In the management of partial epilepsy, it is important to know if there is a single electrical focus and where it is located in the brain. However, in obtaining this information the scalp EEG often has serious shortcomings which are only overcome by surgically implanting electrodes in the brain. Because the magnetoencephalogram (MEG) measures the electrical currents differently than does the EEG, it should help to localize a focus and may make electrode implantation unnecessary in some cases. It should help in two ways. First, when the MEG is used in the same ac bandpass as in the standard EEG, it is complementary to the EEG, hence the combination of MEG-EEG should localize foci more accurately than does the EEG alone. The primary aim of this research is to assess this improvement in accuracy. In the second way, when the MEG is used with the lower limit at dc (the dcMEG), it is far less subject to the skin artifact which makes dcEEG measurements unreliable; hence, the dcMEG offers a new way of measuring any dc produced at a focus, therefore of locating this focus. The secondary aim of this research is to assess this use of the dcMEG. Measurements will be made in two groups of patients. The first group will be partial-seizure patients with a single documented focal lesion due to tumor or recent stroke. MEG and EEG maps (ac bandpass), and dcMEG maps, will be measured of interictal (and ictal) events in these patients. Then, to determine the foci locations, accurate computer modelling will be performed using the combined MEG-EEG maps, the EEG map alone, and the dcMEG map alone. The locations determined from the MEG-EEG combination and the dcMEG alone will be compared with that from the EEG alone and from lesion locations determined from CT and NMR scans. This comparison will show, in these relatively simple cases, the improvements due to the MEG-EEG combination, and the usefulness of the dcMEG. The second group of patients will be those who present with multi-focal problems in one or both hemispheres, and who are candidates for implanted electrodes; in these cases the locations of the primary foci will usually be determined from the implanted electrodes. The same types of maps and modelling will be made as with the first group. Then the locations of the foci determined from the MEG-EEG combination and the dcMEG alone will be compared with that from EEG alone and with the primary foci locations determined from the electrodes. This will allow the assessments to be made in these more complicated cases.
| Melcher, J R; Cohen, D (1988) Dependence of the MEG on dipole orientation in the rabbit head. Electroencephalogr Clin Neurophysiol 70:460-72 |
