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At least 19 recordsLinked to original sources
PubMed1987

Magnetoencephalography and epilepsy research.

Magnetoencephalography is the detection of the magnetic field distribution across the surface of the head, which is generated by a neuronal discharge within the brain. Magnetoencephalography is used in clinical epilepsy to localize the epileptogenic region prior to its surgical removal. A discussion of the instrumentation based on the superconducting quantum interference device that is used for detecting the magnetic field distribution, the analytical techniques, current research, and future directions of magnetoencephalography in epilepsy research is presented.

Electroencephalography↗
PubMed1989

Neocortical propagation in temporal lobe spike foci on magnetoencephalography and electroencephalography.

Propagation of the neuronal population of the interictal epileptic spike was quantified in 5 patients with complex partial epilepsy arising from temporal lobe using electroencephalography and magnetoencephalography. During the spike complex in each patient there was a spike at the deep sphenoidal electrode and a spike at the superficial scalp electrode on spontaneous electroencephalography. In each patient the sphenoidal spike had a different peak latency than the scalp spike, consistent with spike propagation. Electroencephalography was used to trigger two magnetoencephalographic averages of stereotyped spikes during the sphenoidal peak and the scalp peak. Magnetoencephalography discriminated the centers of two cortical spike populations at different latencies, showing deeper localization with sphenoidal trigger and more superficial localization with scalp trigger in each patient (p less than 0.05). Latency differences and propagation distances of spikes were consistent with the conduction velocity of corticocortical fibers. Noninvasive estimates of the cortical surface area of the spikes agreed with estimates obtained by electrocorticography over temporal neocortex. These findings indicate propagation of neuronal populations active during human interictal spikes between deep and superficial cortex of temporal lobe, likely by monosynaptic or oligosynaptic pathways. This interictal system appears to be partly independent of the hippocampal interictal system in complex partial epilepsy.

Cerebral Cortex↗
PubMed1992

Magnetoencephalography of focal cerebral ischemia in rats.

BACKGROUND AND PURPOSE: The purpose of this study was to use magnetoencephalography to record magnetic field changes in the brain during middle cerebral artery occlusion. METHODS: A direct-current electrocorticogram (two channels) and a direct-current magnetoencephalogram (seven channels) were simultaneously recorded from five rats subjected to middle cerebral artery occlusion for 1-2 hours. RESULTS: Direct-current electrocorticographic and direct-current magnetoencephalographic signal deflections were observed after the onset of middle cerebral artery occlusion and occurred repeatedly throughout the ischemic period, with a mean +/- SD time interval of 12 +/- 5 minutes. A one-to-one correspondence of the electrocorticographic and magnetoencephalographic signal deflections was demonstrated. CONCLUSIONS: Direct-current magnetoencephalography can provide a new noninvasive technique for studying depolarization and/or spreading depression in focal cerebral ischemia.

Animals↗
PubMedDate not supplied

Anatomic localization of cerebral cortical function by magnetoencephalography combined with MR imaging and CT.

Magnetoencephalography (MEG) monitors magnetic field amplitudes, which are time averages of evoked neuronal responses. This method can detect magnetic fields emanating from the brain and localize the neuronal source. The location of somatosensory neuronal sources for voluntary right thumb and right index finger flexions were determined in four normal volunteers by using a seven-sensor neuromagnetometer inside a magnetically shielded room. These neuronal sources were then identified on the individual's respective CT or MR scans, and correlation was accomplished by geometric calculations, direct cranial measurement, and surface marker identification. Specific functional magnetic fields were located over the appropriate sensory motor cortex; however, there was considerable variation in the exact site. Magnetoencephalography combined with CT and MR may improve localization of normal and abnormal neurologic function.

Adult↗
PubMed1990

Localization of auditory response sources using magnetoencephalography and magnetic resonance imaging.

Magnetoencephalography offers the possibility of localizing accurately and noninvasively the source of intracranial currents associated with normal and abnormal brain activity. The purpose of this study was to assess the validity and across-subject reliability of localization of cortical sources responding to ipsilateral and contralateral auditory stimulation. Magnetic evoked fields to both stimulation conditions were measured in eight consecutive normal subjects, and the cortical sources of these fields were estimated on the basis of these measurements. Subsequent projection of the source location coordinates onto magnetic resonance images showed that in all subjects the sources were accurately estimated to fall in the vicinity of the auditory cortex and that two separate sources may account for the response to ipsilateral and contralateral stimulation.

Acoustic Stimulation↗
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↗
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↗
PubMed1990

Magnetoencephalography in the study of epilepsy.

A brief review is given about the basic principles of magnetoencephalography (MEG), a noninvasive brain research method in which weak magnetic fields are detected outside the human head with SQUID (Superconducting Quantum Interference Device) magnetometers. The active brain areas, producing the signal, are modelled by current dipoles, which are assumed to be situated in a spherically symmetric volume conductor. The locations of these "equivalent dipoles" can be found, in the optimal case, with a precision of a few millimeters. The new multichannel magnetometers allow measurements of spontaneous brain activity without EEG-triggered averaging. The 3-dimensional locations of superficial epileptogenic foci can be determined with respect to external landmarks on the skull and to known generator areas of evoked responses in the brain. Examples are given about MEG recordings of epileptic patients.

Electricity↗
PubMed1991

Anatomical correlates for magnetoencephalography: integration with magnetic resonance images.

