Ultrasonographic information about the internal structure of a subject body is combined with x-ray or other radiographic information from the same subject body by using an optical correlator to quickly find a transformation which aligns the information from both sources, even if they are acquired in different coordinate systems. Various spatial transformations are applied to the information and cross-correlations are quickly performed. The transformation which gives the best cross-correlation is used to align the two data sets, which can then be displayed visually. The resulting display can be used as an aid in medical diagnosis, for example in diagnosing suspected malignant lesions in a woman's breast.