1. Department of Neurosurgery, Tsukuba Medical Center Hospital, 1‐3‐1 Amakubo, Tsukuba, Ibaraki 305‐8558, Japan
2. Department of Infectious Diseases, Department of Medicine, Tsukuba Medical Center Hospital, 1‐3‐1 Amakubo, Tsukuba, Ibaraki 305‐8558, Japan
3. Miroku Medical Laboratory Inc., 659‐2 Innai, Saku, Nagano 384‐2201, Japan
4. Department of Radiology, Tsukuba Medical Center Hospital, 1‐3‐1 Amakubo, Tsukuba, Ibaraki 305‐8558, Japan
5. Department of Neurosurgery, University of Tsukuba Hospital, 2‐1‐1 Amakubo, Tsukuba, Ibaraki 305‐8576, Japan