Source localization determined by magnetoencephalography and electroencephalography in temporal lobe epilepsy: comparison with electrocorticography: technical case report

Interictal and Ictal MEG in presurgical evaluation for epilepsy surgery

Although presurgical evaluation of patients with pharamacoresistent focal epilepsies provides essential information for successful epilepsy surgery, there is still a need for further improvement. Developments of noninvasive electrophysiological recording and analysis techniques offer additional information based on interictal and ictal epileptic activities. In this review, we provide an overview on the application of ictal magnetoencephalography (MEG). The results of a literature research for published interictal/ictal MEG findings and experiences with own cases are demonstrated and discussed. Ictal MEG may provide added value in comparison to interictal recordings. The results may be more focal and closer to the invasively determined seizure onset zone. In some patients without clear interictal findings, ictal MEG could provide correct localization. Novel recording and analysis techniques facilitate ictal recordings. However, extended recording durations, movement and artifacts still represent practical limitations. Ictal MEG may provide added value regarding the localization of the seizure onset zone but depends on the selection of patients and the application of optimal analysis techniques.

Epileptic MEG Spike Detection Using Statistical Features and Genetic Programming with KNN

Epilepsy is a neurological disorder that affects millions of people worldwide. Monitoring the brain activities and identifying the seizure source which starts with spike detection are important steps for epilepsy treatment. Magnetoencephalography (MEG) is an emerging epileptic diagnostic tool with high-density sensors; this makes manual analysis a challenging task due to the vast amount of MEG data. This paper explores the use of eight statistical features and genetic programing (GP) with the K-nearest neighbor (KNN) for interictal spike detection. The proposed method is comprised of three stages: preprocessing, genetic programming-based feature generation, and classification. The effectiveness of the proposed approach has been evaluated using real MEG data obtained from 28 epileptic patients. It has achieved a 91.75% average sensitivity and 92.99% average specificity.

Reliability of MEG source imaging of anterior temporal spikes: analysis of an intracranially characterized spike focus

OBJECTIVE

To assess the reliability of MEG source imaging (MSI) of anterior temporal spikes through detailed analysis of the localization and orientation of source solutions obtained for a large number of spikes that were separately confirmed by intracranial EEG to be focally generated within a single, well-characterized spike focus.

METHODS

MSI was performed on 64 identical right anterior temporal spikes from an anterolateral temporal neocortical spike focus. The effects of different volume conductors (sphere and realistic head model), removal of noise with low frequency filters (LFFs) and averaging multiple spikes were assessed in terms of the reliability of the source solutions.

RESULTS

MSI of single spikes resulted in scattered dipole source solutions that showed reasonable reliability for localization at the lobar level, but only for solutions with a goodness-of-fit exceeding 80% using a LFF of 3 Hz. Reliability at a finer level of intralobar localization was limited. Spike averaging significantly improved the reliability of source solutions and averaging 8 or more spikes reduced dependency on goodness-of-fit and data filtering.

CONCLUSIONS

MSI performed on topographically identical individual spikes from an intracranially defined classical anterior temporal lobe spike focus was limited by low reliability (i.e., scattered source solutions) in terms of fine, sublobar localization within the ipsilateral temporal lobe. Spike averaging significantly improved reliability.

SIGNIFICANCE

MSI performed on individual anterior temporal spikes is limited by low reliability. Reduction of background noise through spike averaging significantly improves the reliability of MSI solutions.

Magnetoencephalography in pediatric epilepsy

Magnetoencephalography (MEG) records the magnetic field generated by electrical activity of cortical neurons. The signal is not distorted or attenuated, and it is contactless recording that can be performed comfortably even for longer than an hour. It has excellent and decent temporal resolution, especially when it is combined with the patient's own brain magnetic resonance imaging (magnetic source imaging). Data of MEG and electroencephalography are not mutually exclusive and it is recorded simultaneously and interpreted together. MEG has been shown to be useful in detecting the irritative zone in both lesional and nonlesional epilepsy surgery. It has provided valuable and additive information regarding the lesion that should be resected in epilepsy surgery. Better outcomes in epilepsy surgery were related to the localization of the irritative zone with MEG. The value of MEG in epilepsy surgery is recruiting more patients to epilepsy surgery and providing critical information for surgical planning. MEG cortical mapping is helpful in younger pediatric patients, especially when the epileptogenic zone is close to the eloquent cortex. MEG is also used in both basic and clinical research of epilepsy other than surgery. MEG is a valuable diagnostic modality for diagnosis and treatment, as well as research in epilepsy.

Magnetoencephalographic and magnetic resonance spectroscopy evidence of regional functional abnormality in mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (mTLE) with mesial temporal sclerosis (MTS) is a recognized epilepsy syndrome which is successfully treated with mesial temporal lobe resection. However, recent studies suggest that mTLE is more than a "focal" disease process. The objective of our study was to determine the presence and extent of functional abnormalities outside of a defined structural abnormality in epilepsy patients with mTLE. We used a prospective age-matched controlled design to study eight consecutive patients with MTS who were undergoing epilepsy surgery evaluation. Magnetoencephalography was used to localize the sources of electromagnetic abnormality. Proton magnetic resonance spectroscopy ((1)H-MRS) measured integrated peak areas for N-acetyl compounds (NAA) and choline-containing compounds (Cho) to determine regions of metabolic abnormality. All eight subjects had predominant electromagnetic abnormality in the temporal lobe ipsilateral to the MTS. All eight subjects had lower NAA/Cho ratios in the region of electromagnetic abnormality when compared to the homologous contralateral region (P < 0.001). Four subjects had predominant MEG spiking outside the mesial temporal region. Surgery-free outcome for the group with neuroimaging abnormalities outside of the medial temporal lobe is 50%. The region of maximal electromagnetic abnormality is outside the hippocampus in some patients with mTLE. These regions also demonstrate functional abnormalities. Our findings support the concept that mTLE is a more diffuse process than the hippocampal structural abnormality, which may impact surgical outcome.

