1. Department of Neurology, Harvard Medical School, Massachusetts General Hospital, 114 16th Street, Charlestown, Massachusetts 02129, USA
2. Hitachi, Ltd., Research & Development Group, Center for Exploratory Research, Hatoyama, Saitama 350-0395, Japan
3. Athinoula A. Martinos Center for Biomedical Imaging, Harvard Medical School, Massachusetts General Hospital, 149 13th Street, Charlestown, Massachusetts 02129, USA