Ictal MEG onset source localization compared to intracranial EEG and outcome: improved epilepsy presurgical evaluation in pediatrics

Abstract

Purpose: Magnetoencephalography (MEG) has been shown a useful diagnostic tool for presurgical evaluation of pediatric medically intractable partial epilepsy as MEG source localization has been shown to improve the likelihood of seizure onset zone (SOZ) sampling during subsequent evaluation with intracranial EEG (ICEEG). We investigated whether ictal MEG onset source localization further improves results of interictal MEG in defining the SOZ.

Methods: We identified 20 pediatric patients with one habitual seizure during MEG recordings between October 2007 and April 2011. MEG was recorded with sampling rates of 600Hz and 4000Hz for 10 and 2min respectively. Continuous head localization (CHL) was applied. Source localization analyses were applied using multiple algorithms, both at the beginning of ictal onset and for interictal MEG discharges. Ictal MEG onsets were identified by visual inspection and power spectrum using short-time Fourier transform (STFT). Source localizations were compared with ICEEG, surgical procedure and outcome.

Key findings: Eight patients met all inclusion criteria. Five of the 8 patients (63%) had concordant ictal MEG onset source localization and interictal MEG discharge source localizations in the same lobe, but the source of ictal MEG onset was closer to the SOZ defined by ICEEG.

Significance: Although the capture of seizures during MEG recording is challenging, the source localization for ictal MEG onset proved to be a useful tool for presurgical evaluation in our pediatric population with medically intractable epilepsy.

Figure 1

Diagram of selection of patient criteria. Right column (blue) represents number of patients with exclusion criteria.

Figure 2

Patient #1- Initial ictal MEG onset and interictal MEG discharge source localization with different algorithms, intracranial EEG (ICEEG) findings and surgery. (A) The EEG recordings, which recorded synchronously with MEG, showed unclear EEG seizure onset followed by background attenuation with muscle artifacts. Bandwidth was set as 3–70 Hz. (B) Short-time Fourier Transform (STFT) applied to MEG identified the power spectrum that revealed the initial MEG ictal onset. High pass filter was applied as 40 Hz. Source localization for the initial ictal MEG onset using (C1) sLORETA and (D1) MUSIC, showing the focal and similar location close to motor cortex area anatomically. Source localization for an interictal discharge using (C2) sLORETA and (D2) MUSIC, showing wider variation in peak and distribution. (E) Intracranial EEG (ICEEG) findings and surgery. (E1) Overlaid grid placement for invasive presurgical evaluation with the peak of distribution of the initial MEG ictal onset on 3D reconstructed brain based on the patient’s own MRI. (E2) The overlaid MRI with actual resection area (yellow), reconstructed from the patients’ own post-operative MRI. (E3) Resective surgical plan (blue) and functional mapping result: motor cortex (hand/arm: red circle, face: blue circle), language (Broca’s: green circle), and sensory cortex (hand: purple rectangular) on the intra-operative picture.

Figure 3

Patient #5 – Initial MEG onset source localization and its propagation. (A) EEG recordings, recorded synchronously with MEG, showed bilateral diffuse high amplitude burst followed by background attenuation and muscle artifacts. (B) Short-time Fourier Transform (STFT) applied to MEG identified the power spectrum that revealed the initial ictal MEG onset. High pass filter was applied as 35 Hz. (C1–3) Initial ictal MEG onset source localizations and its propagation with sLORETA and (D1–3) with MUSIC.