Diagnostic value of interictal diffusion-weighted imaging in evaluation of intractable temporal lobe epilepsy

OBJECTIVE

To explore the ability of interictal diffusion-weighted imaging (DWI) to localize the temporal lobe of seizure origin and to predict postoperative seizure control in patients with temporal lobe epilepsy (TLE).

METHODS

Twenty-seven patients with intractable TLE considered for surgery and 19 healthy volunteers were studied with conventional magnetic resonance imaging (MRI) and DWI. Apparent diffusion coefficients (ADCs) of bilateral hippocampi in both TLE patients and control subjects were obtained. Lateralization to either temporal lobe with hippocampal ADC was based on the threshold values derived from +/- SD of right/left ratios in normal subjects. And the postoperative pathology was reviewed.

RESULTS

Hippocampal ADCs were higher on the side of surgery compared with those on the contralateral side as well as the ipsilateral side in control subjects [resected side (109.8 +/- 7.3) x 10(-5) cm2/s, contralateral side (91.7 +/- 4.7) x 10(-5) cm2/s, control subjects (81.6 +/- 5.2) x 10(-5) cm2/s, all P < 0.01]. Right/left hippocampal ADC ratio and conventional MRI lateralized to the operated temporal lobe in 21 of 27 (77.8%) and in 18 of 27 (66.7%) patients. Lateralization to the surgical side was not associated with postoperative seizure control with right/left hippocampal ADC ratio (P > 0.05).

CONCLUSIONS

Conventional MRI is a sensitive method to detect hippocampal sclerosis. Accuracy of the right/left hippocampal ADC ratio for lateralizing to the side of surgery is very high, but it isn't a better predictor of surgical outcome.

Benign rolandic epilepsy -- perhaps not so benign: use of magnetic source imaging as a predictor of outcome

The purpose of this study was to evaluate children with benign rolandic epilepsy, a childhood epilepsy characterized by centrotemporal/rolandic spike-wave discharges with infrequent partial seizures that may secondarily generalize. Recently, some investigators have questioned whether benign rolandic epilepsy is indeed "benign" or whether long-term cognitive outcome may be adversely affected. We initiated an ongoing study to identify children with benign rolandic epilepsy. The children were evaluated in the Texas Comprehensive Epilepsy Program using outpatient or continuous video-electroencephalographic monitoring, brain magnetic resonance imaging, magnetoencephalography, and neuropsychological testing. Neuropsychological testing revealed fine motor dysfunction, visuomotor integration deficits, dyscalculia, and/or expressive language deficits in all of the 9 patients evaluated, reaffirming that benign rolandic epilepsy is not necessarily a benign disorder. Our study shows a high concordance of motor and cognitive deficits in benign rolandic epilepsy, as others have previously suggested. Furthermore, magnetic source imaging shows a higher resolution of dipole localization compared with conventional electroencephalography, which may ultimately improve prediction of deficits. This reaffirms that magnetoencephalography is a valuable diagnostic tool in the evaluation of children with benign rolandic epilepsy.

MEG versus EEG: influence of background activity on interictal spike detection

The comparative sensitivity of EEG and magnetoencephalography (MEG) in the visual detection of focal epileptiform activity in simultaneous interictal sleep recordings were investigated. The authors examined 14 patients aged 3.5 to 17 years with localization-related epilepsy. Simultaneous 122-channel whole-head MEG and 33-channel EEG were recorded for 20 to 40 minutes during spontaneous sleep. The EEG and MEG data were separated and four blinded independent reviewers marked the presence and timing of epileptic discharges (ED) in the 28 data segments. EEG and MEG data were matched and spikes identified by at least three reviewers were classified in three categories according to the following criteria: type 1 MEG > EEG, type 2 EEG > MEG (type 1/2: difference of three or more raters), and type 3 EEG = MEG (three or more raters each). The presence of simultaneous sleep changes was visually determined for every single EEG-segment. Spikes with high spatiotemporal correlation were averaged and subjected to single dipole analysis of peak activity in EEG. Out of 4704 marked patterns, 1387 spikes fulfilled the above criteria. In fact, more spikes were unique to MEG (689) than to EEG (136) and to the combination of both modalities (562). ED were detected predominantly by MEG in eight patients and by EEG in two patients. The presence of vertex waves and spindles lead to a significantly higher number of spikes identified only in MEG. Averaging of type 1 spikes produced clear spike activity in EEG in 9 of 12 cases. On the contrary, only 2 of 10 type 2 spikes were visible in MEG after averaging. Dipoles of spikes visible in MEG showed a more tangential orientation compared with more radial dipoles of type 2 spikes. Spike characteristics, e.g., dipole orientation, are a key factor for a sole EEG representation. Exclusive MEG detection is more likely influenced by overlapping background activity in EEG. Because MEG is indifferent to radial activity, i.e., sleep changes, a higher ratio of spikes unique to MEG compared with EEG is detected in the case of overlapping sleep changes.

Combined MEG/EEG analysis of the interictal spike complex in mesial temporal lobe epilepsy

We studied the functional organization of the interictal spike complex in 30 patients with mesial temporal lobe epilepsy (MTLE) using combined magnetoencephalography (MEG)/electroencephalography (EEG) recordings. Spikes could be recorded in 14 patients (47%) during the 2- to 3-h MEG/EEG recording session. The MEG and EEG spikes were subjected to separate dipole analyses; the MEG spike dipole localizations were superimposed on MRI scans. All spike dipoles could be localized to the temporal lobe with a clear preponderance in the medial region. Based on dipole orientations in MEG, patients could be classified into two groups: patients with anterior medial vertical (AMV) dipoles, suggesting epileptic activity in the mediobasal temporal lobe and patients with anterior medial horizontal (AMH) dipoles, indicating involvement of the temporal pole and the anterior parts of the lateral temporal lobe. Whereas patients with AMV dipoles had strictly unitemporal interictal and ictal EEG changes during prolonged video-EEG monitoring, 50% of patients with AMH dipoles showed evidence of bitemporal affection on interictal and ictal EEG. Nine patients underwent epilepsy surgery so far. Whereas all five patients with AMV dipoles became completely seizure-free postoperatively (Class Ia), two out of four patients with AMH dipoles experienced persistent auras (Class Ib). This difference, however, was not statistically significant. We therefore conclude that combined MEG/EEG dipole modeling can identify subcompartments of the temporal lobe involved in epileptic activity and may be helpful to differentiate between subtypes of mesial temporal lobe epilepsy noninvasively.

