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 et al., 2006b; Sun et al., 2006a). DTI has been further extended to provide visual depictions of white matter fiber systems (Stieltjes et al., 2001; Xu et al., 2002; Lehericy et al., 2004) and quantification of the integrity of specific fiber tracks (Gerig et al., 2005; Sullivan et al., 2006b).
