Before image reconstruction can be attempted, a model of photon transport in tissue is required. For diffuse optical imaging, the diffusion equation, in which the propagation of light is assumed to be isotropic, is commonly used. In this section, we discuss the derivation of the diffusion equation from the radiative transport equation. Arridge (1999) provides a more detailed review. . (1) . (2) where photon density , and the diffusion coefficient .