Magnetoencephalographic localization of epileptic cortex--impact on surgical treatment

Electromagnetic source imaging predicts surgical outcome in children with focal cortical dysplasia

OBJECTIVE

To evaluate the diagnostic accuracy of electromagnetic source imaging (EMSI) in localizing spikes and predict surgical outcome in children with drug resistant epilepsy (DRE) due to focal cortical dysplasia (FCD).

METHODS

We retrospectively analyzed magnetoencephalography (MEG) and high-density (HD-EEG) data from 23 children with FCD-associated DRE who underwent intracranial EEG and surgery. We localized spikes using equivalent current dipole (ECD) fitting, dipole clustering, and dynamical statistical parametric mapping (dSPM) on EMSI, electric source imaging (ESI), and magnetic source imaging (MSI). We calculated the distance from the seizure onset zone (DSOZ) and resection (DRES). We estimated receiver operating characteristic (ROC) curves with Youden's index (J) to predict outcome.

RESULTS

EMSI presented shorter DSOZ (15.18 ± 9.06 mm) and DRES (8.56 ± 6.24 mm) compared to ESI (DSOZ: 25.04 ± 16.20 mm, p < 0.009; DRES: 18.88 ± 17.30 mm, p < 0.03) and MSI (DSOZ: 23.37 ± 8.98 mm, p < 0.03; DRES: 15.51 ± 10.11 mm, p < 0.02) for clustering in patients with good outcome. Clustering showed shorter DSOZ and DRES compared to ECD fitting and dSPM (p < 0.05). EMSI had higher performance as outcome predictor (J = 70.63%) compared to ESI (J = 41.27%) and MSI (J = 33.33%) for clustering.

CONCLUSIONS

EMSI provides superior localization and improved predictive performance than individual modalities.

SIGNIFICANCE

EMSI can help the surgical planning and facilitate the localization of epileptogenic foci.

MEG for Greater Sensitivity and More Precise Localization in Epilepsy

Magnetoencephalography is the noninvasive measurement of miniscule magnetic fields produced by brain electrical currents, and is used most fruitfully to evaluate epilepsy patients. While other modalities infer brain function indirectly by measuring changes in blood flow, metabolism, and oxygenation, magnetoencephalography measures neuronal and synaptic function directly with submillisecond temporal resolution. The brain's magnetic field is recorded by neuromagnetometers surrounding the head in a helmet-shaped sensor array. Because magnetic signals are not distorted by anatomy, magnetoencephalography allows for a more accurate measurement and localization of brain activities than electroencephalography. Magnetoencephalography has become an indispensable part of the armamentarium at epilepsy centers.

Magnetoencephalography for localizing and characterizing the epileptic focus

Magnetoencephalography (MEG) is the noninvasive measurement of the miniscule magnetic fields produced by electrical currents flowing in the brain-the same neuroelectric activity that produces the EEG. MEG is one of several diagnostic tests employed in the evaluation of patients with epilepsy, but without the need to expose the patient to any potentially harmful agents. MEG is especially important in those being considered for epilepsy surgery, in whom accurate localization of the epileptic focus is paramount. While other modalities infer brain function indirectly by measuring changes in blood flow, metabolism, oxygenation, etc., MEG, as well as EEG, measures neuronal and synaptic function directly and, like EEG, MEG enjoys submillisecond temporal resolution. The measurement of magnetic fields provides information not only about the amplitude of the current but also its orientation. MEG picks up the magnetic field from neuromagnetometers surrounding the head in a helmet-shaped array of sensors. Clinical whole-head systems currently have 200-300 magnetic sensors, thereby offering very high resolution. The magnetic signals are not distorted by anatomy, because magnetic susceptibility is the same for all tissues, including the skull. Hence, MEG allows for a more accurate measurement and localization of brain activities than does EEG. Because one of its primary strengths is the ability to precisely localize electromagnetic activity within brain areas, MEG results are always coregistered to the patient's MRI. When combined in this way with structural imaging, it has been called magnetic source imaging (MSI), but MEG is properly understood as a clinical neurophysiologic diagnostic test. Signal processing and clinical interpretation in magnetoencephalography require sophisticated noise reduction and computerized mathematical modeling. Technological advances in these areas have brought MEG to the point where it is now part of routine clinical practice. MEG has become an indispensable part of the armamentarium at epilepsy centers where MEG laboratories are located, especially when patients are MRI-negative or where results of other structural and functional tests are not entirely concordant.

