1. A diffusion data processing apparatus comprising: a fiber contents processor arranged to determine the fiber contents per voxel of diffusion data generated by a diagnostic imaging device; a region of interest processor arranged to determine a region of interest in the diffusion data; and a segmentation processor arranged to segment said diffusion data according to at least one segmentation model representing at least part of a fiber bundle to obtain a best fit model of the segmentation model to the fiber contents, the segmentation model including a plurality of segmentation model divisions, each division corresponding to a voxel.