1. From the Division of Neuroradiology (D.M.M., C.C.M., R.I.F., T.K., R.A., K.T.B., R.A.W., D.J.M.), Toronto Western Hospital and the University of Toronto, Toronto, Canada; and the Divisions of Neurology at Sunnybrook Health Sciences Centre (R.H.S.) and the Toronto Western Hospital (F.L.H.), Toronto, Canada.