1. From the Division of Neuroradiology, Department of Radiology (J.E.D.A., A.J.Y., M.J.S., P.W.S., M.H.L., R.G.G., J.M.R.), the Department of Neurology and Center for Human Genetic Research (A.O., H.B.B., J.R.), and the Department of Emergency Medicine (J.N.G.), Massachusetts General Hospital, Harvard Medical School, Boston.