MEG and Magnetic Source Imaging in Epilepsy

Magnetoencephalography (MEG) records the tiny magnetic fields generated by neuronal electrical activity. When MEG signals are localised onto structural MRI, the result is often called magnetic source imaging (MSI).

MEG is mainly used as an advanced localisation tool in drug-resistant focal epilepsy, particularly during presurgical evaluation.

How MEG differs from EEG

EEG measures voltage differences at the scalp.

MEG measures magnetic fields created by intracellular electrical currents in synchronously active neurons.

Why magnetic fields can help localisation

Magnetic fields are distorted less by skull and scalp than electrical potentials.

This can provide complementary localisation information to EEG.

MEG has millisecond resolution

Like EEG, MEG records brain activity with very high temporal resolution.

This allows analysis of the timing and spread of epileptiform activity.

Interictal spikes

Most epilepsy MEG studies analyse interictal epileptiform discharges rather than waiting to capture a clinical seizure.

Clusters of consistent magnetic sources can help identify a likely epileptogenic region.

Magnetic source imaging

MSI places the estimated MEG source onto the person’s MRI.

This allows clinicians to compare source clusters with lesions, cortical anatomy and planned surgical targets.

MRI-negative epilepsy

MEG can be particularly helpful when standard MRI is negative or when MRI shows several possible abnormalities.

A source cluster can direct expert re-review of MRI or guide invasive electrode placement.

Focal cortical dysplasia

MEG can help localise epileptiform activity associated with subtle focal cortical dysplasia and other neocortical epilepsies.

Planning SEEG

MEG results may influence which regions are sampled with stereoelectroencephalography.

The goal is not to replace SEEG but to improve the hypothesis used to design implantation.

What the evidence shows

A 2024 review describes MEG as a clinically useful presurgical tool with millisecond temporal resolution and subcentimetre source-localisation capability under appropriate conditions.

Meta-analysis supports both MEG/MSI and EEG source imaging as useful components of multimodal presurgical evaluation.

MEG and ESI together

Electrical and magnetic source imaging measure related but not identical aspects of neuronal current.

Recent paediatric work suggests that combining high-density EEG and MEG can improve localisation in selected cases.

Limitations

MEG is most sensitive to particular orientations of cortical current and may miss some deep or radially oriented sources.

Interictal spikes also do not always mark the full epileptogenic zone.

Movement and metal artefact

Head movement reduces localisation accuracy.

Metallic dental work, implants and environmental magnetic noise can also create artefact.

MEG is not an MRI scanner

Despite the word “magnetic”, MEG does not produce structural images.

It records magnetic fields from brain activity; MRI supplies the anatomy used for source localisation.

Worldwide access

MEG is one of the least widely available epilepsy investigations because systems are expensive and require highly specialised shielded environments and expertise.

Availability is concentrated in major tertiary epilepsy centres.

The central point

MEG provides a millisecond-scale map of epileptic magnetic activity that complements EEG and MRI. Its main role is refining localisation in difficult presurgical cases rather than diagnosing epilepsy on its own.

Sources

Evidence checked: September 2026.