Crisis del lóbulo temporal registrada mediante magnetoencefalografía: caso clínico

La localización del inicio de las crisis es un factor importante para la evaluación prequirúrgica de la epilepsia. En este trabajo se describe la localización del inicio de una crisis registrada mediante magnetoencefalografía (MEG) en un niño de 12 años que presenta crisis parciales complejas farmacorresistentes. La RM muestra una lesión de 20mm de diámetro en el hipocampo izquierdo. EEG de superficie con ondas theta temporales izquierdas. Registro MEG interictal con punta-onda aislada posterior e inferior a la lesión de la RM. Registro MEG ictal con punta-onda (2 Hz). La localización de los dipolos indica el inicio de la crisis en la circunvolución temporal inferior en la misma localización que la actividad interictal MEG. Esta actividad ictal se propaga bilateralmente a áreas frontales. El registro corticográfico intraquirúrgico confirma los resultados de la localización interictal mediante MEG.

Controversies in clinical neurophysiology. MEG is superior to EEG in the localization of interictal epileptiform activity: Con

OBJECTIVE

To assess whether MEG is superior to scalp-EEG in the localization of interictal epileptiform activity and to stress the 'con' part in this controversy.

METHODS

Advantages and disadvantages of the two techniques were systematically reviewed.

RESULTS

While MEG and EEG complement each other for the detection of interictal epileptiform discharges, EEG offers the advantage of long-term recording significantly increasing its diagnostic yield which is not feasible with MEG. Localization accuracies of EEG and MEG are comparable once inaccuracies for the solution of the forward problem are eliminated. MEG may be more sensitive for the detection of neocortical spike sources. EEG and MEG source localizations show comparable agreement with invasive electrical recordings, can clarify the spatial relationship between the irritative zone and structural lesions, guide the placement of invasive electrodes and attribute epileptic activity to lobar subcompartments in temporal lobe epilepsy and to a lesser extent in extratemporal epilepsy.

CONCLUSIONS

A clear superiority of MEG over EEG for the localization of interictal epileptiform activity cannot be derived from the studies presently available.

SIGNIFICANCE

The combination of EEG and MEG provides information for the localization of interictal epileptiform activity which cannot be obtained with either technique alone.

Areas of Interictal Spiking Are Associated with Metabolic Dysfunction in MRI‐negative Temporal Lobe Epilepsy

PURPOSE

The objective of our study was to determine noninvasively whether metabolic dysfunction is present in focal areas of interictal electrophysiologic abnormality and whether metabolic dysfunction correlates with frequency of spiking.

METHODS

We used a prospective, power analysis-driven, age-matched design to study 20 subjects with nonlesional temporal lobe epilepsy by using magnetoencephalography (MEG) and proton magnetic resonance spectroscopy (1H-MRS). MEG was used to localize the source area of interictal spikes. 1H-MRS measured integrated peak areas for N-acetyl compounds (NAA) and choline-containing compounds (Cho) in both hippocampi, the MEG spike zone, and the region contralateral to the MEG spike zone in all subjects. 1H-MRS was performed in seven controls.

RESULTS

Fifteen of 20 subjects had a lower NAA/Cho ratio in the MEG spike zone compared with the contralateral homologous region. NAA/Cho was significantly decreased in the MEG spike zone (p < 0.01). NAA/Cho ratios were not significantly different between the hippocampus ipsilateral and contralateral to the spike activity, or from control hippocampi. NAA/Cho ratios did not correlate with spike frequency.

CONCLUSIONS

Metabolic dysfunction is present in focal areas of interictal spiking in nonlesional temporal lobe epilepsy. These findings confirm that functional abnormalities can be detected in vivo in radiographically normal-appearing cortex exhibiting abnormal excitability.

Anaesthesia for magnetoencephalography in children with intractable seizures

BACKGROUND

Magnetoencephalography (MEG), a noninvasive technique for evaluation of epileptic patients, records magnetic fields during neuronal electrical activity within the brain. Anaesthesia experience for MEG has not yet been reported.

METHODS

We retrospectively reviewed records of 48 paediatric patients undergoing MEG under anaesthesia. Thirty-one patients (nonprotocol group) were managed according to the anaesthesiologist's discretion. Premedication included oral midazolam, chloral hydrate or fentanyl oralet, intravenous midazolam or inhalational anaesthesia with sevoflurane. Anaesthesia was maintained with propofol, midazolam, fentanyl, alone or in combination. A subsequent protocol group (17 patients) received chloral hydrate as premedication and propofol for maintenance of anaesthesia.

RESULTS

There was an overall 25% failure of interictal activity and localization on the MEG scan. In the nonprotocol group, 11 scans failed (35.5%). Of these, eight (72.7%) received midazolam orally. Only one failure (5.8%) was recorded in the protocol group in a patient who received chloral hydrate as sedation supplemented by sevoflurane.

CONCLUSIONS

In our experience, midazolam premedication resulted in a high MEG failure rate (73%). Chloral hydrate premedication and propofol maintenance resulted in a lower incidence of MEG failure (5.8%). General anaesthesia with a continuous infusion of propofol or sevoflurane appears acceptable, although, lighter levels of anaesthesia might be required to avoid interference with interictal activity of the brain.

