1. From the Department of Radiology, Hospital of the University of Pennsylvania, 3400 Spruce St, 1 Silverstein, Philadelphia, PA 19104-6243 (C.O., L.R.); Department of Radiology, University of Pennsylvania Perelman School of Medicine, Philadelphia, Pa (M.I., S.K., M.H., M.G.A.); and Department of Biostatistics, University of Pennsylvania, Philadelphia, Pa (Q.C.).