1. From the Department of Epidemiology (L.H.K.), University of Pittsburgh, Pittsburgh, Pa; the Departments of Epidemiology and Neurology (W.T.L.), Biostatistics (A.M.A.), and Radiology (N.J.B.), University of Washington, Seattle, Wash; the Division of Neurology (C.B.), University of Nevada, Las Vegas, Nev; and the Department of Radiology (R.N.B.), University of Pennsylvania, Philadelphia, Pa.