Magnetoencephalographic and electroencephalographic evaluation in patients with cryptogenetic partial epilepsy

Magnetoencephalography (MEG) has been applied for more than 20 years to the localization of the epileptic focus in partial epilepsies, but correlation with electroencephalographic (EEG) data in homogeneous groups of patients is scarce.

OBJECTIVE

The aim of our work was to use EEG and MEG for the study of a group of adults and children affected by cryptogenetic partial epilepsy.

METHODS

We analyzed the traces obtained from electroencephalographic and magnetoencephalographic recordings of 10 patients of ages ranging from 7 to 38 years affected by cryptogenetic partial epilepsy. We evaluated the presence of commonly detected or uniquely detected spikes, and, whenever possible, we used MEG for localization of the epileptic focus.

RESULTS

Three patients showed no epileptic activity during the EEG and MEG sessions. Overall agreement between EEG and MEG (presence of concordant spikes with the same localization shown by both techniques) was obtained in five patients. In one patient the spikes detected by EEG and MEG were different, and in another patient interictal activity was demonstrated exclusively by EEG.

CONCLUSIONS

EEG in this series was not inferior to MEG in terms of spike detection. Combination of EEG and MEG is feasible, better than each technique alone, and may be useful for non-invasive diagnosis and monitoring of pediatric and adult patients with partial epilepsies.

Magnetoencephalographic yield of interictal spikes in temporal lobe epilepsy. Comparison with scalp EEG recordings

To compare magnetoencephalography (MEG) with scalp electroencephalography (EEG) in the detection of interictal spikes in temporal lobe epilepsy (TLE), we simultaneously recorded MEG and scalp EEG with a whole-scalp neuromagnetometer in 46 TLE patients. We visually searched interictal spikes on MEG and EEG channels and classified them into three types according to their presentation on MEG alone (M-spikes), EEG alone (E-spikes), or concomitantly on both modalities (M/E-spikes). The M-spikes and M/E-spikes were localized with MEG equivalent current dipole modeling. We analyzed the relative contribution of MEG and EEG in the overall yield of spike detection and also compared M-spikes with M/E-spikes in terms of dipole locations and strengths. During the 30- to 40-min MEG recordings, interictal spikes were obtained in 36 (78.3%) of the 46 patients. Among the 36 patients, most spikes were M/E-spikes (68.3%), some were M-spikes (22.1%), and some were E-spikes (9.7%). In comparison with EEG, MEG gave better spike yield in patients with lateral TLE. Sources of M/E- and M-spikes were situated in the same anatomical regions, whereas the average dipole strength was larger for M/E- than M-spikes. In conclusion, some interictal spikes appeared selectively on either MEG or EEG channels in TLE patients although more spikes were simultaneously identified on both modalities. Thus, simultaneous MEG and EEG recordings help to enhance spike detection. Identification of M-spikes would offer important localization of irritative foci, especially in patients with lateral TLE.

MEG and EEG in epilepsy

Both EEG and magnetoencephalogram (MEG), with a time resolution of 1 ms or less, provide unique neurophysiologic data not obtainable by other neuroimaging techniques. MEG has now emerged as a mature clinical technology. While both EEG and MEG can be performed with more than 100 channels, MEG recordings with 100 to 300 channels are more easily done because of the time needed to apply a large number of EEG electrodes. EEG has the advantage of the long-term video EEG recordings, which facilitates extensive temporal sampling across all periods of the sleep/wake cycle. MEG and EEG seem to complement each other for the detection of interictal epileptiform discharges, because some spikes can be recorded only on MEG but not on EEG and vice versa. Most studies indicate that MEG seems to be more sensitive for neocortical spike sources. Both EEG and MEG source localizations show excellent agreement with invasive electrical recordings, clarify the spatial relationship between the irritative zone and structural lesions, and finally, attribute epileptic activity to lobar subcompartments in temporal lobe and to a lesser extent in extratemporal epilepsies. In temporal lobe epilepsy, EEG and MEG can differentiate between patients with mesial, lateral, and diffuse seizure onsets. MEG selectively detects tangential sources. EEG measures both radial and tangential activity, although the radial components dominate the EEG signals at the scalp. Thus, while EEG provides more comprehensive information, it is more complicated to model due to considerable influences of the shape and conductivity of the volume conductor. Dipole localization techniques favor MEG due to the higher accuracy of MEG source localization compared to EEG when using the standard spherical head shape model. However, if special care is taken to address the above issues and enhance the EEG, the localization accuracy of EEG and MEG actually are comparable, although these surface EEG analytic techniques are not typically approved for clinical use in the United States. MEG dipole analysis is approved for clinical use and thus gives information that otherwise usually requires invasive intracranial EEG monitoring. There are only a few dozen whole head MEG units in operation in the world. While EEG is available in every hospital, specialized EEG laboratories capable of source localization techniques are nearly as scarce as MEG facilities. The combined use of whole-head MEG systems and multichannel EEG in conjunction with advanced source modeling techniques is an area of active development and will allow a better noninvasive characterization of the irritative zone in presurgical epilepsy evaluation. Finally, additional information on epilepsy may be gathered by either MEG or EEG analysis of data beyond the usual bandwidths used in clinical practice, namely by analysis of activity at high frequencies and near-DC activity.

