Abstracts

EEG-fMRI and EEG dipole source localisation are two non-invasive imaging methods that can be applied to the study of the haemodynamic and electrical consequences of epileptic discharges. Using them in combination has the potential to allow imaging with the spatial resolution of fMRI and the temporal resolution of EEG. However, although considerable data is available concerning their concordance in studies with event-related potentials (ERPs), less is known about how well they agree in epilepsy. To this end, seventeen patients were selected from a database of fifty-seven who had undergone an EEG-fMRI scanning session followed by a separate EEG session outside of the scanner for dipole source localisation. Patients were selected with not more than two spike types observed during fMRI scanning and in whom comparable spike types, based on spike morphology and topography, could be identified in the EEGs recorded inside and outside the scanner. Eighteen data sets from seventeen patients were analysed. Patients were selected independently of the EEG-fMRI results. EEG-fMRI images were acquired in one of two 1.5T MR scanners (Vision and Sonata, Siemens, Germany) using echo-planar imaging. An anatomical scan was also acquired. Twenty-one channels of EEG were recorded simultaneously using an EMR amplifier (Schwarzer, Germany). Immediately after the EEG-fMRI scanning session, a second EEG was acquired with additional electrodes giving a total of forty-four. Dipole source localisation was performed using a spatio-temporal approach on averaged spikes using Curry V4.5 (Neuroscan, Hamburg, Germany) with individual boundary element head models created from the anatomical MRI. Four patients had no significant EEG-fMRI responses. In the remaining 13 data sets from 12 patients, the mean distance between the closest activated voxel (positive fMRI response) and the dipoles was 32.5mm, while that to the voxel with the peak positive t value was 58.5mm. The corresponding values for deactivated voxels were 34.0mm and 60.8mm. The distances between dipoles and EEG-fMRI activations measured in the current study are considerably larger than is generally observed when comparing ERP dipole modelling and fMRI. This may be a result of the relatively widespread field associated with epileptic activity, which can lead to artificially deep dipoles, and of the occurrence of EEG-fMRI responses remote from the presumed focus of the epileptic activity. The results suggest that EEG inverse solutions for equivalent current dipole approaches should not be strongly constrained by EEG-fMRI results in epilepsy, and that the use of distributed source modelling may be a more appropriate way of combining EEG-fMRI results with source localisation techniques. (Supported by grant MOP 38079 of the Canadian Institutes of Health Research.)