1. Environmental Management Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8569, Japan
2. Molecular Imaging Program at Stanford, Bio-X Program, Stanford University School of Medicine, Palo Alto, California 94304, USA
3. Department of Engineering Science, Graduate School of Informatics and Engineering, The University of Electro-Communications, Chofu 182-8585, Japan