Lateral Asymmetry of Superior Longitudinal Fasciculus: A White Matter Tractography Study

3, which was interpolated to isotropic dimensions. The total imaging time was 7-8 minutes. Image misregistration from motion and eddy current distortion was corrected using a 2D affine registration algorithm in AIR [7]. Field map correction was subsequently applied to correct for EPI distortions resulting from B0 inhomogeneities. White Matter Tractography: Fiber trajectories were estimated using the streamline algorithm [1] with a second-order Runge-Kutta integration method. A complete set of fiber trajectories in the brain’s diffusion tensor field was obtained by placing seeds in all the voxels with FA>0.4. The propagation of an individual trajectory was terminated when it reached a voxel with FA<0.2 or when the angle between two consecutive steps was greater than 45°. SLF was separated from the complete set of trajectories by retaining those fibers that intersected pre-defined regions of interest (ROIs). The ROIs were chosen to enclose the right and left SLF cross-sections visible in coronal FA and color maps. A set of three cross-sections was used for each hemisphere and each subject, to capture fiber estimates that joined the tract for either shorter distances or for its entire length. SLF sub-segmentation The trajectories that curved inferiorly to enter the temporal lobe were segmented out from the complete set of SLF trajectories. The SLF volumes defined by the fronto-temporal connections were calculated for both left and right hemispheres. Results The estimates of the segmented fiber tracts for six of the subjects are shown in Figure 1. Variability of the shape of the estimated tracts among subjects, and between hemispheres, can be observed. A common feature is the larger volume of the left hemisphere frontal-to-temporal connections, which is apparent by visual inspection of the tractography results. For one subject (Subject 10) no temporal connections of SLF were detected in the right hemisphere, although the cause of this was unclear. The ratios of left to right fronto-temporal connections volumes (depicted in yellow in Figure 1) for the remaining subjects ranged from 1.1 to 6.7, with a median of 1.65.