1. Department of Obstetrics and Gynecology, Faculty of Medicine, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan
2. Department of Radiology, Tokyo Metropolitan Police Hospital, 4-22-1, Nakano, Nakano-ku, Tokyo 164-8541, Japan
3. Department of Radiology, Graduate School of Medicine, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan