Abstracts

Rationale: Diffusion tensor imaging (DTI) delineates the 3D structure of white matter (WM) tracts and measures their microstructural integrity. DTI tractography has shown evidence for tract-specific pathology in temporal lobe epilepsy (TLE) [1,2]. As conventional whole-tract analyses have provided a single value per bundle, it remains unclear whether abnormalities are focal or reflect changes of the entire tract. Our goal was to characterize spatial patterns of water diffusion along WM fascicles carrying temporal lobe connections. Methods: We studied the uncinate, arcuate and inferior longitudinal fasciculi in 30 consecutive patients with drug-resistant TLE and 21 healthy controls matched for handedness, age and gender (Figure 1). To map DTI parameters along tracts, we: 1) created a binary mask of each tract; 2) obtained the tract s skeleton through a distance transformation and interpolation using a B-spline; and 3) parcellated the skeleton into bins. We evaluated bin-wise differences in DTI parameters along each tract using one-tailed Student s t-tests. Significances were corrected for multiple comparisons using random field theory (p<0.05).Results: Results are shown in Figure 2. Ipsilateral to the seizure focus, the uncinate and the inferior longitudinal fasciculi showed abnormally increased mean diffusivity (MD) throughout their length, although abnormalities were more severe in the temporal segments. On the other hand, MD was increased only in the temporal portion of the arcuate fasciculus. In addition, we found reduced fractional anisotropy (FA) in the ipsilateral temporal portion of the uncinate fasciculus. Contralateral increased MD was less marked and mainly present in the temporal portion of the inferior longitudinal and the uncinate fasciculi, whereas the latter displayed also anomalies located in its frontal segment.Conclusions: Our analysis of DTI parameters along tracts localized increased MD restricted to or more prominent within the ipsilateral temporal lobe. These abnormalities tapered off as tracts exited the temporal lobe. This centrifugal pattern of WM microstructural pathology likely reflects astrogliosis related to chronic seizure activity in the vicinity of the focus. Alternatively, axonal and myelin pathology may be consistent throughout the tract's length, but the proportion of altered axons may decrease as intact axons distant from the epileptogenic tissue join the fascicle. References 1. Concha L, et al. Annals of Neurology 2005: 57, 188-196. 2. Yogarajah M, et al. Epilepsia 2008; 49, 189-200.