To this end, this protocol seeks to document the pharmacokinetics of DAT receptor occupancy of d-MPH using PET and C-11 Altropane. It has been estimated that MPH is effective when the CNS DAT occupancy is 50% or greater. This aim of this study will be to measure CNS DAT occupancies at extended time points after administration of d-MPH. This research will provide novel and unique information toward a better understanding of the mechanism of action of long-acting stimulant formulations to enable new drug development.