Developmental Trajectories of White Matter Microstructure

With all data spatially aligned in a similar analysis space, mean developmental MWF, T₁ and T₂ trajectories were obtained for the genu, splenium and body of the corpus callosum, right and left hemisphere cingulum, corona radiata, internal capsule and optic radiation, and right and left hemisphere frontal, occipital, temporal, parietal and cerebellar white matter regions.
Anatomical masks for each of these regions (derived as outlined below) were superimposed onto each infant's dataset, and the mean and standard deviation calculated for each region. Only voxels with MWF values greater than 0.001 were included in the regional means.
For the frontal, occipital, parietal, temporal and cerebellar white matter,1.

A global binary white matter mask was calculated by thresholding the MNI white matter probability image provided within FSL (www.fmrib.ox.ac.uk/fsl) at 180.

2.

Masks of the frontal, occipital, parietal, temporal and cerebellar lobes were obtained from the MNI database (Mazziotta et al., 2001 (link)). These were multiplied by the binary global mask (1) to obtain the regional white matter masks.

3.

The white matter masks for each region were divided by hemisphere.

4.

The registration transformation between the MNI template and the study template was calculated, and each masked transformed to the study space.

For the white matter tract masks, including genu, body and splenium of the corpus callosum, cingulum, corona radiata, internal capsule, and optic radiation, this same process was applied to the John Hopkins University DT-MRI white matter atlas (Oishi et al., 2008 (link)). The corona radiata mask comprised the anterior, superior and posterior portions; the internal capsule mask comprised the anterior, posterior and retrolenticular portions. Each of these regions, superimposed onto the mean study template is shown in Fig. 4.
Pearson correlations between MWF and T₁; and MWF and T₂ were calculated for each white matter tract and region across the full age range; as well as across developmental periods between 1) 0 and 6 months of age; 2) 6–12 months; 3) 12–24 months; 4) 24–36 months; 5) 36–48 months; and 6) 48–60 months of age.