J. Vieth

Sources of spontaneous slow waves associated with brain lesions, localized by using the MEG.

SummaryElectric or magnetic slow wave brain activity can be associated with brain lesions. For an accurate source localization we transformed the magnetoencephalographic (MEG) coordinate system to the magnetic resonance imaging (MRI) system by using a surface fit of the digitally measured head surface and the reconstructed surface of the MRI scan. Furthermore we solved the problem to separate sources of focal activity from other multiple sources by introducing a spatial average, the Dipole Density Plot (DDP). The DDP shows in a quantified manner concentrations of dipoles across time. The DDP uses the single dipole model adequately, because only those signal sections will be analyzed, where one component contributes to the signal predominantly. In all cases, where multiple sources concurrently active are to be localized, a current distribution analysis will be used, the Current Localization by Spatial Filtering (CLSF). All source localization procedures were tested using structural brain lesions, which were verified by imaging techniques (MRI or CT), showing the results in close topographical relation to the lesions. The results so far let us assume, that the DDP and the CLSF are valuable tools to localize sources of focal spontaneous slow wave electrical brain activity.

Estimates of brain activity using Magnetic Field Tomography in a GO/NOGO avoidance paradigm

SummaryThis paper presents the first estimates of three dimensional evolution of activity in the brain associated with a GO/NOGO avoidance (CNV) paradigm. These estimates are continuous probabilistic solutions (Ioannides et al. 1990) to the biomagnetic inverse problem, obtained from averaged multichannel magnetoencephalographic (MEG) recordings (Vieth et al. 1991). The emphasis here is placed on the comparison of the activity associated with the GO and NOGO conditions; estimates of activity are shown for the onset of warning stimulus (S1), the early response half a second after S1, the late response lasting for over one second before S2 (the time between S1 and S2 is 3.5 seconds) and the onset of the imperative stimulus (S2). We find responses in regions of the brain implicated with hearing the stimulus, task engagement and motor output. Differences in the images corresponding to GO and NOGO conditions are significant because they reflect differences in brain function when a motor response is required or must be inhibited.

Three dimensional localization of the pathological area in cerebro-vascular accidents with multichannel magnetoencephalography.

In these .single channel studies we found a sufficient validation of the locahzation of pathological activity, since in 24 patients with venfied bram lesions the estimated single dipoles were located in the vicinity of the lesions (4,5,6,7,8). But using a single channel instruxncni the spatiotemporal dynamics of the bram activity was lost, and the resulting dipoles were onJy show ing the point of gravity of the activity during the time analyzed.

The dipole density plot (DDP), a technique to show concentrations of dipoles.

To analyse spontaneous multichannel slow-wave MEG activity a tool was developed to extract focal abnormal activity with a higher spatial dipole density across time. The first version works on discrete volume units, the second version avoids a possible location error, works continuously and can be adjusted for individual slices three-dimensionally. The first version is used for screening and the second version is used to go into more detail. The dipole density plot (DDP) is not limited to single foci, nor to the single-dipole model; neither is it limited to the MEG. The DDP seems to be a valuable tool for detecting and locating lesions three-dimensionally within the brain.

FOCAL SPONTANEOUS BETA WAVE ACTIVITTY LOCALIZED CLOSE TO STRUCTURAL BRAIN LESIONS

Slow wave brain activity in the ränge of l to 6Hz (delta/theta) is known to arise frora lesions in the brain. We showed, that with MEG measurements this activity can be localized close to the lesions (6). Not so much attention was up to now directed to other frequency bands. Therefore we wanted to know if in addition to delta/theta activity also beta wave activity is associated with structural lesions and where it is located compared to delta/theta activity.

Beta- und Theta/Delta-Schwerpunkte bei strukturellen neuronalen und Faserhirnläsionen, lokalisiert mit Hilfe der Magnetoenzephalographie (MEG)

Kurzlich konnten wir und andere zeigen, das die Quellen spontaner langsamer magnetischer Aktivitat bei kortikalen Lasionen schalenformig um die Lasion angeordnet sind (Vieth et al. 1992; Gallen et al. 1992). Vergleichbare Untersuchungen hinsichtlich spontaner fokaler Beta-Aktivitat (12,5–30 Hz) oder der pathologischen Spontanaktivitat bei Marklagerlasionen wurden unseres Wissens bisher nicht unternommen. Wir untersuchten daher die pathologische Spontanaktivitat im Beta-Frequenzbereich bei Lasionen des Kortex und des Marklagers im Vergleich mit der langsamen magnetischen Aktivitat (2–6 Hz). Die Frage war, wie eng die Schwerpunkte von Beta-Aktivitat mit kortikalen Lasionen zusammenhangen und mit denen der langsamen Aktivitat ubereinstimmen, und wo andererseits sich pathologische Aktivitat bei Marklagerlasionen zeigt.

MAGNETOENCEPHALOPGRAPHY, A METHOD TO LOCALIZE CLINICALLY SILENT TRANSIENT ISCHEMIC ATTACKS (TIA) USING AN IMPROVED DIPOLE-DENSITY-PLOT

Depending on the severity of ischemic lesions caused by embolisrrjjin cerebral arteries the result may be a brain infarction or a transient ischemic attacks (TIA). And depending on the location of the embolis clinical findings are seen or not. The clinically silent infarctions only can be seen in scans of computed tomography (CT) or of magnetic resonance imaging (MRI). In our data (8) we found in 305 patients who were candidates for an endarteriectomy of the internal carotid artery of the neck in CT scans in 61 patients (20%) an infarction with neurological Symptoms and in 76 patients (25%) an infarction without Symptoms (silent). 76 patients of the 305 patients (25%) suffered frora TIAs with neurological Symptoms. activity. At the beginning we used a single channel bioraagnetic System from B.T.i. (Model 601) applying the single dipole s the source model. In most of 22 cases we found the single -current dipole of interictal spike activity or slow wave activity near or close to the verified lesions (tumor, cyst, angioma, scar, brain infarction, hemorrhage) (8,10). Also with the Siemens multichannel bioraagnetic System KRENIKON (3) we continued the validation study with a 37 fold shorter recording time and the preservation of the simultaneous activity. The continuous estimation of single dipoles showed dipole pathways across time and space. Examples are shown elsewhere (7). Up to now we investigated 10 brain infarctions, 4