Electroencephalography in mesial temporal lobe epilepsy: a review

Electroencephalography (EEG) has an important role in the diagnosis and classification of epilepsy. It can provide information for predicting the response to antiseizure drugs and to identify the surgically remediable epilepsies. In temporal lobe epilepsy (TLE) seizures could originate in the medial or lateral neocortical temporal region, and many of these patients are refractory to medical treatment. However, majority of patients have had excellent results after surgery and this often relies on the EEG and magnetic resonance imaging (MRI) data in presurgical evaluation. If the scalp EEG data is insufficient or discordant, invasive EEG recording with placement of intracranial electrodes could identify the seizure focus prior to surgery. This paper highlights the general information regarding the use of EEG in epilepsy, EEG patterns resembling epileptiform discharges, and the interictal, ictal and postictal findings in mesial temporal lobe epilepsy using scalp and intracranial recordings prior to surgery. The utility of the automated seizure detection and computerized mathematical models for increasing yield of non-invasive localization is discussed. This paper also describes the sensitivity, specificity, and predictive value of EEG for seizure recurrence after withdrawal of medications following seizure freedom with medical and surgical therapy.

Surgical treatment of intractable epilepsy associated with focal cortical dysplasia

Focal cortical dysplasias (FCDs) are congenital malformations of cortical development that are a frequent cause of refractory epilepsy in both children and adults. With advances in structural and functional neuroimaging, these lesions are increasingly being identified as a cause of intractable epilepsy in patients undergoing surgical management for intractable epilepsy. Comprehensive histological classification of FCDs with the establishment of uniform terminology and reproducible pathological features has aided in our understanding of FCDs as an epilepsy substrate. Complete resection of FCDs and the associated epileptogenic zone can result in a good surgical outcome in the majority of patients.

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.

Characterizing magnetic spike sources by using magnetoencephalography-guided neuronavigation in epilepsy surgery in pediatric patients

OBJECT

The authors sought to validate magnetoencephalography spike sources (MEGSSs) in neuronavigation during epilepsy surgery in pediatric patients.

METHODS

The distributions of MEGSSs in 16 children were defined and classified as clusters (Class I), greater than or equal to 20 MEGSSs with 1 cm or less between MEGSSs; small clusters (Class II), 6 to 19 with 1 cm or less between; and scatters (Class III), less than 6 or greater than 1 cm between spike sources. Using neuronavigation, the MEGSSs were correlated to epileptic zones from intra- and extraoperative electrocorticography (ECoG), surgical procedures, disease entities, and seizure outcomes. Thirteen patients underwent MEGSSs: nine had clusters; two had small clusters, one with and one without clusters; and three had scatters alone. All 13 had scatters. Clusters localized within and extended from areas of cortical dysplasia and at margins of tumors or cystic lesions. All clusters were colocalized to ECoG-defined epileptic zones. Four of 10 patients with clusters and/or small clusters underwent complete excisions, and six underwent partial excision with or without multiple subpial transections. In the three patients with scatters alone, ECoG revealed epileptic zones buried within MEGSS areas; these regions of scatters were completely excised and treated with multiple subpial transections. Coexisting scatters were left untreated in nine of 10 patients. Postoperatively, nine of 13 patients were seizure free; the four patients with residual seizures had clusters in unresected eloquent cortex. Three patients in whom no MEGSSs were demonstrated underwent lesionectomies and were seizure free.

CONCLUSIONS

Magnetoencephalography spike source clusters indicate an epileptic zone requiring complete excision. Coexisting scatters remote from clusters are nonepileptogenic and do not require excision. Scatters alone, however, should be examined by ECoG; an epileptic zone may exist within these distributions.

Electroclinical and magnetoencephalographic studies in epilepsy patients with polymicrogyria

RATIONALE

Malformations of cortical development (MCDs) are increasingly recognized as important causes of developmental delay, epilepsy, and other neurological disorders. Polymicrogyria, a type of MCD, is characterized by many small microgyria separated by shallow sulci, a slightly thick cortex, neuronal heterotopia and often enlarged ventricles. The present descriptive study analysis the electroclinical and magnetoencephalographic findings of patients with epilepsy and polymicrogyria without schizencephaly.

METHODS

We studied six patients; mean age was 27 years, who had evidence of polymicrogyria in neuroimaging studies. A single equivalent-current dipole (ECD) model was used to estimate the location of epileptiform spike dipole sources. Analysis was performed on selected data segments containing MEG spikes. MEG results were combined with MRI to create magnetic source images (MSI).

RESULTS

In all cases we present results of MRI, MEG, Video-EEG monitoring, and other functional neuroimaging studies if performed.

CONCLUSIONS

MSI can be used to accurately localize sources of epileptiform discharges. As such MSI can play a role of directly determining the functional epileptogenic significance of abnormalities depicted in imaging.

Magnetoencephalography source localization and surgical outcome in temporal lobe epilepsy

OBJECTIVE

We prospectively investigated the role of magnetoencephalography (MEG) in localizing the seizure focus and in predicting outcome to surgical resections for intractable temporal lobe epilepsy (TLE).

METHODS

We performed simultaneous interictal EEG and MEG recording (two 37-channel system) in 26 TLE patients followed by MEG source localization. We correlated early modeling dipoles with intracranial EEG, temporal surgical resection and surgical outcome.

RESULTS

There were 12 patients who had anterior temporal horizontal or tangential dipoles to the anterior infero-lateral temporal tip cortex. Two patients underwent selective amygdalo-hippocampectomy (SAH) and nine patients had antero-medial temporal lobectomy (AMTL). All patients had successful outcome except for one patient who initially failed SAH, but became seizure-free after AMTL. There were 11 patients who demonstrated anterior temporal vertical or tangential oblique dipoles. Five patients had AMTL and three had SAH; all became seizure free. Five of above 23 patients had invasive EEG and demonstrated mesial seizure onset. Three TLE patients had lateral vertical dipoles that were concordant with intracranial EEG and these became seizure free after temporal neocortical resections.

CONCLUSIONS

MEG source analysis produces distinct source patterns that provide useful localizing information, predict surgical outcome, and may aid in planning limited surgical resection in TLE.

Magnetoencephalography in Epilepsy

Magnetoencephalography (MEG)-also known as magnetic source imaging when combined with magnetic resonance imaging-has developed to the point that it has now entered routine clinical application. Epilepsy MEG studies show that it can accurately localize spike sources--both ictal and interictal--as compared to both direct (intracranial EEG) and indirect (imaging abnormalities) measures. Challenges remain with difficulties in detecting complex or deep sources when recording spontaneous cerebral activity. Magnetoencephalography not only provides a novel tool to localize and characterize epileptiform disturbances, it also has an important role in determining the significance of abnormalities seen on both structural and functional imaging. Combined with mapping of normal or eloquent brain function, MEG should ultimately play a major role in the totally noninvasive epilepsy surgery evaluation.

Magnetoencephalography: an investigational tool or a routine clinical technique?

Magnetoencephalography (MEG) is a relatively novel noninvasive technique, with a much shorter history than EEG, that conveys neurophysiological information complementary to that provided by EEG, with high temporal and spatial resolution. Despite its a priori, highly competitive profile, the role of MEG in the clinical setting is still controversial. We briefly review the major obstacles MEG faces in becoming a routine clinical test and the different strategies needed to bypass them. The high cost and complexity associated with MEG equipment are powerful hindrances to wide acceptance of this relatively new technique in clinical practice. The most straightforward advantage is based on the relative facility of MEG recordings in the process of source localization, which also carries some degree of uncertainty, thus partly explaining why the development of clinical applications of MEG has been so slow. Obviously, a decrease in the cost and the elaboration of semiautomatic protocols that could reduce the complexity of the studies and favor the development of consensual strategies, as well as a major effort on the part of clinicians to identify clinical issues where MEG could be decisive, would be most welcome.

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.

128-Channel EEG Source Imaging in Epilepsy: Clinical Yield and Localization Precision

The authors evaluated the feasibility, clinical yield, and localization precision of high-resolution EEG source imaging of interictal epileptic activity. A consecutive series of 44 patients with intractable epilepsy of various causes, who underwent a comprehensive presurgical epilepsy evaluation, were subjected to a 128-channel EEG recording. A standardized source imaging procedure constrained to the individual gray matter was applied to the averaged spikes of each patient. In 32 patients, the presurgical workup identified a focal epileptogenic area. The 128-channel EEG source imaging correctly localized this area in 30 of these patients (93.7%). Imprecise localization was explained by simplifications of the recordings and analysis procedure, which was accepted for the benefit of speed and standardization. In a subgroup of 24 patients who underwent operations, the sublobar precision of the 128-channel EEG source imaging was evaluated by calculating the distance of the source maximum to the resected area. This analysis revealed zero distance in 19 cases (79%). The authors conclude that high-resolution interictal EEG source imaging is a valuable noninvasive functional neuroimaging technique. The speed, ease, flexibility, and low cost of this technique warrant its use in clinical practice.

High-resolution source imaging in mesiotemporal lobe epilepsy: a comparison between MEG and simultaneous EEG

Magnetic source imaging is claimed to have a high accuracy in epileptic focus localization and may be a guide for epilepsy surgery. Non-lesional mesiotemporal lobe epilepsy (MTLE), the most common form of epilepsy operated on, has different etiologies, which may affect the choice of surgical approach. The authors compared whole-head magnetoencephalography (MEG) with high-resolution EEG for source identification in MTLE. Nineteen patients with unilateral, nonlesional MTLE underwent a simultaneous 151-channel CTF MEG (CTF Systems, Inc., Port Coquitlam, British Columbia, Canada) and 64-channel EEG recordings with sleep induction. Three independent observers selected spikes from the EEG and MEG recordings separately. Only when there was interobserver agreement (kappa>0.4) on the presence of spikes in recordings were consensus spikes averaged. EEG and MEG equivalent current dipoles (ECD) were then integrated in the head model of the patient reconstructed from MRI. The results were compared with intraoperative electrocorticography findings. Spikes were detected in 32% of MEGs and 42% of EEGs. No patient showed MEG spikes only. Equivalent current dipole modeling correctly localized the source to the temporal lobe in four out of five MEG and three out of eight EEG recordings. MEG localized sources were more superficial and EEG localized sources were deeper. Unfortunately, basal temporal lobe areas were only partially covered by the sensor helmet of the MEG setup. Best correlation between EEG or MEG findings and electrocorticography findings was between horizontal EEG dipole orientation and prominent neocortical spiking; these patients also had a less favorable prognosis. Magnetic source imaging is currently unlikely to alter the surgical management of MTLE. The yield of spikes is too low, and ECD modeling shows only partial correlation with electrocorticography findings. Moreover, the whole-head MEG helmet provides insufficient coverage of the temporal lobe.

Ictal Magnetoencephalography in Temporal and Extratemporal Lobe Epilepsy

PURPOSE

We evaluated visual patterns and source localization of ictal magnetoencephalography (MEG) in patients with intractable temporal lobe epilepsy (TLE) and extratemporal epilepsy (ETE).

METHODS

We performed spike and seizure recording simultaneously with EEG and MEG on two patients with TLE and five patients with ETE. Scalp EEG was recorded from 21 channels (10-20 international system), whereas MEG was recorded from two 37-channel sensors. We compared ictal EEG and MEG onset, frequency, and evolution and performed MEG dipole source localization of interictal spikes and early ictal discharges and co-registered dipoles to brain magnetic resonance imaging (MRI). We correlated dipole characteristics with intracranial EEG, surgical resection, and outcome.

RESULTS

Ictal MEG lateralized seizure onset in both TLE patients and demonstrated ictal onset, frequency, and evolution in accordance with EEG. Ictal MEG source analysis revealed tangential vertical dipoles in the anterolateral angle in one patient, and anterior dipoles with anteroposterior orientation in the other. Intracranial EEG revealed regional entorhinal seizure onset in the first patient. Both patients became seizure free after temporal lobectomy. In ETE, ictal MEG demonstrated visual patterns similar to ictal EEG and had concordant localization with interictal MEG in all five patients. Two patients underwent surgery. Ictal MEG localization was concordant with intracranial EEG in both cases. One patient had successful outcome after surgery. The second patient did not improve after limited resection and multiple subpial transections.

CONCLUSIONS

Ictal MEG can demonstrate ictal onset frequency and evolution and provide useful localizing information before epilepsy surgery.

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.

Magnetoencephalography: clinical application in epilepsy

Magnetoencephalography (MEG) has developed to the point that it has now entered routine clinical application. Epilepsy MEG studies show that it can accurately localize spike sources--both ictal and interictal--as compared with both directly. Limitations involve difficulties in detecting complex or deep sources when recording spontaneous cerebral activity. MEG not only provides a novel tool to localize and characterize epileptiform disturbances, it also has an important role in determining the significance of abnormalities seen on both structural and functional imaging. Ultimately, MEG should play a major role in totally noninvasive epilepsy surgery evaluation.

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.

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.

Feasibility and limitations of magnetoencephalographic detection of epileptic discharges: simultaneous recording of magnetic fields and electrocorticography

Magnetoencephalography (MEG) is considered clinically useful in localizing the epileptogenic focus in partial epilepsy. However, the relationship between the extent of the brain involved in paroxysmal activities and the magnetic field changes at the scalp has not been fully clarified. Furthermore, whether paroxysmal activities generated in deep brain structures such as the hippocampus can be detected magnetically is uncertain. Eight patients with temporal lobe epilepsy and two with extratemporal lobe epilepsy underwent chronic recording from subdural electrodes. Magnetic and electrocorticographic discharges representing epileptic activity were recorded simultaneously. MEG recorded magnetic field changes originating from paroxysmal activity in the superiolateral cerebral cortex when the amplitudes of the electrical paroxysmal activities exceeded 100 microV and extended over more than 3 cm2 of cortical surface. MEG failed to record paroxysmal activity localized to the medial temporal lobe. MEG is often useful in identifying a spike focus in the superiolateral aspects of the cerebral hemisphere, but not discharges arising from the medial temporal lobe. Rapid decay of the magnetic field is likely to be the reason for this limited sensitivity to medial discharges.

