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PubMed1992

Magnetoencephalography: a tool for functional brain imaging.

At present, one of the most promising windows to the functional organization of the human brain is magnetoencephalography (MEG). By mapping the magnetic field distribution outside the head the sites of neural events can be located with an accuracy of a few millimeters and the temporal evolution of the activation can be traced with a millisecond resolution. This paper reviews some forward field calculation approaches suitable for the interpretation of the brain's electromagnetic signals. Inverse modelling with multiple dipoles is described in detail. An example of the analysis of the somatosensory evoked-responses illustrates the potential of multiple signal classification (MUSIC) algorithm in finding optimal dipole positions.

Algorithms↗
PubMed1990

Magnetoencephalography in clinical epileptology and epilepsy research.

This article reviews the application of magnetoencephalography (MEG) in clinical epileptology and epilepsy research. MEG recordings of interictal as well as ictal epileptiform discharges helped to improve non-invasive localization of epileptic foci in patients with focal epilepsy. Several studies showed good agreement of the localizations obtained from MEG compared with those from invasive electrical recordings. Thus, MEG may become a potentially useful technique in the pre-surgical evaluation of epilepsy patients. As evidenced from studying the penicillin focus in animals and spike propagation in humans, MEG also may contribute to further understand the basic mechanisms of epilepsy and thus may be useful in epilepsy research. Directions of future research include recording from a large number of channels covering a wide area of the head, long-term recording to study mechanisms involved in the transition of interictal to ictal state, and recording of slow magnetic field shifts associated with interictal and ictal epileptiform discharges.

Epilepsy↗
PubMed1995

Precision of dipole localization in a spherical volume conductor: a comparison of referential EEG, magnetoencephalography and scalp current density methods.

In this study, we determined the influence of dipole orientation, dipole location, and number of recording sites on the precision of dipole localization in a spherical volume conductor. We compared localization from referential EEG (R-EEG), scalp current density EEG (SCD-EEG) and magnetoencephalography (MEG). Dipole orientation had a small influence on the precision of dipole localization for R-EEG and SCD-EEG. Dipole location relative to the recording sites, dipole depth, and number of recording channels strongly influenced the precision of dipole localization. Assuming equal signal to noise conditions for each recording method, MEG and SCD-EEG had a similar precision for dipole localization of a single tangential dipole source and both methods were more precise than R-EEG. However, SCD-EEG was inferior to MEG for distinguishing a single tangential current source from a pair of deeper radial current sources. In summary, these results suggest that the MEG will be most useful for localization of multiple simultaneous dipole sources.

Brain↗
PubMed2005

Reduction of noise from magnetoencephalography data.

A noise reduction method for magnetoencephalography (MEG) data is proposed. The method is a combination of Kalman filtering and factor analysis. A state-space model for a Kalman filter was constructed using the forward problem in MEG measurement. Factor analysis provide estimations of noise covariances required by the Kalman filter to eliminate independent additive sensor noise. The proposed method supports independent component analysis (ICA), which is difficult to use in MEG analysis owing to the sensor noise. Numerical experiments were conducted to investigate the performance of the proposed method. In a single dipole case where the maximum signal-to-noise ratio (SNR) was -10 dB, approximately equivalent to raw MEG data, noise-free signals were successfully estimated from noisy data; a 0.02 s delay of the peak latency and 15-40% of attenuation of the peak amplitude were observed. Moreover, in a multiple dipole case, independent components preprocessed with the proposed method had high correlation, 0.88 at the lowest, with correlation of 0.69 and 0.52 for those preprocessed with conventional bandpass filters. The results show that the noise reduction method reduces sensor noise effectively. High SNR-independent components are obtained by the proposed method. Real MEG data analysis was also demonstrated. The proposed method extracted auditory evoked responses from unaveraged single-trial data.

Algorithms↗
PubMed2002

[Use of magnetoencephalography and functional neuronavigation in planning and surgery of brain tumors].

