Anatomical Connectivity Mapping – Measuring connectivity changes in Multiple Sclerosis

Introduction Anatomical Connectivity Mapping (ACM) based on diffusion MRI generates a scalar map that reflects the connectivity of each voxel with the rest of the brain [1, 2]. The value of an ACM voxel reflects white mater integrity both locally and globally and the ACM is therefore able to reveal effects not seen with traditional diffusion maps such as Mean Diffusivity (MD) or Fractional Anisotropy (FA), specifically with a disease that results in a varying degree of local and global pathology changes along white matter axons. Multiple Sclerosis (MS) is a disease that fits this profile with its heterogeneous disease pattern where lesions along a given tract may have varying impact on the global conductivity/connectivity of the underlying axons. The ACM map is estimated using probabilistic tractography performed within a brain mask which is usually made by manual delineation by a clinician or obtained from automatic brain segmentation tools. We suggest a variation of ACM particularly suited for group studies that avoid brain masks defined in individual subject space as done in previously [2] and demonstrate it on a group of patients with MS. It is known that MS patients with higher Expanded Disability Status Scale (EDSS) have an increased motor handicap and our aim is to show that this effect is measurable using ACM by testing the hypothesis that a group of patients with high motor impairment (High EDSS) has a significant decreased connectivity profile along the corticospinal tract (CST) compared to a group of patients with low motor impairment (Low EDSS). The results demonstrate that ACM is able to reveal group wise differences not seen in FA.