High angular resolution diffusion imaging probabilistic tractography of the auditory radiation

Abstract

Background and purpose: The auditory radiation crosses other white matter tracts and cannot reliably be delineated or quantitatively assessed with DTI fiber tracking. This study investigates whether HARDI fiber tracking can be used to robustly delineate the full extent of the tract.

Materials and methods: HARDI (64-direction, b=3000 s/mm²) and DTI (30-direction, b=1000 s/mm²) were acquired from 25 control participants between 8 and 26 years old. Probabilistic HARDI and DTI fiber tracking of the auditory radiation was performed with starting and filter regions automatically generated from the FreeSurfer white matter parcellation. DTI fiber tracking was performed with both the 64-direction and the 30-direction datasets. Fiber-tracking trials demonstrating connectivity from the Heschl gyrus to the medial geniculate nucleus were considered successful.

Results: The HARDI fiber tracking success rate was 98% and was significantly higher than the 64-direction DTI rate of 50% or the 30-direction DTI rate of 42% (P < .001). The success rates of HARDI fiber tracking for the left and right auditory radiations were not significantly different. In contrast, the left auditory radiation was successfully delineated with DTI fiber tracking at a higher rate than the right auditory radiation.

Conclusions: HARDI can discriminate the complex white matter pathways at the junction of the auditory radiation and the ILF. HARDI fiber tracking can reliably delineate the auditory radiation.

Fig 1.

The starting region of interest for fiber tracking within the Heschl gyrus is shown on the left. The target region of interest on the inferior surface of the thalamus is shown on the right. The regions are shown overlaid on axial sections through the b=0 s/mm² echo-planar volume from the 64-direction HARDI acquisition.

Fig 2.

Q-ball reconstructions of the HARDI data are shown for each voxel on an axial section. HARDI fiber tracks (red streamlines) course from the AC to the thalamus. The ILF (green fiber peaks and orientation indicated with arrow) intersects the auditory radiation.

Fig 3.

HARDI fiber tracks (red) and DTI fiber tracks (blue) are visualized in the left and right hemispheres. DTI fiber tracks are from the 64-direction dataset. The left hemisphere is on the left side of the figure. Both HARDI and DTI fiber tracks were launched from the same starting regions in the Heschl gyrus. The HARDI fiber tracks reaching the thalamic target regions are retained. For this figure, all DTI fiber tracks are retained regardless of destination. The DTI fiber tracks emerge from the Heschl gyrus and follow the ILF in either the anterior or posterior direction. No DTI fiber tracks cross the ILF to reach the thalamus in the right hemisphere.

Fig 4.

HARDI and DTI fiber tracks connecting the AC to the thalamic target region are shown in a case where both methods were successful in each hemisphere. The number of fiber trajectories passing through each voxel is encoded with the overlay color. The yellow voxels have the highest probability of being within the auditory radiation.

Fig 5.

The threshold for successful fiber-tracking is varied between 1 and 200 trajectories connecting the AC to the thalamus. The percentage of successful HARDI and DTI fiber-tracking trials is shown at each threshold. An asterisk indicates that the HARDI rate of success is significantly higher than the respective DTI fiber-tracking rate of success (P < .01).