1. a Department of Neurosurgery, Institute of Clinical Medicine, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305 Japan
2. b Department of Neurosurgery, Institute of Clinical Medicine, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305 Japan
3. c Department of Neurosurgery, Institute of Clinical Medicine, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305 Japan
4. d National Institute of Bioscience and Human Technology, 1-1 Higashi, Tsukuba, Ibaraki, 305 Japan
5. e National Institute of Bioscience and Human Technology, 1-1 Higashi, Tsukuba, Ibaraki, 305 Japan