Robert W Hurst MD is Professor of Radiology, Neurosurgery, and Neurology at the University of Pennsylvania Medical School, Philadelphia, and Director of Interventional Neuroradiology at the Hospital of the University of Pennsylvania, Philadelphia. Robert H Rosenwasser is the Jewell L. Osterholm Professor and Chair of Neurological Surgery, Professor of Radiology, and Director of the Division of Neurovascular Surgery, Interventional Neuroradiology at Jefferson Medical School, Thomas Jefferson University, Philadelphia, Pennsylvania.