Magnetic resonance imaging in Parkinson's disease: from routine imaging to molecular biomarkers - an update for radiologists

Magnetic resonance imaging (MRI) is becoming an increasingly important tool in the diagnostic approach to Parkinson's disease (PD), extending its role beyond the exclusion of secondary causes of parkinsonism. Recent developments in high-resolution imaging techniques - including neuromelanin-sensitive MRI, susceptibility-weighted imaging, quantitative susceptibility mapping, diffusion tensor imaging, and functional MRI - have enabled in vivo assessment of substantia nigra degeneration, iron accumulation, and functional connectivity alterations. These imaging findings correspond to core pathophysiological mechanisms of PD and support efforts toward earlier and more precise diagnosis. This narrative review outlines the current capabilities and limitations of MRI in the context of the evolving biological definition of PD, which emphasizes the presence of pathological α-synuclein as the defining feature, regardless of clinical manifestations. We discuss the utility of MRI across different disease stages - from prodromal to advanced - and its value in differentiating PD from atypical parkinsonian syndromes such as multiple system atrophy, progressive supranuclear palsy, and corticobasal degeneration. Special attention is paid to age-related diagnostic strategies, including recommendations for imaging protocols and adjunctive testing (e.g., dopamine transporter single photon emission computed tomography, positron emission tomography, genetic testing) for different age groups. We also review the role of radiomics and machine learning in enhancing diagnostic accuracy and consider practical limitations related to MRI field strength, reader expertise, and standardization. This article aims to serve as a practical primer for radiologists involved in the imaging of parkinsonian syndromes and provides a framework for integrating MRI findings into multimodal diagnostic pathways.