In this study we analyze the limits of the diffusion approximation to the Boltzmann transport equation for photon propagation in the human brain. Two dimensional slices through the head are obtained with the method of magnetic resonance imaging (MRI). Based on these images we assign optical properties to different regions of the brain. A finite–difference transport/diffusion code is then used to calculate the fluence throughout the head. Differences between diffusion and transport calculations occur especially in void–like spaces and regions where the absorption coefficient is comparable to the reduced scattering coefficient.