser welding process, the welding time can be reduced by 30–40%. The tensile strength of the incision after welding is up to 0.38 MPa, to meet the strength requirements. The adaptability is strong and the quality is stable.
Conventional photodynamic therapy is severely constrained by the limited light-penetration depth in tissue. Here, we show efficient photodynamic therapy (PDT) mediated by bioluminescence resonance energy transfer (BRET) that overcomes the light-penetration limitation. A photosensitizer Rose Bengal (RB) was loaded in biodegradable poly(lactic-co-glycolic acid) (PLGA) nanoparticles, which were then conjugated with firefly luciferase. Spectroscopic characterizations indicated that BRET effectively activated RB to generate reactive oxygen species (ROS). In vitro studies of the cellular cytotoxicity and photodynamic effect indicated that cancer cells were effectively destroyed by BRET-PDT treatment. In vivo studies in a tumor-bearing mouse model demonstrated that tumor growth was significantly inhibited by BRET-PDT in the absence of external light irradiation. The BRET-mediated phototherapy provides a promising approach to overcome the light-penetration limitation in photodynamic treatment of deep-seated tumors.
Photothermal effect has been proved to mediate the interaction of near-infrared laser with biological tissue. However, the generation and transformation mechanism of the photothermal effect is still unclear. In this paper, we combine a patch clamp technique with the laser simulation to figure out the chromophores, which are responsible for the photothermal effect generation. This method is based on the fact that temperature dependence of solution can be measured as resistance changes. A dual-wavelength infrared light irradiating the open pipette in extracellular solution is designed to study the relation between the photothermal effect and the absorption property of solution. The principle is based on that the nearly ten times difference in the magnitude of the optical absorption coefficient in water (0.502 cm-1 at 980 nm and 0.0378 cm-1 at 845 nm), makes the corresponding proportional absorption-driven temperature rise. The photothermal effect in laser-tissue interaction can be assessed in two stages: the establishment and the dissipation of the temperature rise. In the establishment stage, an open pipette method is employed to measure the temperature rise by fabricating a glass pipette which is filled with electrolyte solution. In the dissipation stage, the electrophysiological function of a living neuron cell is studied based on a patch clamp. Theoretical calculation and experimental results show that the optical absorption properties of solution determine the photothermal effect. The results can be used to study the photothermal effect in laser-tissue interaction.
A new theoretical model of an RC circuit is proposed for prediction problem of laser induced biological tissue temperature rise. The RC circuit system function and unit impulse response are deduced based on Kirchhoff's voltage law (KVL). Then RC circuit zero state response model is deduced from the convolution of unit impulse response and rectangle input signal. The two model constant parameters are calculated from experimental results of the laser irradiated bio-tissues temperature. Two model parameter calculation methods are proposed and simulated. Theoretical calculation and experimental results show that the temperature response curves are consistent. Relative error ranges of liver and muscle tissue peak temperature are -0.0557°C~-0.0025°C and 0.0139°C~0.0641°C respectively, and average relative error ranges of the temperature curve are 0.55%~2.39% and 0.38%~0.99% respectively. This method needs less parameters and is more precise than classical Pennes bio-heat transfer equation model, which provides a new method for laser and bio-tissues photothermal effect research.
Retinal-temperature increases produced by circularly symmetric exposures to electromagnetic radiation between 400 and 1200 nm were calculated for a wide range of image sizes. Temporal, axial, and radial temperature distributions are described. Constant and exponentially decreasing source strengths are considered. Solutions of the heat-conduction equation are expressed in terms of both retinal irradiance and the total power entering the eye. Application of solutions to the prediction of thresholds for retinal damage is discussed, and excellent agreement is noted between theoretical predictions and experimental observations of the spectral dependence of chorioretinal damage thresholds.
Reflectance measurement of breast tissue is influenced by the underlying chest wall, which is often tilted as seen by the detection probe. We develop an analytical solution of light propagation in a two-layer tissue structure with tilted interface and refractive index difference between the layers. We validate the analytical solution with Monte Carlo simulations and phantom experiments, and a good agreement is seen. The influence of varying the tilting angle of the interface on the reflectance is discussed for two types of layered structures. Further, we apply the developed analytical solution to obtain the optical properties of breast tissue and chest wall from clinical data. Inverse calculation using the developed solution applied to the data obtained from Monte Carlo simulations shows that the optical properties of both layers are obtained with higher accuracy as compared to using a simple two-layer model ignoring the interface tilt. This is expected to improve the accuracy in estimating the optical properties of breast tissue, thus enhancing the accuracy of optical tomography of breast tumors.
