1. Departments of Neurological Surgery and
2. Departments of Neurological Surgery and Radiology, University of Miami Hospital, Miami, Florida
3. Department of Epidemiology, University of Iowa College of Public Health, Iowa City;
4. Department of Neurological Surgery, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania; and
5. Department of General Surgery, Mercy Medical Center, Des Moines, Iowa;
6. Radiology, University of Iowa Hospitals and Clinics, Iowa City;