Magnetoencephalography in pediatric neurology and in epileptic syndromes

In recent years, great advances in the knowledge of neuromagnetism have permitted the application of Superconducting Quantum Interference Devices to the pathophysiologic study of the human brain. In particular, in pediatric neurology, the integration of biomagnetism with magnetic resonance imaging and other techniques for medical imaging have allowed for precise neuromagnetic measurements of the human brain. The more frequently used technique is magnetoencephalography. Recent data have illustrated the usefulness of magnetoencephalography in mapping activity of sensory and motor areas and in studying the spatiotemporal pattern of brain activation specific to somatosensory function. Moreover, magnetoencephalography is an important tool to localize epileptic activity; magnetic source imaging superimposes magnetoencephalographic localizations on the magnetic resonance imaging and yields improved spatial resolution as compared with surface electroencephalography. The role of magnetoencephalography in evaluating patients with epilepsy continues to evolve; in fact, it seems to be very useful in the localization of the epileptogenic zone in patients with partial epilepsy. This application of magnetoencephalography is essential in the selection of epileptic children candidates to surgical treatment of seizures.

Consistency of interictal and ictal onset localization using magnetoencephalography in patients with partial epilepsy

OBJECT

The aim of this study was to evaluate the spatial accuracy of interictal magnetoencephalography (MEG) in localizing the primary epileptogenic focus in comparison with alternative MEG-derived estimates such as ictal onset recording or sensory mapping of the periphery where seizures manifest.

METHODS

During this retrospective study of 12 patients with epilepsy who had undergone successful magnetic source (MS) imaging with the aid of a dual 37-channel biomagnetometer as well as simultaneous MEG/electroencephalography (EEG) recordings, ictal events were observed in five patients and quantitative comparisons of interictal spike and ictal seizure onset source localizations were made. In the eight patients who had presented with sensorimotor seizure, source localization of cortical sites concordant with seizure foci was determined using somatosensory functional mapping, and the results were quantitatively compared with interictal spike source localizations. Interictal spike sources demonstrated on MEG localized to the same region as the corresponding ictal event or somatosensory source localizations. The mean distance between the ictal foci and interictal spike sources was 1.1 +/- 0.3 cm. Results of functional somatosensory mapping in patients with sensorimotor seizures demonstrated that seizure sources consistently colocalized with interictal MEG spike sources, with a mean distance of 1.5 +/- 0.4 cm. No systematic directional bias was observed. Interictal sources tended to be tightly clustered, and the mean ellipsoid volume, defined by one standard deviation of the source spatial coordinates, was 1 cm3.

CONCLUSIONS

Interictal spike localizations on MEG were concordant with ictal and, where relevant, functional somatosensory mapping localizations. These findings support the interpretation of interictal spikes on MEG as a useful and effective noninvasive method for localizing primary seizure foci.

Non-supervised spatio-temporal analysis of interictal magnetic spikes: comparison with intracerebral recordings

OBJECTIVE

Our main goal was to evaluate the accuracy of an original non-supervised spatio-temporal magnetoencephalography (MEG) localization method used to characterize interictal spikes generators.

METHODS

MEG and stereotactic intracerebral recordings (stereo-electro-encephalographic exploration, SEEG) data were analyzed independently in 4 patients. MEG localizations were performed with and without anatomical constraints.

RESULTS

We analyzed 1326 interictal spikes recorded using MEG. For each patient, 2-3 typical source patterns were described. These source configurations were compared with SEEG. SEEG findings and MEG spatio-temporal localization results were remarkably coherent in our 4 patients. Most of the MEG patterns were similar to interictal SEEG patterns from a spatio-temporal point of view.

CONCLUSIONS

We were able to evaluate the usefulness of our non-invasive localization method. This approach described correctly the part of the epileptogenic network involved in the generation of interictal events. Our results demonstrate the potential of MEG in the non-invasive spatio-temporal characterization of generators of interictal spikes.

Multidipole analysis of simulated epileptic spikes with real background activity

This simulated magnetoencephalographic study was designed to determine the variability in source parameters with real subject background activity when applying multidipole spatial-temporal dipole analyses, for which the correct model was compared with undermodeled and overmodeled cases. The simulated sources were created from patches of the cortical surface of each subject's MRI. One- and two-source frontal lobe spikes were generated in two cortical regions seen commonly in frontal lobe epilepsy patients tested at our site (orbital frontal and premotor cortex). In general, the modeling results were adequate for the correct model order and the correct model order plus one. In addition, if the localization error was less than 10 mm from the simulated source, the peak latency of the spike and orientation were very reliable, but the peak amplitude was not. The additional source in the overmodeled condition, on the other hand, was not localized reliably across the different epochs within subjects. The results suggest that consistency of the spike localization and inconsistency of other sources will allow one to determine reliably the appropriate model order in real data, and therefore determine single and multifocal spike generators.

Characteristics of dipoles in clustered individual spikes and averaged spikes

The aim of this study is to analyze the characteristics of dipoles in clustered individual spikes and averaged spikes, we compared electroencephalography (EEG) dipole localizations from patients with intractable extratemporal lobe epilepsy (IETLE) and from patients with benign epilepsy with centrotemporal spikes (BECTS). We studied 10 patients; five with IETLE who underwent epilepsy surgery after subdural EEG and five with BECTS. We recorded 19-channel digital scalp EEGs and used clustering analysis for individual spikes to characterize interictal spikes. We selected and averaged one representative spike group at the maximum negative peak electrode. We used a single dipole method with three-shell spherical head model. We compared dipole localizations of both averaged and individual spikes.IETLE data had more identifiable spike clusters and fewer spikes in each cluster than BECTS (P<0.05). Dipole sources with goodness-of-fit >or=95% in averaged spikes were less frequent in IETLE than in BECTS (P<0.05). For IETLE, averaged spikes showed no dipoles (two patients), while individual spikes gave dipole sources reliably in the epileptic region. For BECTS, individual and averaged spike sources were clustered. More than 80% of dipoles in averaged spikes were stable, in close proximity, for prolonged periods in BECTS. More spike groups after clustering and fewer acceptable dipoles from averaged spikes in IETLE reflect variable spike activity over extensive epileptic regions. Fewer spike groups producing more acceptable dipoles in BECTS correlate with stable spike sources within the isolated epileptic central region. Characteristics of clustered interictal spikes need careful examination before the use of dipole analysis of averaged spikes for epilepsy evaluation.

