For a large class of intramodality problems, intensity-based registration poses an adequate retrospective solution to the registration problem. As demands for precise quantification of small differences grow, differences in partial volume effects and the associated interpolation errors will need to be addressed more effectively. The best strategy for dealing with these problems is to move the use of intensity-based cost functions from the realm of retrospective methods into the realm of prospective methods. Using a scout scan, it is possible to perform a preliminary registration and then acquire new images in an orientation that will match that of a previous scanning session . This eliminates many of the problems associated with partial volume effects and inter polation errors and greatly facilitates comparative interpretation of the images. Future functional MRI sequences may well modify themselves on-the-fly to compensate for subject movements detected within the sequence of images. An appropriate long-term goal for the future of the field would be to advance to a state where within-modality image registration is so robust and so omnipresent that it is routinely taken for granted.
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