Rationale:
Studies suggest that the temporal pole (TP) has an important role in the genesis and propagation of seizures in nonlesional temporal lobe epilepsy (TLE-NL) (Chabardès et al., 2005). Better understanding of the role of the TP in TLE-NL, and it’s connections, is important for informing resective surgery. The purpose of this study was to characterize the microstructure of white matter fibre bundles proximal to the TP, namely the inferior longitudinal fasciculus (ILF) and uncinate fasciculus (Unc) using diffusion kurtosis imaging (DKI), a technique able to quantify diffusion in complex tissue environments.
Method:
TLE subjects (n = 10) with normal clinical MRI scans and detailed electroclinical findings showing 4 left, 5 right and 1 uncertain temporal seizure onset zones, along with healthy controls (n = 21) were recruited for this study. All subjects underwent the same imaging protocol performed on a 3T MRI scanner. Diffusion-weighted images (DWI) were obtained via multiband echo-planar sequence with 3 diffusion weightings (b = 0, 1300, and 2600 s/mm2) along 130 diffusion-encoding directions acquired twice with left-right, right-left phase encoding directions respectively. The acquired DWIs were corrected for echo planar imaging, eddy current distortion and Gibbs’ ringing. DKI measurements including mean kurtosis (MK) were obtained using the diffusion kurtosis estimator tool v2.6. Anatomically constrained tractography (Smith et al., 2012) was performed to extract the ILF and Unc using MRtrix v3.0. An automated fiber bundle quantification method (Yeatman et al. 2012) was used to quantify diffusion kurtosis measurements at 100 equidistant points along the two fiber bundles. In addition, using a novel DKI microstructural tissue modeling approach (McKinnon et al. 2018), we estimated axonal water fraction (AWF) as a surrogate marker for axonal loss within the two fiber bundles.
Results:
We found significant MK changes between the groups (patients vs controls) in the anterior left ILF compared to the right ILF (p-value < 0.05) (Figure 1). We also noticed significant MK changes between the groups at the anterior and posterior segment of the left Unc. The measured AWF did not show any significant changes between the groups in the ILF. On the other hand, left Unc showed significant AWF changes at the anterior segment and around the middle part of the right Unc (Figure 2).
Conclusion:
These findings demonstrate that DKI has potential for detecting microstructural changes in ILF and Unc proximal to the TP in TLE-NL patients. Identifying abnormalities at specific locations along the ILF and Unc prior to surgery in TLE-NL could be used to guide the procedure by more completely identifying the region to be resected, potentially resulting in improved outcomes.
Funding:
:This work was supported by the CIHR Foundation, NSERC Discovery, the Canada First Research Excellence Fund, Brain Canada, and the Ontario Brain Institute Epilepsy Program (EpLink).
FIGURES
Figure 1