Benefit of simultaneous recording of EEG and MEG in dipole localization

PURPOSE

In this study, we tried to show that EEG and magnetoencephalography (MEG) are clinically complementary to each other and that a combination of both technologies is useful for the precise diagnosis of epileptic focus.

METHODS

We recorded EEGs and MEGs simultaneously and analyzed dipoles in seven patients with intractable localization-related epilepsy. MEG dipoles were analyzed by using a BTI Magnes 148-channel magnetometer. EEG dipoles were analyzed by using a realistically shaped four-layered head model (scalp-skull-fluid-brain) built from 2.0-mm slice magnetic resonance imaging (MRI) images.

RESULTS

(a) In two of seven patients, MEG could not detect any epileptiform discharges, whereas EEG showed clear spikes. However, dipoles estimated from the MEG data corresponding to the early phase of EEG spikes clustered at a location close to that of the EEG-detected dipole. (b) In two of seven patients, EEG showed only intermittent high-voltage slow waves (HVSs) without definite spikes. However, MEG showed clear epileptiform discharges preceding these EEG-detected HVSs. Dipoles estimated for these EEG-detected HVSs were located at a location close to that of the MEG-detected dipoles. (c) Based on the agreement of the results of these two techniques, surgical resection was performed in one patient with good results.

CONCLUSIONS

Dipole modeling of epileptiform activity by MEG and EEG sometimes provides information not obtainable with either modality used alone.

Fusiform gyrus epilepsy: the use of ictal magnetoencephalography. Case report

The authors report successful presurgical identification of an epileptic focus in the fusiform gyrus by using ictal magnetoencephalography (MEG), which was performed with the aid of an advanced whole-brain neuromagnetometer. A 22-year-old man had suffered from medically refractory complex partial seizures since he was 10 years of age. Seizure symptoms, magnetic resonance imaging, and ictal single-photon emission computerized tomography examinations indicated right temporal lobe epilepsy; however, ictal electroencephalography, including sphenoidal recordings, failed even to lateralize the seizure focus. The MEG studies revealed that equivalent current dipoles of interictal activities were scattered bilaterally around the medial temporal structures, but those of ictal onset and postictal activities formed a cluster in the left fusiform gyrus. After confirmation of each ictal and interictal MEG finding by using long-term electrocorticography recordings, focal cortical resection of the left inferior temporal and fusiform gyri was performed. The histopathological diagnosis was cortical dysplasia, and the patient has achieved a good seizure outcome, now 15 months after the operation. Ictal and also postictal MEG may be more specific than interictal MEG for identifying the ictal onset zone.

Monte Carlo simulation studies of EEG and MEG localization accuracy

Both electroencephalography (EEG) and magnetoencephalography (MEG) are currently used to localize brain activity. The accuracy of source localization depends on numerous factors, including the specific inverse approach and source model, fundamental differences in EEG and MEG data, and the accuracy of the volume conductor model of the head (i.e., the forward model). Using Monte Carlo simulations, this study removes the effect of forward model errors and theoretically compares the use of EEG alone, MEG alone, and combined EEG/MEG data sets for source localization. Here, we use a linear estimation inverse approach with a distributed source model and a realistic forward head model. We evaluated its accuracy using the crosstalk and point spread metrics. The crosstalk metric for a specified location on the cortex describes the amount of activity incorrectly localized onto that location from other locations. The point spread metric provides the complementary measure: for that same location, the point spread describes the mis-localization of activity from that specified location to other locations in the brain. We also propose and examine the utility of a "noise sensitivity normalized" inverse operator. Given our particular forward and inverse models, our results show that 1) surprisingly, EEG localization is more accurate than MEG localization for the same number of sensors averaged over many source locations and orientations; 2) as expected, combining EEG with MEG produces the best accuracy for the same total number of sensors; 3) the noise sensitivity normalized inverse operator improves the spatial resolution relative to the standard linear estimation operator; and 4) use of an a priori fMRI constraint universally reduces both crosstalk and point spread.

Modality-specific spike identification in simultaneous magnetoencephalography/electroencephalography: a methodological approach

Epileptiform spikes may have a different morphology and signal-to-noise ratio in simultaneously recorded EEGs and magnetoencephalograms (MEGs) that may lead to differences in the identification of spikes if both the modalities are presented separately. Moreover, there are no criteria for MEG spikes. It is unknown to which extent the visual assessment of MEG data yields consistent and meaningful results. Nineteen patients were selected with mesial temporal lobe epilepsy who underwent whole-head simultaneous MEG/EEG. These data were split into MEG and EEG files and were assessed independently by three observers for the occurrence of spikes. Interobserver kappa values were calculated. A mean kappa value greater than 0.5 was taken as a criterion for the presence of unequivocal spikes. Index cases from the resulting four subgroups were studied further. One patient had unequivocal spikes in both modalities, one in EEG only, one in MEG only, and one did not show any unequivocal spike. Spikes on which at least two observers agreed were then subjected to a template match algorithm to test for equal morphology and distribution. Equal spikes were averaged and electrical and magnetic field maps were plotted. Unequivocal spikes were found in both MEG and EEG in one patient, in MEG only in two patients, in EEG only in two patients, and no spikes in either modality were seen in 14 patients. In the four index patients, MEG showed 50 to 80% more spikes than EEG. After averaging identical consensus spikes, MEG spikes revealed a concomitant spike in the EEG, but the reverse was not always true. Even in the patient with MEG and EEG spikes that met all selection criteria, simultaneous field maps showed unexpected inconsistencies. In most patients with mesial temporal lobe epilepsy, there are no unequivocal spikes during MEG/EEG. In some cases, however, experienced electroencephalographers can identify MEG spikes reliably. Because of a better signal-to-noise ratio, more spikes could be identified in MEG than in EEG. Simultaneous MEG/EEG recordings do not simply ensure the best of both, but one modality may improve the identification of spikes in the other. In addition, different aspects of a complex source can be revealed. Our three-step approach to combined data ensures a reproducible selection of spikes for source modeling.

