[1] Dept. of Biomed. Eng. and Med. Phys., Keele University, North Staffordshire Hospital, Stoke-on-Trent ST4 7QB, United Kingdom
[2] Dept. of Elec. and Info. Engineering, Faculty of Engineering, Yamagata University, Yonezama 992, Japan
[3] Inst. for Med. and Dent. Engineering, Tokyo Medical and Dental University, Tokyo 101, Japan