An approximate numerical technique for modeling optical pulse propagation through weakly scattering biological tissue is developed by solving the photon transport equation in biological tissue that includes varying refractive index and varying scattering/absorption coefficients. The proposed technique involves first tracing the ray paths defined by the refractive index profile of the medium by solving the eikonal equation using a Runge-Kutta integration algorithm. The photon transport equation is solved only along these ray paths, minimizing the overall computational burden of the resulting algorithm. The main advantage of the current algorithm is that it enables to discretise the pulse propagation space adaptively by taking optical depth into account. Therefore, computational efficiency can be increased without compromising the accuracy of the algorithm.
The general two-layer inverse problem in biomedical photon migration is to estimate the absorption and scattering coefficients of each layer as well as the top-layer thickness. We attempted to solve this problem, using experimental and simulated spatially resolved frequency-domain (FD) reflectance for optical properties typical of skin overlying muscle or skin overlying fat in the near infrared. Two forward models of light propagation were used: a two-layer diffusion solution [Appl. Opt. 37, 779 (1998)] and a hybrid Monte Carlo (MC) diffusion model [Appl. Opt. 37, 7401 (1998)]. MC-simulated FD reflectance data were fitted as relative measurements to the hybrid and the pure diffusion models. It was found that the hybrid model could determine all the optical properties of the two-layer media studied to ~5%. Also, the same accuracy could be achieved by means of fitting MC-simulated cw reflectance data as absolute measurements, but fitting them as relative ones is an ill-posed problem. In contrast, two-layer diffusion could not retrieve the top-layer optical properties as accurately for FD data and was ill-posed for both relative and absolute cw data. The hybrid and the pure diffusion models were also fitted to experimental FD reflectance measurements from two-layer tissue-simulating phantoms representative of skin-on-fat and skin-on-muscle baseline optical properties. Both the hybrid and the diffusion models could determine the optical properties of the lower layer. The hybrid model demonstrated its potential to retrieve quantitatively the transport scattering coefficient of skin (the upper layer), which was not possible with the pure diffusion model. Systematic discrepancies between model and experiment may compromise the accuracy of the deduced top-layer optical properties. Identifying and eliminating such discrepancies is critical to practical application of the method.
The known optical properties (absorption, scattering, total
attenuation, effective attenuation, and/or anisotropy coefficients) of
various biological tissues at a variety of wavelengths are reviewed. The
theoretical foundations for most experimental approaches are outlined.
Relations between Kubelka-Munk parameters and transport coefficients are
listed. The optical properties of aorta, liver, and muscle at 633 nm are
discussed in detail. An extensive bibliography is provided
Most of the laser applications in medicine and biology involve thermal effects. The laser-tissue thermal interaction has therefore received more and more attentions in recent years. However, previous works were mainly focused on the case of laser heating on normal tissues (37 °C or above). To date, little is known on the mechanisms of laser heating on the frozen biological tissues. Several latest experimental investigations have demonstrated that lasers have great potentials in tissue cryopreservation. But the lack of theoretical interpretation limits its further application in this area. The present paper proposes a numerical model for the thawing of biological tissues caused by laser irradiation. The Monte Carlo approach and the effective heat capacity method are, respectively, employed to simulate the light propagation and solid-liquid phase change heat transfer. The proposed model has four important features: (1) the tissue is considered as a nonideal material, in which phase transition occurs over a wide temperature range; (2) the solid phase, transition phase, and the liquid phase have different thermophysical properties; (3) the variations in optical properties due to phase-change are also taken into consideration; and (4) the light distribution is changing continually with the advancement of the thawing fronts. To this end, 15 thawing-front geometric configurations are presented for the Monte Carlo simulation. The least-squares parabola fitting technique is applied to approximate the shape of the thawing front. And then, a detailed algorithm of calculating the photon reflection/refraction behaviors at the thawing front is described. Finally, we develop a coupled light/heat transport solution procedure for the laser-induced thawing of frozen tissues. The proposed model is compared with three test problems and good agreement is obtained. The calculated results show that the light reflectance/transmittance at the tissue surface are continually changing with the progression of the thawing fronts and that lasers provide a new heating method superior to conventional heating through surface conduction because it can achieve a uniform volumetric heating. Parametric studies are performed to test the influences of the optical properties of tissue on the thawing process. The proposed model is rather general in nature and therefore can be applied to other nonbiological problems as long as the materials are absorbing and scattering media.
