Localization of fast MEG waves in patients with brain tumors and epilepsy

It was investigated if single dipole analysis of spontaneous fast waves (>8 Hz) can be used to determine the location of the epileptic focus. Automatic dipole analysis was applied to MEG data of 25 patients with intracranial tumors and epilepsy. The frequency range of 8-50 Hz was divided into standard EEG bands. MEG results were overlaid on the MRI scans of the patients. Dipoles describing fast wave fields were located in the parietal/ occipital cortex, and not at tumor border zones. In the cases that the dipoles were lateralized there was no clear preference to be located ipsi or contralateral to the tumor. However the generators of epileptic activity in these patients are thought to be located in the border areas of the tumors. Therefore it seems unlikely that the dipole locations describing fast waves are related to the epileptic zones in patients with brain tumors and epilepsy. A remarkable finding is that lateralized dipoles tend to be located in the right hemisphere and not in the left hemisphere. This appears to reflect an asymmetry of possibly normal background activity.