Objective: The first purpose of this study was to compare the degree of anisotropy in compact white matter and noncompact white matter in each of three pediatric age groups using diffusion tensor imaging. We hypothesized that anisotropy would be higher in compact white matter than in noncompact white matter in each age group. The second purpose of our study was to compare the increase in anisotropy over time in compact versus noncompact white matter during early childhood. We hypothesized that increases in anisotropy would be higher in noncompact white matter.
Materials and methods: We retrospectively analyzed anisotropy maps derived from diffusion tensor imaging studies performed in 66 pediatric patients (age range, 4 days-71 months; mean age, 18.6 months) who underwent clinical MR imaging and were found to have no abnormalities on conventional MR images. Anisotropy was measured in three compact white matter structures (corpus callosum, internal capsule, cerebral peduncle) and two regions of noncompact white matter (corona radiata and peripheral white matter). Patients were assigned to one of the three following groups on the basis of age: group 1, younger than 12 months (n = 40); group 2, 12-35 months (n = 11); and group 3, 36-71 months (n = 15). First, we compared anisotropy values of noncompact white matter with those of compact white matter for each age group. Second, we compared the increase over time in anisotropy of noncompact white matter regions with that seen in compact white matter structures.
Results: Among all three age groups, anisotropy measurements in compact white matter structures were higher than those in noncompact white matter (p < 0.01). The mean anisotropy values in noncompact white matter for groups 1, 2, and 3, respectively, were 0.349, 0.480, and 0.531. The mean anisotropy values in compact white matter for groups 1, 2, and 3, respectively, were 0.494, 0.646, and 0.697. When age groups were compared, a statistically significant increase in anisotropy was seen in both compact white matter and noncompact white matter (p < 0.01). However, the increase in anisotropy was significantly greater in non-compact white matter regions than in compact white matter structures when comparing group 1 with group 3 (p < 0.01) as well as group 1 with group 2 (p < 0.01).
Conclusion: Although anisotropy measurements were higher in compact than non-compact white matter in all three age groups, the increase in anisotropy was greater in non-compact white matter across each of the three groups. These data suggest that although myelination is initially greater in compact white matter, the change in myelination may be greater in noncompact white matter during the first few years after infancy.