Advanced Neuroimaging for Modern Epilepsy Surgery
neuroimaging method that can be combined with MRI to generate magnetic source imaging (MSI) to localise epileptogenic zones (see Figure 5). MEG has the advantage over conventional EEG because magnetic signals can pass through skull and other tissues without significant distortion. In addition, MEG spikes are usually shorter in duration with a steeper ascending slope than EEG spikes, leading to a larger signal-to-noise ratio and greater source localisation.53,54
One
disadvantage of MEG, however, is that it only detects tangential components of a current source. Therefore, MEG selectively measures sulcal activity whereas EEG can detect both sulcal and cortical activity. Another major disadvantage is that MEG is mostly restricted to the detection of interictal spiking, as it is not practical or feasible to continuously monitor patients in the MEG device.
The use of MEG in the clinical setting has largely been limited to mapping of the eloquent cortex for surgery; its diagnostic yield in detecting focal seizure foci remains uncertain. In the largest series, consisting of 455 epilepsy patients, 131/455 (28.8 %) underwent surgical treatment and MEG succeeded in identifying the epileptogenic zone in 70 % of patients and lobar localisation in 89 % of patients. MEG supplied additional information in 5 % of patients and crucial information for the final decision for surgery in 10 % of patients.35
Interictal MEG used in the
pre-surgical evaluation of patients with non-localising lesions has shown general agreement with invasive EEG recording, with a sensitivity of 58–64 % and specificity of 79–88 %.35,36
In a prospective blinded study,
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Sutherling and colleagues found MSI to yield non-redundant information in 33 % of 69 patients with suspected neocortical epilepsy; this information consisted of additional areas to sample intracranial EEG or modifications of the surgical decision.55
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demonstrated that the improved spatial resolution of MEG could localise the epileptogenic source in patients with EEG-confirmed secondary bilateral synchrony that otherwise appeared as generalised.56 Altogether, these studies and suggest that MEG/MSI has an important role in guiding the decision-making process for invasive monitoring and surgical resection for epilepsy surgery, especially when other localising evidence is lacking or discordant.
Conclusion
With the aid of these neuroimaging tools, epilepsy surgeons are no longer operating on ‘invisible’ lesions. Ultrahigh-resolution MRI will be able to provide greater anatomical and structural detail that will reveal more information regarding the pathophysiology of epilepsy. The realm of functional imaging will also continue to grow as we discover more specific molecular tracers that will not only map the functional connectivity of epileptogenic networks but will also confer greater prognostic value in predicting seizure outcome. In the future, MEG may become a routine part of pre-surgical evaluation for epileptogenic foci localisation and functional mapping. These techniques, among other emerging imaging methods, will undoubtedly push the field of epilepsy surgery to new frontiers. n
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