An adjunctive ultrasound mammography system and associated methods are described in which an ultrasound image being displayed near an x-ray mammogram image may be superimposed thereon or thereunder by a user for facilitating rapid comprehension of breast structures and detection of abnormalities therein. In one preferred embodiment, the x-ray mammogram image corresponds to a standard x-ray mammogram view, and the ultrasound image is a thick-slice image representing a thick-slice or slab-like portion of the breast volume substantially parallel to that standard x-ray mammogram view. In another preferred embodiment, the user is permitted to manually manipulate the registration of the ultrasound image with the x-ray mammogram image. It has been found that the manual registration process, which involves manual vernier adjustments responsive to perceived registration differences, can rapidly increase the radiologist's perception of the breast structures being displayed by both component images. Even though ultrasound images tend to have substantially different textures and feature emphases than x-ray images, the bimodal thick-slice/x-ray image, alone or in conjunction with the vernier registration process, can often expose or clarify tissue structures hidden in the separate component images, and can often obviate or explain certain noticed structures in the component images.