1. A method of comparing a test image with a set of reference images comprising the steps of: defining a set of regions of interest (ROIs) in the test image and a corresponding set of ROIs in each of the reference images; for each ROI in each image, calculating the value of a chosen linear feature for each image, calculating a set of ratios given by dividing the value of said linear feature in an ROI with the value of said linear feature in another ROI within the same image and performing statistical analysis of the distribution of ratios to compare the test image to the reference images.