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 and to convert each of the first and second data sets to form first and second volumetric (voxel) data sets, respectively; wherein the second data set is contrast-enhanced data, and a cerebral blood volume (CBV) is computed from the first and the second volumetric data sets.