Using multiparametric MRI in 486 healthy adults, the study maps age-related structural, diffusion, and hemodynamic alterations across nigrostriatal nuclei and tract, and derives a composite nigrostriatal aging index (NAI) that rises after age 60 and correlates with motor and cognitive decline.

Offers a noninvasive imaging biomarker (NAI) to detect early nigrostriatal vulnerability and stratify/monitor at-risk older adults for PD-related research and trials, though it lacks direct molecular therapeutic targets.

Aging is an important risk factor for Parkinson's disease (PD). Characterizing age-related alterations in the nigrostriatal system may help identify early vulnerability prior to overt neurodegeneration. We aimed to delineate aging trajectories of structure and hemodynamics of the nigrostriatal system and examine their associations with motor and cognitive functions. We analyzed 486 healthy adults from Human Connectome Project-Aging dataset, stratified into younger (≤ 60 years) and older (> 60 years) groups. Motor, cognitive, and motor cognition functions were assessed. Multiparametric MRI included T1- and T2-weighted, multi-delay arterial spin labeling, and multi-shell diffusion imaging. Volumes, T1/T2 ratio, arterial transit time (ATT), and cerebral blood flow (CBF) were quantified in the nigrostriatal nuclei. The nigrostriatal tract (NST) was reconstructed and segmented along the nigra-to-striatum axis. Diffusion metrics and quantitative anisotropy were derived. A composite nigrostriatal aging index (NAI) was generated using principal component analysis. Older adults exhibited reduced substantia nigra and putamen volumes, increased caudate volume, prolonged ATT, and reduced CBF across nigrostriatal nuclei. The NST showed segment-specific age trajectories, with increased diffusivity after age 60. Imaging alterations in nigrostriatal nuclei and tract segments correlated with declines in motor, cognitive, and motor-cognitive performance. The NAI increased more steeply after age 60 and predicted poorer behavioral performance exclusively in older adults. Healthy aging is characterized by coordinated structural and hemodynamic alterations within the nigrostriatal system associated with functional decline. The composite NAI provides a sensitive framework for detecting early nigrostriatal vulnerability in older adults prior to overt neurodegeneration.