1. A method for detecting a halo surrounding an ultrasound image of a tumor, the steps comprising: a) locating the boundary of the tumor in the image; b) defining a first annular region surrounding the tumor and adjacent the tumor boundary; c) defining a second annular region surrounding the tumor and adjacent the first annular region; and d) determining the presence of a halo by comparing the value of an image parameter of image pixels in the first annular region with the value of the image parameter of pixels in the second annular region.