Diffusion anisotropy measurement of brain white matter is affected by voxel size: Underestimation occurs in areas with crossing fibers

Abstract

BACKGROUND AND PURPOSE: Voxel size/shape of diffusion tensor imaging (DTI) may directly affect the measurement of fractional anisotropy (FA) in regions where there are crossing fibers. The purpose of this article was to investigate the effect of voxel size/shape on measured FA by using isotropic and nonisotropic voxels. MATERIALS AND METHODS: Ten healthy adult volunteers had MR imaging by using a 1.5T clinical imager. DTI was performed with 2 different voxel sizes: a 2-mm-section isotropic voxel (2 x 2 x 2 mm3) and a 6-mm-section nonisotropic voxel (2 x 2 x 6 mm3). Images were obtained by using a single-shot echoplanar imaging technique with motion-probing gradients in 15 orientations and a b-value of 1000 s/mm2. FA and the apparent diffusion coefficient (ADC) were measured at different sites of the brain. RESULTS: When smaller isotropic voxels were used, the FA was greater in areas with crossing fibers, including the superior longitudinal fasciculus, the thalamus, and the red nucleus; the FA was not significantly different in areas without crossing fibers, such as the corpus callosum, the posterior limb of the internal capsule, and the corticospinal tract at the level of the centrum semiovale (P > .05). The ADC values were not affected by voxel size/shape at any of the areas of the brain that were measured. CONCLUSION: FA values that are measured in regions containing crossing fibers are underestimated when using nonisotropic DTI.