Analysis of corpus callosum diffusion tensor imaging parameters in infants.

Radial diffusivity is a diffusion tensor imaging (DTI) metric that has received increased attention in recent studies as a parameter that may better reflect myelination than the more commonly-used fractional anisotropy (FA). This study compared rates of radial diffusivity decrease against FA increase and axial diffusivity decrease on DTI maps in the corpus callosum of normal infants during the first postnatal year. Fifty-three normal infants (range: 0-52 weeks adjusted for gestational age) underwent six-direction DTI on a 1.5 Tesla scanner (b= 1,000 s/mm(2), one excitation). A single individual placed regions of interest on FA maps in the genu 1) and radial diffusivity (i.e., λ and splenium to obtain axial (i.e., 3)/2]), FA and ADC. We calculated mean and median values for FA, λ 2+λ[ ADC, radial diffusivity and axial diffusivity in each of four 13-week epochs and measured the percent change over the first year of life. Within the genu, radial diffusivity decreased 36%, FA increased 25%, ADC decreased 22% and axial diffusivity decreased 10%. Within the splenium, radial diffusivity decreased 53%, FA increased 43%, ADC decreased 38%, and axial diffusivity decreased 23%. For both genu and splenium, the greatest difference was seen in radial diffusivity values, followed in order by FA, ADC and axial diffusivity. Furthermore, decreases in radial diffusivity were on the order of two to threefold greater than those in axial diffusivity. The high rate of radial diffusivity decrease compared to axial diffusivity decrease is consistent with myelination. Decreases in radial diffusivity were greater than increases in FA values. This finding is further support of the concept that radial diffusivity and FA values represent two different types of microstructural change during development of white matter.