In optical tomography the high degree of scattering associated with human tissue leads to the diffusion approximation being a good model for the transport of photons through this medium.
However, there are situations when the diffusion approximation breaks down and hence it is necessary to examine ways to improve the model. Indeed it is well known that the diffusion approximation is inaccurate when the medium contains low scattering regions as well as in the vicinity of sources, especially if they are directed.
In these regions it is possible to keep the higher order terms associated with a Pn formulation of the Boltzmann equation. It will be shown that higher order corrections to the diffusion approximation decay rapidly in high scattering regions but cannot be neglected near low scattering regions or sources.
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