This is a Phase 2 study evaluating the positron-emitting radiopharmaceutical 18F-mFBG as an imaging agent for quantification of the effect of neurodegenerative diseases on myocardial sympathetic innervation. Effectiveness of 18F-mFBG imaging of the heart will be judged in terms of the quantitative difference between results for subjects with Lewy body and non-Lewy body neurologic disease as compared to historical data for healthy control subjects.