The role of magnetoencephalography (MEG) in neurology has been established for basic research, epilepsy, and functional brain mapping. The presurgical localization of functionally important brain areas has evolved as an important application of MEG. Both neurologists and neurosurgeons can use this method for decision-making and planning of nonsurgical or surgical treatment in brain tumors. The integration of functional brain mapping data into neuronavigation systems may help to minimize postoperative morbidity. This is especially important in low-grade gliomas, in which a potential benefit of surgery is only achieved when the tumor has been resected completely, whereas neurological deterioration means a substantial loss of quality of life during the survival time. This report addresses the utility of MEG combined with neuronavigation in the treatment of brain tumors adjacent to eloquent brain areas.

Adolescent↗
PubMed1997

Frequency and periodicity are represented in orthogonal maps in the human auditory cortex: evidence from magnetoencephalography.

Timbre and pitch are two independent perceptual qualities of sounds closely related to the spectral envelope and to the fundamental frequency of periodic temporal envelope fluctuations, respectively. To a first approximation, the spectral and temporal tuning properties of neurons in the auditory midbrain of various animals are independent, with layouts of these tuning properties in approximately orthogonal tonotopic and periodotopic maps. For the first time we demonstrate by means of magnetoencephalography a periodotopic organization of the human auditory cortex and analyse its spatial relationship to the tonotopic organization by using a range of stimuli with different temporal envelope fluctuations and spectra and a magnetometer providing high spatial resolution. We demonstrate an orthogonal arrangement of tonotopic and periodotopic gradients. Our results are in line with the organization of such maps in animals and closely match the perceptual orthogonality of timbre and pitch in humans.

Acoustic Stimulation↗
PubMed2001

Postoperative changes on functional mapping of the motor cortex in patients with brachial plexus injury: comparative study of magnetoencephalography and functional magnetic resonance imaging.

Using magnetoencephalography (MEG) and functional magnetic resonance imaging (f-MRI), we investigated the areas of the cerebral cortex that were activated when patients with brachial plexus injuries performed elbow flexion, a motion re-acquired through nerve transfer surgery. In all patients, elbow flexion on the operated side and on the unaffected side led to the activation of an area in the motor cortex, with these areas being located almost symmetrically on either side of the sagittal midline. These findings suggest that the activity center for the transferred intercostal nerves shifted to the motor cortex for the elbow, from the original intercostal nerve site.

Adolescent↗
PubMed2002

Magnetoencephalography in presurgical epilepsy evaluation.

The introduction of whole-head magnetoencephalography (MEG) systems facilitating simultaneous recording from the entire brain surface has led to a major breakthrough of MEG in presurgical epilepsy evaluation. Localizations of the interictal spike zone with MEG showed excellent agreement with invasive electrical recordings, were useful to clarify the spatial relationship of the irritative spike zone to structural lesions, and could attribute epileptic activity to lobar subcompartments both in temporal lobe and extratemporal epilepsy. MEG was especially useful for the study of patients with non-lesional neocortical epilepsy and of patients with large lesions, where it provided unique information on the epileptogenic zone. It could reliably localize sensorimotor cortex prior to surgical procedures adjacent to central fissure. MEG language mapping yielded concordant results with the Wada test and cortical stimulation studies. MEG localizations of epileptic activity and essential brain regions were successfully integrated into frameless stereotaxy systems providing accurate functional information intraoperatively. Because MEG and EEG yield both complementary and confirmatory information, combined MEG-EEG recordings in conjunction with advanced source modeling techniques will further improve the noninvasive evaluation of epilepsy patients and constantly reduce the need for invasive procedures.

Brain↗
PubMed2002

Practicability of magnetoencephalography-guided neuronavigation.

Magnetoencephalography (MEG) is a noninvasive option for localizing electroneurophysiological activity on the human cortex. The purpose of this study was to evaluate the practicability and reliability of MEG imaging integrated into a neuronavigation system to identify the sensorimotor cortex intraoperatively in patients with brain tumors in or near the central motor strip. It was performed prior to surgery in 30 patients with space-occupying lesions in or around the central region to localize the primary somatosensory cortex. These functional brain maps were superimposed on MR images obtained prior to surgery and transferred in the operating room for intraoperative functional neuronavigation. During surgery, the phase reversal technique identified a generator which coincided with the somatosensory cortex as displayed by the MEG-based functional neuronavigation system. Following surgery, the motor deficit improved in seven patients, was unchanged in five, and showed a slight transient deterioration in five. One patient suffered a deterioration of motor function with incomplete recovery. The MEG-based functional neuronavigation was found to be practicable and useful in finding a safe approach to tumors in or adjacent to the central region. The accuracy of MEG was concluded to be reliable as verified by the phase reversal technique.

