1. Department of Neurosurgery, Tsukuba Memorial Hospital, Tsukuba, Ibaraki, Japan
2. Department of Neurosurgery, Faculty of Medicine, University of Tsukuba, Tsukuba, Ibaraki, Japan
3. Division of Stroke Prevention and Treatment, Department of Neurosurgery, Faculty of Medicine, University of Tsukuba, Tsukuba, Ibaraki, Japan