A medical viewing system with an X-ray image acquisition device for acquiring angiograms and interventional live images of vessels is adapted for generating a region of interest border into which an object referenced by an object-based registration process must extend in order to achieve an accurate registration of vessel trees extracted from the angiogram and the live images. The region of interest border is then overlaid onto the vessel tree images and the live images. The medical viewing system reminds the person accomplishing the intervention of the importance of pushing the object far enough into the image, while being discrete enough to be ignored if preferred, thus yielding in a reliable and precise road mapping processing.