The first of these we found successful was the third order moment of the histogram of the feature space. Our work up to this stage was discussed in more detail in chapter 7 of Derek Hill's PhD , completed at the end of 1993, and awarded in 1994. We then introduced a formalism to describe several existing voxel similarity measures in terms of the joint probability distribution, and developed a robust multiresolution optimisation technique. It is clear, however, that mutual information is the only voxel similarity measure that has yet been demonstrated to accurately register both modality combinations. Both The Leuven group, and ourselves demonstrated this independently as part of the Retrospective Registration Evaluation project run by Mike Fitzpatrick at Vanderbilt University.
Mutual information is described in more detail here .
687 Springer-Verlag, Berlin. Visualisation in Biomedical Computing 1994. June 1995 ed. Information Processing in Medical Imaging, 287-298, Ile de Berder, France. Real, Robot. in Med.