We have implemented an initial version of software for the integration of magnetic resonance imaging (MRI) and magnetoencephalography (MEG). The package displays MRI images and performs basic image processing. The coordinate systems of the two methods are matched with the help of markers fixed on known head landmarks and a 3D digitiser. The spherical conductor model can be individually fitted to the shape of the brain in the region of interest. The computed source locations can be instantly superimposed on the MRI images during the analysis. Examples of localisation results from a healthy subject and an epileptic child are discussed.

Brain↗
PubMed1991

Principles of magnetoencephalography.

Magnetoencephalography (MEG) is a new, noninvasive functional test equivalent to EEG. It has been used to localize the sources of evoked responses and interictal and ictal epileptiform discharges and to study patients with psychiatric illnesses, cerebrovascular accidents, and migraine. In epilepsy research, it is hoped that MEG will provide information similar to that yielded by depth or subdural electrode recording, or that the combination of these methods will provide more information than either one alone. The application of MEG appears to be widening, although it is not yet a routine clinical diagnostic tool. The utility of MEG is limited by technological problems, but new and more efficient systems are becoming available. Within several years, advances in the technology and understanding of MEG may modify the course of its application.

Epilepsy↗
PubMed1991

Techniques for DC magnetoencephalography.

DC shifts are known to occur in association with a number of physiologic phenomena including spreading depression, hypoxia, epilepsy, and hypercapnia and possibly in migraine, closed head injury, and ischemia. Magnetoencephalography (MEG) makes it possible to record these shifts by prolonged DC monitoring of brain activity and offers several advantages over DC EEG and DC electrocorticography. Among the advantages of MEG is its non-invasive nature and the lack of impedance changes at the electrode-tissue interface that produce baseline shifts in DC EEG. In DC MEG measurements, great care must be taken in dealing with a variety of artifactual signals. Environmental noise can be reduced by magnetic shielding and recognized by use of reference magnetometers. Patient-generated artifacts are numerous and can be recognized and limited by a variety of methods.

Humans↗
PubMed1990

Magnetoencephalography and epilepsy research.

The theory of magnetoencephalography (MEG) and its application to epilepsy research are reviewed briefly. The MEG prediction appears to agree in general with regions where epileptiform discharges are found on the electrocorticogram. MEG appears to have a somewhat better localizing capability than EEG, although MEG may well miss the tangential component of magnetic fields. Thus, the combination of MEG and EEG may be more fruitful than either one alone.

Brain↗
PubMed1991

Localization of implanted dipoles by magnetoencephalography.

We attempted to validate the location of sources predicted by magnetoencephalography (MEG) by studying 19 specially designed dipole electrodes implanted in six patients with intractable partial seizures who were undergoing subdural electrode recording. We used a seven-channel magnetometer to measure the magnetic fields produced by passing through the dipoles a 40-microA, 5-msec square-wave pulse followed 40 msec later by a pulse of opposite polarity; 200 pulses were averaged for each magnetometer position. The actual dipole locations were measured from skull radiographs, and we based MEG localization on a spherical head model with the inclusion of volume currents. MEG estimates of the sources were within several centimeters (mean, 1.69 cm) of the measured locations. We conclude that MEG localization was promising.

Electrodes, Implanted↗
PubMed1991

[Preoperative and postoperative magnetoencephalography in a patient with partial epilepsy].

Magnetoencephalography (MEG) is a non-invasive method with a potential of clinical diagnostic use to localize epileptogenic foci in the brain. The aim of this study was to investigate whether the pathological focus localized by MEG was concurrent with the conventional preoperative examinations in a patient with medically intractable epilepsy who underwent surgery (left side uncohippocampectomy). A conventional preoperative test battery had already documented a left-hemisphere fronto-temporal epileptogenic focus. Postoperatively the patient was seizure free. MEG was performed three months before and ten months after the operation. The analysis of the MEG data indicated a left hemisphere fronto-temporal focus in agreement with results obtained by means of the conventional methods used for locating epileptic foci. Postoperatively the focus had disappeared.

Adult↗
PubMed1990

Magnetoencephalography and late component ERPs.

The recent emergence of magnetoencephalography is a development that is already yielding results. Its intrinsic sensitivity to the actual source activity, rather than to volume currents, means that its particular value lies in source localization. As well as having proven value in research relating to early evoked response and epilepsy, biomagnetic measurements have been applied to later components. For example, results for the auditory P300 potential indicate the source to lie either in the temporal cortex or the hippocampus. More definite conclusions await further development of the instrumentation. Also, there is much scope for more sophisticated analysis of the magnetic fields, of the electric potentials, or ideally of both together. This is likely to elucidate the largely unknown functional circuitry corresponding to the late components of the response.

Brain Diseases↗
PubMed1991

Localization of the P3 sources using magnetoencephalography and magnetic resonance imaging.

In this study, two related issues were addressed: first, whether the P3 component of auditory evoked responses, obtained in the context of an oddball paradigm, and its magnetoencephalographically recorded counterpart (P3m) are generated by the same intracranial sources; and, second, whether these sources, modeled as equivalent current dipoles, can be localized in particular brain structures using magnetic resonance imaging. The study involving 8 normal adult subjects resulted in the following findings. (1) Both the similarities and differences in wave form characteristics of the simultaneously recorded P3 and P3m can be best accounted for by common intracranial sources. (2) Several successively activated single-dipolar sources, rather than a single source, account for the entire evolution of the P3m component. (3) Most of these sources were localized in the vicinity of the auditory cortex in all subjects, although some sources appeared to be in deeper structures, possibly the lateral thalamus. (4) The successive activation of sources followed an orderly medial-to-lateral course. These results suggest that activity responsible for the surface-recorded P3 (and P3m) component may be initiated in deep structures, but it quickly spreads over and is sustained in areas near the auditory cortex.

Adult↗