**(EN)** An image processing system and method employs a registration processor (**130**) to calculate a statistical measure of likelihood for two volumetric images (**110, 112**). The likelihood is calculated based on an assumption that the voxel values in two images in registration are probabilitically related. The likelihood is calculated for a plurality of relative transformations in iterative fashion until a transformation that maximizes the likelihood is found. The transformation that maximizes the likelihood provides an optimal registration and the parameters for the optimized transform are output to memory (**150**) for use by a display system (**160**) in aligning the images for display as a fused or composite image. If statistics about the relationship between the involved images are known, a mutation probability can be derived and used in the likelihood calculation. If there is no such prior knowledge, the mutation probability can be estimated purely from the image data. All voxel pairs in the overlapping volume or, alternatively, a portion of it can be used to compute the likelihood. The optimized likelihood has a simple and less abstract interpretation and can be further used to provide an indication of the quality of the registration.