Localization of ictal and interictal bursting epileptogenic activity in focal cortical dysplasia: agreement of magnetoencephalography and electrocorticography

Focal cortical dysplasia (FCD) is often associated with severe partial epilepsy. In such cases, interictal frequent rhythmic bursting epileptiform activity (FBREA) on both scalp electroencephalography (EEG) and electrocorticography (ECoG) is generally accepted to be identical to the ictal epileptiform activity. We used magnetoencephalography (or Magnetic Source Imaging (MSI)) to determine the epileptogenic zone in a 6-year-old patient with histopathologically proven FCD and normal magnetic resonance imaging (MRI). MSI was used to localize the sources of both ictal activity and FRBEA, which was then compared with ECoG findings. The intracranial sources of both types of activity co-localized in the left inferior frontal and superior temporal gyri. The location and extent of the epileptogenic area determined by MSI was essentially identical to that determined directly through extra-operative ECoG. In the absence of structural abnormalities detectable on MRI, the noninvasive method of MSI provided valuable information regarding the location and extent of the primary epileptogenic area. This was critical for pre-surgical planning regarding placement of intracranial electrodes and for risk-benefit evaluation.

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.

Detection and significance of focal, interictal, slow-wave activity visualized by magnetoencephalography for localization of a primary epileptogenic region

OBJECT

Magnetoencephalography (MEG) is a novel noninvasive diagnostic tool used to determine preoperatively the location of the epileptogenic zone in patients with epilepsy. The presence of focal slowing of activity recorded by electroencephalography (EEG) is an additional indicator of an underlying pathological condition in cases of intractable mesial temporal lobe epilepsy (MTLE). In the present study the authors examined the significance of focal, slow-wave and interictal spike activity detected using MEG in 29 patients who suffered from MTLE that was not associated with structural brain lesions.

METHODS

All patients underwent resective surgery after MEG and EEG monitoring. Equivalent single-dipole modeling was applied to focal low-frequency magnetic activity (LFMA) and interictal paroxysmal activity. Lateralized LFMA was defined as trains of rhythmic activity over the temporal area, with frequencies lower than 7 Hz, which were easily distinguished from background activity. Lateralized LFMA was found in 17 patients (58.6%); it always occurred on the side ipsilateral to the side of resection and displayed a maximum amplitude over the temporal area. Dipolar sources of magnetic flux computed during slow-wave trains were found in the majority of cases in the posterior superior temporal region and, occasionally, in mesial temporal structures that were subsequently resected. With respect to lateralization there was never disagreement between LFMA and MEG interictal spike sources. Thus, in patients with MTLE that is not associated with a mass lesion LFMA is topographically related to the epileptogenic area and, therefore, has value for reliable determination of the side and, possibly, the location of this area.

CONCLUSIONS

Although focal slowing of EEG background activity is generally considered to be a nonspecific sign of functional disturbance, interictal LFMA in patients with MTLE should be conceptualized as a distinct electrographic phenomenon that is directly related to the epileptogenic abnormality. Analyzing the interictal MEG distribution of LFMA and sharp activity improves the diagnostic utility of MEG in patients with suspected TLE who are undergoing surgical evaluation.

Two magneto-encephalographic epileptic foci did not coincide with the electrocorticographic ictal onset zone in a patient with temporal lobe epilepsy

To evaluate the usefulness and limitations of magneto-encephalography (MEG) for epilepsy surgery, we compared 'interictal' epileptic spike fields on MEG with ictal electrocorticography (ECoG) using invasive chronic subdural electrodes in a patient with intractable medial temporal lobe epilepsy (MTLE) associated with vitamin K deficiency intracerebral hemorrhage. A 19-year-old male with an 8-year history of refractory complex partial seizures, secondarily generalized, and right hemispheric atrophy and porencephaly in the right frontal lobe on MRI, was studied with MEG to define the interictal paroxysmal sources based on the single-dipole model. This was followed by invasive ECoG monitoring to delineate the epileptogenic zone. MEG demonstrated two paroxysmal foci, one each on the right lateral temporal and frontal lobes. Ictal ECoG recordings revealed an ictal onset zone on the right medial temporal lobe, which was different from that defined by MEG. Anterior temporal lobectomy with hippocampectomy was performed and the patient has been seizure free for two years. Our results indicate that interictal MEG does not always define the epileptogenic zone in patients with MTLE.

MEG predicts epileptic zone in lesional extrahippocampal epilepsy: 12 pediatric surgery cases

PURPOSE

To discover whether the spatial distribution of spike sources determined by magnetoencephalography (MEG) provides reliable information for planning surgery and predicting outcomes in pediatric patients with lesional extrahippocampal epilepsy.

METHODS

We retrospectively studied 12 children with extrahippocampal epilepsy secondary to cortical dysplasia (CD), tumor, or porencephalic cyst. We compared interictal MEG spike source locations and somatosensory evoked fields derived from equivalent-current dipole modeling with intraoperative or extraoperative electrocorticography (ECoG).

