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A number of neurodegenerative diseases can manifest as parkinsonian disorders. Structural imaging, such as CT and MRI, is of limited value for differentiating these diseases. PET can demonstrate the selective patterns of disruption of regional cerebral metabolism and neurotransmitter systems associated with subcortical degenerations, such as Parkinson's disease, striatonigral degeneration, progressive supranuclear palsy, and corticobasal degeneration. It can also determine, where underlying Parkinson's disease may be suspected, whether nigral dysfunction is present in patients with isolated tremor or drug-associated rigidity. Finally, PET can detect the presence of subclinical disruption of the dopaminergic system in at-risk subjects, such as relatives of patients with Parkinson's disease, or subjects exposed to nigral toxins, such as MPTP. With the advent of putative neuroprotective agents for Parkinson's disease, PET can help identify patients with early disease who might benefit from therapy with these agents and monitor their disease progression.