NEUROIMAGING IN THE DIAGNOSIS OF ALZHEIMER'S DISEASE: FROM STRUCTURAL MRI TO MOLECULAR IMAGING AND THE BIOLOGICAL AT(N) FRAMEWORK

Neuroimaging has become an indispensable component of the diagnostic pathway in Alzheimer’s disease (AD), a condition increasingly defined by its underlying biology rather than by clinical syndrome alone. Structural magnetic resonance imaging (MRI) remains the first-line modality: it excludes surgically relevant or potentially reversible causes of cognitive decline, characterises the pattern of medial temporal and posterior cortical atrophy, and quantifies the vascular burden that frequently coexists with AD. Standardised visual rating scales—including the medial temporal atrophy (Scheltens) scale, the Koedam posterior atrophy scale, and the Fazekas scale for white-matter change—improve the reproducibility of these assessments, while automated morphometry and machine-learning tools are expanding the precision and scalability of image analysis. Molecular and functional imaging, chiefly [18F]fluorodeoxyglucose, amyloid, and tau positron emission tomography (PET), enables the pathological hallmarks of AD to be detected in vivo, often years before overt atrophy appears. In parallel, cerebrospinal fluid and, more recently, accurate plasma biomarkers such as phosphorylated tau 217 have transformed access to biological confirmation. These advances underpin a shift from syndromic to biological diagnostic criteria, from the 2011 National Institute on Aging–Alzheimer’s Association recommendations and the International Working Group criteria to the AT(N) system and the 2024 revised criteria. This review synthesises the contemporary evidence on structural, functional, and molecular neuroimaging in AD, integrates it with fluid biomarkers and current diagnostic frameworks, and outlines a practical, imaging-anchored diagnostic approach together with its limitations and future directions.