Effects of anisotropic optical parameters on the penetration of photons into a turbid medium

Abstract

There are by now many applications of methods based on near- infrared radiation (NIR) used for optical imaging and  therapeutic purposes in medical settings. Such optical techniques are appealing in not requiring potentially harmful ionizing radiation, being non-invasive, and generally being easily implementable. Since photons are randomly scattered by cell components, successful use of NIR requires knowledge of the photon trajectories expressed in statistical terminology. Until now the necessary analysis has been based on diffusion theory assuming that the scattering coefficient is an isotropic material property. We analyze the properties of the penetration depth when this assumption is violated. By penetration depth will be meant the depth attained in the turbid medium, given its ultimate emission at the planar surface at a time T , as a function of the degree of anisotropy of the scattering coefficient. Our analysis will be based on a continuous-time random walk formalism. Properties of both time-gated and continuous-wave experiments will be derived.