RESULTS

MEG spike sources were found in proximity to the lesion in all patients and extended from lesions in five patients with CD. Marginal spike sources were noted in three patients with tumors, one patient with a cyst, and one with CD, and extramarginal sources in three patients with tumors. Three patients with tumors underwent lesionectomy only; two had further cortical excisions. One patient with CD underwent lesionectomy only, three had lesionectomy and cortical excisions, and two had lesionectomy and multiple subpial transection. Asymmetric MEG spike sources correlated with ECoG findings in all patients. Residual epileptiform discharges on postexcisional ECoG corresponded to spike sources in three patients with tumors and one patient with a cyst. Eleven patients have been seizure free for 1-6 years (mean, 4 years). One patient had residual seizures after incomplete excision of right temporal CD.

CONCLUSIONS

MEG delineated asymmetric epileptogenicity surrounding lesions and the eloquent cortex. Complete tumor resection produced favorable outcomes despite residual postexcisional ECoG spikes and extramarginal MEG spike sources. CD characterized by clusters of MEG spike sources within and extending from lesions seen on magnetic resonance imaging (MRI) should be removed to prevent seizures.

Interictal and ictal magnetoencephalographic study in patients with medial frontal lobe epilepsy

PURPOSE

To determine whether magnetoencephalography (MEG) has any clinical value for the analysis of seizure discharges in patients with medial frontal lobe epilepsy (FLE).

METHODS

Four patients were studied with 74-channel MEG. Interictal and ictal electroencephalographic (EEG) and MEG recordings were obtained. The equivalent current dipoles (ECDs) of the MEG spikes were calculated.

RESULTS

In two patients with postural seizures, interictal EEG spikes occurred at Cz or Fz. The ECDs of interictal MEG spikes were localized around the supplementary motor area. In the other two patients with focal motor or oculomotor seizures, interictal EEG spikes occurred at Fz or Cz. The ECDs of interictal MEG spikes were localized at the top of the medial frontal region. The ECDs detected at MEG ictal onset were also localized in the same area as those of the interictal discharges.

CONCLUSIONS

In medial FLE patients, interictal and ictal MEG indicated consistent ECD localization that corresponded to the semiology of clinical seizures. Our findings demonstrate that MEG is a useful tool for detecting epileptogenic focus.

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.

EEG in epilepsy: current perspectives

The electroencephalogram (EEG) plays an important diagnostic role in epilepsy and provides supporting evidence of a seizure disorder as well as assisting with classification of seizures and epilepsy syndromes. Emerging evidence suggests that the EEG may also provide useful prognostic information regarding seizure recurrence after a single unprovoked attack and following antiepileptic drug withdrawal. Continuous EEG video telemetry monitoring has an established role in the diagnosis of non-epileptic pseudo-seizures and in localizing the seizure focus for epilepsy surgery. Newer tools such as EEG mapping and magneto-encephalogram, although still investigational, appear potentially useful for defining the seizure focus in epilepsy. This review examines the traditional concepts of clinical EEG in the light of newly available data.

A comparison of magnetoencephalography, MRI, and V-EEG in patients evaluated for epilepsy surgery

PURPOSE

To determine the efficacy and relative contribution of several diagnostic methods [ictal and interictal scalp and intracranial EEG, magnetic resonance imaging (MRI), and magnetoencephalography (MEG)] in identifying the epileptogenic zone for resection.

METHODS

This was a prospective study using a masked comparison-to-criterion standard. Fifty-eight consecutive patients with refractory partial epilepsy from two university comprehensive epilepsy programs were studied. Patients who were evaluated for and underwent epilepsy surgery were recruited. The main outcome measure was the efficacy of each diagnostic method to identify the resected epileptogenic zone, when referenced to surgical outcome.

RESULTS

MEG (52%) was second only to ictal intracranial V-EEG in predicting the epileptogenic zone for the entire group of patients who had an excellent surgical outcome (seizure free or rare seizure). In a subanalysis, for patients who had temporal lobe surgery, this same relation was seen (MEG, 57%, ictal intracranial V-EEG, 62%). With extratemporal resection, ictal (81%) and interictal (75%) intracranial EEG were superior to MEG (44%) in predicting the surgery site in those patients with an excellent outcome. Finally, for all patients who had a good surgical outcome, MEG (52%) was better than ictal (33%) or interictal (45%) scalp VEEG in predicting the site of surgery.

CONCLUSIONS

These results indicate that MEG is a very promising diagnostic method and raise the possibility that it may obviate the need for invasive EEG in some cases or reduce the length of scalp EEG evaluation in others.

Confirmation of two magnetoencephalographic epileptic foci by invasive monitoring from subdural electrodes in an adolescent with right frontocentral epilepsy

PURPOSE

To report our evaluation of interictal two epileptic spike fields on magnetoencephalography (MEG) by using invasive intracranial monitoring in a patient without lesion on magnetic resonance imaging (MRI).

