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 Emergency Medicine (J.N.G.), and the Department of Neurology and Center for Human Genetic Research (J.R., A.O.), Massachusetts General Hospital, Harvard Medical School, Boston, Mass.