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 1.5 T clinical imager. DTI was performed with 2 different voxel sizes: a 2-mm-section isotropic voxel (2 x 2 x 2 mm(3)) and a 6-mm-section nonisotropic voxel (2 x 2 x 6 mm(3)). Images were obtained by using a single-shot echo-planar imaging technique with motion-probing gradients in 15 orientations and a b-value of 1000 s/mm(2). 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.

Fig 1.

The regions of interest used in the data analysis are superimposed on the transverse single-shot EPI performed with a b-value of 0 s/mm² and the FA maps. 1 indicates red nucleus; 2, thalamus; 3, genu of the corpus callosum; 4, splenium of the corpus callosum; 5, centrum semiovale; 6, posterior limb of the internal capsule; 7, CST; 8, SLF.

Fig 2.

FA measurement in a nonisotropic voxel. It was assumed that there are 3 fiber bundles traversing this voxel, each with an FA of 0.5. When all fibers are aligned in a single direction (A), the measured FA of the voxel is 0.5. When there is a crossing fiber within this voxel (B), then the measured FA of the voxel is <0.5.

Fig 3.

The graphs present the measured mean FA values in the nonisotropic voxel (2 × 2 × 6 mm³) and in the isotropic voxel (2 × 2 × 2 mm³). A, Statistically significant FA differences were noted at the centrum semiovale (P < .0001), SLF (P < .0001), thalamus (P < .0001), and red nucleus (P < .0001). B, The areas without statistically significant differences.

Fig 4.

Graph shows ADC measurements of the 2- and 6-mm-thick sections. The ADC values were not affected by voxel size/shape at any of the areas of the brain that were measured.

Fig 5.

Fiber tracts of the CST (green) and SLF (yellow) are superimposed on the vector maps. Note that the SLFs obtained from 2-mm-section images (A, C) are more robust than those obtained by 6-mm-section images (B, D). Depiction of the CST did not substantially differ between 2- and 6-mm-section images.

Fig 6.

The graphs show the results of the simulation study in which 1 (A) or 2 (B) of the 3 fiber bundles contained within a nonisotropic voxel were rotated 180°. A, When only 1 of the fiber bundles was rotated from its original location, FA underestimation occurred, which was lowest when the fiber bundle was oriented perpendicular (90°) to its original location. B, Similarly, when 2 of 3 fiber bundles were rotated simultaneously, FA was underestimated to various degrees depending on the extent of the rotation, becoming zero when both bundles were perpendicular to each other (represented by the central part of this 3D graph).

Fig 7.

B = 0 (A) and color-coded FA maps (B) through the red nucleus are shown. The circles represent the location where the FA measurements were taken. Note that there are many different colors within this region of interest, indicating that there may be crossing fibers contained in this small area of the brain.