We are in the midst of major advances in medical imaging, converting static pre sentation of anatomic information into near real-time interactive imaging and displays capable of depicting both structure and function. This will have pro found impact on diagnosis and image-guided therapies. These advances have been most notable in the modalities of computed tomog raphy and magnetic resonance. These technologies have become capable of ac quiring volumetric images in short periods of time. Speed gains in MRI were very significant in the early 1990s with the development of clinically usable pulse...