1. An image data processing system for automatically indicating an image of a digitally subtracted Angiography (DSA) image sequence is associated with at least one of, arterial, venous, or capillary phases of blood flow, comprising: an interface for acquiring a sequence of images of patient vessels both prior to and following introduction of contrast agent into the vessels and subtracting a mask image representing background detail in the absence of a contrast agent to produce data representing a DSA sequence of digitally subtracted images enhancing vessel structure; an image data processor for automatically indicating an image of the DSA sequence is associated with at least one of, arterial, venous, or capillary phases of blood flow by determining individual minimum luminance intensity level values of individual images of said DSA sequence and using the determined individual minimum luminance intensity level values in identifying images of said DSA sequence are associated with at least one of, arterial, venous, or capillary phases of blood flow; and an output processor for automatically assigning an attribute to image data to identify vessel phase associated with individual images of the identified images of said DSA sequence.