In this work, the finite-element method (FEM) was used to predict the temperature distribution, and the thermal damage volume in human liver tissue subjected to laser in laser-induced interstitial thermotherapy (LITT). The effect of laser power, blood perfusion, and thermal and optical properties on maximum temperature and thermal damage volume were predicted using the finite-element method. A computer program was written in visual basic language, which was verified by comparing its result with data published elsewhere. The bio-heat equation together with the effect of linear laser source were used to simulate heat transfer through tissue from which the temperature distributions, and the subsequent thermal damage, were obtained based on Arrhenius equation. In this mathematical model for LITT, it was found that increasing laser power, absorption, and scattering coefficient increased the damage zone while increasing tissue water content, perfusion rate, and tissue anisotropy factor decreased the damage zone. These findings are important aspects for doctors in the pre-estimation of the damage zone before starting the therapy so as to kill only the desired zone.

In this work, the finite element analysis has been used to predict the temperature distribution in Nd: YAG laser rod; double end-pumped by two methods Gaussian or top hat beam. The rod is cooled by water passing through annular, which surrounds the active media. The temperature distribution has been used to predict numerically, the nodal displacements, strain and stress based on the principle of virtual work. The main task is to determine the temperature distribution in Nd: YAG laser rod, the subsequent value and location of maximum tensile hoop stress associated with the two types of the double end pumping for different absorption power. Some conclusions are obtained; as the radius pumping ratio increases the location of maximum hoop stress will move toward the periphery and vice-versa. Small reduction is observed in the location of maximum hoop stress when pumping method change from the top-hat beam to Gaussian beam, especially at low radius pumping ratio and high absorption power. Top hat beam end pumping will cause more intense tension hoop stress at the facets of the rod than that of Gaussian beam even the later may produce high center temperature. This work may be important for designer while choosing the type of pumping, maximum produced tensile hoop stress and its location, especially when hoop stress is ultimate.

The analytical solution of transient temperature distribution and Tresca failure stress in cw-end-pumped laser rod has been derived using integral transform method. The analytical result is compared with numerical solutions presented by other works and good agreement has been found. Analytical solution with its clear physical meaning and its explicit form permits to predict the influence of various factors on the solution. The optical path difference which gives a valuable means to quantify the optical properties of laser material such as designed beam quality, will converge to a constant value as steady-state temperature distribution is reached. One can obtain the dominate factors which affect the laser response to bring the laser rod to the thermal equilibrium; it has been found that fast response can be achieved by reducing pumping power, increasing extracted heat from the rod, choosing a crystal having high thermal diffusivity and decreasing laser rod radius while its volume remains constant. One final advantage of the analytical solution is that a fast result can be obtained where the numerical solution usually is a time consuming technique.

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