1. Laboratory of Neuroimaging, National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, Bethesda, Maryland 20892-1013, United States
2. Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0810, United States
3. Department of Mathematics, Howard University, Washington, D.C. 20059, United States
4. Molecular Imaging Branch, National Institute of Mental Health, National Institutes of Health, Bethesda, Maryland 20892, United States
5. Drug Design and Synthesis Section, National Institute on Drug Abuse, National Institutes of Health, Rockville, Maryland 20850, United States
6. Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts 02129, United States
7. National Institute on Drug Abuse, National Institutes of Health, Bethesda, Maryland 20892-1013, United States