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Case Reports
. 2013 Sep;124(9):1915-8.
doi: 10.1016/j.clinph.2013.03.016. Epub 2013 May 2.

Localization of the ictal onset zone with MEG using minimum norm estimate of a narrow band at seizure onset versus standard single current dipole modeling

Rafeed Alkawadri, Balu Krishnan, Yosuke Kakisaka, Dileep Nair, John C Mosher, Richard C Burgess, Andreas V Alexopoulos
Case Reports

Localization of the ictal onset zone with MEG using minimum norm estimate of a narrow band at seizure onset versus standard single current dipole modeling

Rafeed Alkawadri et al. Clin Neurophysiol. 2013 Sep.
No abstract available

PubMed Disclaimer

Conflict of interest statement

Dr. Alexopoulos has received personal compensation for activities with UCB Pharma as a speaker. The other authors have no conflicts of interest to declare.

Figures

Fig. 1
Fig. 1
Summary of pre-surgical findings and localization of seizure onset zone by intracranial depth electrodes. MRI, PET, SPECT, and MEG images follow the neurological orientation (‘right is right’). (A) Several clinical seizures were recorded and had the same electro-clinical behavior. EEG onset (left – red vertical line) was marked by general attenuation of background and appearance of paroxysmal fast activity at the vertex with subsequent involvement of the right parasagittal chains (middle). The activity became more widespread shortly before its offset (right – blue vertical line). (B) Axial FLAIR and sagittal T1-weighted images showed two prior resection cavities caudal and rostral to the right primary motor cortex, which was preserved in prior resections. (C) Axial FDG–PET image (left) showed decreased uptake (arrow) in the right frontal region and subtraction ictal SPECT co-registered to MRI scan (right) showed increased blood flow anterior and medial to the prior anterior resection. (D) Seizure onset zone was delineated by stereotactically implanted depth EEG electrodes and was localized to the anterior edge of the previous anterior resection cavity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2
Fig. 2
MRI, PET, SPECT, and MEG images follow the neurological orientation (‘right is right’). (A) 10-second MEG data from the right frontal and vertex sensors at the time of onset. The blue frame highlights 400-millisecond window at onset, which is magnified in the box (arrow). The vertical blue line marks the time point at which sECD analysis was performed. The red frame highlights 100-millisecond window on which L2-MNE was subsequently performed. The black-rimmed box shows the distribution of the magnetic field, which remained stable for several seconds following ictal onset. (B) Morlet wavelet transformation of the same 10-second window shown in (A) after averaging power spectra of the involved electrodes. This step is important for visual identification of the dominant rhythm at onset in terms of signal power (in this case 15 Hz, as highlighted by the red arrow). (C) Mapping of current density of the narrow band identified in B on a 2D helmet using 200-millisecond intervals for 1 s to demonstrate signal relative stationarity (in space and time), focality (in space), and evolution (in time and power) prior to implementing L2-MNE solution. This step is important to confirm stability of the ictal rhythm and to rule out temporal overlap with other ‘contaminating’ rhythms such as physiologic beta rhythms (illustrated in the lower box). Such rhythms tend to show bilateral distribution across the midline. (D) sECD analysis (upper row) at the time point shown in A (vertical blue line) localized the ‘point’ of activation in the precentral gyrus. On the other hand, L2-MNE solution (lower row) of a narrow band of interest (14–16 Hz) localized the seizure onset in the anterior edge of the prior anterior resection. (E) Post-surgical T1-weighted images confirmed the resection of the area delineated by L2-MNE of the narrow band of interest. (F) L2-MNE is biased to signals with high power. This figure illustrates L2-MNE solution of the signal prior to filtering. The active areas shown do not overlap with the seizure onset zone. Rather they represent distant areas of slowing in the setting of an active seizure. This illustration highlights the significance of selection of a narrow band tailored to the ictal rhythm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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