1. From the Neuroradiology Section (A.C., A.A.T., V.M.T., D.Z., R.T.), Department of Radiology, Headache Section (M.B., D.W., C.O.B., J.E.), Department of Neurology, and Neurosurgery (F.G.), University of Colorado Anschutz Medical Campus, Aurora; Department of Biostatistics & Informatics (J.P.), Colorado School of Public Health, University of Colorado-Denver Anschutz Medical Campus, Aurora and Department of Anesthesiology (I.C.), Perioperative and Pain Medicine, Stanford University, CA.