with a precision higher than the width of a slice since the stet image is a true three dimensional image one of the two images can be u201c re sliced u201d so that the image slices of one stet image are exactly aligned to the slices of the other stet image in the case of registering a stet image to a x ray ct image the preferred way to find the correct matching ct and sptct slices is to have the physician choose the slice pair based on his understanding of the images and his knowledge of human anatomy once the closest matching slices are found the stet image can be re sliced so that the stet image slices fall on boundaries of the ct slices for images derived from different modalities the z scale may be different a slice scale factor may be derived based on matching a plurality of structural features in different slices in an additional preferred embodiment of the invention steps a and b of the registration process are replaced by a single step of correlating the two images as a whole additionally three dimensional images may also be correlated as wholes without first slicing them and correlating the slices in order to facilitate manual finding and matching or marking of prominent body structures between images it is useful to display.