Structural Myelin Mapping in Individuals with a History of Head Injury.

Head injury has emerged as a significant environmental risk factor for dementia. Demyelination, defined as the loss of the fatty protective layer surrounding axons, may be associated with neurodegenerative processes following head injury, especially in cortical regions. Although many neuroimaging studies have investigated demyelination in deep subcortical white matter tracts following head injury, these methods do not readily capture myelinated axons that also extend to the cortical surface. To address this gap, the present study used a structural neuroimaging method (T1-weighted [T1w]/T2-weighted [T2w] ratio) as a proxy measure for intracortical myelin content and related these values to history of traumatic brain injury (TBI) and/or repetitive head impacts (RHIs). The sample included 73 older adults (42% male; mean age = 64.8 years; range: 51-86 years) categorized based on TBI (n = 31) and RHI (n = 23) exposure. Rank-transformed analyses of covariance compared whole-brain and regional T1w/T2w values between exposed and nonexposed groups. Partial Spearman's correlations tested monotonic associations between continuous numbers of TBI and RHI exposures and T1w/T2w values. Linear regression models explored interaction effects between head injury and age on T1w/T2w values. Covariates in all analyses included sex, education, and image signal intensity variables. Results revealed that RHI exposure and head injury frequency, including lifetime RHI estimates and number of TBI events, were associated with lower global T1w/T2w values. Regionally, RHI was linked to lower T1w/T2w values in the lateral occipital gyrus. Age further moderated these effects, with stronger associations between RHI and whole-brain myelination observed in younger individuals. These findings provide in vivo evidence linking repetitive impacts to cortical-level myelination and highlight the modifying role of age on these processes.