1. a Department of Precision Machinery Engineering, Faculty of Engineering, University of Tokyo, 7-3- 1, Hongo, Bunkyo-ku, Tokyo 113, Japan
2. b Department of Precision Machinery Engineering, Faculty of Engineering, University of Tokyo, 7-3- 1, Hongo, Bunkyo-ku, Tokyo 113, Japan
3. c Department of Neurosurgery, Neurological Institute, Tokyo Women's Medical College, Japan
4. d Department of Neurosurgery, Neurological Institute, Tokyo Women's Medical College, Japan
5. e Department of Precision Machinery Engineering, Faculty of Engineering, University of Tokyo, 7-3- 1, Hongo, Bunkyo-ku, Tokyo 113, Japan
6. f Department of Precision Machinery Engineering, Faculty of Engineering, University of Tokyo, 7-3- 1, Hongo, Bunkyo-ku, Tokyo 113, Japan