1. Department of Radiology, University of Michigan Medical School, Ann Arbor, Michigan 48109, United States
2. Department of Medicinal Chemistry, University of Michigan, Ann Arbor, Michigan 48105, United States
3. E.I. du Pont de Nemours and Company, DuPont Haskell Global Center for Health Sciences, P.O. Box 30, Newark, Delaware 19714, United States