1. An image data processing system, 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 for 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 determining individual minimum luminance intensity level values of individual images of said DSA sequence and using the determined individual minimum luminance intensity level values to determine that a first one or more of the individual images of said DSA sequence depicts an arterial phase of blood flow, that a second one or more of the individual images of said DSA sequence depicts a venous phase of blood flow, and that a third one or more of the individual images of said DSA sequence depicts a capillary phase of blood flow; and an output processor for automatically assigning an attribute to each of the individual images of said DSA sequence to identify a vessel phase depicted by the individual images of said DSA sequence.