Magnetic resonance imaging volumetry in atypical parkinsonism: current evidence and neuroradiological perspectives

Atypical parkinsonism, including progressive supranuclear palsy (PSP), multiple system atrophy (MSA), and corticobasal syndrome (CBS), represents a major diagnostic challenge, particularly in the early stages of disease. Magnetic resonance imaging (MRI) plays a pivotal role in the differential diagnosis of these disorders. Beyond conventional qualitative assessment, quantitative MRI techniques, especially volumetry and planimetry, have gained increasing importance as objective tools for evaluating disease specific patterns of brain atrophy. This narrative review summarizes current evidence on MRI-based morphometric approaches in atypical parkinsonism, with a particular focus on brainstem, basal ganglia, cerebellar, and cortical measurements. The diagnostic value of established indices, including the Magnetic Resonance Parkinsonism Index (MRPI) and its updated version, MRPI 2.0, is discussed, along with advances in automated segmentation and machine learning based approaches. The potential role of MRI volumetry as both a diagnostic and progression biomarker is highlighted, together with current limitations and future perspectives relevant to neuroradiological practice.