Nuclear Medicine Imaging Biomarkers in Parkinson's Disease: Past, Present, and Future Directions.

Parkinson's disease (PD) is a multifaceted neurodegenerative disorder characterized by dopaminergic neuronal loss and widespread α-synuclein pathology. Nuclear medicine imaging offers essential in vivo tools for early diagnosis, differential assessment, and monitoring disease progression. This review summarizes key PET and SPECT radiotracers targeting dopaminergic synthesis and transport, vesicular storage, post-synaptic receptors, neuroinflammation, and protein aggregation, highlighting their roles in clinical evaluation and phenotyping. Clinically, these modalities support earlier recognition of PD, distinction from atypical parkinsonian syndromes, and assessment of non-motor involvement. Future directions include the development of selective α-synuclein tracers and multimodal imaging strategies to refine prodromal detection and guide personalized therapeutic interventions.