1. A computer implemented image processing method for tracking a site in a series of images, the method comprising identifying a site to be tracked through the series of images; selecting at least three non-collinear locations distributed around the site in a reference image of the series of images; selecting, from the reference image, a region of image data around each of the at least three non collinear locations; comparing each selected region of image data with image data from a second image of the series of images 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, and using the single common affine transformation for tracking the site from the reference image to the second image.