Subjects will participate in 5 study visits over the course of the study period. The screening visit will include an assessment of eligibility. The second visit will be the collection of the community neurologist's diagnostic assessment. The third visit, during which all subjects receive a single intravenous (IV) injection of 123I-ALTROPANE® and single photon emission computed tomography (SPECT) imaging, will include appropriate safety assessments before and after dosing. The fourth visit, to occur 24 to 72 hours after SPECT imaging, will include follow up safety assessments as well as the first Movement Disorder Specialist's (MDS) evaluation. The fifth visit, to occur 6 months (±7 days) after SPECT imaging, will include follow-up safety assessments as well as the MDS reevaluation and truth standard diagnosis. The subject's participation in the study will be up to 7 months.