1. Geriatric Research, Educational and Clinical Center, Veterans Affairs Pittsburgh Health Care System, Pittsburgh, Pennsylvania, USA
2. Department of Neurology and Center of Cerebrovascular Disease Research, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA
3. Cerebrovascular Diseases Research Institute, Xuanwu Hospital of Capital Medical University, and Key Laboratory of Neurodegenerative Diseases, Ministry of Education, Beijing, China
4. Department of Radiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA