Magnetoencephalography (MEG) is a non-invasive neuroimaging technique that maps the tiny magnetic fields generated by the brain’s electrical activity in real-time. This capability makes MEG an important tool for evaluating complex neurological conditions, particularly those involving abnormal electrical signaling. For patients with epilepsy, MEG offers unique insights into the origin and spread of seizures that other imaging modalities may not provide. The resulting high-resolution maps are used to inform treatment strategies, including surgical planning.
Defining Magnetoencephalography
Magnetoencephalography records the minute magnetic fields that are a natural byproduct of the electrical currents flowing within the brain’s neurons. Neurons communicate by generating postsynaptic currents, and this flow of electricity naturally produces an external magnetic field. These magnetic fields are extremely weak, billions of times smaller than the Earth’s magnetic field. Specialized, highly sensitive sensors known as SQUIDs (Superconducting Quantum Interference Devices) are necessary to detect these faint signals.
The fundamental difference between MEG and its electrical counterpart, electroencephalography (EEG), lies in how the signals are affected by the skull and scalp. EEG measures electrical voltage at the scalp, and this electrical signal is significantly distorted and “smeared” by the layers of tissue, bone, and fluid it must pass through. In contrast, the magnetic fields measured by MEG pass through these layers almost completely undistorted. This allows MEG to achieve better spatial resolution and more accurately pinpoint the source of activity deep within the brain compared to standard scalp EEG.
The Patient Experience
A MEG scan typically takes between one and four hours, depending on the specific type of examination being performed. Before the scan begins, patients must remove all metal objects, such as jewelry, hearing aids, and certain clothing items, because metal can interfere with the measurement of the brain’s faint magnetic fields. The environment for the scan is a specially constructed, magnetically shielded room designed to block out environmental magnetic noise.
Once inside the shielded room, the patient lies comfortably on an examination bed or sits in a chair, with their head positioned inside a large, helmet-like device. This helmet contains the array of magnetic sensors that will record the brain activity. The patient is alone during the recording, but the technologist can see, hear, and speak with them at all times using a two-way intercom and video system. Depending on the goal of the scan, the patient may be asked to lie quietly, or they may perform simple tasks like listening to sounds or moving a finger.
MEG’s Role in Epilepsy Evaluation
MEG is used in the pre-surgical evaluation of patients with focal epilepsy that has not responded to medication. The goal is to precisely localize the epileptogenic zone, which is the small area of brain tissue where seizures originate. MEG accomplishes this by detecting and mapping interictal spikes—abnormal bursts of electrical activity that occur between actual seizure events.
MEG allows clinicians to determine the exact coordinates of the spike generation with sub-centimeter accuracy. This information, when overlaid onto a patient’s structural MRI scan to create a magnetic source image (MSI), provides a detailed map of the seizure focus. Identifying this focus is crucial because successful epilepsy surgery often depends on removing or disconnecting the tissue responsible for initiating the seizures.
Beyond localizing the seizure focus, MEG is also used for functional mapping of the cortex. This process identifies “eloquent cortex,” which refers to areas responsible for functions like language, motor control, and sensation. During a functional mapping scan, the patient performs tasks such as speaking or moving a limb, and MEG records the activated brain areas.
Mapping the eloquent cortex is an important step in pre-surgical planning, as it helps neurosurgeons plot a path to the epileptic focus that avoids damaging functional areas. This dual capability—precise localization of seizure activity and identification of functional areas to preserve—allows for highly targeted and individualized surgical strategies. The data provided by MEG can significantly influence the treatment plan and, in some cases, may determine a patient’s candidacy for surgery.