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, a data processing unit calculating the value of a chosen linear feature; for each image, said data processing unit calculating a set of ratios given by dividing the value of said linear feature in every ROI by the value of said linear feature in every other ROI within the same image; and said data processing unit performing statistical analysis of the distribution of ratios to compare the test image to the reference images.