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.