Adult↗
PubMed2005

Dynamic brain activation patterns for face recognition: a magnetoencephalography study.

The present study used magnetoencephalography (MEG) to investigate the spatiotemporal profile of neurophysiological activity associated with recognition of recently encountered human faces in seventeen healthy right-handed adults. Activity sources modeled as instantaneous equivalent current dipoles were found in ventral occipito-temporal regions during the early stages of stimulus processing and in lateral temporal cortices during later stages. Hemispheric asymmetries in regional activity were restricted to ventral occipitotemporal areas. The magnitude of magnetic flux originating in these regions was greater in the right hemisphere during the first 350 ms post-stimulus onset. In addition, the duration of neurophysiological activity was greater in the right hemisphere after 600 ms post-stimulus onset. The results indicate right hemisphere predominance in the degree of engagement of neurophysiological processes involved in both the pre- and post-recognition phases of face processing.

Adolescent↗
PubMed2003

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.

Animals↗
PubMed2006

Magnetoencephalography in epilepsy: tailoring interpretation and making inferences.

Although the ability of magnetoencephalography (MEG) to detect epileptiform discharges noninvasively has long been known, only recently has it become a common tool in clinical settings. Whether MEG or electroencephalography (EEG) is superior has been controversial; MEG has a theoretic edge over EEG for precise localization, but EEG has many practical advantages. Experience has shown that they often provide different and complementary information. Although the results of careful MEG analysis can be quite precise, MEG interpretation, like that of EEG, is partly subjective and reader dependent. Therefore, the appearance of well-defined foci on MEG should not reflexively be regarded as conclusive, but weighed by judgment, experience, and an understanding of the assumptions and behavior of the localization model. We review selected studies in the past 2 years that are relevant to epilepsy. In particular, studies are described that provide insights into MEG's relation to EEG, its contribution to preoperative decision making, its application to benign Rolandic epilepsy, and analysis of secondary generalization.

Electroencephalography↗
PubMed1989

Subdural electrode as a dipole source for magnetoencephalography.

A subdural electrode was designed and constructed with 3 pairs of contacts to make 3 dipoles. The dipoles well approximate the expected magnetic behavior for a current dipole and can be used clinically to test localization capabilities of magnetoencephalography.

Electrodes↗
PubMed1996

Frontal mental theta wave recorded simultaneously with magnetoencephalography and electroencephalography.

Frontal mental theta waves were recorded simultaneously with magnetoencephalography (MEG) and electroencephalography (EEG) in normal subjects. MEG analyses demonstrated that theta burst activities distribute widely in the frontal cortices of both the cerebral hemispheres during concentration on mental calculation. On the other hand, EEG showed the theta activities seemed maximal on the frontal midline part (Fz), as if they might have been generated mainly in the frontal midline cortical area.

Adult↗
PubMed2004

Learning new names for new objects: cortical effects as measured by magnetoencephalography.

We tracked the evolvement of naming-related cortical dynamics with magnetoencephalography when five normal adults successfully learned names and/or meanings of unfamiliar objects. In all subjects, the learning of new names was associated with pronounced cortical effects. The learning effect was of long latency and emerged as a change of activation in the same cortical network that was active during naming of familiar items. In four out of five subjects, the cortical learning effect occurred in the inferior parietal lobe. In three of these subjects, the cortical effect was left-sided. These results suggest that the inferior parietal lobe plays an important role in the acquisition of novel words, presumably as a part of working memory systems.

Adult↗
PubMed2006

Magnetoencephalography and diffusion tensor imaging in gelastic seizures secondary to a cingulate gyrus lesion.

