Analytical simulation of thermal processes under laser light interaction with multilayered biological tissue

An analytical procedure to study linear thermal processes in multilayered biological tissues irradiated by a laser beam is proposed. The procedure is based on the simplified representation of the heat source function after the irradiation. The Green function of both temporal and radial coordinates of the thermal problem is analytically derived in the paper. It enables one to easy simulate spatial and temporal temperature distributions in tissue by a simple convolution. Pulsed or continuous-wave laser beams with any radial structures can be treated. The validity of the approximations assumed to get the Green function is verified by comparing the simulations with published data on both light and thermal fields in tissues. Rather a good agreement is shown. Specific attention is paid to the most critical approximation, namely to the separation of radial and depth coordinates in the thermal source function. The applicability of such an approach is discussed and quantitatively checked by evaluating the temporal dynamics of radial light and heat spot spreading. The procedure is used to study thermal processes in irradiated skin layers under wide variations of optical parameters of the problem. Sample results on the heating effects of a sensitizer used for photodynamic therapy are illustrated.