1. From the Department of Radiology (S.P., L.C.S.S., P.W.S., E.F.H., R.G.G., M.H.L.), Massachusetts General Hospital and Harvard Medical School, Boston, MA; Schulich School of Medicine and Dentistry (Y.W.), University of Western Ontario, London, Ontario, Canada; Department of Neurology (K.L.F.), Massachusetts General Hospital and Harvard Medical School, Boston, MA.