Application to biomedical signals - Practical Biomedical Signal Analysis Using MATLAB - page 137

These rules in fact should be mandatory for any kind of method aiming at prediction
or classification based on the analysis of EEG/MEG signals.
4.1.6.4.3 Localization of an epileptic focus In case of drug resistant epilepsy
surgery is a treatment option. Its goal is to remove a minimum volume of brain tissue
responsible for seizure generation; therefore a seizure focus should be determined
with maximal precision. To this aim presurgical evaluations are performed involv-
ing EEG, video-EEG monitoring, high resolution MRI, SPECT. In the last years
magnetoencephalography has also contributed to the presurgical techniques applied
for focus localization. The advantage of MEG technique over EEG is that the mag-
netic field is very weakly influenced by the tissues of the head. On the other hand
EEG combined with video monitoring provides long-term recordings including pre-,
post-, and seizure periods.
The application of to the MEG technique for presurgical evaluation of epilepsy
activity was reported by [Fischer et al., 2005]. In this study authors used single
equivalent dipole model for source reconstruction. The localizations obtained for
epileptic structures were subjected to a hierarchical cluster analysis and ICA was
used to construct an ellipsoid representing the epileptic focus. The ellipsoid volume
was compared voxelwise with the resection volume generated from pre- and post-
operative MRI scans. A low distance between the mass centers of both volumes and
high coverage of the MEG ellipsoid by the resection volume correlated with the pos-
itive outcome of the surgical treatment.
MEG was compared with long-term EEG by [Paulini et al., 2007], who reported
that in a studied group of 105 patients MEG was inferior to long-term EEG in
recording interictal spikes, but yielded more circumscribed localizations if spikes
were present. In another study [Wheless et al., 1999] the site of surgery was cor-
rectly predicted by MEG in 52%, in interictal scalp video-EEG in 45%, in ictal scalp
video-EEG in 33%, and in the invasive EEG in 70%.
The surgical treatment is usually preceded by application of subdural and/or depth
electrodes in the brain area delineated as responsible for seizures by the EEG, MEG,
and imaging techniques. Intracranial EEG allows for more precise localization of
epileptic focus. However some seizures are difficult to localize even with intracra-
nial probes. It was reported by [Matsuoka and Spencer, 1993] that in patients with
partial seizures originating from lateral temporal or extratemporal regions only 15%
of seizures could be localized by visual analysis, because of the rapid regional spread
of epileptic activity. The method which helps in this respect, providing insights into
sources of EEG signal, is directed transfer function. DTF is a measure of the prop-
agation of activity as a function of frequency. The method has been successfully
applied by [Franaszczuk et al., 1994] to the localization of mesial temporal seizures
foci. The authors demonstrated that the DTF method can determine whether during
seizure development the initial focus continues to be the source of a given activity or
whether the other more remote areas become secondary generators.