METHODS

A 15-year-old left-handed boy with a 9-year history of refractory simple partial seizures, secondarily generalized, and a normal MRI, was studied with MEG to define magnetic spike sources, followed by invasive intracranial monitoring with subdural electrodes to delineate the epileptogenic zone and eloquent function pursuant to focal cortical excision.

RESULTS

MEG demonstrated two spike foci on the right middle frontal and inferior rolandic areas adjacent to the sensory area. Ictal recordings during prolonged invasive monitoring from subdural electrodes revealed two epileptogenic zones in the same locations as those defined by MEG. Focal cortical excision was performed of each epileptogenic zone. The patient has been seizure free for 24 months without neurologic deficit.

CONCLUSIONS

Magnetic source imaging is a valuable adjunct in the planning of subdural grid placement in epilepsy surgery, particularly in patients in whom conventional imaging fails to reveal a lesion.

Intrinsic epileptogenicity of focal cortical dysplasia as revealed by magnetoencephalography and electrocorticography

Focal cortical dysplasia (FCD) is often associated with severe partial epilepsy. In this study, we performed magnetoencephalography (MEG) and electrocorticogrsphy (ECoG) on four patients with FCD-associated epilepsy to confirm the 'intrinsic' epileptogenicity of FCD. In all patients, we determined the three-dimensional locations of the magnetic sources of the interictal paroxysmal activities by a single dipole model, and then the estimated dipole localization was superimposed on the magnetic resonance image. The dipole clusters were located in the T2-prolonged lesions, namely in the FCD lesions themselves. All patients underwent surgery for their medically intractable epilepsy, and the acute and/or chronic ECoG were thereafter recorded. Either frequent or continuous paroxysmal activities were recorded from the ECoG electrodes which were placed over the surface of the FCD lesion, while few paroxysmal activities were observed on the normal appearing adjacent cortex. Intraoperative depth recordings were performed in a patient with the needle electrode inserted into the FCD lesion and they revealed these paroxysmal foci to be located not on the cortical surface but at a depth of 15 mm from the cortical surface where both abnormal giant neurons and bizarre large eosinophilic cells (so-called balloon cells) were also prominently observed on the postoperative histological sections. Following a lesionectomy combined with the removal of the underlying white matter, three patients demonstrated a favorable seizure outcome. Our findings thus suggest the FCD lesions to be highly and intrinsically epileptogenic lesions.

Three-dimensional localization of subclinical ictal activity by magnetoencephalography: correlation with invasive monitoring

BACKGROUND

Although magnetoencephalography (MEG) provides accurate information on the spatial distribution and temporal patterns of the "interictal" epileptic activities, it is interictal in nature and therefore also prone to all the problems associated with interictal data.

METHODS

We investigated the subclinical "ictal" epileptic activity with a 37-channel, large-array biomagnetometer and mapped the data onto a three-dimensional image in a patient with intractable frontal lobe epilepsy. Dipole source localization was calculated based on magnetic fields for both the interictal and subclinical ictal activities.

RESULTS

The current dipoles of the interictal MEG spikes (MEG irritative zone) were revealed to be scattered in the left anterior frontal lobe, whereas that of the subclinical ictal onset (MEG subclinical ictal onset zone) was surrounded by the interictal dipole cluster. The dipole source localization of the propagating activities was not calculated with a single dipole model. The MEG subclinical ictal onset zone correlated well with the ictal onset zone subsequently recorded by invasive subdural electrophysiological monitoring. After multiple subpial transection of the deduced epileptogenic area, a dramatic reduction of the seizures occurred.

CONCLUSION

These results illustrate the potential of MEG for localizing the epileptogenic foci with high spatial and temporal resolution.

Magnetoencephalography in partial epilepsy: clinical yield and localization accuracy

The goals of this study were to determine (1) the yield of magnetoencephalography (MEG) according to epilepsy type, (2) if MEG spike sources colocalize with focal epileptogenic pathology, and (3) if MEG can identify the epileptogenic zone when scalp ictal electroencephalogram (EEG) or magnetic resonance imaging (MRI) fail to localize it. Twenty-two patients with mesial temporal (10 patients), neocortical temporal (3 patients), and extratemporal lobe epilepsy (9 patients) were studied. A 37-channel biomagnetometer was used for simultaneously recording MEG with EEG. During the typical 2-3-hour MEG recording session, interictal epileptiform activity was observed in 16 of 22 patients. MEG localization yield was greater in patients with neocortical epilepsy (92%) than in those with mesial temporal lobe epilepsy (50%). In 5 of 6 patients with focal epileptogenic pathology, MEG spike sources were colocalized with the lesions. In 11 of 12 patients with nonlocalizing (ambiguous abnormalities or normal) MRI, MEG spike sources were localized in the region of the epileptogenic zone as ultimately defined by all clinical and EEG information (including intracranial EEG). In conclusion, MEG can reliably localize sources of spike discharges in patients with temporal and extratemporal lobe epilepsy. MEG sometimes provides noninvasive localization data that are not otherwise available with MRI or conventional scalp ictal EEG.