Cartilage laser thermoforming, also known as laser reshaping, is a new surgical procedure that allows in-situ treatment of deformities in the head and neck with less morbidity than traditional approaches. During laser irradiation, cartilage becomes sufficiently subtle or deformable for stretching and shaping into new stable configurations. This study describes the experimental and theoretical characterization of the thermal response of porcine cartilage to laser irradiation (Nd:YAG). The surface temperature history of cartilage specimens was monitored during heating and thermal relaxation; using laser exposure times ranging between 1 and 15 s and laser powers of 1 to 10 W. The experimental results were then used to validate a finite element model, which accounts for heat diffusion, light propagation in tissue, and heat loss due to water evaporation. The simultaneous solution of the energy and mass diffusion equations resulted in predictions of temperature distribution in cartilage that were in good agreement with experiments. The model simulations will provide insights to the relationship between the laser treatment parameters (exposure time, laser beam diameter, and power) and the onset of new molecular arrangements and cell thermal injury in the material, thus conceiving basic guidelines of laser thermoforming.
Laser heating of a two-layer system is studied and the temperature dependence of the optical absorptance of the irradiated target is considered. The Laplace integral method is used to solve the heating problem. Expressions for the temperature profiles in the thin film and the substrate are obtained. Such profiles, together with the surface temperature are computed for four two-layer systems: aluminum-glass, silver-glass, copper-glass and gold-glass. Each target is subjected at the front surface to incident laser irradiance q0 of value 1012 W m−2. The thickness of the metallic film in each system is 5 μm. The critical timetm required to initiate melting for the four systems considered is computed and compared with the case when the absorptance is constant. Lower values of tm are obtained when considering such an effect. For constant absorptance, computations revealed that tm = 0.0673,2.1312,1.8657 and 1.7440 μs while for temperature-dependent absorptance tm = 0.0453,1.1085,1.0108 and 1.0203 μs for the four elements considered respectively.
Laser heating of a two-layer system is studied using the Laplace integral transform method. Expressions for the temperature profiles in the thin film and the substrate are obtained. The front surface temperature is also obtained. As an illustrative example the critical time required to initiate melting is computed for four systems. These systems are aluminum-glass, silver-glass, copper-glass and gold-glass. The obtained profiles in these systems are represented graphically. The obtained results reveal the linear dependence of the considered profiles on the heat power absorbed at the front surface. They also depend strongly on the thermal properties of the considered materials. Computations make it possible to decide whether one laser pulse can initiate melting in the considered target or not.
Results of experimental investigations to determine threshold levels for doubled neodymium and ruby lasers are presented. The experimental animals were rhesus monkeys. Threshold levels for retinal damage caused by the doubled long-pulsed and Q-switched neodymium laser are reported, and results of experiments to determine retinal damage and lens damage caused by the Q-switched ruby laser are presented. In addition, theoretical considerations and calculations for retinal damage are discussed and compared with experimental results. (Author)
In this work small angle coherent scattering data are presented for some biological tissues. The effect of low angle diffusion on the micro-beams dose distributions are assessed by means of a dedicated version of the EGS4 code. The atomic form factor tabulations were upgraded in EGS4 to include experimental values of compounds instead of obtaining the linear differential scattering coefficient from a simple weighted sum of the elemental components. In this manner, one includes the effects induced by both the large-scale arrangement of the sample structure and the molecular interference. It is shown that the inclusion of the measured form factors and cross-sections in the photon transport calculations induce not negligible effects in the dose deposited around the microbeams. The fact of not taking into account small angle scattering cross-sections may lead to underestimating the number of scattered photons reaching zones not directly illuminated. At the center of a microbeam planar array the effect can be as much as 100% for X-ray photons of 50 keV.
Optical properties of porcine myocardium tissue in the exposure temperature range of 37-80degC were determined for 700, 750, 800, 850, 900 nm wavelengths of a Ti: sapphire laser. A neural network were trained with diffuse reflectance data from Monte Carlo simulations based on the semi-infinite tissue model, and then it was used to assess the absorption coefficient and reduced scattering coefficient from the spatially resolved relative reflectance which derived from diffuse reflectance image. The result showed that there was slight change in the absorption coefficient; the average for fresh is 0.11 mm-<sup>1</sup>, which increased to 0.17mm-<sup>1</sup> for 80degC. The average reduced scattering coefficient of the five wavelengths for fresh samples is 0.14 mm-<sup>1</sup>, which increased obviously to 0.66 mm-<sup>1</sup> for 80degC, and the average for all samples is 0.13 mm-<sup>1</sup>. The reduced scattering coefficient at the longer wavelength increased slowly compared with the shorter wavelength in the wavelength range of 700-900 nm. The average optical penetration depth of the five wavelengths for fresh samples is 3.37 mm, which reduced obviously to 1.54 mm for 80degC. In conclusion, thermal effect during photothermal treatment of tissue is an important factor altering optical properties of tissue.
We investigated the ability of mathematical models to predict temperature rises in biological tissue duringlaser irradiation by comparing calculated values with experimental measurements. Samples of normal human aorta, beef myocardium, and polyacrylamide gel were irradiated in air with an argon laser beam, while surface temperatures were monitored with an IR camera. The effects of different surface boundary conditions in the model predictions were examined and compared with the experimental data. It was observed that, before a temperature of 60 degrees C was reached, the current mathematical models were capable of predicting tissue-surface temperature rises with an accuracy of 90% for a purely absorbing medium and with an accuracy of 75% for biological tissue (a scattering medium). Above 60 degrees C, however, the models greatly overestimated temperature rises in both cases. It was concluded that the discrepancieswere mainly a result of surface water vaporization, which was not considered in current models and which was by far the most significant surface-heat-loss mechanism for laser irradiation in air. The inclusion of surface water vaporization in the mathematical models provided a much better match between predicted temperatures and experimental results.
The current safety standards for lasers operating in the 1,400- to 2,000-nanometer (nm) wavelength region are based on only a few observations at specific wavelengths. On the basis of experimental results conducted with Yorkshire pigs (Sus scrofa domestica), these standards may not accurately reflect the potential for laser injury when humans are exposed to these wavelengths. It is our belief that one of the damage mechanisms involved in these laser injuries results from energy absorption by skin pigmentation (melanin), and a more highly pigmented animal model, the Yucatan hairless minipig, may be a more suitable subject for laser exposure studies.
Skin specimens were collected from Yorkshire pigs and Yucatan minipigs for histologic examination, and the thickness of the epidermis was measured. Epidermal thickness of human skin also was determined, and a qualitative assessment of the melanin content in the epidermal layers was conducted.
Mean +/- SD thicknesses of the Yucatan minipig flank and dorsal neck epidermis were 68 +/- 34 and 68 +/- 25 microm, respectively. Thicknesses of the Yucatan minipig skin were closely comparable to the thicknesses of human epidermis from the face (68 +/- 26 microm), neck (65 +/- 24 microm) and arms (68 +/- 21 microm). The Yorkshire pig lacks substantial melanin in the epidermis, whereas the skin of the Yucatan minipig is more similar to that of humans.
On the basis of epidermal skin thickness measurements and melanin assessment, the flank and dorsal neck of the Yucatan minipig are better suited to laser injury studies than are the Yorkshire pig models of human skin.
The time-dependent temperature distributions produced within thermally homogeneous media heated by a moving laser beam with Gaussian and uniform power density profiles are examined using a time-domain method based on Green's functions. Regions of finite length, width, and depth within the medium having exponential power absorption are considered. The temperature distribution is written as a single integral with respect to time of simple functions and the resulting expressions have been used to model the heating of blood vessels for birthmark (port-wine stain) removal. The temperature distributions obtained are in good agreement with those produced using Monte Carlo optical and finite difference thermal models.
Several closed form analytical solutions to the bioheat transfer problems with space or transient heating on skin surface or inside biological bodies were obtained using Green's function method. The solutions were applied to study several selected typical bioheat transfer processes, which are often encountered in cancer hyperthermia, laser surgery, thermal comfort analysis, and tissue thermal parameter estimation. Thus a straightforward way to quantitatively interpret the temperature behavior of living tissues subject to constant, sinusoidal, step, point or stochastic heatings etc. both in volume and on boundary were established. Further solution to the three-dimensional bioheat transfer problems was also given to illustrate the versatility of the present method. Implementations of this study to the practical problems were addressed.
