This research has been supported by the National Science Foundation under grants CCR-9205047 and DCA 9303181 and by the Department of Energy under the PICS grant. Cláudio Silva acknowledges partial support from CNPq-Brazil under a PhD fellowship. We thank Tzi-cker Chiueh, Steve Dawson, Juliana Freire, Ron Peierls, and Amitabh Varshney for useful discussions about the PVR system and for help with this paper. The MRI data set is courtesy of Siemens, Princeton, NJ. The high potential iron protein is courtesy of Scripps Clinic, La Jolla, CA.