1. An image data subtraction system to enhance visualization of vessels subject to movement, comprising: an image data processor for, automatically identifying temporally corresponding pairs of images comprising a mask image and a contrast enhanced image from first and second image sets respectively and for said corresponding pairs, automatically determining a shift of a contrast enhanced image relative to a mask image to compensate for motion induced image mis-alignment and automatically shifting a contrast enhanced image relative to a mask image in response to the determined shift and subtracting data representing a mask image of a corresponding pair from a shifted contrast enhanced image of said corresponding pair, to provide a plurality of subtracted images showing enhanced visualization of vessels.