Enhancement of deep epileptiform activity in the EEG via 3-D adaptive spatial filtering

The detection of epileptiform discharges (ED's) in the electroencephalogram (EEG) is an important component in the diagnosis of epilepsy. However, whe n the epileptogenic source is located deep in the brain, the ED's at the sc alp are often masked by more superficial, higher-amplitude EEG activity. A noninvasive technique which uses an adaptive "beamformer" spatial filter ha s been investigated for the enhancement of signals from deep sources in the brain suspected of containing ED's. A forward three-layer spherical model was used to relate a dipolar source to recorded signals to determine the be amformer's spatial response constraints. The beamformer adapts, using the l east-mean-squares (LMS) algorithm, to reduce signals from sources distant t o some arbitrarily defined location in the brain. The beamformer produces t hree outputs, being the orthogonal components of the signal estimated to ha ve arisen at or near the assumed location. Simulations were performed by using the same forward model to superimpose r ealistic ED's on normal EEG recordings. The simulations show the beamformer 's ability to enhance signals emanating from deep foci by way of an enhance ment ratio (ER), being the improvement in signal-to-noise ratio (SNR) to th at observed at any of the scalp electrodes. The performance of the beamform er has been evaluated for 1) the number of scalp electrodes, 2) the recordi ng montage, 3) dependence on the background EEG, 4) dependence on magnitude , depth, and orientation of epileptogenic focus, and 5) sensitivity to inac curacies in the estimated location of the focus. Results from the simulations show the beamformer's performance to be depend ent on the number of electrodes and moderately sensitive to variations in t he EEG background. Conversely, its performance appears to be largely indepe ndent of the amplitude and morphology of the ED. The dependence studies ind icated that the beamformer's performance was moderately dependent oh eccent ricity with the ER increasing as the dipolar source and the beamformer were moved from the center to the surface of the brain (1.51-2.26 for radial di poles and 1.17-2.69 for tangential dipoles). The beamformer was also modera tely dependent on variations in polar or azimuthal angle for radial and tan gential dipoles. Higher ER's tended to be seen for locations between electr ode sites. The beamformer was more sensitive to inaccuracies in both polar and azimuth al location than depth of the dipolar source. For polar locations, an ER > 1.0 was achieved when the beamformer was located within +/-25 degrees of a radial dipole and +/-35 degrees of a tangential dipole. Similarly, angular ranges of +/-37.5 degrees and +/-45 degrees, respectively, for inaccuracies in azimuthal locations. Preliminary results from real EEG records, compris ing 12 definite or questionable epileptiform events, from four patients, de monstrated the beamformer's ability to enhance these events by a mean 100% (52%-215%) for referential data and a mean 104% (50%-145%) for bipolar data .