DTI offers a non-invasive technique for the assessment of WM structures by quantifying the diffusion of water molecules within the brain (Mori & Zhang, 2006). If unconstrained, water molecules will randomly diffuse in all directions. In contrast, non-random diffusion can be used to infer constraints placed upon the motion of water by physical features such as cell membranes or interactions with large molecules (Le Bihan et al., 2001). Fractional anisotropy (FA) is a measure used to indicate the degree of non-randomness of diffusion, providing information on the microstructure of WM and the axons contained within it. Mean diffusivity (MD) is an index of the overall magnitude of diffusion irrespective of direction and therefore tends to be decreased by these same factors. In typical development, reflecting increasing myelination, a pattern of overall FA increasing and MD decreasing during childhood and adolescence is generally observed (Barnea-Goraly et al., 2005; Bava et al., 2011; Lebel, Walker, Leemans, Phillips, & Beaulieu, 2008). High FA is interpreted as reflecting coherently bundled myelinated axons and axonal pruning, and has been associated with more efficient neuronal signalling (Suzuki, Matsuzawa, Kwee, & Nakada, 2003) and improved cognitive performance (Beaulieu et al., 2005; Schmithorst, Wilke, Dardzinski, & Holland, 2005).
