An image data processing system automatically indicates 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. The system includes an interface for acquiring data representing a DSA sequence of digitally subtracted images enhancing vessel structure. An image data processor automatically indicates 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 the DSA sequence and using the determined individual minimum luminance intensity level values in identifying images of the DSA sequence are associated with at least one of, arterial, venous, or capillary phases of blood flow. An output processor automatically assigns an attribute to image data to identify vessel phase in response to the identifying images of the DSA sequence.