1. From the Department of Radiology, Perelman School of Medicine, The University of Pennsylvania, 3400 Spruce St, Philadelphia, PA 19104-4283 (K.M.S.); Department of Radiology, Stanford University School of Medicine, Stanford, Calif (K.M.S., J.R., A.B.S., S.T.C., A.M.L.); and Department of Radiology, Veterans Affairs Palo Alto Healthcare System, Palo Alto, Calif (S.T.C.).