Ictal Magnetic Source Imaging in Presurgical Assessment

Abstract

Ictal MEG recordings constitute rare data. The objective of this study was to evaluate ictal magnetic source localization (MSI), using two algorithms: linearly constrained minimum variance (LCMV), a beamforming technique and equivalent current dipole (ECD). Ictal MSI was studied in six patients. Three of them were undergoing post-operative re-evaluation. For all patients, results were validated by the stereoelectroencephalographic (SEEG) definition of the epileptogenic zone (EZ). EZ was quantified using the epileptogenicity index (EI) method, which accounts for both the propensity of a brain area to generate rapid discharges and the time for this area to become involved in the seizure. EI values range from 0 (no epileptogenicity) to 1 (maximal epileptogenicity). Levels of concordance between ictal MSI and EZ were determined as follows: A: ictal MSI localized the site whose value EI = 1, B: MSI localized a part of the EZ (not corresponding to the maximal value of EI = 1), C: a region could be identified on ictal MSI but not on SEEG, D: a region could be identified on SEEG but not on MSI, E: different regions were localized on MSI and SEEG. Ictal MEG pattern consisted of rhythmic activities between 10 and 20 Hz for all patients. For LCMV (first maxima), levels of concordance were A (two cases), B (two cases) and E (two cases). For ECD fitted on each time point separately (location characterized by the best goodness-of-fit value), levels of concordance were A (one case), B (one case), D (three cases) and E (one case). For ECD calculated for the whole time window, levels of concordance were A (two cases) and D (four cases). Source localization methods performed on rhythmic patterns can localize the EZ as validated by SEEG. In terms of concordance, LCMV was superior to ECD. In some cases, LCMV allows extraction of several maxima that could reflect ictal dynamics. In a medial temporal lobe epilepsy case, ictal MSI indicated an area of delayed propagation and was non-contributory to the presurgical assessment.

Source localization of ictal epileptic activity based on high‐density scalp EEG data

“…By applying ESI of the focal spikes that precede and occur at the onset of the seizure, localized ictal onsets close to the same region were identified by intracranial monitoring in 8 of 10 cases. Ictal magnetic source imaging has also been described, 36,37 but ictal MEG is limited to patients with a high seizure frequency and hypomotor seizures. 38 The analysis of interictal scalp EEG with ESI appears highly relevant for presurgical evaluation.…”

Source localization of the seizure onset zone from ictal EEG/MEG data

“…The vast majority of previous study on ictal source localization were based on small series of less than 10 patients [Assaf et al, ; Badier et al, ; Ding et al, ; Eliashiv et al, ; Fujiwara et al, ; Ishibashi et al, ; Ishii et al, ; Kakisaka et al, ; Kovac et al, ; Oishi et al, ; Shiraishi, 2001, ; Stefan et al, ; Sutherling et al, ; Tanaka et al, ; Tilz et al, ; Yagyu et al, ; Yoshinaga et al, ]. Encouraging ictal source localization results were obtained by applying techniques originally developed to perform MSI/ESI‐Spikes reconstructions for patients showing ictal patterns characterized by repetitive spikes [Eliashiv et al, ; Koessler et al, ].…”

“…The vast majority of previous study on ictal source localization were based on small series of less than 10 patients [Assaf et al, 2003;Badier et al, 2016;Ding et al, 2007;Eliashiv et al, 2002;Fujiwara et al, 2012;Ishibashi et al, 1998;Ishii et al, 2008;Kakisaka et al, 2012;Kovac et al, 2014;Oishi et al, 2002;Shiraishi, 2001Shiraishi, , 2011Stefan et al, 1992;Sutherling et al, 1987;Tanaka et al, 2009;Tilz et al, 2002; r wMEM for Ictal Source Imaging r r 2539 r PA03. MSI/ESI-SOZ is shown in the left panel.…”

“…In spite of this, ictal source imaging has been mainly performed applying the equivalent current dipole technique [Assaf et al, 2003;Eliashiv et al, 2002;Medvedovsky et al, 2012;Shiraishi et al, 2001;Stefan et al, 1992;Tilz et al, 2002;Yoshinaga et al, 2004] which assumes spatially restricted generators, does not model the rhythmicity, and can be inaccurate if not even misleading [Catarino et al, 2012;Kobayashi et al, 2005;Yagyu et al, 2010]. A few studies have used distributed source models for ictal EEG [Beniczky et al, 2013;Koessler et al, 2010;Tanaka et al, 2009;Worrell et al, 2000] or MEG [Badier et al, 2016;Tanaka et al, 2009], but they only included small cohorts or a low number of sensors.…”

Facing the hidden wall in mesial extratemporal lobe epilepsy

“…Iwasaki et al 2005 andKnake et al 2006). difficulty to maintain the patient in the magnetic shielded room for a prolonged time) (Medvedovsky et al, 2012;Badier et al, 2016). magnetic source imaging [MSI]).…”