least one year. Smoking status was confirmed by plasma cotinine levels >150 ng/mL, urine cotinine levels >100 ng/mL and carbon monoxide levels >11 on the day of intake. Smokers were helped to quit smoking using Clinical Practice Guidelines and contingency management45-47. Briefly, contingency management is a behavioral therapy in which reinforcement is provided contingent upon a successful response. In this study, monetary reinforcement was provided contingent upon abstinence from smoking measured by urine cotinine and breath carbon monoxide levels. Breath carbon monoxide and urine cotinine levels were monitored daily for the first 8 days of smoking cessation and a minimum of twice weekly thereafter. Subjects were instructed that they could not use any form of nicotine replacement therapy or medication throughout the study. All control subjects were nonsmokers (defined as <100 cigarettes in lifetime) and had no history of significant medical illness or major head trauma. Nonsmoking status, was confirmed by plasma cotinine levels <15 ng/mL, urine cotinine levels <100 ng/mL and carbon monoxide levels <11 ppm on the day of intake and day of scan. All women were required to have a negative pregnancy test during the screening process and prior to radiotracer injection on each study day. Plasma
