1. From the Carl E. Ravin Advanced Imaging Laboratories (P.S., W.P.S., E.S.) and Departments of Radiology (P.S., D.M., R.C.N., E.S.), Medical Physics Graduate Program (W.P.S., E.S.), and Biomedical Engineering, Electrical and Computer Engineering, and Physics (E.S.), Duke University Medical Center, 2424 Erwin Rd, Suite 302, Durham, NC 27705; and Department of Physics, NC State University, Raleigh, NC (P.S.).