1. Department of Radiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
2. Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
3. Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63110, United States
4. Department of Neurology, Washington University School of Medicine, St. Louis, Missouri 63110, United States