We study light propagation in biological tissue containing an absorbing obstacle. In particular, we solve the infinite-domain problem in which an absorbing plate of negligible thickness prevents a portion of the light from the source from reaching the detector plane. Inasmuch as scattering in the medium is sharply peaked in the forward direction, we replace the governing radiative transport equation with the Fokker-Planck equation. The problem is solved first by application of the Kirchhoff approximation to determine the secondary source distribution over the surface of the plate. That result is propagated to the detector plane by use of Green's function. The Green's function is given as an expansion of plane-wave modes that are calculated numerically. The radiance is shown to obey Babinet's principle. Results from numerical computations that demonstrate this theory are shown.
A two-layer model using different properties for the pathological tissue and the normal tissue was developed to describe the spatial photon, temperature and thermal damage distributions during laser-induced interstitial thermo-therapy (LITT). The photon distribution was simulated using the Monte Carlo method. The optical tissue parameters and the blood perfusion were derived based on the Arrhenius rate process formulation of thermal damage and kinetics of vasodilatation. The corresponding temperature distribution was numerically calculated using the Pennes bio-heat equation. The calculated results showed that the two-layer model predicted different results on the temperature variation and distribution, the thermal damage distribution and the thermal damage volume etc. from the one-layer model. As a more reasonable physical model, the two-layer model can be used to optimize the therapeutic parameters for improved LITT treatments.
Dendritic cell-like cells (Mo-DCs) generated from peripheral blood monocytes with interleukin-4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM-CSF) have been used as tools to treat cancer patients (DC-vaccines). Because Mo-DCs have multiple antigen presentation-related functions, including phagocytosis, migration, cytokine production, and T cell stimulation, establishment of a method for simultaneously evaluating the various functions of Mo-DCs is important. We developed a new in vitro three-dimensional two-layer collagen matrix culture model that consists of a collagen gel containing Mo-DCs as the lower layer and a collagen gel containing necrotic GCTM-1 tumor cells and/or T cells as the upper layer. We used this system to observe simultaneously multiple functions of Mo-DCs by phase-contrast or fluorescence microscopy and to assess IL-12 secretion during more than 2 weeks of culture. We also observed interactions between Mo-DCs and necrotic GCTM-1 or T cells on an individual cell basis by time-lapse videomicroscopy. In addition, we collected Mo-DCs from the collagen gels by collagenase treatment and analyzed the expression of antigen presentation-related molecules such as HLA-DR, CD80, CD83, and CD86 on Mo-DCs. This model may be a useful tool for evaluation of the various functions of Mo-DCs used as DC vaccines and for studies of the complex behaviors of Mo-DCs in vivo.
We study theoretically light backscattered by tissues using the radiative transport equation. In particular we consider a two-layered medium in which a finite slab is situated on top of a half space. We solve the one-dimensional problem in which a plane wave is incident normally on the top layer and is the only source of light. The solution to this problem is obtained formally by imposing continuity between the solutions for the upper and lower layers. However, we are interested solely in probing the top layer. Assuming that the optical properties in the lower layer are known, we remove it from the problem yielding a finite slab problem by prescribing an alternate boundary condition. This boundary condition is derived using the theory of Green's functions and is exact. Hence, one needs only to solve the transport equation in a finite slab using this alternate boundary condition. We derive an asymptotic solution for the case when the slab is optically thin. We extend these results to the three-dimensional problem using Fourier transforms. These results are validated by comparisons with numerical solutions for the entire two-layered problem.
A two-dimensional model was developed to model the effects of dynamic changes in the physical properties on tissue temperature and damage to simulate laser-induced interstitial thermotherapy (LITT) treatment procedures with temperature monitoring. A modified Monte Carlo method was used to simulate photon transport in the tissue in the non-uniform optical property field with the finite volume method used to solve the Pennes bioheat equation to calculate the temperature distribution and the Arrhenius equation used to predict the thermal damage extent. The laser light transport and the heat transfer as well as the damage accumulation were calculated iteratively at each time step. The influences of different laser sources, different applicator sizes, and different irradiation modes on the final damage volume were analyzed to optimize the LITT treatment. The numerical results showed that damage volume was the smallest for the 1,064-nm laser, with much larger, similar damage volumes for the 980- and 850-nm lasers at normal blood perfusion rates. The damage volume was the largest for the 1,064-nm laser with significantly smaller, similar damage volumes for the 980- and 850-nm lasers with temporally interrupted blood perfusion. The numerical results also showed that the variations in applicator sizes, laser powers, heating durations and temperature monitoring ranges significantly affected the shapes and sizes of the thermal damage zones. The shapes and sizes of the thermal damage zones can be optimized by selecting different applicator sizes, laser powers, heating duration times, temperature monitoring ranges, etc.
