Effects of anisotropic optical properties on photon migration in structured tissues

It is often adequate to model photon migration in human tissue in terms of isotropic diffusion or random walk models. A nearly universal assumption in earlier analyses is that anisotropic tissue optical properties are satisfactorily modelled by using a transport-corrected scattering coefficient which then allows one to use isotropic diffusion-like models. In the present paper we introduce a formalism, based on the continuous-time random walk, which explicitly allows the diffusion coefficients to differ along the three axes. The corrections necessitated by this form of anisotropy are analysed in the case of continuous-wave and time-resolved measurements and for both reflectance and transmission modes. An alternate model can be developed in terms of a continuous-time random walk in which the times between successive jumps differ along the three axes, but is not included here.