Diffusion Anisotropy Measurement of Brain White Matter Is Affected by Voxel Size: Underestimation Occurs in Areas with Crossing Fibers
H. Oouchia,
K. Yamadaa,
K. Sakaib,
O. Kizua,
T. Kubotaa,
H. Itoa and
T. Nishimuraa
a Department of Radiology, Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kyoto City, Kyoto, Japan
b Center for the Promotion of Excellence in Higher Education, Kyoto University, Kyoto City, Kyoto, Japan
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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.
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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.
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Fig 3. The graphs present the measured mean FA values in the nonisotropic voxel (2 x 2 x 6 mm3) and in the isotropic voxel (2 x 2 x 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.
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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.
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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.
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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).
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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.
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