1. A system for obtaining perfusion images comprising: a C-arm X-ray device; and a computer operative to: accept a plurality of projection X-ray images representing a first data set of a patient and a second data set of a patient; convert each of the first data set and the second data set to form a first volumetric data set and a second volumetric data set, respectively; process the second volumetric data set to form a data set of segmented contrast-enhanced blood vessels; excise the segmented contrast-enhanced blood vessels from the second volumetric data set to form a third volumetric data set; and subtract the first volumetric data set from the third volumetric data set to form a fourth volumetric data set that represents a cerebral blood volume (CBV) on a voxel-by-voxel basis, wherein the second data set is contrast-enhanced data.