1. A prior probability MRI based atlas embodied on a computer-readable medium comprising: a plurality of nodes corresponding to a plurality of voxels representing spatial locations of a subject, each of the nodes configured to store: at least two magnetic property values for each of the voxels as determined by magnetic resonance imaging of a plurality of subjects, wherein the at least two magnetic property values correspond to tissue type at one or more voxels; and prior probability data relating to tissue type at one or more voxels, wherein the tissue type prior probability is determined from measurements of at least one subject.