Diffusion imaging has been used extensively over the last decade or so to study healthy brain maturation during childhood and adolescence. Methods vary greatly across studies, but studies consistently report nonlinear maturation that continues into young adulthood, with the most protracted development occurring in frontal-temporal connections. These diffusion changes suggest increasing myelination, axonal packing, and/or coherence with age. Less consistent findings have been reported for specific timing of development (e.g., age at peak), and sex differences. Emerging new methods and large longitudinal or multi-site studies will greatly add to our understanding of brain development over the next few years.
Over the last decade or so, dozens of studies have used diffusion imaging to study healthy brain maturation during childhood and adolescence. The vast majority use diffusion tensor imaging (DTI) to examine correlations with age in a cross-sectional sample. More recently, longitudinal DTI studies, and others using advanced diffusion models have added significantly to the literature.
Acquisition protocol, quality control, preprocessing steps, analysis methods, and choice of model vary across studies and can significantly impact conclusions. Nonetheless, several consistent findings have emerged. First, fractional anisotropy (FA) increases and mean diffusivity (MD) decreases with age during childhood and adolescence. This maturation occurs nonlinearly, with more rapid changes in childhood and slower changes during adolescence. Longitudinal studies confirm that brain changes continue within individuals in some areas into young adulthood. Second, there is substantial regional variation in the timing of maturation, with commissural and projection tracts maturing first, and frontal-temporal connections demonstrating a more protracted development course. These diffusion changes suggest increasing myelination, axonal packing, and/or coherence with age. More advanced diffusion models, specifically neurite orientation dispersion and density imaging (NODDI), have added to our understanding of brain changes, suggesting that axon coherence is relatively stable across childhood and adolescence, and that brain development is driven by changes in myelin and/or axonal packing.
The specific timing of maturation (e.g., age of peak FA) varies considerably across studies, likely due to methodological considerations. Longitudinal studies of brain development offer great promise, and studies with larger sample sizes may be able to further elucidate the nature of development trajectories.
Differences between males and females are reported in some studies, but no consistent findings have emerged. Larger studies and longitudinal data may help further clarify differences between males and females.