Side matters: diffusion tensor imaging tractography in left and right temporal lobe epilepsy

Abstract

Background and purpose: Noninvasive imaging plays a pivotal role in lateralization of the seizure focus in presurgical patients with temporal lobe epilepsy (TLE). Our goal was to evaluate the utility of diffusion tensor imaging (DTI) tractography in TLE.

Materials and methods: Twenty-one patients with TLE (11 right, 10 left TLE) and 21 controls were enrolled. A 1.5T MR imaging scanner was used to obtain 51 diffusion-gradient-direction images per subject. Eight pairs of white matter fiber tracts were traced, and fiber tract fractional anisotropy (FA) was calculated and compared with controls. Fiber tract FA asymmetry and discriminant function analysis were evaluated in all subjects and fiber tracts respectively.

Results: Compared with controls, patients with TLE demonstrated decreased FA in 5 ipsilateral fiber tracts. Patients with left TLE had 6 ipsilateral and 4 contralateral fiber tracts with decreased FA. Patients with right TLE had 4 ipsilateral but no contralateral tracts with decreased FA compared with controls. Right-sided FA asymmetry was demonstrated in patients with right TLE for 5 fiber tracts, and left-sided asymmetry, for patients with left TLE for 1 fiber tract. Discriminant function analysis correctly categorized patients into left-versus-right TLE in 90% of all cases (100% correct in all patients without hippocampal sclerosis) by using uncinate fasciculus and parahippocampal fiber tracts.

Conclusions: We found widespread reductions in fiber tract FA in patients with TLE, which were most pronounced ipsilateral to the seizure focus. Patients with left TLE had greater, more diffuse changes, whereas patients with right TLE showed changes that were primarily ipsilateral. Disease was lateralized to a high degree independent of identifiable hippocampal pathology noted on conventional MR imaging.

Fig 1.

Traced fiber tracts in a control subject. A, Bilateral cingulum fibers within the CG. B, Bilateral PH. C, Bilateral ATR. D, Bilateral IFOF. E, Right UF. F, Left ILF. G, Left SLF. H, Bilateral FORX. I, Fibers in a 29-year-old control subject.

Fig 2.

A, For CG, “OR” regions of interest were drawn in the coronal plane in the region of CG at the level just posterior to the genu of the CC and anterior to the splenium of CC, with “AND” argument placed at the midpoint of CC. B, Order of “OR” region-of-interest selection in a coronal section to obtain right FORX fiber.

Fig 3.

Fiber FA values in all patients with TLE ipsilateral and contralateral to the seizure focus were calculated by using z scores based on the mean of the control group in a given hemisphere. Note the widespread decrease in FA, especially ipsilateral to the focus. Asterisk indicates P < .05; double asterisks, P < .01.

Fig 4.

FA in fibers from right or left hemispheres in TLE subgroups were compared with the corresponding fiber and hemisphere in the control population. Orange represents fiber FA in patients with right TLE; blue, patients with left TLE; and green, controls. Asterisk indicates P < .05; double asterisks, P < .01. Error bars represent standard error.

Fig 5.

Graphic presentation of fiber asymmetries in left (blue) and right (orange) TLE groups as calculated by subtracting right from left hemisphere FA. A positive value represents a right-sided asymmetry with right-hemisphere FA less than left, as seen in the patients with right TLE. Asterisk indicates P < .05; double asterisks, P < .01. Error bars represent standard error.