1. From the Department of Radiology, Division of Neuroradiology, Perelman School of Medicine at the University of Pennsylvania, 3400 Spruce St, Philadelphia, PA 19104 (J.D.R., C.D., S.M.); Department of Radiology & Biomedical Imaging, University of California, San Francisco, San Francisco, Calif (A.M.R.); and Department of Diagnostic Medicine, Dell Medical School, University of Texas, Austin, Tex (R.N.B.).