1. Department of Radiology, Otaru General Hospital, Otaru 047-0152, Japan
2. Graduate School of Health Sciences, Hokkaido University, Sapporo 060-0812, Japan
3. Department of Neurosurgery, Otaru General Hospital, Otaru 047-0152, Japan
4. Faculty of Health Sciences, Hokkaido University, Sapporo 060-0812, Japan