At least 2 weeks following the initial imaging visit subjects (up to 2 PD subjects, up to 2 AD subjects and up to 5 HC subjects) may be asked to return for a second injection and scanning procedure to evaluate the reproducibility of the imaging measure using this procedure. Subjects may decline participation in the second scan. Subjects will be requested to return for this second scan until enough second scan data is collected to evaluate the imaging outcome measure for reproducibility.