1. Firda Medical Center, Sandane, Bergen, Norway.
2. Section for Neurosurgery, Department of Surgical Sciences, University of Bergen, and Department of Neurosurgery, Haukeland University Hospital, Bergen, Norway.
3. Section for Radiology, Department of Surgical Sciences, University of Bergen, and Department of Radiology, Haukeland University Hospital, Bergen, Norway.
4. Section for Epidemiology and Medical Statistics, Department of Public Health and Primary Health Care, University of Bergen, Bergen, Norway.