Simulation Of The Internal Temperature Distribution In Biological Tissue Under Laser Irradiation

The lasers appear opened up a new way of life science research for clinical diagnosis and therapy, and provides a new means. The interaction between laser and tissue has become very important research topics. A new subject called'tissue optics'has thus been motivated. The laser techniques have been widely applied. However, the theory research, especially on how to set the light irradiation exposal dose, has been far behind the clinic applications. In practice, the light irradiation exposal dose is set blindly and tentatively in a sense which usually leads to the problems of security and validity. Therefore the studies of laser and biological organization interactions will become very important.This paper works out the temperature distribution in tissues under the laser irradiation, by using analytic method and numerical method, and comparing the existing experimental results to testify the reliability of the research results. Provide the theory basis for the safety and efficacy of laser application.Firstly, the basic theory work, including the interaction between the light and biology tissues, the light propagation and the heat transmission in tissues, are introduced. Solution conditions of heat transfer equation were summarized entirely, which include tissue optical properties, tissue thermal properties, blood perfusion rate, boundary conditions and initial conditions.Secondly, by the physical and mathematical method, according to the limited tissue laser heat conduction of the actual situation, solution for biological heat transfer equation, in the initial condition and convection heat transfer boundary conditions. The one dimensional temperature analytical expression is obtained. Without considering the convection heat transfer boundary conditions, the simplified solution of biological heat conduction equation is worked out.Then the bio-heat transfer equation was solved numerically. The Finite Element Method (FEM) realized by Matlab is used to solve the bio–heat transfer nonlinear Partial Differential Equation (PDE).The tissue temperature distribution was calculated. For the convenience of the contrast, choose the same organization and laser parameters as experiment, numerically simulate the model of Nd: YAG laser irradiating in vivo murine skin. The simulation results show that parallel beam irradiate skin, the surface organization absorb more heat. If the laser irradiation time is short, there is a maximum temperature near the illuminate point center, along the radial distribution, temperature rise in waveform , with the increase of radial distance, the temperature wave amplitude decrease gradually, with the increase of irradiation time, the radial temperature wave amplitude increase; Along the axial distribution, temperature increases rapidly with the irradiation time increased, but the temperature distribution of deep tissue under the illuminate point center ( z > 0.2cm) compared with the steady-state temperature distribution are nearly identical, it is not affected by laser irradiation. At the same time and the different laser power, temperature distributions rule unchanged. With the increase of laser power, the radial temperature wave amplitude increase. The results of simulation and analytical solution is basic anastomotic with the experiment. In a certain extent, it explains the rationality of theory and numerical simulation. On this basis, the thesis simulates the model of Nd:YAG laser irradiate liver tissue. Results showed that the blood perfusion rate is a factor that can not be neglected.Results show that the blood perfusion rate of real living tissue and air convection boundary conditions is considered, it is concluded that the rule of tissue temperature change depends not just on the incident laser intensity and organization of optical parameters, but on the thermal physical parameters organization; The highest temperature irradiated by laser is on the center point of the surface tissue, and decreases in the radial and axial direction, the temperature degrade more seriously in axial than that in radial.