1. A method of deriving a quantitative measure of a degree of calcification of a blood vessel by processing an image of at least a part of the blood vessel containing said calcification, which method comprises: taking a starting set of digital data representative of an image of at least part of a blood vessel containing an area of calcification, said area of calcification being set against a background area; using an inpainting method to replace digital data in said starting set of digital data representing the calcification with data extrapolating the background to extend over the area of calcification, and so generating an inpainted set of digital data; and computing the difference between the starting set of digital data and the inpainted set of digital data to obtain a result representative of the degree of calcification of the blood vessel.