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 of the balloon from the sequence of medical angiograms in a phase of balloon expansion, wherein the balloon is inflated with contrast agent; computing means for automatically defining and storing coordinates of a Region of Interest (ROI) based on the automatically extracted balloon image data of the balloon inflated with contrast agent, said Region of Interest being defined as a limited region around an automatically extracted expanded balloon zone of a corresponding image of the sequence of medical angiograms; processing means for using an automatically previously defined and stored coordinates in order to apply the previously defined ROI to another sequence of medical angiograms, for extracting balloon markers associated to a deflated balloon, in another phase of stent implantation in the artery, where a stent is wrapped around the deflated balloon, and for registering the ROI based on the markers location; and display means for displaying the images of the sequence of medical angiograms.