1 Cerebral Imaging Centre, Douglas Mental Health University Institute, Montreal, Canada.
2 Integrated Program in Neuroscience, McGill University, Montreal, Canada.
3 Center for the Studies in the Prevention of Alzheimer's Disease, Douglas Mental Health University Institute, Montreal, Canada.
4 Centre d'études avancées en médecine du sommeil, Université de Montreal, Montreal, Canada.
5 Département de pharmacologie et physiologie, Université de Montréal, Montreal, Canada.
6 Centre de recherche de l'Institut universitaire de gériatrie de Montréal, Montreal, Canada.
7 Department of Anatomy and Cell Biology, McGill University, Montreal, Canada.
8 Département d'Anatomie, Université du Québec à Trois-Rivières, Trois-Rivières, Canada.
9 Department of Neurology and Neurosurgery, McGill University, Montreal, Canada.
10 Department of Psychiatry, McGill University, Montreal, Canada.
11 Department of Psychology, Concordia University, Montreal, Canada.
12 Department of Neurology, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany.
13 McConnell Brain Imaging Centre, Montreal Neurological Institute, Montreal, Canada.
14 Department of Biomedical Engineering, McGill University, Montreal, Canada.

White matter hyperintensities (WMHs) are neuroimaging markers widely interpreted as caused by cerebral small vessel disease, yet emerging evidence suggests that a subset may have a neurodegenerative etiology. Current imaging methods have lacked the specificity to disentangle biological processes underlying WMHs in vivo. Here, we used voxel-level normative modeling and seven microstructural MRI markers with complementary biophysical sensitivities to generate single-subject high-resolution WMH pathophysiology maps in a large cohort (n=32,526). We calculated data-driven spatial patterns of similar WMHs, revealing distinct periventricular, posterior, and anterior clusters. We identified a reproducible WMH signature linked to dementia and Alzheimer's disease, characterized by a posterior predominance and a pathophysiological pattern indicative of selective fiber degeneration. Posterior WMHs connected cortical regions vulnerable to tau pathology. Our framework distinguishes vascular and neurodegenerative contributions of WMHs in vivo, which could alter the course of treatment strategies and provide nuanced interpretations of research findings.