The application of PET-MR image registration in the brain.

The coregistration in three-dimensional space of positron emission tomography (PET) and magnetic resonance (MR) image volumes has, over the last decade, become a matter of routine in the analysis of brain PET studies. The ability to objectively localize small regions of interest in PET using images more closely correlated to tissue structure has itself improved the effective resolution of PET. There are a number of highly effective software packages for image coregistration available in the public domain. Voxel-by-voxel coregistration, involving little or no intervention from the user can, on today's computing hardware, provide fast and accurate registration with little or no pre-processing and algorithms based on mutual information measures now seem to be the mathematical method of choice. Registration may be applied in a number of ways. Rigid body registration is used to match a single subject's brain scanned using either different imaging modalities or as serial scans with the same modality. Increasingly, this technique is being extended to studies of disease involving regional atrophy, where location and extent of tissue loss can be identified. Non-linear registration can be used to warp a subject's brain onto a template, atlas or other standardized guide. While numerous examples are available of the added value produced by image registration in the brain, similar examples are not yet available from registration in the torso, where the problem is much more complex. It is here that newly emerging hardware such as combined PET/CT scanners may prove their worth.