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Comparative Study
. 2004 Nov 30:2004:74.

Localization of interictal spikes using SAM(g2) and dipole fit

S E Robinson  1 , S S Nagarajan, M Mantle, V Gibbons, H Kirsch
Affiliations
Comparative Study

Localization of interictal spikes using SAM(g2) and dipole fit

S E Robinson et al. Neurol Clin Neurophysiol. .

Abstract

SAM(g2) is an automated analysis that transforms the MEG data into a functional image of spike-like activity, giving the source waveforms for those locations. Since the source waveforms estimated by SAM have higher signal-to-noise ratio (SNR) than does the raw MEG data, it is possible to automatically mark the location and timing of each spike for comparisons with dipole fit procedures. Both SAM(g2) and equivalent current dipole (ECD) fits were used to analyze MEG interictal spike recordings in 10 patients with cortical dysplasias and medial temporal lobe epilepsy. The ECD fit locations obtained by manual spike classification and latency marking were compared with those found by automated SAM(g2) procedures. When the SNR of interictal activity was high (compared to the background) with a clear single focus, there was excellent agreement between the ECD cluster location and the SAM(g2) maximum. However, when the SNR of spikes was low, manual single ECD location scatter was larger than SAM(g2) reconstructions. When multiple independent interictal spike loci were present, there was some disagreement between SAM(g2) and ECD scatter in the cases of low SNR spikes. When SAM(g2) indicated multiple coupled spike loci, the residual variance for the dipole fit was high and its scatter unacceptably large--even for multiple dipole models. This study demonstrates that SAM(g2) is equivalent to ECD fit for localizing interictal spikes when there is a single locus and good SNR. Further studies are required to validate cases in which there are multiple spike loci or poor SNR.

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Figures

Figure 1
Figure 1
A single interictal spike locus is seen in both SAM(g2) (a) and dipole fit (b). The dipole fit scatter is small (for dipoles with < 10 % residual variance). However, its centroid is slightly deeper than that indicated by SAM(g2).
Figure 2
Figure 2
In one of the patients, SAM(g2) found three spike loci, labelled V0, V1, and V2 (top row). Spikes were marked in each source waveform for these loci. Single dipole fits were then computed around each spike. The scatterplots (residual variance < 10 %) for spikes corresponding to each source are shown in the bottom row. The dipole scatterplots are unable to resolve the different loci.
Figure 3
Figure 3
Three SAM virtual sensors, corresponding to the three maxima in the SAM(g2) image, show independent source waveforms with some spike correlation.
See this image and copyright information in PMC

References

    1. Robinson SE, Vrba J, Otsubo H, Ishii R. In: Nowak H, Haueisein J, Giessler F, Huonker R, editors. Finding epileptic loci by nonlinear parameterization of source waveforms; Proceedings of the 13th International Conference on Biomagnetism; Jena. 2002.Aug, pp. 220–222.

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