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      Interictal magnetoencephalography in parietal lobe epilepsy – Comparison of equivalent current dipole and beamformer (SAMepi) analysis

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          Highlights

          • •

            MEG may aid in the localization of the epileptogenic zone in the parietal lobe.

          • •

            SAMepi – a novel kurtosis beamformer – results in localizations similar to those of the ECD analysis.

          • •

            A unifocal result in both the ECD and the SAMepi analysis is associated with a good clinical outcome.

          Abstract

          Objective

          To evaluate a novel analysis method (SAMepi) in the localization of interictal epileptiform magnetoencephalographic (MEG) activity in parietal lobe epilepsy (PLE) patients in comparison with equivalent current dipole (ECD) analysis.

          Methods

          We analyzed the preoperative interictal MEG of 17 operated PLE patients utilizing visual analysis and: (1) ECD with a spherical conductor model; (2) ECD with a boundary element method (BEM) conductor model; and (3) SAMepi – a kurtosis beamformer method. Localization results were compared between the three methods, to the location of the resection and to the clinical outcome.

          Results

          Fourteen patients had an epileptiform finding in the visual analysis; SAMepi detected spikes in 11 of them. A unifocal finding in both the ECD and in the SAMepi analysis was associated with a better chance of seizure-freedom (p = 0.02). There was no significant difference in the distances from the unifocal MEG localizations to the nearest border of the resection between the different analysis methods.

          Conclusions

          Localizations of unifocal interictal spikes detected by SAMepi did not significantly differ from the conventional ECD localizations.

          Significance

          SAMepi – a novel semiautomatic analysis method – is useful in localizing interictal epileptiform MEG activity in the presurgical evaluation of parietal lobe epilepsy patients.

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          Most cited references20

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          Spatiotemporal signal space separation method for rejecting nearby interference in MEG measurements.

          S Taulu, J Simola (2006)
          Limitations of traditional magnetoencephalography (MEG) exclude some important patient groups from MEG examinations, such as epilepsy patients with a vagus nerve stimulator, patients with magnetic particles on the head or having magnetic dental materials that cause severe movement-related artefact signals. Conventional interference rejection methods are not able to remove the artefacts originating this close to the MEG sensor array. For example, the reference array method is unable to suppress interference generated by sources closer to the sensors than the reference array, about 20-40 cm. The spatiotemporal signal space separation method proposed in this paper recognizes and removes both external interference and the artefacts produced by these nearby sources, even on the scalp. First, the basic separation into brain-related and external interference signals is accomplished with signal space separation based on sensor geometry and Maxwell's equations only. After this, the artefacts from nearby sources are extracted by a simple statistical analysis in the time domain, and projected out. Practical examples with artificial current dipoles and interference sources as well as data from real patients demonstrate that the method removes the artefacts without altering the field patterns of the brain signals.
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            Comparison of three-shell and simplified volume conductor models in magnetoencephalography.

            Matti Stenroos, Alexander Hunold, Jens Haueisen (2014)
            Experimental MEG source imaging studies have typically been carried out with either a spherically symmetric head model or a single-shell boundary-element (BEM) model that is shaped according to the inner skull surface. The concepts and comparisons behind these simplified models have led to misunderstandings regarding the role of skull and scalp in MEG. In this work, we assess the forward-model errors due to different skull/scalp approximations and due to differences and errors in model geometries. We built five anatomical models of a volunteer using a set of T1-weighted MR scans and three common toolboxes. Three of the models represented typical models in experimental MEG, one was manually constructed, and one contained a major segmentation error at the skull base. For these anatomical models, we built forward models using four simplified approaches and a three-shell BEM approach that has been used as reference in previous studies. Our reference model contained in addition the skull fine-structure (spongy bone). We computed signal topographies for cortically constrained sources in the left hemisphere and compared the topographies using relative error and correlation metrics. The results show that the spongy bone has a minimal effect on MEG topographies, and thus the skull approximation of the three-shell model is justified. The three-shell model performed best, followed by the corrected-sphere and single-shell models, whereas the local-spheres and single-sphere models were clearly worse. The three-shell model was the most robust against the introduced segmentation error. In contrast to earlier claims, there was no noteworthy difference in the computation times between the realistically-shaped and sphere-based models, and the manual effort of building a three-shell model and a simplified model is comparable. We thus recommend the realistically-shaped three-shell model for experimental MEG work. In cases where this is not possible, we recommend a realistically-shaped corrected-sphere or single-shell model.
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              Parietal lobe epilepsy: the semiology, yield of diagnostic workup, and surgical outcome.