Visualisation method of spatial interictal discharges in temporal epilepsy patients using magneto-encephalogram

The aim of the study was to develop a method for investigating how interictal epileptic discharges in temporal epilepsy patients are activated spatially. The activity was measured using magneto-encephalography (MEG). The MEG data were used to produce a current-arrow map that reflected the topographic distribution of the electrical current for each peak epileptic waveform. A large current distribution was obtained that appeared to be contained in the limbic structure, in each temporal lobe. The large current orientation indicated two opposite directions. Furthermore, the decrease in the maximum strength of the current-arrow, depending on the medication (e.g. the decrease from 11 to 6 pT m-1 in the left temporal lobe (contralateral stimuli)), suggested that the discharge distributions could be used to verify the efficacy of medication. Thus the topographical visualisation method could be a new strategy for diagnosis in temporal epilepsy patients.

Surgical implications of neuromagnetic spike localization in temporal lobe epilepsy

PURPOSE

To investigate the clinical usefulness of magnetoencephalography (MEG) as a guide to the surgical treatment of temporal lobe epilepsy (TLE).

METHODS

Preoperative spike localization by MEG was compared with seizure outcome and postoperative spike localization at 12 months after resective surgery in 16 patients with TLE. Spike localization was classified into anterior temporal (AT) and non-AT localization in 11 patients without neocortical lesion treated with anterior temporal lobectomy (ATL); and lesion and lobar localization in five patients with neocortical lesion treated with lesionectomy (n = 3) or lesionectomy with medial temporal resection (n = 2).

RESULTS

All five patients with AT localization became seizure free and spike free after surgery. Among the six patients with non-AT localization, two became seizure free and spike free, two became seizure free with residual spikes, one had residual seizures but no spikes, and one had both residual seizures and spikes. All three patients with lesion localization and two with lobar localization had favorable seizure outcome and became spike free after surgery.

CONCLUSIONS

MEG spike localization can identify neocortical sources remote from the presumed epileptogenic area. Favorable seizure outcome can be expected in patients with AT localization after ATL and patients with lesion localization after lesionectomy. In contrast, non-AT localization indicates either nonmedial TLE or spike propagation to the posterior and extratemporal neocortex. Similarly, lobar localization indicates spike propagation from an epileptogenic lesion or extensive epileptogenicity. Patients with non-AT localization or lobar localization should undergo intensive evaluations, such as intracranial EEG, for improved seizure outcome.

Comparison of electroencephalographic dipoles of interictal spikes from prolonged scalp video-electroencephalography and magnetoencephalographic dipoles from short-term recording in children with extratemporal lobe epilepsy

We retrospectively compared electroencephalographic (EEG) dipoles of interictal spikes from prolonged video-EEG monitoring with magnetoencephalographic dipoles from short-term recording in four children with extratemporal lobe epilepsy. We analyzed both sets of dipoles using individual interictal spikes and single moving dipole modeling and evaluated the profiles of spike appearance, dipole position, and orientation in EEG and magnetoencephalography. We obtained more than 100 magnetoencephalographic spikes in two patients who manifested frequent interictal EEG spikes throughout both day and night but fewer than 40 magnetoencephalographic spikes in two patients who had interictal EEG spikes mainly during sleep. The dipole positions of EEG and magnetoencephalography were in close proximity and included in the surgical resection area. Most of the dipoles between EEG and magnetoencephalography were oriented perpendicularly. A combination of EEG dipole analysis from prolonged video-EEG monitoring and magnetoencephalographic dipole analysis provides complementary information for presurgical evaluation in children with intractable extratemporal lobe epilepsy.

Dipole localization for identification of neuronal generators in independent neighboring interictal EEG spike foci

PURPOSE

We evaluated dipole localizations of independent neighboring interictal spike foci using scalp electroencephalogram (EEG) to identify neuronal generators of epileptic discharges.

METHODS

Three pediatric patients with extratemporal lobe epilepsy who had two independent neighboring interictal spike foci on scalp EEG were studied. Prolonged video EEG was digitally recorded from 19 scalp electrodes, whose positions were registered using a three-dimensional digitizer. Interictal spikes were visually selected based on negative phase reversals on bipolar montages. We analyzed the dipole position and moment of each spike using a single moving dipole and three-shell spherical head model. The dipoles were overlaid onto magnetic resonance (MR) images and divided into two groups based on two spike foci.

RESULTS

The dipoles of the two groups were oriented either tangentially or radially to the scalp in close proximity to each other. The dipoles oriented radially were located underneath the electrode with a negative peak; those oriented tangentially were between electrodes with a negative and positive peak. The positions of tangential dipoles were more concentrated than those of radial dipoles. The epileptogenic regions corresponded to the dipole localizations. Surgical excisions were performed based on the results of electrocorticography. After surgery, two patients were seizure free, and one had rare seizures (follow-up period, 13-31 months).

CONCLUSIONS

We showed that dipoles in close proximity but with different orientations projected two negative maxima on scalp EEG in three patients with extratemporal localization-related epilepsy. Equivalent current dipole analysis of individual interictal spikes can provide useful information about the epileptogenic zone in these patients.