Gelastic seizures are relatively uncommon and rarely observed secondary to frontal lobe lesions. This report presents magnetoencephalography (MEG) and diffusion tensor imaging (DTI) findings in an adolescent with gelastic seizures secondary to a left anterior cingulate gyrus lesion. Ictal scalp video EEG showed bilateral frontal 4 Hz theta discharges. Interictal EEG showed left fronto-temporal spikes or sharp waves. Interictal MEG showed spike sources over bilateral temporal regions. DTI and tractography delineated slightly shifted corpus callosum posterior to the lesion, unaffected uncinate and inferior longitudinal fasciculi. The patient became seizure free for 12 months after surgical excision of a pleomorphic xanthoastrocytoma in the left anterior cingulate region. In our patient, MEG and EEG did not localize the deep-seated epileptogenic zone. The combination of DTI and neurophysiologic studies, however, possibly disclosed neuronal connections within the epileptic network and indicated that epileptic discharges propagated via the uncinate fibers from the primary epileptogenic zone in the anterior cingulate region to the mesial temporal region in this case with gelastic seizures secondary to a cingulate lesion.

Adolescent↗
PubMed2004

Co-registration of magnetoencephalography with magnetic resonance imaging using bite-bar-based fiducials and surface-matching.

OBJECTIVE: To introduce a new technique for co-registration of Magnetoencephalography (MEG) with magnetic resonance imaging (MRI). We compare the accuracy of a new bite-bar with fixed fiducials to a previous technique whereby fiducial coils were attached proximal to landmarks on the skull. METHODS: A bite-bar with fixed fiducial coils is used to determine the position of the head in the MEG co-ordinate system. Co-registration is performed by a surface-matching technique. The advantage of fixing the coils is that the co-ordinate system is not based upon arbitrary and operator dependent fiducial points that are attached to landmarks (e.g. nasion and the preauricular points), but rather on those that are permanently fixed in relation to the skull. RESULTS: As a consequence of minimizing coil movement during digitization, errors in localization of the coils are significantly reduced, as shown by a randomization test. Displacement of the bite-bar caused by removal and repositioning between MEG recordings is minimal ( approximately 0.5 mm), and dipole localization accuracy of a somatosensory mapping paradigm shows a repeatability of approximately 5 mm. The overall accuracy of the new procedure is greatly improved compared to the previous technique. CONCLUSIONS: The test-retest reliability and accuracy of target localization with the new design is superior to techniques that incorporate anatomical-based fiducial points or coils placed on the circumference of the head.

Brain↗
PubMed2004

Automated interictal spike detection and source localization in magnetoencephalography using independent components analysis and spatio-temporal clustering.

OBJECTIVE: Magnetoencephalography (MEG) dipole localization of epileptic spikes is useful in epilepsy surgery for mapping the extent of abnormal cortex and to focus intracranial electrodes. Visually analyzing large amounts of data produces fatigue and error. Most automated techniques are based on matching of interictal spike templates or predictive filtering of the data and do not explicitly include source localization as part of the analysis. This leads to poor sensitivity versus specificity characteristics. We describe a fully automated method that combines time-series analysis with source localization to detect clusters of focal neuronal current generators within the brain that produce interictal spike activity. METHODS: We first use an ICA (independent components analysis) method to decompose the multichannel MEG data and identify those components that exhibit spike-like characteristics. From these detected spikes we then find those whose spatial topographies across the array are consistent with focal neural sources, and determine the foci of equivalent current dipoles and their associated time courses. We then perform a clustering of the localized dipoles based on distance metrics that takes into consideration both their locations and time courses. The final step of refinement consists of retaining only those clusters that are statistically significant. The average locations and time series from significant clusters comprise the final output of our method. RESULTS AND SIGNIFICANCE: Data were processed from 4 patients with partial focal epilepsy. In all three subjects for whom surgical resection was performed, clusters were found in the vicinity of the resectioned area. CONCLUSIONS: The presented procedure is promising and likely to be useful to the physician as a more sensitive, automated and objective method to help in the localization of the interictal spike zone of intractable partial seizures. The final output can be visually verified by neurologists in terms of both the location and distribution of the dipole clusters and their associated time series. Due to the clinical relevance and demonstrated promise of this method, further investigation of this approach is warranted.

Action Potentials↗