Abstract
Previous diffusion tensor imaging tractography studies have demonstrated exponential patterns of developmental changes for diffusion parameters such as fractional anisotropy (FA) and mean diffusivity (MD) averaged over all voxels in major white matter (WM) tracts of the human brain. However, this assumes that the entire tract is changing in unison, which may not be the case. In this study, a surface model based tract-specific analysis was applied to a cross-sectional cohort of 178 healthy subjects (83 males/95 females) aged from 6 to 30 years to spatially characterize the age-related changes of FA and MD along the trajectory of seven major WM tracts – corpus callosum (CC) and six bilateral tracts. There were unique patterns of regions that showed different exponential and linear rates of increasing FA or decreasing MD and age at which FA or MD levels off along each tract. Faster change rate of FA was observed in genu of CC and frontal-parietal part of superior longitudinal fasciculus (SLF). Inferior corticospinal tract (CST), posterior regions of association tracts such as inferior longitudinal fasciculus, inferior frontal occipital fasciculus and uncinate fasciculus also displayed earlier changing patterns for FA. MD decreases with age also exhibited this posterior-to-anterior WM maturation pattern for most tracts in females. Both males and females displayed similar FA/MD patterns of change with age along most large tracts; however, males had overall reached the FA maxima or MD minima later compared with females in most tracts with the greater differences occurring in the CST and frontal-parietal part of SLF for MD. Therefore, brain WM development has spatially varying trajectories along tracts that depend on sex and the tract.
Highlights
Quantitative magnetic resonance imaging (MRI) has demonstrated significant brain changes during development from infancy to adulthood in vivo
It has been suggested that those developmental changes in gray matter (GM) and white matter (WM) observed at the macroscopic MRI level may reflect synaptic pruning and myelination at the microscopic neuronal level (Gogtay et al, 2004)
In neurodevelopmental studies with a limited age range from childhood to adulthood, fractional anisotropy (FA) or mean diffusivity (MD) versus age curves have been fit with either an exponential trajectory to account for FA/MD changing faster at younger ages and leveling off at a plateau (Lebel et al, 2008; Colby et al, 2011; Taki et al, 2013b) or linear model that assumes the same rate of diffusion parameter changes across the given age span (Schmithorst et al, 2002; Barnea-Goraly et al, 2005; Bonekamp et al, 2007)
Summary
Quantitative magnetic resonance imaging (MRI) has demonstrated significant brain changes during development from infancy to adulthood in vivo. Using region-of-interest (ROI) by manual placement/tractography or voxel based analysis, brain development studies of children and adolescents using DTI have consistently demonstrated agerelated increases of fractional anisotropy (FA) and decreases of overall diffusion mean diffusivity (MD; Klingberg et al, 1999; Morriss et al, 1999; Schmithorst et al, 2002; Schneider et al, 2004; Barnea-Goraly et al, 2005; Snook et al, 2005; Ashtari et al, 2007; Bonekamp et al, 2007; Eluvathingal et al, 2007; Lebel et al, 2008; Tamnes et al, 2010) that differ between tracts with some data suggesting a posteriorto-anterior developmental trend (Colby et al, 2011). In neurodevelopmental studies with a limited age range from childhood to adulthood (typically up to 30 years), FA or MD versus age curves have been fit with either an exponential trajectory to account for FA/MD changing faster at younger ages and leveling off at a plateau (Lebel et al, 2008; Colby et al, 2011; Taki et al, 2013b) or linear model that assumes the same rate of diffusion parameter changes across the given age span (Schmithorst et al, 2002; Barnea-Goraly et al, 2005; Bonekamp et al, 2007)
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