Identification of the Epileptogenic Lobe in Neocortical Epilepsy with Proton MR Spectroscopic Imaging

Summary: Purpose: The aim of this study was to evaluate the usefulness of multislice magnetic resonance spectroscopic imaging (MRSI) in combination with tissue segmentation for the identification of the epileptogenic focus in neocortical epilepsy (NE).

Methods: Twenty patients with NE (10 with MRI-visible malformations, 10 with normal MRI) and 19 controls were studied. In controls, N-acetylaspartate NAA/Cr and NAA/Cho of all voxels of a given lobe were expressed as a function of white matter, and thresholds were determined by calculating the 95% prediction intervals (PIs) for NAA/Cr and NAA/Cho. Voxels with NAA/Cr or NAA/Cho values less than the 95% PI were defined as “pathological.” Z-scores were calculated. Depending on the magnitude of those z-scores, we used two different methods (score-localization or forced-localization) to identify in a given subject the lobe with the highest percentage of pathological voxels, which was supposed to represent the epileptogenic lobe.

Results: MRSI correctly identified the lobe containing the epileptogenic focus as defined by EEG in 65% of the NE patients. MRSI localization of the focus was correct in 70% of the patients with an MRI-visible malformation and in 60% of the patients with normal MRI. Of the patients, 15% had metabolically abnormal brain regions outside the epileptogenic lobe, and 35% of the patients had evidence for secondary hippocampal damage.

Conclusions: MRSI may be helpful for the identification of the epileptogenic focus in NE patients, even in NE with normal MRI.