Abstract
In recent years, the use of source analysis based on electroencephalography (EEG) and magnetoencephalography (MEG) has gained considerable attention in presurgical epilepsy diagnosis. However, in many cases the source analysis alone is not used to tailor surgery unless the findings are confirmed by lesions, such as, e.g., cortical malformations in MRI. For many patients, the histology of tissue resected from MRI negative epilepsy shows small lesions, which indicates the need for more sensitive MR sequences. In this paper, we describe a technique to maximize the synergy between combined EEG/MEG (EMEG) source analysis and high resolution MRI. The procedure has three main steps: (1) construction of a detailed and calibrated finite element head model that considers the variation of individual skull conductivities and white matter anisotropy, (2) EMEG source analysis performed on averaged interictal epileptic discharges (IED), (3) high resolution (0.5 mm) zoomed MR imaging, limited to small areas centered at the EMEG source locations. The proposed new diagnosis procedure was then applied in a particularly challenging case of an epilepsy patient: EMEG analysis at the peak of the IED coincided with a right frontal focal cortical dysplasia (FCD), which had been detected at standard 1 mm resolution MRI. Of higher interest, zoomed MR imaging (applying parallel transmission, ‘ZOOMit’) guided by EMEG at the spike onset revealed a second, fairly subtle, FCD in the left fronto-central region. The evaluation revealed that this second FCD, which had not been detectable with standard 1 mm resolution, was the trigger of the seizures.
Highlights
Despite considerable advancements in electroencephalography (EEG) and magnetoencephalography (MEG) source analysis, these techniques’ sensitivity, specificity, and spatial resolution to completely replace invasive recordings are still under discussion
EEG and MEG have high temporal resolution in the range of milliseconds and measure the electrical activity of neurons directly without using indirect phenomena like hemodynamics or metabolism, which is the case for functional magnetic resonance imaging or positron emission tomography (PET)
In order to check if the mismatch between the standardized low resolution brain electromagnetic tomography (sLORETA) results near the peak of the spike, MRI and seizure semiology was due to propagated activity, we have investigated time points prior to the spike peak
Summary
Despite considerable advancements in electroencephalography (EEG) and magnetoencephalography (MEG) source analysis, these techniques’ sensitivity, specificity, and spatial resolution to completely replace invasive recordings are still under discussion. Instead of solving the EEG and MEG inverse problems independently, performing combined EEG/MEG (EMEG) source analysis leads to significant improvements (Aydin et al 2015; Chowdhury et al 2015). These improvements are significant for scenarios with low signal-to-noise ratio (SNR), such as for deep sources or at the spike onset (Aydin et al 2015)
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