Light and temperature distribution in laser irradiated tissue: The influence of anisotropic scattering and refractive index

The rigorous method of discrete ordinates was used to evaluate the effects of anisotropic scattering and optical discontinuity at the boundaries on light and temperature distribution in tissue. The influence of optical parameters of tissue on its thermal response was examined by using a finite element solution of the heat conduction equation. Calculations were performed for wide ranges of scattering albedo, the anisotropy factor, as well as interface reflectivities. This study shows that the presence of an optical discontinuity due to an air-tissue interface forces the maximum peak intensity to move from subsurface to the surface for tissue with high scattering albedo, which leads to a higher fluence rate in the near surface region. Temperature field calculations show a higher subsurface temperature for a highly scattering medium during tissue coagulation. Neglecting the anisotropic properties of tissue as well as the optical discontinuity at the boundaries would result in considerable error in the calculated temperature rises. Additionally the accuracy of the photon diffusion theory for predicting light and temperature distribution near the tissue surface is examined.