1. An image data subtraction system suitable for use in Angiography or other medical procedure to enhance vessel visualization, comprising: an imaging processor for, excluding images prior to regulation of X-ray radiation dose emission, from a set of temporally sequential individual images of patient vessels, subtracting data representing a first mask image from data representing images of said set to provide data representing first digitally subtracted images, said sequential individual images encompassing introduction of a contrast agent; automatically processing said data representing said first digitally subtracted images to identify a first image indicating presence of said contrast agent and a second image preceding said first image, said second image being substantially exclusive of an indication of presence of said contrast agent, by determining a measure representing a net presence of contrast agent in an individual digitally subtracted image, automatically selecting said second image as a second mask image and substituting said second mask image for said first mask image in automatically subtracting data representing said second mask image from the data representing images of said set to provide second digitally subtracted images; and a user interface for presenting said second digitally subtracted images for display while the patient is undergoing a medical procedure.