1. Division of Neurological Surgery, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona
2. Current Address: Norton Neuroscience Institute, Norton Hospital, Louisville, Kentucky
3. Division of Neurosurgery, University of California at San Diego, San Diego, California
4. Department of Neurosurgery and Division of Interventional Neuroradiology, University of Pennsylvania, Philadelphia, Pennsylvania