Magnetoencephalography and magnetic source imaging in children

Magnetoencephalography is a technique that detects the magnetic fields associated with the intracellular current flow within neurons, unlike electroencephalography, which measures extracellular volume currents. Superconducting quantum interference devices are used to amplify these very small magnetic field signals. Magnetic source imaging is the combination of functional data derived from magnetoencephalographic recordings coregistered with structural magnetic resonance imaging (MRI). The utility of magnetic source imaging lies in the combination of the submillisecond temporal resolution of magnetoencephalography with the precise anatomic images provided by magnetic resonance imaging. As such, magnetic source imaging is a useful tool for noninvasive localization of the epileptogenic zone in children who are candidates for epilepsy surgery. Similarly, using magnetoencephalographic recordings with evoked and event-related potentials, magnetic source imaging holds great promise as a noninvasive method for precise localization of somatosensory, motor, language, visual, and auditory cortex. Finally, magnetic source imaging is proving a valuable research tool in the investigation of epilepsy, head trauma, brain plasticity, and disorders of language, memory, cognition, and executive function in children.

Magnetoencephalography in focal epilepsy

The introduction of whole-head magnetoencephalographic (MEG) systems facilitating simultaneous recording from the entire brain surface has led to a major breakthrough in the MEG evaluation of epilepsy patients. MEG localizations estimates of the interictal spike zone showed excellent agreement with invasive electrical recordings and were useful to clarify the spatial relationship of the irritative zone and structural lesions. MEG appears to be especially useful for study of patients with neocortical epilepsy, and helped to guide the placement of subdural grid electrodes in patients with nonlesional epilepsies. MEG could differentiate between patients with mesial and lateral temporal seizure onset. Spike propagation in the temporal lobe and the spatio-temporal organization of the interictal spike complex could be studied noninvasively. MEG was useful to delineate essential brain regions before surgical procedures adjacent to the central fissure. MEG appears to be more sensitive than scalp EEG for detection of epileptic discharges arising from the lateral neocortex, whereas only highly synchronized discharges arising from mesial temporal structures could be recorded. A major limitation of MEG has been the recording of seizures because long-term recordings cannot be performed on a routine basis with the available technology. Because MEG and EEG yield both complementary and confirmatory information, combined MEG-EEG recordings in conjunction with advanced source modeling techniques should improve the noninvasive evaluation of epilepsy patients and further reduce the need for invasive procedures.

Neuromagnetic recordings in temporal lobe epilepsy

The introduction of whole-head magnetoencephalography (MEG) systems facilitating simultaneous recording from the entire brain surface has established MEG as a clinically feasible method for the evaluation of patients with temporal lobe epilepsy (TLE). In mesial TLE, two types of MEG spike dipoles could be identified: an anterior vertical and an anterior horizontal dipole. Dipole orientations can be used to attribute spike activity to temporal lobe subcompartments. Whereas the anterior vertical dipole is compatible with epileptic activity in the mediobasal temporal lobe, the anterior horizontal dipole can be explained by epileptic activity of the temporal tip cortex. In nonlesional TLE, medial and lateral vertical dipoles were found which could distinguish between medial and lateral temporal seizure onset zones as evidenced from invasive recordings. In lesional TLE, MEG could clarify the spatial relationship of the structural lesion to the irritative zone. Evaluation of patients with persistent seizures after epilepsy surgery may represent another clinical important application of MEG because magnetic fields are less influenced than electric fields by the prior operation. Simultaneous MEG and invasive EEG recordings indicate that epileptic activity restricted to mesial temporal structures cannot reliably be detected on MEG and that an extended cortical area of at least 6 to 8 cm2 involving also the basal temporal lobe is necessary to produce a reproducible MEG signal. In lateral neocortical TLE MEG seems to be more sensitive than scalp-EEG which further underlines the potential role of MEG for the study of nonlesional TLE. Whole-head MEG therefore can be regarded as a valuable and clinically relevant noninvasive method for the evaluation of patients with TLE.

Systematic approach to dipole localization of interictal EEG spikes in children with extratemporal lobe epilepsies

OBJECTIVES

To assess the reliability of dipole localization based on residual variances (RV), using equivalent current dipole analysis of interictal EEG spikes in children with extratemporal lobe epilepsy.

METHODS

Four pediatric patients with extratemporal lobe epilepsy were studied. Digital EEG was recorded from 19 scalp electrodes. Computer programs for spike detection and clustering analysis were used to select spikes. Dipoles were calculated 5 times for each spike using different initial guesses by the moving dipole model. Standard deviation (SD) of the dipole positions was calculated at each time point in the 5 trials.

RESULTS

We analyzed the dipoles at 1097 time points from 4 patients. Among 106 time points with RV < 2%, the SD was < 1 mm in 78 (74%), while in those with SD > 1 mm the dipole positions varied between 2.8 and 52.6 mm. Of dipoles with RV < 1%, 26 of 27 (96%) had an SD < 1 mm; the one dipole with SD > 1 mm varied within 2.5 mm. The dipole localizations with RV < 2% corresponded to the epileptogenic zones identified on intracranial invasive video EEG and intraoperative ECoG.

CONCLUSIONS

The systematic approach of equivalent current dipole analysis using spike detection, clustering analysis, and an RV < 2% as a standard is useful for identifying extratemporal epileptic regions.

Combined MEG and EEG source imaging by minimization of mutual information

Though very frequently assumed, the necessity to operate a joint processing of simultaneous magnetoencephalography (MEG) and electroencephalography (EEG) recordings for functional brain imaging has never been clearly demonstrated. However, the very last generation of MEG instruments allows the simultaneous recording of brain magnetic fields and electrical potentials on the scalp. But the general fear regarding the fusion between MEG and EEG data is that the drawbacks from one modality will systematically spoil the performances of the other one without any consequent improvement. This is the case for instance for the estimation of deeper or radial sources with MEG. In this paper, we propose a method for a cooperative processing of MEG and EEG in a distributed source model. First, the evaluation of the respective performances of each modality for the estimation of every dipole in the source pattern is made using a conditional entropy criterion. Then, the algorithm operates a preprocessing of the MEG and EEG gain matrices which minimizes the mutual information between these two transfer functions, by a selective weighting of the MEG and EEG lead fields. This new combined EEG/MEG modality brings major improvements to the localization of active sources, together with reduced sensitivity to perturbations on data.