Abstracts

Rationale: EEG and functional MRI (fMRI) have been successfully combined to help localize changes in brain activity associated with epileptiform discharges. The superior temporal resolution of EEG is complemented by the high spatial resolution of fMRI. Despite the success of scalp EEG-fMRI experiments in the study of epilepsy, it has limited ability to detect discharges in small cortical areas or from deep structures. The ability to detect low amplitude, deeply originating discharges would greatly increase the yield of EEG-fMRI studies and thereby facilitate the study of the spatiotemporal mechanisms of seizure generation. Intracranial EEG (ICE) provides an opportunity to improve the power of EEG-fMRI. We have previously confirmed the safety of using intracranial electrodes in a high-field (3.0 T) MR environment. In this report, we provide data from the first simultaneous ICE-fMRI study of human subjects at 3.0 T. Methods: We recruited 7 patients undergoing intracranial video-EEG monitoring for seizure characterization. Of the implanted electrodes, two electrode strips were selected as the most active and monitored during data collection. Subjects were connected to a MR compatible EEG system and monitored in real time throughout the study with simultaneous fMRI. Functional images were collected using a series of single-shot GRE-EPI volumes, providing T2*-W contrast. In addition to the functional scans, the protocol included low and high resolution anatomical scans and a FLAIR study. Epileptiform discharges were identified and BOLD fMRI responses were modeled by convolving the timing of events with a hemodynamic response function. Regressors were included in a general linear model to generate maps of statistical significance (Z-score) and register functional images to the anatomical data.Results: Simultaneous ICE-fMRI studies were performed without incident in all patients. Interpretable EEG data was obtained in 4 studies with 3 EEG recordings uninterpretable due to persistent EEG artifact. Subject 1 had 105 epileptiform discharges in 10 minutes of fMRI scanning, Subject 2 had 478 independent bitemporal discharges in 20 minutes, Subjects 5 and 7 had 952 and 1373 discharges in 60 minutes, respectively. Activation maps of subjects 2 and 7 are shown in Figures 1 and 2. Of the successful studies, subjects 1 and 7 had malformations of cortical development (MCD) and the remaining had no obvious lesion on MRI. Subjects with MCDs had more widespread fMRI activation than those without lesions. Specifically, activity was found in both ipsi- and contralateral hemispheres at locations distant from the suspected focus. Conversely, patients with non-lesional epilepsy had maximal activity at the location where interictal discharges were recorded.Conclusions: ICE-fMRI is safe at 3.0 T and is an effective tool for the assessment of pre-surgical patients with epilepsy. We have also shown that the networks underlying the generation of interictal discharges may be different between patients with no obvious lesion and those with MCDs.