              Kwang Kim, An Kee Chang, Christopher S D Lee … (2004)
              To characterize the clinical features, the prognostic value, and diagnostic sensitivities of various presurgical evaluations and the surgical outcomes in parietal lobe epilepsy (PLE), we describe 40 patients who were diagnosed as having PLE, including 27 surgically treated patients. The diagnosis was established by means of a standard presurgical evaluation, including magnetic resonance imaging (MRI), fluorodeoxyglucose-positron emission tomography (FDG-PET), ictal single-photon emission tomography (SPECT), and scalp video-electroencephalography (EEG) monitoring, with additional intracranial EEG monitoring in selected cases. Among the 40 patients, 27 experienced at least one type of aura. The most common auras were somatosensory (13 patients), followed by affective, vertiginous, and visual auras. The patients had diverse manifestations. Eighteen patients showed simple motor seizure, followed by automotor seizure, and dialeptic seizure. Two patients manifested generalized tonic-clonic seizures only, and 19 patients experienced more than one type of seizure. The surgical outcome was favorable in 22 of 26 patients including 14 who were seizure free. Patients with localized MRI abnormality had a higher probability to be seizure free, with marginal significance (p = 0.062), whereas other diagnostic modalities failed to predict the surgical outcome. In the seizure-free group, localization sensitivity was 64.3% by MRI, 50% by PET, 45.5% by ictal SPECT, and 35.7% by ictal EEG. The concordance rate of the various diagnostic modalities was higher in the seizure-free group than in the non-seizure-free group, although it did not reach statistical significance. Seizures, in the case of PLE, can manifest themselves in a wider variety of ways than was previously thought. Surgical outcome was favorable in most of the patients. MRI abnormality and concordance of different diagnostic modalities were associated with high seizure-free rate.
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                Author and article information

                Contributors
                Juha Wilenius
                Journal
                Journal ID (nlm-ta): Clin Neurophysiol Pract
                Journal ID (iso-abbrev): Clin Neurophysiol Pract
                Title: Clinical Neurophysiology Practice
                Publisher: Elsevier
                ISSN (Electronic): 2467-981X
                Publication date PMC-release: 10 March 2020
                Publication date Collection: 2020
                Publication date (Electronic): 10 March 2020
                Volume: 5
                Pages: 64-72
                Affiliations
                [a ]HUS Medical Imaging Center, Clinical Neurophysiology, University of Helsinki and Helsinki University Hospital, Finland
                [b ]HUS Medical Imaging Center, BioMag Laboratory, University of Helsinki and Helsinki University Hospital, Finland
                [c ]Pediatric Neurology, Children’s Hospital, University of Helsinki and Helsinki University Hospital, Finland
                Author notes
                [* ]Corresponding author at: Department of Clinical Neurophysiology, New Children's Hospital, PO Box 347, 00029 HUS, Finland. juha.wilenius@ 123456hus.fi
                Article
                Publisher ID: S2467-981X(20)30008-1
                DOI: 10.1016/j.cnp.2020.02.003
                PMC ID: 7118275
                SO-VID: bba8a1dc-d299-4676-84c7-5fddee3fc9d4
                Copyright © © 2020 International Federation of Clinical Neurophysiology. Published by Elsevier B.V.
                License:

                This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

                History
                Date received : 17 July 2019
                Date revision received : 31 October 2019
                Date accepted : 2 February 2020
                Categories
                Subject: Clinical and Research Article

                Keywords: epilepsy surgery,magnetoencephalography,interictal,parietal lobe,equivalent current dipole,beamformer
                Data availability:
                Keywords: epilepsy surgery, magnetoencephalography, interictal, parietal lobe, equivalent current dipole, beamformer

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