Brain Advance Access published online on June 4, 2009
Brain, doi:10.1093/brain/awp137
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Interictal magnetoencephalography and the irritative zone in the electrocorticogram
Department of Neurology and Clinical Neurophysiology, Rudolf Magnus Institute of Neuroscience, University Medical Centre Utrecht, Utrecht, The Netherlands
Correspondence to:
Dr Z. Agirre-Arrizubieta, Department of Neurology and Clinical Neurophysiology, hp F02.230, University Medical Centre, Utrecht, PO Box 85500, 3508 GA, Utrecht, The Netherlands, E-mail: Z.AgirreArrizubieta{at}umcutrecht.nl and zagirre{at}hotmail.com
Magnetoencephalography (MEG) is considered a useful tool for planning electrode placement for chronic intracranial subdural electrocorticography (ECoG) in candidates for epilepsy surgery or even as a substitute for ECoG. MEG recordings are usually interictal and therefore, at best, reflect the interictal ECoG. To estimate the clinical value of MEG, it is important to know how well interictal MEG reflects interictal activity in the ECoG. From 1998 to 2008, 38 candidates for ECoG underwent a 151-channel MEG recording and 3D magnetic resonance imaging as a part of their presurgical evaluation. Interictal MEG spikes were identified, clustered, averaged and modelled using the multiple signal classification algorithm and co-registered to magnetic resonance imaging. ECoG was continuously recorded with electrode grids and strips for 
1 week. In a representative sample of awake interictal ECoG, interictal spikes were identified and averaged. The different spikes were characterized and quantified using a combined amplitude and synchronous surface–area measure. The ECoG spikes were ranked according to this measure and plotted on the magnetic resonance imaging surface rendering. Interictal spikes in MEG and ECoG were allocated to a predefined anatomical brain region and an association analysis was performed. All interictal MEG spikes were associated with an interictal ECoG spike. Overall, 56% of all interictal ECoG spikes had an interictal MEG counterpart. The association between the two was 
90% in the interhemispheric and frontal orbital region, 75% in the superior frontal, central and lateral temporal regions, but only 25% in the mesial temporal region. MEG is a reliable indicator of the presence of interictal ECoG spikes and can be used to plan intracranial electrode placements. However, a substantial number of interictal ECoG spikes are not detected by MEG, and therefore MEG cannot be considered a substitute for ECoG.
Key Words: interictal; magnetoencephalography; electrocorticography; intracranial EEG monitoring; irritative zone
Abbreviations: AROM, anatomical region of match; ECoG, electrocorticography; EEG, electroencephalography; MEG, magnetoencephalography; MUSIC, multiple signal classification; RMS_bg, root mean square amplitude of background; RMS_base, root mean square amplitude of baseline; RMS_spike, root mean square amplitude of spike; RMS_int, root mean square integrated value; SPECT, single photon emission computed tomography
Received February 9, 2009. Revised April 22, 2009. Accepted April 24, 2009.