Structural and resting-state brain connectivity of motor networks after stroke.

Abstract

Focal ischemic brain lesions primarily affect circumscribed brain regions and fiber tracts resulting in acute neurological deficits. Secondary damage because of apoptosis, inflammation, diaschisis, and neurodegeneration, however, can also affect remote brain areas and may result in a more widespread perturbation of entire functional networks even in the nonischemic hemisphere. Such network-wide effects may play a role in the development of poststroke cognitive impairment and may impose limits on functional recovery. Assessing the total effect of all these mechanisms onto the damaged and recovering brain may be clinically useful for better outcome prediction and identification of therapeutic targets because some of those processes (diaschisis and inflammation) are in principle reversible and the strengthening of alternative pathways by targeted intervention may promote recovery. From a practical clinical point of view, imaging methods which can be performed on clinical MRI scanners and do not require active patient participation are ideal candidates to perform such assessments. This topical review thus focuses on diffusion tensor imaging (DTI) MRI and resting-state functional MRI (RS-fMRI) as the most promising methods to this regard (Table). We briefly introduce the physiological background of each method and review current evidence for assessing connectivity in the context of motor recovery. View this table: Table. Comparison of Resting-State fMRI and Diffusion Tensor Imaging ### Principles of DTI Diffusion-based MRI methods are techniques that are sensitive to signals originating from freely moving water molecules in the tissue. Unrestricted movement in all 3 spatial directions results in isotropic diffusion. In the healthy human brain, highly organized white matter pathways consisting of tightly packed axons present a barrier to isotropic diffusion. This diffusion anisotropy is characterized by the preferential diffusion of water molecules parallel to the long axis of the fiber bundles with restricted diffusion perpendicular to the long axis.1 During DTI acquisitions, pairs of balanced diffusion-sensitizing field gradients are applied …