Imaging biomarkers in neurodegenerative diseases: advances and challenges.

Neurodegenerative diseases (NDDs), including Alzheimer's disease (AD), Parkinson's disease (PD), frontotemporal dementia (FTD), and amyotrophic lateral sclerosis (ALS), represent a major global health burden. Imaging biomarkers have emerged as important tools for improving the diagnosis, monitoring, and biological characterization of neurodegenerative diseases. Structural MRI, diffusion tensor imaging (DTI), functional MRI (fMRI), positron emission tomography (PET), hybrid PET/MRI and molecular imaging have transformed our ability to investigate neurodegeneration in vivo non-invasively. This review highlights updated information on how each imaging modality offers a unique window into different disease pathophysiology including regional atrophy, amyloid-β, tau, dopaminergic terminal degeneration, synaptic density (SV2A), and neuroinflammation. We also focused on the translational and evidence supporting biomarkers, appropriate use criteria for amyloid and tau PET imaging, and standardized quantification methods such as the Centiloid scale. The growing role of multimodal fusion, where imaging is increasingly integrated with scalable fluid biomarkers to enable "blood-first" strategies where high-risk patients are selectively referred to advanced imaging, improving feasibility and equity. Despite tremendous progress, there are still issues with their standardization, sensitivity, specificity, and clinical translation. Moreover, the review emphasizes the frontiers of α-synuclein and glial state-specific PET ligands, advanced diffusion models, and dynamic connectivity analysis to support precision medicine and mechanism-based trial design for NDDs.