1. A system for registering a floating image with a reference image, the system comprising: a reference image memory configured to store a reference image voxel set representing the reference image; a floating image memory configured to store a floating image voxel set the floating image; a voxel enumerator unit configured to count the voxels in sequential order along an axis of the reference image voxel set; a voxel coordinate transformation unit configured to transform the voxels of the floating image voxel set with respect to the voxel in the reference image voxel set indicated by the voxel enumerator unit and to produce a transformed floating image voxel set; a mutual histogram accumulator unit configured to accumulate a mutual histogram representing the transformed floating image voxel set interpolated with the reference image voxel set; a volumetric interpolator unit configured to store in the mutual histogram accumulator unit the contribution to the mutual histogram of respective voxels of the transformed floating image voxel set interpolated with respective voxels of the reference image voxel set; and a mutual information calculator unit configured to calculate the mutual information between the reference image voxel set and the transformed floating image voxel set based on the mutual histogram.