1. A method of selecting one or more x-ray difference images from a group of x-ray images, which comprises the steps of: (a) obtaining first and second pluralities of x-ray images of a patient or subject, each x-ray image being obtained at a different time; (b) obtaining a sample difference image representing the difference between one of the images of the first plurality of images and one of the images of the second plurality of images; (c) selecting a region of the sample difference image, the region being less than the entire area of the sample difference image; (d) pairing images selected from the first plurality of images with images selected from the second plurality of images; (e) obtaining a plurality of difference images each representing the difference between the images of one of the pairs of images; (f) searching regions of the difference images, the searched regions corresponding, respectively, to the selected region of the sample difference image; and (g) selecting one or more difference images, the selected difference images having searched regions that satisfy a predetermined condition or index.