3D fiber tractography with susceptibility tensor imaging

Abstract

Gradient-echo MRI has revealed anisotropic magnetic susceptibility in the brain white matter. This magnetic susceptibility anisotropy can be measured and characterized with susceptibility tensor imaging (STI). In this study, a method of fiber tractography based on STI is proposed and demonstrated in the mouse brain. STI experiments of perfusion-fixed mouse brains were conducted at 7.0T. The magnetic susceptibility tensor was calculated for each voxel with regularization and decomposed into its eigensystem. The major eigenvector is found to be aligned with the underlying fiber orientation. Following the orientation of the major eigenvector, we are able to map distinctive fiber pathways in 3D. As a comparison, diffusion tensor imaging (DTI) and DTI fiber tractography were also conducted on the same specimens. The relationship between STI and DTI fiber tracts was explored with similarities and differences identified. It is anticipated that the proposed method of STI tractography may provide a new way to study white matter fiber architecture. As STI tractography is based on physical principles that are fundamentally different from DTI, it may also be valuable for the ongoing validation of DTI tractography.

Figure 1

Orientation dependence of the apparent magnetic susceptibility (AMS). (A) The contrast between gray and white matter depends on the fiber angle. The right limb of the anterior commissure (blue ROI) becomes brighter as the fiber angle increases (arrows). Red circles illustrate ROI in gray matter for susceptibility reference. (B) AMS increases monotonously as the fiber angle decreases following a sine squared relationship. Maximum AMS is achieved when the fiber is aligned with the external field.

Figure 2

Elements of susceptibility tensor demonstrating susceptibility anisotropy. The six independent elements of the susceptibility tensor are shown for a representative dorsal slice. Off diagonal terms (χ₁₂, χ₁₃ and χ₂₃) are significant only in the white matter. The varying contrast in the hippocampal commissure (red arrows) among the tensor elements demonstrates strong susceptibility anisotropy.

Figure 3

Principal susceptibility determined by eigenvalue decomposition of two dorsal slices. The three principal susceptibilities vary significantly in the white matter such as the hippocampal commissure (red arrows) and the anterior commissure (blue arrows). The principal susceptibilities are relatively homogeneous in the gray matter. Both principal susceptibilities and mean susceptibility offer excellent contrast between gray and white matter. Hippocampal commissure and anterior commissure are also highlighted in |χ₁ − χ₃|, which indicated stronger susceptibility anisotropy than gray matter.

Figure 4

Susceptibility index (SI) compared to DTI diffusion anisotropy (FA). (A) SI of representative dorsal, sagittal and coronal slices. (B) Diffusion FA at the same three slices.

Figure 5

Comparison of color-coded STI SI and DTI FA. Red represents the anterior-posterior direction; green represents the left-right direction; blue represents the dorsal-ventral direction. Overall, the colors in major fiber bundles (arrows) show good consistency while differences are also present. Similar to DTI, STI color maps demonstrate the striking capability of separating fiber bundles in close contact such as the corpus callosum (green) and the cingulum bundle (red) shown in the third column (arrows). Abbreviations: hc – hippocampal commissure; cc – corpus callosum; ac – anterior commissure; cg – cingulum.

Figure 6

Comparison of STI and DTI fiber tracts in selected fiber bundles. (A) The anterior commissure; (B) the hippocampal commissure; (C) the posterior corpus callosusm. In general, similar fiber tracts are reconstructed by both techniques while DTI tracts appear to be smoother at the edges of the fiber bundle. In all cases, a single starting ROI was chosen in the middle of each bundle (e.g. the yellow oval in (A)).

Figure 7

Angles between the major eigenvectors of STI and DTI in voxels of high SI value. χ̄ was used as the background image for anatomical information. The threshold for SI was 0.2. Three slices are shown from left to right that encompass major fiber bundles (black arrows) including the corpus callosum, the hippocampal commissure and the anterior commissure. The angles between STI and DTI eigenvectors are small in the major fiber bundles while significantly larger in more complex and smaller fiber structures.