Fig. 6. Physical basis for diffusion spectrum imaging (DSI). DSI is a high resolution diffusion-weighted imaging technique which determines the complete spin displacement function by acquiring a large number of images with varying diffusion weighting and angularity. This results in the generation of a probability density function (PDF) for the entire set of possible molecular displacements, and thus depicts the set of principal fiber orientations based on the properties of the diffusion maxima. (A) Two sets of model fibers drawn with arbitrary angular separation. (B) In order to simplify the visualization of 3-D diffusion, the PDF is converted to an orientational diffusion function (ODF), which provides a probability distribution for diffusion for a set of angular directions, weighted by the magnitude of the diffusion.