1Department of Radiology, Beijing Tsinghua Changgung Hospital, Beijing, China, People's Republic of, 2Center for Biomedical Imaging Research, Department of Biomedical Engineering, Tsinghua University, Beijing, China, People's Republic of, 3Department of Neurology, Beijing Tiantan Hospital, Beijing, China, People's Republic of, 4Department of Radiology, University of Washington, Seattle, WA, United States