Abstracts

Rationale: Intracranial EEG (iEEG), in comparison to scalp EEG, can show very focal epileptiform discharges, such as mesial temporal discharges, which can not be seen on scalp EEG. We have previously confirmed the safety of using intracranial electrodes in a high-field (3.0 T) MR environment. This project was designed to assess the Blood Oxygen Level Dependent (BOLD) response to focal iEEG discharges.Methods: We recruited 10 patients undergoing intracranial video-EEG monitoring for seizure focus localization as pre-surgical assessment. All patients had subdural strip implantation, one in combination with a unilateral grid and one in combination with depth electrodes. Subjects were connected to a MR compatible EEG system and monitored in real time throughout the study with simultaneous fMRI. Up to 64 channels of EEG in eight electrodes can be recorded during fMR imaging. Therefore, the most active electrodes based on long-term EEG monitoring were selected for the study. Functional images were collected using a series of single-shot GRE-EPI volumes, providing T2* contrast. Epileptiform discharges were identified and marked by two experienced electroencephalographers familiar with EEG recorded inside MR scanner. BOLD fMRI responses were modeled by convolving the timing of interictal events with multiple hemodynamic response functions of varying time-to-peak (0-12s). The data were cluster corrected, and the largest and most statistically significant clusters identified and used for analysis (Z-score 3.1, p value <0.001). We did not assess the deactivation (negative BOLD response) in this study.Results: Ten patients completed the simultaneous iEEG-fMRI studies with no incident or adverse effect. Three studies were excluded due to excessive artifacts leading to poor quality EEG. Seven patients (5 female) with an average age of 25 yrs had analyses based on the location of the interictal epileptiform discharges. One patient had bilateral independent spikes, which were analyzed separately leading to a total of eight different analyses. The location of spikes was in the mesial temporal in four, neocortical temporal in two, frontal in one, and parietal in one. All had BOLD responses with variable peak HRF with max zstat from 2 to 9 seconds after the EEG events. The locations of BOLD responses matched with the spike locations in 6 analyses (75%). In the remaining cases, the maximum BOLD response was seen in distant areas.Conclusions: This study showed that iEEG-fMRI could be safely performed in our 3.0 T MR scanner with high (75%) concordance between the locations of epileptic spikes and BOLD responses. This opens a new avenue in better understanding of hemodynamic changes of locally recorded focal epileptogenic activity.