Abstract
MEG and EEG source analysis is frequently used for the presurgical evaluation of pharmacoresistant epilepsy patients. The source localization of the epileptogenic zone depends, among other aspects, on the selected inverse and forward approaches and their respective parameter choices. In this validation study, we compare the standard dipole scanning method with two beamformer approaches for the inverse problem, and we investigate the influence of the covariance estimation method and the strength of regularization on the localization performance for EEG, MEG, and combined EEG and MEG. For forward modelling, we investigate the difference between calibrated six-compartment and standard three-compartment head modelling. In a retrospective study, two patients with focal epilepsy due to focal cortical dysplasia type IIb and seizure freedom following lesionectomy or radiofrequency-guided thermocoagulation (RFTC) used the distance of the localization of interictal epileptic spikes to the resection cavity resp. RFTC lesion as reference for good localization. We found that beamformer localization can be sensitive to the choice of the regularization parameter, which has to be individually optimized. Estimation of the covariance matrix with averaged spike data yielded more robust results across the modalities. MEG was the dominant modality and provided a good localization in one case, while it was EEG for the other. When combining the modalities, the good results of the dominant modality were mostly not spoiled by the weaker modality. For appropriate regularization parameter choices, the beamformer localized better than the standard dipole scan. Compared to the importance of an appropriate regularization, the sensitivity of the localization to the head modelling was smaller, due to similar skull conductivity modelling and the fixed source space without orientation constraint.
Highlights
In about 30% of patients suffering from focal epilepsy, pharmacotherapy with antiepileptic drugs is insufficiently effective [1]
The calibrated skull compacta (SC) conductivity was determined as 0.0167 S/m for patient 1
The beamformer power is not focused on a single spot, but positions with nearly the same power are distributed around it, see Figure 4 or Figure 5, for an example
Summary
In about 30% of patients suffering from focal epilepsy, pharmacotherapy with antiepileptic drugs is insufficiently effective [1]. The relative insensitivity of the MEG localization to the variability of skull conductivity [6] makes it a promising modality, allowing early identification of surgery candidates [7]. It improves planning and results of invasive recordings [8,9], yields non-redundant information in up to about 30% of cases, and is confirmatory in an additional 50% [10,11]. While EEG measures the same underlying activity, it gives complementary information to the MEG [12,13,14]. Analyzing both MEG and EEG together may give a more complete picture than the single modalities
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