1. A method of registering images, the method comprising: for each image, determining an optimum intensity threshold set from a plurality of intensity threshold sets that maximizes a variance between classes of each set by a computer; segmenting each image using the corresponding determined optimum intensity threshold set by a computer; generating mutual information from a joint histogram of at least two of the segmented images by a computer; and registering the at least two images based on the mutual information using a computer.