Regions with synchronous BOLD signal, electrical or magnetic fluctuations often (but not always) feature some form of direct physical connection. DTI assesses the structural integrity of brain connections (i.e. axons and fiber tracts) by measuring changes in the diffusion of water molecules through tissues.35 Two markers of structural integrity are commonly investigated: fractional anisotropy, a marker of white matter (WM) fiber disruption (loss of fiber coherence, demyelination, axonal loss), and mean diffusivity, a marker for cell density.35 Axial and radial diffusivity may provide more specific markers of axonal damage and demyelination.35 Common methods to investigate structural disruption are voxel-wise, DTI tractography and ROI-based techniques.35 DTI tractography may be preferable on an individual subject basis, allowing one to reconstruct and visualize specific WM connections between cortical nodes (Figure 4).36 Graph theoretical analysis can be used to build structural networks and study their topology, in a way similar to that used to investigate resting-state fMRI and EEG/MEG-derived functional networks.
