1. Cerebrovascular Research Center, University of Pennsylvania, Philadelphia
2. Departments of Neurology, University of Pennsylvania, Philadelphia
3. Departments of Radiology, University of Pennsylvania, Philadelphia
4. Department of Chemistry, Brookhaven National Laboratory, Upton, Long Island
5. Department of Radiology, Health Sciences Center, State University of New York at Stony Brook, Stony Brook, New York, U.S.A.