1. Department of Radiology, The University of Tokyo Hospital, Tokyo 113-8655, Japan
2. Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan
3. National Institutes for Quantum and Radiological Science and Technology, Tokai, Ibaraki 319-1195, Japan
4. Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
5. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan