1. Key Laboratory of Radiopharmaceuticals, Ministry of Education, Department of Chemistry; Beijing Normal University; Beijing 100875 China
2. Chinese PLA General Hospital; Beijing 100853 China
3. PET Center, Department of Diagnostic Radiology; Yale University School of Medicine; New Haven CT 06520-8048 USA
4. Department of Neuropharmacology, National Institute on Drug Dependence; Peking University; Beijing 100191 China