Magnetoencephalography and Magnetic Source Modeling

Fig. 23.1

Dipolar sources of epileptic spikes. The panels above show an example of dipolar source models of epileptic spike activity. The panel on the far left shows MEG traces from a subset of magnetometers with the cursor marking an epileptic spike event. A sensor level topographic representation of the event is shown in the top right panel, along with dipolar sources of a collection of such events on axial and coronal planes through the dipole cluster in the bottom right panel

Fig. 23.2

Dipolar sources of ictal sharp rhythms. The panels above show an example of ictal source modeling using MEG. The traces on the left panel show seizure onset as recorded in a subset of MEG sensors. Dipolar sources of successive peaks of the ictal waveform are shown on planar views in the panels on the right

There is ample evidence that magnetic source modeling of evoked responses can reliably localize primary sensory cortices (visual, auditory, and somatosensory). Figure 23.3 shows an example of source modeling of somatosensory evoked response to median nerve stimulation using dipolar modeling and dSPM [3]. Localization of primary motor cortex using dipolar modeling of motor preparation potentials is, however, less reliable [4]. Alternative methods to localize changes in beta band oscillatory activity in the motor cortex have been explored with greater success [5], although yet to be widely adopted.

Fig. 23.3

Dipolar and distributed source models (dSPM) for somatosensory evoked responses. The left upper panel shows the evoked responses to somatosensory stimulation of the right median nerve in a subset of MEG sensors in the left central region. Dipolar sources at the peak of the response are shown in the top right and bottom left panels. A distributed source model for the same point in time is shown in the bottom right panel (dSPM with a threshold of p < 0.001). Both the dipole model and maxima of the dSPM activation localize to the post-central gyrus in an area consistent with anatomically predicted hand somatosensory representation

For lateralizing language, neuromagnetic responses to auditory language stimuli have been found to be concordant with the Wada test in 87% of patients [6]. Using the same methods, Doss et al. [7] found language representation in the hemisphere to be treated with a concordance rate of 86% with the Wada test in 35 patients, with a sensitivity of 80% and specificity of 100%. Several smaller studies have reported MEG–Wada concordance rates between 69 and 100% using a variety of paradigms and analysis methods.

The Role of MEG in Presurgical Evaluations

Unlike EEG, MEG is not indicated for the initial evaluation of new-onset seizures but can provide valuable localization of epileptic pathology in patients with medically refractory epilepsy who are undergoing evaluations for epilepsy surgery. MEG primarily localizes interictal epileptic abnormalities which help identify “irritative zones” in the brain. Source modeling of interictal spikes using MEG may be particularly useful in patients with normal MR imaging, large or cystic lesions, lesions of indeterminate significance to the patient’s epilepsy, or with multifocal or rapidly propagated spikes.

MEG is also clinically indicated for localizing primary motor or sensory cortices (somatosensory, visual, or auditory) to guide surgical planning for epilepsy, tumors, or vascular lesions, and can also be used to determine hemispheric language dominance.

The spatial accuracy of MEG and magnetic source modeling for localizing “irritative zones” is second only to invasive EEG [8]. MEG-guided review of MRI data has also been reported to identify subtle abnormalities that were previously missed, especially focal cortical dysplasia [9–11]. However, MEG should not be viewed as a tool that replaces invasive EEG or other noninvasive tests such as PET or SPECT. There is now sufficient evidence that MEG can provide significant non-duplicative information to improve surgical outcomes or preempt expensive invasive intracranial EEG studies [12–15]. While MEG may not eliminate the need for intracranial EEG studies, it can help generate better hypotheses about seizure onset zones, and thereby guide electrode placement for invasive EEG studies [16, 17].

Some Limitations of Current Clinical MEG Methodologies

Localizing “irritative zones” is often insufficient to predict seizure onset zones, especially when they are multifocal. Unfortunately, ictal MEG studies are not the norm since it is impractical to monitor patients in a MEG scanner for an extended period of time in order to capture seizures. In about 20% of MEG studies, no epileptic spikes may be observed during the recording. In these cases, MEG is unable to provide useful localizing information about epileptic pathology. Alternative interictal biomarkers of epilepsy such as focal slow waves or pathological high-frequency oscillations are therefore of interest.

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