1. Department of Neurosurgery, Faculty of Medicine and Graduate School of Medicine, Hokkaido University, Sapporo, Hokkaido, Japan
2. Department of Diagnostic and Interventional Radiology, Hokkaido University Hospital, Sapporo, Hokkaido, Japan
3. Department of Biostatistics, Graduate School of Medicine, Hokkaido University, Sapporo, Hokkaido, Japan
4. Department of Neurosurgery, Sapporo Azabu Neurosurgical Hospital, Sapporo, Hokkaido, Japan