Abstract
Simultaneous EEG–fMRI acquisitions in patients with epilepsy often reveal distributed patterns of Blood Oxygen Level Dependant (BOLD) change correlated with epileptiform discharges. We investigated if electrical source imaging (ESI) performed on the interictal epileptiform discharges (IED) acquired during fMRI acquisition could be used to study the dynamics of the networks identified by the BOLD effect, thereby avoiding the limitations of combining results from separate recordings.Nine selected patients (13 IED types identified) with focal epilepsy underwent EEG–fMRI. Statistical analysis was performed using SPM5 to create BOLD maps. ESI was performed on the IED recorded during fMRI acquisition using a realistic head model (SMAC) and a distributed linear inverse solution (LAURA).ESI could not be performed in one case. In 10/12 remaining studies, ESI at IED onset (ESIo) was anatomically close to one BOLD cluster. Interestingly, ESIo was closest to the positive BOLD cluster with maximal statistical significance in only 4/12 cases and closest to negative BOLD responses in 4/12 cases. Very small BOLD clusters could also have clinical relevance in some cases. ESI at later time frame (ESIp) showed propagation to remote sources co-localised with other BOLD clusters in half of cases. In concordant cases, the distance between maxima of ESI and the closest EEG–fMRI cluster was less than 33 mm, in agreement with previous studies.We conclude that simultaneous ESI and EEG–fMRI analysis may be able to distinguish areas of BOLD response related to initiation of IED from propagation areas. This combination provides new opportunities for investigating epileptic networks.
Highlights
In patients with epilepsy, non-invasive imaging techniques can offer precious information regarding the localisation and dynamics of epileptic activity
In 2 of these cases, the closest positive Blood Oxygen Level Dependant (BOLD) cluster was in the same lobe as ESI at IED onset (ESIo) but more distant than the cluster corresponding to BOLD decrease
Good concordance between electrical source imaging (ESI), EEG-fMRI and subdural grid EEG recording in a single patient has been reported in an early EEG-fMRI paper (Seeck, Lazeyras et al 1998) and, more recently, a study of 5 patients with focal epilepsy concluded that the results of ESI using multiple equivalent dipoles and clusters of significant BOLD responses were concordant both with each other and with intracranial recording (Benar, Grova et al 2006)
Summary
Non-invasive imaging techniques can offer precious information regarding the localisation and dynamics of epileptic activity. EEG-correlated functional Magnetic Resonance Imaging (EEG-fMRI) is a rapidly developing non-invasive imaging technique that can map regional changes in the cerebral oxygenation and blood flow (BOLD signal (Ogawa, Lee et al 1990)) that are time-locked to Interictal Epileptiform Discharges (IED) identified on the simultaneously recorded EEG. Electric Source Imaging (ESI) is the reconstruction of the three-dimensional brain electrical activity derived from scalp EEG data, relying on various mathematical algorithms and constraining neurophysiological hypotheses (see (Michel, Murray et al 2004) for a review). This technique benefits from a very high temporal resolution corresponding to the EEG sampling frequency and from the fact that EEG reflects neural activity (local synchrony) more directly than fMRI.
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