Six is enough? Comparison of diffusion parameters measured using six or more diffusion-encoding gradient directions with deterministic tractography.

Diffusion tensor imaging tractography is commonly used to quantify white matter tracts in the human brain via parameters such as fractional anisotropy and mean diffusivity. Simulation studies recommend the use of more than six directions for robust parameter estimates; however, no study has examined the impact of the number of gradient directions on deterministic tractography-derived diffusion parameters in human brain. Here, for 10 major white matter tracts in 11 healthy volunteers at 1.5 T, six-direction diffusion tensor imaging data were compared to 30- or 60-direction data, keeping scan time and number of b = 0 images constant within each test. Mean diffusivity was systematically lower for six-direction protocols (20/40 comparisons); six-direction data had higher fractional anisotropy in the superior longitudinal fasciculus and smaller tract volume for the genu of the corpus callosum. In general, parameter differences due to the number of directions were smaller than those from intersubject variation or signal-to-noise ratio. Despite some absolute differences, standard deviations were significantly different for only one of 160 comparisons. Thus, six-direction data provide diffusion measures with comparable robustness to 30- or 60-direction data and yield appropriate parameter values for most white matter tracts, although there are clear advantages in acquiring higher angular resolution data.