Magneto-encefalografie

A magnetoencephalogram (MEG) is the registration of the magnetic field in points near the head. Because MEG's are weak fields, they have to be measured by means of superconducting sensors. The electric active population of neurons can be computed from the distribution of the magnetic field at a certain instant of time. This is called the inverse problem. In order to solve this probem, both the generators and the head have to be modelled. Usually, a patch of active neurons is modelled as a current dipole. Commonly, the head is described by three compartments, representing the brain, the skull and the scalp. The compartments may have the shape of spheres or they may have a realistic shape. Integration of EEG and MEG with MRI leads to a technique for functional imaging of the brain with a time resolution of one millisecond and a spatial resolution of one centimetre. Clinical applications are the non-invasive localization of an epileptic focus or the presurgical mapping of the sensorimotor cortex.

Information processing in the human brain: magnetoencephalographic approach

Rapid progress in effective methods to image brain functions has revolutionized neuroscience. It is now possible to study noninvasively in humans neural processes that were previously only accessible in experimental animals and in brain-injured patients. In this endeavor, positron emission tomography has been the leader, but the superconducting quantum interference device-based magnetoencephalography (MEG) is gaining a firm role, too. With the advent of instruments covering the whole scalp, MEG, typically with 5-mm spatial and 1-ms temporal resolution, allows neuroscientists to track cortical functions accurately in time and space. We present five representative examples of recent MEG studies in our laboratory that demonstrate the usefulness of whole-head magnetoencephalography in investigations of spatiotemporal dynamics of cortical signal processing.

Potential contribution of bilateral magnetic source imaging to the evaluation of epilepsy surgery candidates

The current procedures that are used to evaluate candidates for epilepsy surgery are time-consuming, costly, and often invasive. Magnetic source imaging (MSI), the combination of magnetoencephalography and anatomic imaging modalities, has shown promise as an efficient noninvasive means of localizing and characterizing seizure sources for possible resection. However, MSI has been limited by the inability to conduct simultaneous bilateral monitoring. In this study, a newly developed dual-magnetometer system was employed to record bilaterally the interictal activity in 30 candidates for epilepsy surgery. A standard monitoring protocol that included concurrent electroencephalographic recording and required a 2- to 3-hour examination period for each patient was developed. As a first step in a series of studies, the resultant MSI indications were compared with the information available from standard magnetic resonance imaging and concurrent electroencephalographic results. In 83% of the cases, this MSI protocol provided new information about the location of interictal epileptic activity that could be directive for subsequent patient care. Based on these results, it seems that MSI may become a cost-effective early step in epilepsy surgery evaluation. To continue the development on this basis, a study intended to validate the accuracy of MSI indicated by comparison with invasive electroencephalography has been initiated.

Brain glucose utilisation in acquired childhood aphasia associated with a sylvian arachnoid cyst: recovery after shunting as demonstrated by PET

Regional brain glucose utilisation was investigated with PET and fluorodeoxyglucose (FDG) in a case of epileptic aphasia (Landau-Kleffner syndrome) associated with a left sylvian arachnoid cyst. CT and MRI had failed to disclose any mass effect of the cyst on surrounding brain structures. Sequential metabolic measurements showed a comparable pronounced hypometabolism in cortical regions around the cyst, involving speech areas, and suggested mild but chronic compression of the developing brain. After placement of a cyst-peritoneal shunt system, significant metabolic improvement occurred in all cortical regions, especially the inferior frontal gyrus and the perisylvian area, with predominant residual deficit in the left superior temporal gyrus. These findings were correlated with a pronounced increase in word fluency and slower progress in verbal auditory comprehension. This report suggests that PET is able to evaluate the functional disturbances associated with expanding arachnoid cysts, and to follow the neurological improvement after drainage.

Magnetoencephalographic evaluation of children and adolescents with intractable epilepsy

Magnetoencephalographic (MEG) discharges were recorded with multichannel superconducting quantum interference device (SQUID) gradiometers in 13 young candidates for epilepsy surgery. The sources of epileptic activity were related to generators of somatosensory and auditory evoked cortical responses and projected on magnetic resonance imaging (MRI) scans. Seven subjects had restricted or regional MEG foci, located in the frontoopercular (1), sensorimotor (3), perisylvian (1), mesiotemporal (1), or temporooccipital cortex (1). The MEG foci in the 3 patients who underwent operation agreed with the intracranial findings. Findings in the other patients emphasize the need to collect further data to define the ultimate role of MEG in preoperative evaluation of epilepsy.