1. From the Translational Neuroradiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda (D.S.R.), and the Departments of Neurology and Neuroscience, Johns Hopkins School of Medicine, Baltimore (P.A.C.) — both in Maryland; and the Department of Neurology, Mayo Clinic, Rochester, MN (C.F.L.).