COMPUTERIZED DETECTION OF MASSES IN DIGITAL MAMMOGRAMS - AUTOMATED ALIGNMENT OF BREAST IMAGES AND ITS EFFECT ON BILATERAL-SUBTRACTION TECHNIQUE

An automated technique for the alignment of right and left breast imag es has been developed for use in the computerized analysis of bilatera l breast images. In this technique, the breast region is first identif ied in each digital mammogram by use of histogram analysis and morphol ogical filtering operations. The anterior portions of the tracked brea st border and computer-identified nipple positions are selected as lan dmarks for use in image registration. The paired right and left breast images, either from mediolateral oblique or craniocaudal views, are t hen registered relative to each other by use of a least-squares matchi ng method. This automated alignment technique has been applied to our computerized detection scheme that employs a nonlinear bilateral-subtr action method for the initial identification of possible masses. The e ffectiveness of using bilateral subtraction in identifying asymmetries between corresponding right and left breast images is examined by com paring detection performances obtained with various computer-simulated misalignments of 40 pairs of clinical mammograms. Based on free-respo nse receiver operating characteristic and regression analyses, the det ection performance obtained with the automated alignment technique was found to be higher than that obtained with simulated misalignments. D etection performance decreased gradually as the amount of simulated mi salignment increased. These results indicate that automatic alignment of breast images is possible and that mass-detection performance appea rs to improve with the inclusion of asymmetric anatomic information bu t is not sensitive to slight misalignment.