1. From the Department of Radiology, University of Pennsylvania, 3400 Spruce St, 1 Silverstein Bldg, Philadelphia, PA 19103 (J.G.F.); Departments of Neurology (A.S.A.) and Radiology (D.A.P.), Dartmouth-Hitchcock Medical Center, Lebanon, NH; and Geisel School of Medicine, Dartmouth College, Hanover, NH (N.Y.R.).