Magnetoencephalography was developed with the hope that it, too, might provide unique and nonredundant localization data. Because magnetic field strength is not significantly attenuated or dispersed by the skull and scalp, MEG, theoretically, can detect activity within the brain with better resolution than that provided by EEG.23,33 Magnetic source imaging is the term used to describe the imaging modality in which MEG-identified dipoles are coregistered with an MR image and then superimposed on the MR image to correlate dipole localization with anatomy. Because data obtained using MS imaging can be directly incorporated into surgical planning either by using visual inspection or by incorporation into a surgical navigation system, MS imaging has facilitated the utility of data provided by MEG as an intraoperative surgical adjunct.7,18,27 Despite the potential promise of MEG and MS imaging, this technique has remained relatively underused, largely due to the great expense and technical demands required to maintain and refine the associated technology.
Recently several groups reported a significant correlation between MS imaging—determined interictal spike dipoles and seizure foci in patients with epilepsy, which are principally based on surgical outcomes.8,9,19,35 These publications are the first population studies to suggest regional correlation of MS imaging with intracranial ECoG and the potential use of MS imaging for guiding the placement of intracranial electrodes. Because MEG typically measures interictal activity, whereas ECoG is used to localize both interictal and ictal activity, the true accuracy of MEG can only be ascertained if there is a precise correlation of seizure zone with MEG localization of interictal spikes in the individual patient. This is important because interictal activity may be seen in many locations, depending on when it is sampled, and may not directly correlate with seizure onset zone. Thus it is often uncertain whether an MEG study provided information that was essential to localize a seizure.
We now report our experience with whole-head MEG—MS imaging in the management of epilepsy. Careful correlation of the quality and location of MS imaging—determined interictal spike activity with invasive electrode ECoG-determined seizure onset was performed to identify those situations in which MS imaging provided valuable localization data and those situations in which it did not. Furthermore, we identified cases in which MS imaging provided nonredundant localization data crucial to surgical management.
We thank Mireya Sanchez for preparation and J. Michael Tyska, Ph.D., for critical review of this manuscript.
This work was supported by Public Health Service Grant No. NIH NS20806 from the Epilepsy Branch and No. NCRR RR13176, a shared instrumentation grant, and by a generous gift from Robert S. and Denise Zeilstra.