A method has been described for deriving contour data in X-Ray images for vessels with differential absorption through applying a contour-finding algorithm on a shadow image and finding the vessel borders through segmentation based on image intensities. In particular, the method uses the following steps: finding a densitometric area of an above mentioned vessel, and displacing one or both of the borders inward until the densitometric measurement result between the borders after such displacing will start to change significantly. Furthermore, a specific procedure is introduced to automatically determine the conversion factor to equate the densitometrically based diameter to the contour based diameter of the vessel and to discriminate bifurcating or parallel vessels.