Magnetoencephalography in the study of epilepsy

Atypical cortical language organization in epilepsy patients: evidence for divergent hemispheric dominance for receptive and expressive language function

The central goal of presurgical language mapping is to identify brain regions that subserve cortical language function to minimize postsurgical language deficits. Presurgical language mapping in patients with epilepsy presents a key challenge because of the atypical pattern of hemispheric language dominance found in this population, with higher incidences of bilateral and right-biased language dominance than typical. In this prospective study, we combine magnetoencephalography with a panel of tasks designed to separately assess receptive and expressive function to provide a sensitive measure of language function in 15 candidates for resective surgery. We report the following: 4 of 15 patients (27%) showed left hemisphere dominance across all tasks, 4 of 15 patients (27%) showed right hemisphere dominance across all tasks, and 7 of 15 (46%) showed discordant language dominance, with right-dominant receptive and left-dominant expressive language. All patients with discordant language dominance showed this right-receptive and left-expressive pattern. Results provide further evidence supporting the importance of using a panel of tasks to assess separable aspects of language function. The clinical relevance of the findings is discussed, especially about current clinical operative measures for assessing language dominance, which use single hemisphere procedure (intracarotid amobarbital procedure and awake intraoperative stimulation) for determining language laterality.

Pediatric Magnetoencephalography and Magnetic Source Imaging

Magnetoencephalography (MEG) and magnetic source imaging (MSI) together represent a uniquely powerful functional imaging modality because of their capabilities of directly observing the electrophysiologic activity of neurons with exquisite temporal detail and accurately localizing corresponding neuromagnetic field sources onto high-resolution MR images. These features have and should continue to advance our understanding of the complex spatiotemporal basis of normal and abnormal brain function and development in children. By more clearly delineating and characterizing epileptogenic foci and their relation to eloquent cortex, MSI enables earlier and more effective neurosurgery to be performed, thus resulting in improved seizure outcomes. Although MEG and MSI cannot replace scalp electroencephalography, neuropsychologic testing, and the need for meticulous intraoperative cortical mapping in patients undergoing excision of epileptogenic lesions, their increasing availability should ultimately persuade many clinicians of their key, if not essential, role in the evaluation and treatment of children with epilepsy.

Magnetoencephalography (MEG) and magnetic source imaging (MSI)

BACKGROUND

Real-time, direct assessment of brain electrophysiology is critical for noninvasive functional mapping and for the identification of paroxysmal epileptiform abnormalities in the evaluation of patients for epilepsy surgery. Historically, electroencephalography (EEG) and evoked potentials (EPs) have performed these functions. However, both often required direct intracranial recording for precise localization. Magnetoencephalography (MEG) takes advantage of the fact that neuromagnetic signals penetrate the skull and scalp without distortion. The magnetic source image (MSI) is created when the MEG data is superimposed on a magnetic resonance image (MRI).

REVIEW SUMMARY

MEG performs noninvasive functional imaging by recording the magnetic flux on the head surface associated with electrical currents in activated sets of neurons. MEG has rapidly evolved in the last 2 decades because of the introduction of whole head systems and advances in computer technology. MEG is now the imaging modality of choice where a precise and high degree of localization is required. MEG can be used to localize the primary sensory cortices (visual, auditory, or somatosensory), areas involved with receptive language function, the irritative zone in epilepsy patients, and identify children with anomalous language development. This article reviews the basis of MEG, the instrumentation used, the clinical applications and current limits of the technology.

CONCLUSION

MEG studies can now be performed on a routine basis as a clinical tool. MEG is now indicated for: 1) localization of the irritative zone in lesional and nonlesional epilepsy surgery patients, 2) functional mapping of eloquent cortex, and 3) assessment of normal and abnormal language development. In the future MEG may help the understanding of normal development and reorganization after brain injury. The neurologist can use MEG data to complement structural and metabolic imaging techniques.

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.

[18F]FDG-PET and whole-scalp MEG localization of epileptogenic cortex

PURPOSE

To evaluate combined [18F]fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) and 122-channel whole-scalp magnetoencephalography (MEG) in lateralizing the epileptogenic cortex in patients whose routine presurgical evaluations gave discordant results about the location of the epileptic focus.

METHODS

Nine patients (five women, four men) aged 13-40 years were studied. Subdural EEG (SEEG) was recorded from eight patients. Six patients were operated on.

RESULTS

In seven of nine patients, PET and MEG agreed in localizing the epileptogenic cortex. When PET and MEG were in congruence, SEEG agreed with the findings. In five of six operated-on patients, PET and MEG results were congruent, and the outcome of the operation was successful. Two patients had discordant PET and MEG results. In one patient, PET showed bitemporal hypometabolism, whereas MEG showed epileptiform activity in the right parietal lobe. The surgical outcome of the palliative temporal lobectomy was poor. Another patient had unilateral temporal hypometabolism in PET and bitemporal activity in MEG. She was not operated on.

CONCLUSIONS

In most patients, PET and MEG were congruent in locating the epileptogenic cortex. Thus the combination of these techniques may provide useful support for the localization of the seizure onset and reduce the need for invasive procedures.

Functional organization of the human first and second somatosensory cortices: a neuromagnetic study

Multichannel neuromagnetic recordings were used to differentiate signals from the human first (SI) and second (SII) somatosensory cortices and to define representations of body surface in them. The responses from contralateral SI, peaking at 20-40 ms, arose mainly from area 3b, where representations of the leg, hand, fingers, lips and tongue agreed with earlier animal studies and with neurosurgical stimulations and recordings on convexial cortex in man. Representations of the five fingers were limited to a cortical strip of approximately 2 cm in length. Responses from SII peaked 100-140 ms after contra- and ipsilateral stimuli and varied considerably from one subject to another. Signs of somatotopical organization were seen also in SII. Responses of SII were not fully recovered at interstimulus intervals of 8 s.

Models of source currents in the brain

Electroencephalography (EEG) and magnetoencephalography (MEG) provide signals that are weighted integrals of source currents in the brain. In addition to technical aspects, the two methods differ in their sensitivities to various cerebral sources. Moreover, it is more difficult to determine the lead fields of EEG than of MEG. If it can be assumed that only one localized source is active at a particular time, the source location, direction, and amplitude can be found with the dipole model. However, if the assumption of a single localized source is violated, erroneous results are obtained. If a few sources are responsible for the measured fields, multiple-dipole models can be used. In the general case one must start from the fundamentals of estimation theory. The use of a priori information, together with experimental data, will provide the best possible solution to the inverse problem. In the case of minimal prior information, the so-called minimum-norm solution is obtained. With the help of supplementary information, the resolution can be further improved.

Development of multichannel neuromagnetic instrumentation in Finland

In this paper we describe the instrumentation for biomagnetic measurements available in our laboratory. The focus is on our 24-channel planar gradiometer system. In addition, a 122-channel system under construction will be discussed.