1. A method for deriving data characterizing the contour of a vessel portion in at least one x-ray image, the method performed by a data processing system with access to image data representing the at least one x-ray image, the method comprising: a) processing the image data to detect by segmentation borders of the vessel portion and a centerline of the vessel portion; b) processing the image data to derive densitometric areas for centerline positions along the length of the vessel portion; and c) displacing positions of at least one border of the vessel portion as derived in a) relative to corresponding centerline positions as derived in a) based upon the densitometric areas of the corresponding centerline positions as derived in b).