LASER BEAM PROPAGATION IN BIOLOGICAL TISSUE WITH STRONG ANISOTROPY.

Recent measurements of the phase function of biological tissue such as blood and skin for visible and near-IR wavelengths indicate a highly anisotropic scattering behavior. This suggests that the photon diffusion approximation is not valid for thin layers of an anistropic medium or in the near-surface regions of optically thick tissues. Instead, a combination of small-angle scattering approximation to account for the anisotropic component of the phase function and the diffusion approximation has been used to study the off-axis propagation in biological tissues with a high degree of anisotropy. The former computes the near-axis radiance, and the latter estimates the off-axis light spatial distribution using the on-axis radiance as an external source. The results of the computation of total light intensity as a function of axial as well as radial distance for a 2-mm spot size show the existence of two regions. In the first region, proximal to the surface, scattering is confined to a narrow forward angle. This is followed by a second region of more diffuse scattering. These results indicate that for highly anisotropic scattering the combined model gives a better estimation of the spatial light distribution than each model independently.