quality and architecture (Conturo et al. 1999; Pierpaoli and Basser 1996; Shimony et al. 1999). High FA reflects greater fiber organization and coherence, myelination and/or other structural components of the axon, and low MD values suggest greater white matter density (Roberts and Schwartz 2007). Studies of typically developing adolescents show increases in FA and decreases in MD. These trends continue through early adulthood in a nearly linear manner (Barnea-Goraly et al. 2005; Bonekamp et al. 2007; Mukherjee et al. 2001; Schmithorst et al. 2002), though recent data suggest an exponential pattern of anisotropic increase that may plateau during the late-teens to early twenties (Lebel et al. 2008).