An optical-thermal-damage model of the skin under laser irradiation is developed by using finite-element modeling software (FEMLAB 3.1, Comsol, Incorporated, Burlington, Massachusetts). The general model simulates light propagation, heat generation, transient temperature response, and thermal damage produced by a radically symmetric laser beam of normal incidence. Predictions from the model are made of transient surface temperatures and the thermal damage on a pigskin surface generated by 2000-nm laser irradiation, and these predictions are compared to experimental measurements. The comparisons validate the model predictions, boundary conditions, and optical, thermal, and rate process parameters. The model enables the authors to verify the suitability of the American National Standards Institute (ANSI) maximum permissible exposure (MPE) standard for a wavelength of 2000 nm with exposure duration from 0.1 to 1 s and 3.5-mm beam diameter. Compared with the ANSI MPE standard, however, the MPE values predicted by the model are higher for exposure durations less than 0.1 s. The model indicates that it may be necessary to modify the ANSI MPE standard for cases in which the laser-beam diameter is larger than 3.5 mm when a "safety factor" of ten is used. A histopathological analysis of the skin damage is performed to determine the mechanisms of laser-induced damage in the skin.
Light and heat distributions are measured in a rat glioma model used in photodynamic therapy. A fiber delivering 632-nm light is fixed in the brain of anesthetized BDIX rats. Fluence rates are measured using calibrated isotropic probes that are positioned stereotactically. Mathematical models are then used to derive tissue optical properties, enabling calculation of fluence rate distributions for general tumor and light application geometries. The fluence rates in tumor-free brains agree well with the models based on diffusion theory and Monte Carlo simulation. In both cases, the best fit is found for absorption and reduced scattering coefficients of 0.57 and 28 cm(-1), respectively. In brains with implanted BT(4)C tumors, a discrepancy between diffusion and Monte Carlo-derived two-layer models is noted. Both models suggest that tumor tissue has higher absorption and less scattering than normal brain. Temperatures are measured by inserting thermocouples directly into tumor-free brains. A model based on diffusion theory and the bioheat equation is found to be in good agreement with the experimental data and predict a thermal penetration depth of 0.60 cm in normal rat brain. The predicted parameters can be used to estimate the fluences, fluence rates, and temperatures achieved during photodynamic therapy.
The paper presents problems connected with thermal radiation of
human bodies in the microwave range in respect of diagnosis of breast
carcinoma. A mathematical model of transmission of thermal radiation
through tissues is introduced and methods of measurement of temperature,
depth and size of heat source, by means of multifrequency microwave
thermography, are described. Theoretical considerations are supplemented
with presentation of the results of experiments
Thermal model of laser-induced eye damage. Engineering Mechanics Division
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15s Temperature increase /K Fig. 4. When t ¼ 0:05, 0.1, 0.15 s, the temperature increase of the cubital fossa
- Guan
t = 0.05s 2. t = 0.1s 3. t = 0.15s Temperature increase /K Fig. 4. When t ¼ 0:05, 0.1, 0.15 s, the temperature increase of the cubital fossa. K.-W. Guan et al. / Optics & Laser Technology 43 (2011) 425–429
Modeling the thermal response of porcine cartilage to laser irradiation. In: Laser-tissue interaction XIII: photochemical, photothermal, and photomechanical
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- G Aguilar
- R Basu
- Ej Lavernia
- Wong
Diaz-Valdes SH, Aguilar G, Basu R, Lavernia EJ, Wong BJ. Modeling the thermal response of porcine cartilage to laser irradiation. In: Laser-tissue interaction XIII: photochemical, photothermal, and photomechanical, 2002. p. 47–56.
Ocular laser threshold investigations
- Christian Hc Dedrick
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Vassiliadis A, Christian HC, Dedrick KG. Ocular laser threshold investigations. USAF School of Aerospace Medicine, Stanford Research Institute, Brooks Air Force Base, TX, 1971.
Engineering Mechanics Division, IIT Research Institute, 10 West 35th
- A N Takata