1. A method for performing image registration, comprising the steps of: obtaining a reference image dataset and at least one other target image dataset; defining an image mask for a region of interest in the reference image dataset; differently weighting respective sub-regions within the image mask, wherein each sub-region corresponds to a different respective anatomical structure; and registering a corresponding region of interest in the at least one other target image dataset with the image mask, using a similarity metric, wherein the similarity metric is computed based on one or more voxels in the region of interest defined by the image mask and using the respective weights associated with each voxel.