Regions of reduced cortical magnetization transfer ratio detected in MS patients using surface-based techniques

Introduction: The multiple sclerosis imaging community still struggles with the in vivo detection of cortical grey matter lesions. While newer sequences can help identify a small fraction of the cortical pathology present, subpial demyelination appears to be largely undetectable [1]. This is despite the fact that these cortical lesions are the most common subtype when sampled at autopsy and tend to extend over multiple gyri, following the shape of the cortical mantle [2]. The problem with conventional imaging of subpial cortical demyelination is that these lesions do not appear to be associated with an appreciable influx of inflammatory cells or edema, and consequently show little alteration of T2 or T1 relaxation times. An additional challenge is the complex geometry and thinness of the cortex, which induce variable amounts of partial volume at the cortical boundaries. Magnetization transfer imaging has been shown to be sensitive to changes in myelin content in white matter [3]. Based on this, we quantified the extent of subpial decreases of magnetization transfer ratio (MTR) of the cortical grey matter, which may indicate regions of cortical demyelination, in groups of MS patients and healthy controls. To increase our sensitivity, we exploited the knowledge gained from pathological studies of the unique geometry of these lesions, and carried out our analyses on two-dimensional parametric surface models of the cortex, instead of the three-dimensional voxel-wise analyses traditionally used.