1Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Charlestown, MA, United States, 2Department of Radiology, Key Laboratory of Obstetric & Gynecologic and Pediatric Diseases and Birth Defects of Ministry of Education, West China Second University Hospital, Sichuan University, Chengdu, China, People's Republic of, 3Cerebrovascular Diseases Research Institute, Xuanwu Hospital of Capital Medical University, Beijing, China, People's Republic of, 4Neuroprotection Research Laboratory, Department of Radiology and Neurology, Massachusetts General Hospital and Harvard Medical School, Charlestown, MA, United States