1Center for Cybernics Research, University of Tsukuba, Tsukuba, Japan, 2Department of Radiology, Chiba University Hospital, Chiba, Japan, 3Medical Technology Department, Kameda General Hospital, Kamogawa, Japan, 4Department of Neurosurgery, University of Tsukuba, Tsukuba, Japan, 5Faculty of Engineering, Information and Systems, University of Tsukuba, Tsukuba, Japan