Clinical yield of magnetoencephalography distributed source imaging in epilepsy: A comparison with equivalent current dipole method

Abstract

Objective: Source localization of interictal epileptic discharges (IEDs) is clinically useful in the presurgical workup of epilepsy patients. It is usually obtained by equivalent current dipole (ECD) which localizes a point source and is the only inverse solution approved by clinical guidelines. In contrast, magnetic source imaging using distributed methods (dMSI) provides maps of the location and the extent of the generators, but its yield has not been clinically validated. We systematically compared ECD versus dMSI performed using coherent Maximum Entropy on the Mean (cMEM), a method sensitive to the spatial extent of the generators. Methods: 340 source localizations of IEDs derived from 49 focal epilepsy patients with foci well-defined through intracranial EEG, MRI lesions, and surgery were analyzed. The comparison was based on the assessment of the sublobar concordance with the focus and of the distance between the source and the focus. Results: dMSI sublobar concordance was significantly higher than ECD (81% vs 69%, P < 0.001), especially for extratemporal lobe sources (dMSI 5 84%; ECD 5 67%, P < 0.001) and for seizure free patients (dMSI 5 83%; ECD 5 70%, P < 0.001). The median distance from the focus was 4.88 mm for ECD and 3.44 mm for dMSI (P < 0.001). ECD dipoles were often wrongly localized in deep brain regions. Conclusions: dMSI using cMEM exhibited better accuracy. dMSI also offered the advantage of recovering more realistic maps of the generator, which could be exploited for neuronavigation aimed at targeting invasive EEG and surgical resection. Therefore, dMSI may be preferred to ECD in clinical practice. Hum Brain Mapp 39:218-231, 2018.V C 2017 Wiley Periodicals, Inc.

Reproducibility ofEEG‐MEGfusion source analysis of interictal spikes: Relevance in presurgical evaluation of epilepsy

“…Spike averaging has been adopted in most studies (Bast et al, ; Hara et al, ; Heers et al, , Pellegrino et al, ; Tanaka et al, ; Wennberg & Cheyne, ) to increase the SNR and improve the reliability of the source localization solutions. However, it is well known that waveforms of individual spikes might actually be quite variable in epileptic patients (Aydin et al, ; Köhling et al, ).…”

“…Brainstorm software (Tadel, Baillet, Mosher, Pantazis, & Leahy, ) was used to preprocess the EEG/MEG data offline. Data were downsampled to 600 Hz, synthetic third‐order gradient noise correction was applied to MEG data, DC offset was removed, EEG was re‐referenced to average montage, and 60 Hz notch filter (and its harmonics) was applied (Heers et al, ; Pellegrino et al, ). EEG and MEG data were visually inspected and inter‐ictal spikes were marked using the DataEditor software (MISL, Vancouver, Canada) by two experienced neurophysiologists (GP and EK).…”

“…Our standard clinical investigation involves averaging spikes with similar morphology and topography to improve the SNR of the EEG/MEG signals and then applying cMEM source localization method (Heers et al, ; Pellegrino et al, ). We first presented a comparison between standard EEG and MEG source localization of averaged spike for all 34 studies (range, 10–287 spikes per study) from the 26 selected patients.…”

“…EEG and MEG source localization techniques are used to localize the generators of epileptic discharges, but the main challenge lies not only in localizing the generators but also in accurately recovering their spatial extension. Therefore, several methods have been designed for localizing spatially extended generators (Chang, Nummenmaa, Hsieh, & Lin, ; Cheyne et al, ; Chowdhury, Lina, Kobayashi, & Grova, ; Chowdhury et al, ; Grova et al, ; Huiskamp, Agirre‐Arrizubieta, & Leijten, ; Jung et al, ; Sohrabpour, Lu, Worrell, & He, ; Zhu, Zhang, Dickens, King, & Ding, ), and source localization with the coherent maximum entropy on the mean (cMEM) method is one such method that has been extensively studied on simulated and clinical data, and provided reliable and accurate localization of the sources of EEG and MEG discharges together with their spatial extent (Chowdhury et al, ; Grova et al, ; Heers et al, ; Pellegrino et al, ). In fact, recently we have compared cMEM with the standard dipole fitting approach on a large cohort of epilepsy patients and showed the reliability of cMEM on MEG source localization of clinical data (Pellegrino et al, ).…”

Characterizing hippocampal dynamics with MEG: A systematic review and evidence‐based guidelines

“…For instance, many early clinical studies used equivalent dipole methods to localize epileptiform spike activity to a seizure focus, and initially, these methods were also used in general research studies as well. However, while suitable for modeling early evoked fields located near the cortical surface, localization methods using a simple dipole are now considered to be an oversimplification, whereas minimum‐norm and beamforming‐based inverse methods are more appropriate for components that are likely to be from deeper or less dominant sources, have longer, later, or more variable duration, are generated by nonfocal distributed sources, or simply are largely uncharacterized and unknown based on current understanding (Chatani et al, ; Gramfort et al, ; Henson, Mattout, Phillips, & Friston, ; Lalancette, Quraan, & Cheyne, ; Pellegrino et al, ).…”