1. An image data subtraction system suitable for use in Angiography or other medical procedure to enhance vessel visualization, comprising: an imaging system for acquiring, during a medical procedure, data representing a plurality of temporally sequential individual images of vessels of a portion of patient anatomy, said sequential individual images encompassing introduction of a contrast agent; an image processor for automatically, processing said data representing said plurality of temporally sequential individual images to identify, a first image indicating presence of said contrast agent, and a second image preceding said first image, by comparing a difference between measures representative of luminance content of the first and second image, with a threshold, said image processor identifying a first plurality of images preceding said first image and being substantially exclusive of an indication of presence of said contrast agent and selecting said second image from the first plurality of images and in response to said difference exceeding said threshold, selecting said second image as a mask image and subtracting data representing said mask image from data representing images of said temporally sequential individual images to remove background image detail and emphasize vessel structure in providing processed image data for display; and a user interface for presenting said processed image data for display while said patient is undergoing said medical procedure.