Abstracts

Rationale: MRI studies in temporal lobe epilepsy (TLE) have provided evidence for atrophy and signal changes within the temporal lobe white matter (Bernasconi, Neuroimage 2004) that may be tract-specific, as shown by diffusion tensor imaging (DTI) and tractography (Concha, Ann Neurol 2005; Rodrigo, Eur Radiol 2007; Focke, Neuroimage 2008). Since conventional analyses of DTI tractography have provided a single value per tract, it remains unclear whether these abnormalities involve the entire tract or are focal. The goal of this study was to characterize spatial patterns of water diffusion along the trajectory of white matter fascicles carrying connections of the temporal lobe in TLE. Methods: We studied 13 patients with pharmaco-resistant TLE (11 males, mean age: 37±11; 7/6 right/left TLE) and 12 age- and sex-matched healthy controls. DTI was performed on a 1.5 T Siemens scanner (54 slices, 30 diffusion gradient directions, b=1000 s/mm², voxel size 2.5×2.5×2.5 mm³). Tractography was performed in native space in all voxels with fractional anisotropy (FA) values >0.2. The uncinate fasciculus (UF), inferior longitudinal fasciculus (ILF) and arcuate fasciculus (AF), as well as the supra-callosal portion of the cingulum (Cg) were virtually dissected (Figure 1). All tracts were converted to binary volumes and the distance at every voxel with respect to the origin of the tract was determined. To achieve inter-subject anatomical correspondence of tract segments, this distance was partitioned into bins. FA and mean diffusivities (MD) were assessed at each bin and converted into z-scores (based on the values of the corresponding hemisphere in controls). Between-group comparisons were performed at each bin using two-tailed Student's t-tests (α=0.05). Correction for multiple comparisons was performed using random field theory, which identifies clusters of abnormalities providing robust control for family-wise error (Worsley, HBM 1999). We also performed standard whole-tract analyses by examining between-group differences. Results: Whole-tract analysis showed increased MD in the ipsilateral uncinate (p=0.005), inferior longitudinal (p=0.004) and arcuate (p=0.05) fascicles. The spatial analysis located the increased MD exclusively in the temporal portions of the UF and AF (Figure 2), whereas changes were distributed all along the ILF. No abnormalities were found in the cingulum. Small clusters of increased MD were found in the temporal portion of the UF and the occipital portion of the ILF contralateral to seizure focus. Conclusions: We provide the first evidence that TLE fascicles carrying connections of the temporal lobe show focal diffusion abnormalities. These alterations of water diffusion are located in the portions of the tracts in the temporal lobes ipsilateral to the focus. The ability to detect these subtle changes may help lateralizing the seizure focus, particularly in patients with normal conventional MRI.