Measuring Anisotropic Brain Diffusion in Three and Six Directions: Influence of the Off-Diagonal Tensor Elements

Abstract

Diffusion tensor imaging (DTI) has been proposed for examination of cerebral white matter. However, this measurement needs sophisticated postprocessing and is susceptible to eddy current artefacts. The aim of this study was to examine whether diffusion measured in three anatomically well defined directions provides full and reliable information on diffusion anisotropy. Measurements were performed in water and gelatine phantoms and in 14 healthy volunteers. Diffusion was measured in six independent directions. For the full tensor DTI we diagonalised the diffusion tensor, for measurements with three directions we used only the diagonal elements. We then calculated the diffusion trace and the linear anisotropy index (AI_l). We measured a slight anisotropy in the phantoms, which was larger in the case of the full tensor DTI (AI_l = 0.022) then for the three orthogonal diffusion directions (0.008–0.019). The linear anisotropy index measured in white matter regions within the right and left hemisphere ranged between 0.33 and 0.42. AIl values were moderately correlated between right and left hemisphere regions (correlation factor: 0.35–0.64). DTI using the full tensor information is more susceptible to systematic errors resulting from eddy current effects than the measurement of diffusion in three orthogonal directions. However, in the latter method the anisotropy index is systematically underestimated in anatomical structures that do not exhibit a principal diffusion parallel to one of the diffusion directions. Therefore it is recommended to use the full tensor method together with sophisticated methods for eddy current correction.