Both the electroencephalogram (EEG) and the magnetoencephalogram (MEG) are caused by the neuronal postsynaptic currents. Both can be measured at the surface of the skull. Because the EEG is highly influenced by the different conductivities between the source and the measuring points, it is much easier to localize the underlying sources by the MEG than by the EEG. Another practical advantage of the MEG is: The spatial coordinates of the MEG measuring points can be obtained by only one measurement, which is important in multichannel recordings.

In this longitudinal study multichannel MEG was used to localize and to quantify focal pathological spontaneous neuromagnetic activity in six patients with transient ischemic attacks (TIA) and two patients with transient global amnesia (TGA). Slow (2–6 Hz) and beta (14–30 Hz) activity were monitored up to 10 weeks. Results were compared with normative data, and changes over time were statistically analyzed. MEG detected pathological activity that persisted clinical symptoms. Focal slow activity originating from sensorimotor (TIA) and mesiotemporal (TGA) cortices exceeded normal values up to 14 times during the first hours after the attack and recovered to normal within 11 days. Focal beta activity was not useful to monitor the time course of TIA or TGA.

Cerebrovascular emboli may cause irreversible deficits (brain infarcts) or reversible ones (transient ischemic attacks (TIA)). If the reason is a high grade stenosis (>70%) of the cervical internal carotid artery, this symptomatic stenosis should be treated surgically according to large North-American and European joint studies [3]. According to the location in the brain clinical symptoms may or may not occur.

Brain lesions may alter the surrounding brain tissue structurally by moving or compressing it, or by infiltration or edema. Electrophysiological signs of the deteriorated function in these border zones can be focal slow (and fast) wave and epileptic activity in MEG (and EEG). Clinically it is of great interest to know the localization and the extent of these focal sources, but also the location and the extent of the evoked and event related activity of functionally important areas around brain lesions. So we developed, adapted and established clinical applications in neurosurgery and neurology with special reference to clinical relevance and acceptance.

Article Functional Mapping of Speech Evoked Brain Activity by Magnetoencephalography and its Clinical Application was published on January 1, 1999 in the journal Biomedical Engineering / Biomedizinische Technik (volume 44, issue s2).

Brain lesions may influence their border zones by moving, compressing, infiltration, edema or by a mixture of all. Sources of evoked and spontaneous activity in border zones can be localized by magnetoencephalography (MEG) giving information on the function of these areas. The inherent multisource problem of the MEG can be handled by using spatial average techniques (DipoleDensity-Plot (DDP) and Current-Density-Plot (CDP)). The reliability was tested with structural lesions (tumors and infarctions). The localizing of the somatosensory, the motor, and the auditory cortex and the speech related areas in relation to adjacent tumors is established in our clinical routine. The MEG results will be fused into the neuronavigator for image guided neurosurgery. In two studies with subjcets and patients our MEG results have been compared to the results of the functional MRI (fMRI). In the presurgical evaluation of epileptic patients the localization of the epileptogenic lesion is an additional valuable tool, especially when the lesion is not visible in the MRI. In patients with multiple sclerosis we could demonstrate that the white matter lesions produce abnormal activity only in the adjacent neuronal areas. In the field of cerebrovascular accidents the penumbra can be localized in infarctions and TIAs. The feasibility to localize the penumbra by the MEG was demonstrated by our two comparisons of MEG results and those of the proton magnetic resonance spectroscopic imaging ( 1 H MRSI) of N-acetyl and lactate in patients with brain infarction and tumors.

FUNCTIONAL magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) were performed in six subjects during self-paced finger movement performance, tactile somatosensory stimulation and binaural auditory stimulation using identical stimulation paradigms. Both functional imaging modalities localized brain activity in adjacent areas of anatomically correct cortex. The mean distances measured between fMRI activity and the corresponding MEG dipoles were 10.1 mm (motor), 10.7 mm (somatosensory), 13.5 mm (auditory right hemisphere) and 14.3 mm (auditory left hemisphere). The distances found may reflect the correlation between electrophysiological and hemodynamic responses due to the different underlying substrates of neurophysiology measured by fMRI and MEG: BOLD contrast vs neuronal biomagnetic activity.

Background. Brain lesions may influence their border zones by moving, compressing, infiltration, edema or by a mixture of all the above. Sources of evoked and spontaneous activity in border zones can be localized by the MEG.Methods. The inherent multisource problem can be handled by using spatial average techniques (dipole density plot (DDP) and current density plot (CDP)). The reliability was tested with structural lesions (tumors and infarctions).Results and Conclusions. The localizing of the somatosensory, motor, auditory cortex and the speech-related areas in relation to adjacent tumors, is used in clinical routine. The results will be fused into the neuronavigator for image-guided neurosurgery. In the presurgical evaluation of epileptic patients, the localization of the epileptogenic lesion is an additional valuable finding, especially when the lesion is not visible in the MRI. In the field of cerebrovascular accidents the penumbra can be localized in infarctions and TIAs. The feasibility to localize the penumbra by the MEG was demonstrated by our comparison of MEG results and those of the proton magnetic resonance spectroscopic imaging (H-1 MRSI).

INTRODUCTION: Magnetoencephalography (MEG) offers a non-invasive way to map evoked cortical activity with a high spatiotemporal resolution. The localization of the precentral motor cortex (Brodmann's area 4) is of great clinical interest because, especially in cases of presurgical diagnostics of pericentral masses in the brain, imaging procedures such äs MRI often cannot render sufficient Information about the lesion's exact local relation to the precentral gyrus [1]. For clinical usefulness, e. g. in a neuronavigating System, MEG results of measuring motor activity have to be consistently reproducible (reliable) when localized in the precentral gyrus (valid). The purpose of the present study was to establish a motor field source localization protocol in a neurologically normal subject population and to use this protocol in a clinical environment with patients undergoing presurgical magnetic source imaging (MSI).

INTRODUCTION: Multiple sclerosis (MS) is a demyelinating disease of the central nervous System. The underlying inflammatory infiltrations cause white matter lesions which can be visualized äs "plaques" in magnetic resonance imaging (MRI). Magnetoencephalography (MEG) offers the opportunity to localize pathological activity in neuronal areas with a high spatio-temporal resolution [1]. A preliminary investigation of our laboratory using a single-dewar biomagnetic System showed abnormal magnetic activity associated with white matter lesions (MS, lacunar infarctions) to be concentrated in adjacent cortical areas [2]. In this study, we investigated patients with MS simultaneously recording biomagnetic activity from both hemispheres to find if abnormal cortical activity is caused by MS lesions and, in case of such acti-