A tractography comparison between turboprop and spin-echo echo-planar diffusion tensor imaging

Abstract

The development of accurate, non-invasive methods for mapping white matter fiber-tracts is of critical importance. However, fiber-tracking is typically performed on diffusion tensor imaging (DTI) data obtained with echo-planar-based imaging techniques (EPI), which suffer from susceptibility-related image artifacts, and image warping due to eddy-currents. Thus, a number of white matter fiber-bundles mapped using EPI-based DTI data are distorted and/or terminated early. This severely limits the clinical potential of fiber-tracking. In contrast, Turboprop-MRI provides images with significantly fewer susceptibility and eddy-current-related artifacts than EPI. The purpose of this work was to compare fiber-tracking results obtained from DTI data acquired with Turboprop-DTI and EPI-based DTI. It was shown that, in brain regions near magnetic field inhomogeneities, white matter fiber-bundles obtained with EPI-based DTI were distorted and/or partially detected, when magnetic susceptibility-induced distortions were not corrected. After correction, residual distortions were still present and several fiber-tracts remained partially detected. In contrast, when using Turboprop-DTI data, all traced fiber-tracts were in agreement with known anatomy. The inter-session reproducibility of tractography results was higher for Turboprop than EPI-based DTI data in regions near field inhomogeneities. Thus, Turboprop may be a more appropriate DTI data acquisition technique for tracing white matter fibers near regions with significant magnetic susceptibility differences, as well as in longitudinal studies of such fibers. However, the intra-session reproducibility of tractography results was higher for EPI-based than Turboprop DTI data. Thus, EPI-based DTI may be more advantageous for tracing fibers minimally affected by field inhomogeneities.

Figure 1

A) Cross-section of the fibers of the forceps-minor with a coronal plane. The white dots represent voxels that belong to fiber-lines. These voxels are referred to in the text as “fiber-points”. B) The “fiber-points” of the forceps-minor that belong to the left and right hemispheres are assigned to different clusters. The cluster corresponding to the right hemisphere is shaded darker grey and the one corresponding to the left hemisphere is shaded lighter grey. C) The vertices of the minimum perimeter polygons that enclose all the “fiber-points” in each cluster. D) The complex lines that connect the “fiber-points” closer to the sides of the minimum perimeter polygons. E) The voxels that were eventually considered to be part of the volumetric representation of this fiber-bundle.

Figure 2

Each row in this figure corresponds to a different slice of a normal volunteer. The first column contains FA maps produced from SE-EPI-DTI₁₂ data (after correction of eddy-current distortions). The second column contains the corresponding unwrapped phase-maps. The third column contains FA maps produced from SE-EPI-DTI₁₂ data that have been corrected for susceptibility-related distortions. The last column contains FA maps produced from Turboprop-DTI data. Distortions, signal loss and signal hyperintensities are visible in the SE-EPI-DTI₁₂ images near tissue-bone and tissue-air interfaces, even after correction using the phase maps. In regions distant to significant magnetic field inhomogeneities, the distortions in SE-EPI-DTI₁₂ images are not as pronounced. Turboprop-DTI images are free of eddy-current and susceptibility related artifacts.

Figure 3

Fibers of the AC, reconstructed from SE-EPI-DTI₁₂ (before distortion correction) (A) and Turboprop-DTI (B) data from the same subject. Tracking based on SE-EPI-DTI₁₂ data mapped only part of the AC (A). Tracking based on Turboprop-DTI data produced a more complete representation of the AC (B). The part of the AC that is included in the axial slice shown in C and D is characterized by increased curvature in SE-EPI-DTI₁₂ (C), compared to Turboprop-DTI (D), due to magnetic susceptibility-related distortions. The cross-section of the traced AC fibers with this axial plane is shown in red. Although the curvature of the AC is increased due to the distortions, the diffusion orientation information within the AC remains the same. Thus, the estimated pathway was not curved enough during the fiber-tracking procedure, reached the walls of the bundle, and was terminated prematurely.

Figure 4

Images of the fornix (A), CC (B), FM (C), CST (D), ILF (E), SLF (F), IOF (G), UF (H), produced using SE-EPI-DTI₁₂ (first column), SE-EPI-DTI₁₃₈ (second column), and Turboprop-DTI (third column) datasets. FA maps from the corresponding datasets are displayed in the background. The projection of all fibers assigned to a bundle is shown, independent of if the fibers are in front or behind the 2D plane of the background FA map. Bright colors correspond to fibers with high FA values and dark colors to fibers with low FA values. All data were acquired in the same scan session and on the same normal subject.

Figure 5

Boxplots of the intra-session and inter-session reproducibility for the fibers of the UF, FM, fornix, CST, IFO, ILF, produced from SE-EPI-DTI₁₂, SE-EPI-DTI₁₃₈, and Turboprop-DTI datasets of subject 1.

Figure 6

Images of the cross-section of all copies of the CST (A), ILF (B), FM (C), fornix (D), produced with wild bootstrap for two separate scan sessions, with various slices of a healthy human brain. The fiber tracts were produced from data acquired with SE-EPI-DTI₁₂ (first column), SE-EPI-DTI₁₃₈ (second column), and Turboprop-DTI (third column). MPRAGE images from the corresponding slices are displayed in the background. Blue fibers were generated from DTI data acquired in a different position of the head than red/yellow fibers. The overlap of fibers generated from data acquired in different scan sessions is higher for Turboprop-DTI than SE-EPI-DTI₁₂, and SE-EPI-DTI₁₃₈.