Abstracts

Rationale: We investigate the contribution of Diffusion Tensor Imaging (DTI) and Diffusion Tensor Tractography (DTT) in identifying abnormalities in MRI negative epilepsy surgery candidates with cryptogenic extramesiotemporal focal epilepsies. Methods: 14 patients with cryptogenic extramesiotemporal focal epilepsy were examined. DTI data was acquired on a GE Signa HDx 3T Scanner, using an acquisition scheme with 64 diffusion weighted directions, a b-value of 1000m/s2, 2.4 mm slice thickness and 2 mm in-plane resolution. Fractional anisotropy (FA) maps were investigated for focal changes and asymmetries. Streamline DTT of the whole brain was used as an exploratory method in the absence of a structural lesion and the number of reconstructed streamlines in homologous anatomical areas of the left and right hemisphere were compared. Asymmetries of more than 10% for FA maps and more than 20% for the streamline count were rated as a significant finding.Results: Asymmetries in the number of reconstructed streamlines were found in nine of the 14 patients (64%). In eight of them, these changes were consistent with the clinically suspected seizure onset zone, based on video-EEG-monitoring and nuclear medicine data, however, in two patients DTT indicated more widespread, hemispheric changes, beyond the seizure onset zone. FA maps showed asymmetries beyond 10% in only one patient. In two patients, the seizure onset zone was confirmed in the area of DTT abnormalities by intracranial electrodes, the other patients are still awaiting invasive evaluation, including the one with discrepant DTI findings.Conclusions: These preliminary data show the potential role of DTI and DTT as complementary lateralizing and localizing imaging modality in the presurgical evaluation of cryptogenic extramesiotemporal epilepsy patients. We hypothesize, the observed changes reflect migration disorders, where heterotopic neurons disrupt the microstructural order of white matter underlying the seizure onset zone. Follow ups with intracranial electrodes and correlation with histopathology are required for further interpretation of these findings. DTT appears to be more sensitive than FA maps, and this method may be less sensitive in patients with small circumscribed focal pathologies compared to patients with a more widespread pathology.