1. School of Biomedical Engineering Tsinghua University Beijing 100084 P. R. China
2. Wellcome Centre for Integrative Neuroimaging, FMRIB, Nuffield Department of Clinical Neurosciences University of Oxford Oxford OX3 9DU UK
3. Athinoula A. Martinos Center for Biomedical Imaging Massachusetts General Hospital Charlestown MA 02129 USA
4. Harvard Medical School Boston MA 02129 USA
5. Department of Engineering Physics Tsinghua University Beijing 100084 P. R. China