Abstracts

Rationale: It has been shown that EEG-fMRI can be useful in the pre-operative work-up of some difficult epilepsy cases, and this research technique has the potential to move to the clinical field. However, the reproducibility of EEG-fMRI findings still needs to be established. In this study, the intra-subject reproducibility of EEG-fMRI and possible superiority of higher magnetic field strength were studied in epilepsy patients who were scanned twice, first with 1.5T scanner, and later with 3.0T MRI machine. Methods: Patients who had one 1.5T and one 3.0T EEG-fMRI study with more than 10 occurrences of the same interictal epileptic discharges (IEDs) in both studies were included. For both studies runs with an approximately equal number of the same type of IEDs were selected, IEDs identified on the EEG were convolved with a regressor with four hemodynamic response functions with peaks at 3, 5, 7 and 9s. Significant responses were defined with a |t|>3.1 and a spatial extent of 4 voxels (p=0.05, corrected). Results were reviewed and maximum absolute t-score with anatomical position and cluster sizes were reported, comparing the two scanning sessions. Results: Seven patients matched the inclusion criteria. Five patients had similar results in the two scans. Four of these had better detection of BOLD activation or deactivation in terms of higher maximum t-score and cluster size with 3.0T than with 1.5T. The fifth patient in this group had very similar pattern and maximum t-score in both activation and deactivation. The two remaining patients had no significant activation/deactivation in both studies, despite an acceptable number of EEG events. Conclusions: This study compared EEG-fMRI results from patients with focal epilepsy in 1.5 and 3.0T MRI scanners. When IED type and frequency are similar in two studies, interictal continuous EEG-fMRI appears to be a reproducible method. The absence of BOLD response also appears reproducible, although its cause is unknown. EEG-fMRI of IEDs thus fulfills a pre-requisite to be a clinically applicable method. As expected from an fMRI study, we found that a stronger magnetic field seems to result in better sensitivity. Further studies using more subjects are suggested to evaluate more solidly this reproducibility and to see if applies equally to specific clinical, electrographic, or anatomical findings. Supported by CIHR grant MOP-38079