A method of deriving a quantitative measure of a degree of calcification of a blood vessel such as an aorta by processing an image such as an X-ray image of at least a part of the blood vessel containing said calcification comprises: taking a starting set of digital data representative of an image of at least part of a blood vessel containing a calcification set against a background; estimating the boundary of the calcification; using inpainting to replace digital data in said starting set representing the calcification with data extrapolating the boundary of 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 quantitative measure of the degree of calcification of the blood vessel.