Whereas structural MRI provides measurement of regional tissue expressed as a volume over multiple image slices and voxels, MR diffusion tensor imaging (DTI) provides a qualitative assessment of the microstructure of tissue, typically white matter, within voxels (Basser and Pierpaoli, 1996). DTI image acquisition and data analysis are complex, and details of these methods are available in numerous reviews (e.g., Le Bihan, 2003; Jones, 2010). In short, white matter fiber integrity is commonly measured in terms of fractional anisotropy (FA), which is usually higher in fibers with a homogeneous or linear structure, such as healthy white matter, and bulk mean diffusivity (MD), for which higher values, commonly due to larger presence of mobile water molecules in a tissue sample (Pierpaoli et al., 2001; Pfefferbaum et al., 2003; Pfefferbaum and Sullivan, 2003), reflect diminished fiber integrity. MD can be decomposed into two components: axial (longitudinal) diffusivity (λL), which can be altered with disruption of axonal integrity and axonal deletion; and radial (transverse) diffusivity (λT), which increases selectively with decline in myelin integrity (Song et al., 2002; Song et al., 2005; Sun
