1. From the Departments of Radiology (L.W., D.C., L.C., J.V., C.C., D.S.H., M.M.B.) and Neurologic Surgery (C.W., R.H.B.), University of Washington, 1959 NE Pacific St, Seattle, WA 98195-0005; and Department of Surgery, University of Colorado, Denver, Colo (C.W.).