Dependence of brain DTI maps of fractional anisotropy and mean diffusivity on the number of diffusion weighting directions

The rotational variance dependence of diffusion tensor imaging (DTI) derived parameters on the number of diffusion weighting directions (N) has been investigated by several Monte Carlo simulation studies. However, the dependence of fractional anisotropy (FA) and mean diffusivity (MD) maps on N, in terms of accuracy and contrast between different anatomical structures, has not been assessed in detail. This experimental study further investigated in vivo the effect of the number of diffusion weighting directions on DTI maps of FA and MD. Human brain FA and MD maps of six healthy subjects were acquired at 1.5T with varying N (6, 11, 19, 27, 55). Then, FA and MD mean values in high [Formula: see text] and low [Formula: see text] anisotropy segmented brain regions were measured. Moreover, the contrast‐to‐signal variance ratio [Formula: see text] between the main white matter and the surrounding regions was calculated. Analysis of variance showed that [Formula: see text] and [Formula: see text] significantly [Formula: see text] depend on N. In particular, FAL decreased (6%–11%) with N, whereas [Formula: see text] (1.6%–2.5%) and [Formula: see text] (4%–6.5%) increased with [Formula: see text] and [Formula: see text] did not significantly [Formula: see text] depend on N. Unlike MD values, FA values significantly vary with N. It is noteworthy that the observed variation is opposite in low and high anisotropic regions. In clinical studies, the effect of N may represent a confounding variable for anisotropy measurements and the employment of DTI acquisition schemes with high [Formula: see text] allows an increased CVR and a better visualization of white matter structures in FA maps. PACS number: 87.61.Tg, 82.56.Lz