MRI-Based Measures of Intracortical Myelin Content are Sensitive to the Severity of a Traumatic Brain Injury

Abstract

Traumatic brain injuries (TBIs) induce persistent behavioral and cognitive deficits via diffuse axonal injury. Such axonal injuries are often examined in vivo using diffusion MRI, which identifies demyelinated regions in deep white matter. However, TBI patients can exhibit impairment in the absence of diffusion-measured abnormalities, suggesting that axonal injury and demyelination may occur outside the deep white matter. Importantly, myelinated axons are also present within the cortex. Cortical myelination cannot be measured using diffusion imaging, but can be assessed using the T1/T2 ratio myelin mapping method. Here, we conducted the first work examining effects of TBI on intracortical myelin in living humans by applying the myelin mapping technique to 26 US Military Veterans with varying histories of TBI severity. We observed that myelin maps could be created in individual TBI patients that were similar to known distributions of cortical myelin. Critically, after controlling for participant age, a history of more severe TBI was associated with progressively reduced intracortical myelin in a broad, diffuse pattern across the cortex. Such significant but diffuse reductions in myelin were observed in high-myelin but not low-myelin regions. By contrast, the presence of blast TBI did not affect intracortical myelin above and beyond TBI severity in either a diffuse or focal pattern. These findings suggest that intracortical myelin may be a biomarker of TBI that is anatomically complimentary to diffusion MRI measures of deep white matter. Thus, myelin mapping could potentially be combined with diffusion imaging measures to improve MRI-based diagnostic tools for TBI.