1. A medical viewing system for acquiring and displaying a sequence of medical angiograms representing an artery with a balloon, moving in the artery, this system comprising processing means including: extracting means for automatically extracting balloon image data in a phase of balloon expansion, and computing means for automatically defining and storing coordinates of a Region of Interest (ROI) based on the expanded balloon image data, said Region of Interest being defined around the expanded balloon zone of image, display means for displaying the images.