1. A computer implemented method for tracking a site in a series of images, the method comprising identifying a site to be tracked; selecting at least three non-collinear locations distributed around the site in the reference image; selecting, from the reference image, a region of image data around each of the at least three non collinear locations; comparing each selected region with a second image and, based on these comparisons, identifying for each selected region a corresponding location in the second image, determining a single common affine transformation to map the at least three locations from the reference image onto the corresponding locations in the second image.