HDEEG - MEG in clinical practice
What is MEG?
Magnetoencephalography (MEG) works by detecting the magnetic fields created by the brain's electric signals. These fields are a billion times smaller than the Earth's magnetic field, so MEG has to be carried out in a heavily shielded room. The person sits inside a 'helmet' of special sensors that detect the tiny magnetic signals produced by the brain. Magnetic fields are detected by extremely sensitive devices, using super-conducting detectors and amplifiers, also known as SQUIDs. The scanner, used to detect and amplify magnetic signals produced by the brain, does not emit radiation or use strong magnetic fields.
How is the MEG used?
MEG can be combined with scanning techniques to build up a detailed picture of functioning brain and can be used to help better plan for brain surgery for patients with refractory focal epilepsy. In particular, MEG can provide accurate information about the cortex (the outer layer of the brain) because it is less sensitive to signals generated from deep within the brain, unlike an EEG. MEG can also 'see through' the skull and scalp more clearly, unlike for EEG where these impact the signals observed.
wHY combine hIGH DENSITY eeg WITH the MEG?
The specialised equipment and recording considerations required for MEG studies make it a less widely available and applied technique globally. However, the ability to record abnormal brain activity directly without needing to apply EEG electrodes, and the large number of MEG sensors in the helmet also provides access to information with a higher spatial resolution to better localise sources more effectively. Traditional EEG is much more readily available and waveform patterns have been much better characterised for discriminating features relevant to interictal discharges. The use of high density EEG offers more information and enhances confidence in the electric fields that are then projected to the scalp, although still suffering from the interference to the signal when passing through different tissues in the head.
The simultaneous acquisition of MEG and EEG increases some of the technical challenges but it provides a richer insight into the nature of the sources. What we have demonstrated is that while the same source of abnormality may be observed equally well in either modality most of the time, there are clear instances where one modality may “see” earlier features than the other. This may in part be related to the sensitivities of electrical potentials recorded by EEG as they propagate through different tissues unlike magnetic fields, but some source orientations from magnetic fields are less clear in the MEG. This is the value of combining information from these two non-invasive modalities to enhance their complementarity, and safely explore the earliest features of the epileptiform discharges which are better predictors of the seizure onset zone - the real target for epilepsy surgery.
Is an MEG necessary for my patient?
This is something that is best discussed with our expert team, but we have established a detailed questionnaire to help guide us to making that decision with you. We focus on complex cases that require special attention before surgery can be considered. Our approach involves a detailed clinical consultation with the patient at the time of the study to best understand their epilepsy when interpreting results. A sample of the questionnaire items can be found here.
What does the report provide me with?
The report is based upon a information captured during the recording and our careful analysis of the data when isolating sources of seizures. This is commonly done by modelling early features of interictal epileptiform spikes which have been shown to be a valuable predictor when compared to modelling an actual seizure. The report generated provides details of events detected during the high density EEG and MEG recording after source analysis, based on their complementary information to best isolate the true seizure generator.
Who do i contact to refer a patient for an MEG study?
Enter relevant information for a patient’s referral via the High Density EEG-MEG Clinical Questionnaire below which will be reviewed carefully by our team. Once this has been done, our neurologist will contact you to discuss options and next steps. If you would like to request just a High-Density EEG-only screener study, please complete the Clinical Questionnaire Screener items using this link instead.