Noninvasive localization of epileptogenic zones with ictal high-frequency neuromagnetic signals

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Recent reports suggest that high-frequency epileptic activity is highly localized to epileptogenic zones. The goal of the present study was to investigate the potential usefulness of noninvasive localization of high-frequency epileptic activity for epilepsy surgery.


Data obtained in 4 patients, who had seizures during routine magnetoencephalography (MEG) tests, were retrospectively studied. The MEG data were digitized at 4000 Hz, and 3D MR images were obtained. The magnetic sources were volumetrically localized with wavelet-based beamformer. The MEG results were subsequently compared with clinical data.


The 4 patients had 1–4 high-frequency neuromagnetic components (110–910 Hz) in ictal and interictal activities. The loci of high-frequency activities were concordant with intracranial recordings therein 3 patients, who underwent presurgical evaluation. The loci of high-frequency ictal activities were in line with semiology and neuroimaging in all 4 of the patients. High-frequency epileptic activity was highly localized to the epileptogenic zones.


High-frequency epileptic activity can be volumetrically localized with MEG. Source analysis of high-frequency neuromagnetic signals has the potential to determine epileptogenic zones noninvasively and preoperatively for epilepsy surgery.

Abbreviations used in this paper: ECoG = electrocorticography; EEG = electroencephalography; HFBS = high-frequency brain signal; MEG = magnetoencephalography.

Article Information

Address correspondence to: Jing Xiang, M.D., Ph.D., Division of Neurology, Cincinnati Children's Hospital Medical Center, 3333 Burnet Avenue, Cincinnati, Ohio 45220. email:

© AANS, except where prohibited by US copyright law.



  • View in gallery

    Waveforms and spectrograms from the same data set illustrating the basic principle of high-frequency signal analysis. Left: Waveforms with conventional band-pass filters. Right: Spectrograms with our new time-frequency analysis method. The waveform filtered with band-pass filters of 1–1000 Hz shows both low- (Spike) and high- (HFBS) frequency signals. The waveform filtered with band-pass filters of 70–1000 Hz shows only high-frequency signals (HFBS), whereas the waveform filtered with 3–70 Hz shows only a low-frequency component (Spike). High-frequency components are barely identifiable in waveforms. However, our new method reveals the high-frequency components clearly. Of note, our new method also reveals frequency components in low-frequency ranges. For example, the conventional spike (Spike) in the waveforms is clearly identifiable as strong brain activity around 26 Hz (F26). The F340, F170, and F26 indicate increases of spectral power around 340, 170, and 26 Hz, respectively.

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    Spectrograms, magnetic source image (MSI), and intracranial ECoG image demonstrating the frequency and spatial features of interictal and HFBSs. Ictal HFBSs (230 Hz; Ictal spectrogram) are localized to the left frontal temporal cortex (red area). This irritable area has been confirmed by intracranial ECoG recording (green area). Interestingly, the interictal HFBSs are also localized to the same area. The MSI is a summary of a 2-minute recording. In the accumulated spectrograms, the y axis indicates frequency range in 100–1000 Hz, and the x axis indicates the time window for multiple epochs of data.

  • View in gallery

    Spectrograms, MSI, and intracranial recordings showing the frequency and spatial features of interictal and ictal HFBSs. The spectrogram of interictal MEG data (Interictal) and the spectrogram of ictal MEG data (Ictal) show the main high-frequency components between the seizures and during the seizures. Noticeably, a component around 310 Hz is identifiable in both ictal and interictal spectrograms. The MSI shows the source location of neuromagnetic signals in 310 Hz in the left frontal cortex. There is no significant difference between the ictal and interictal activity in source localization. The intracranial ECoG recording shows the location of ictal-onset zone (green area) in the left frontal cortex. The results indicate that the localization of the high-frequency epileptic activity is concordant with the intracranial recording. In the accumulated spectrograms, the y axis indicates frequency range in 100–1000 Hz, and the x axis indicates the time window for multiple epochs of data.

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    Spectrograms, MSI, and intracranial recordings demonstrating the frequency and spatial features of interictal and ictal high HFBSs. The patient had tremulousness of the fingers of the right hand during MEG. Ictal HFBSs (910 Hz, ictal spectrogram) are localized to the left central-frontal region (F arrow in the MSI). This irritable area has been confirmed by intracranial recording (the green area: 47, 48 and 55 in the photograph). Interestingly, the interictal HFBSs are also localized to the left hemisphere (P and F arrows in the photograph) with neuromagnetic signals in 110 Hz and 350 Hz. In the MSI, R indicates right side and L indicates left side. The MSI is a summary of a 2-minute recording. In the photograph, F (without the arrow) indicates frontal region and T indicates temporal region. In the accumulated spectrograms, the y axis indicates frequency range in 100–1000 Hz, and the x axis indicates the time window for multiple epochs of data.



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