Traumatic Brain Injury (TBI) is associated with accelerated neurodegeneration and is a recognized cause of late-life dementia, a type of Alzheimer’s Disease Related Dementia (ADRD). Although the driving pathological mechanisms are incompletely understood, preclinical evidence increasingly points to microvascular injury as a key component of TBI neuropathology, indicating that TBI-related Neurodegeneration (TreND) shares important features including a vascular contribution to cognitive impairment and dementia (VCID) with other ADRDs. While neuroimaging investigations of TBI to date have primarily focused on structural damage to neuronal cell bodies and axons, there remains an unmet need to characterize the evolution of microvascular dysfunction after TBI in humans and establish the role of TBI-related VCID. An improved understanding of how microvascular injury contributes to long-term neurodegeneration holds potential to improve patient outcome prediction and help overcome persistent barriers to the development of effective therapies to ameliorate this disease process. Thus, there is a great need to develop reliable imaging biomarkers of TBI-related VCID to better inform prognosis, classify injury endophenotypes, monitor long-term recovery, and identify treatment targets. The central hypothesis of this proposal is that chronic microvascular dysfunction as a consequence of TBI represents a type of VCID and links the initial brain injury to subsequent slowly progressive neuronal loss occurring in the ensuing months to years, ultimately contributing to brain atrophy and long-term cognitive decline. To test this hypothesis, we propose a multi-timepoint neuroimaging study to determine the natural history of microvascular dysfunction over the first 3 years post-TBI, in which we will establishing the pattern of longitudinal change in MRI-based measures of microvascular function including cerebral blood flow, cerebrovascular reactivity, blood-brain-barrier dysfunction, and extracellular free water in relation to brain atrophy. We expect that TBI patients will exhibit brain atrophy and microvascular dysfunction in excess of that associated with normal aging, which will be determined by comparison to a longitudinally examined healthy control group. We additionally hypothesize that microvascular imaging measures assessed in the early postinjury period will predict the magnitude of subsequent brain atrophy and cognitive decline during the first 3 years after TBI. Ultimately, this project will yield novel insights into the role of VCID in TBI pathophysiology, a potentially treatable yet understudied endophenotype of TBI.