In addition, the investigators will invite six subjects who have completed the two PET scans described above to complete a third PET scan. During this third PET scan, the investigators plan to measure α4β2* receptor availability using the PET radio-ligand 2-[18F]FA, administered as bolus plus constant infusion after 24 hours of abstinence. The purpose will be to compare α4β2* nAChR binding potential data from the bolus 2-[18F]FA infusion protocol used in the main study to the bolus plus constant infusion protocol used in this third PET scan.