A medical viewing system for processing and displaying a sequence of medical angiograms representing a balloon, moving in an artery, this system comprising 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, located around the expanded balloon; and display means for displaying the images. Contrast agent may be used as agent of balloon expansion. The system may have means to detect and keep track of balloon markers and means to look around those markers for further balloon image data extraction.