MEG Test

Magnetoencephalography (MEG) is the newest, most advanced method of recording and evaluating the brain while it is actively functioning.

This recording provides a direct measurement of the ongoing function of normal neurons and can pinpoint the location of malfunctioning neurons. MEG can be used either to evaluate the brain’s spontaneous activity (e.g., for epilepsy) or to check its response to specific external stimuli (e.g., for mapping motor and sensory areas, language, vision and other functions).

MEG can localize epileptic activity more accurately than any other noninvasive modality can without the smearing and blurring that affect the electroencephalogram (EEG). Due to a very large number of sensors, as well as the absence of any effect from skull or scalp, MEG has an inherently high resolution. When we combine MEG with the high resolution anatomic images obtained via MRI, we can localize the neuronal activity to a specific sublobar area, usually to a specific gyrus or sulcus.

Brain cells (neurons) interact with each other by generating tiny electrical voltages. The flow of electrical current produces a magnetic field, which can then be recorded using sensitive magnetic sensors. Because the strength of the magnetic field produced by the brain is so small, very specialized instrumentation is required to pick up the signal.

These sensing systems consist of small, high-resolution coils, coupled to devices called SQUIDs (superconducting quantum interference devices). More than 300 of these specialized sensors are arrayed inside a helmet, providing whole-head coverage with high resolution capabilities. By analyzing the patterns of the signals recorded by all of these sensors, the location, strength and orientation of the sources can be inferred.

A MEG scan is noninvasive and painless. With no injections, radioactivity or strong magnetic fields, MEG is safe for children and adults. Unlike with some imaging tests, the machinery is quiet and almost never produces a feeling of claustrophobia. During MEG testing, brain activity is ordinarily recorded in both wakefulness and sleep.

Diagnostic methods for imaging the brain generally divide between two categories: anatomic and functional. CT and MRI are most common for anatomic imaging, while PET and fMRI are examples of functional imaging. Like EEG, MEG records the electrophysiological effect of neuronal activity over time; however, with its higher sensor count and simpler modeling physics, MEG has a higher source resolution.

Additionally, recordings with MEG are reference-free; its signals are not attenuated by bone and multichannel, whole-head, high spatial-density recordings are easily obtained. By its very nature, MEG shows areas of function: It localizes the signals generated by neurons as they are activated, as they communicate and as activity spreads through them.

MEG is sometimes called a functional imaging test, but it differs in significant ways from other such tests:

• The functional tests available at most centers are indirect measurements, dependent on changes in oxygen consumption (fMRI), glucose uptake (PET) and blood flow (SPECT). Conversely, MEG measures neuronal activity directly.
• While PET and fMRI measure changes in metabolism and blood flow, respectively, over many seconds, MEG measures electrical activity millisecond by millisecond.

Localizing the entire sequence of activation as it evolves over time is what MEG does superbly. Hence, the activity of the whole chorus of neurons required for everyday actions (pressing on the accelerator) or abnormal episodes (an epileptic aura) — not just the maximally involved area — can be mapped in space chronologically as it changes.

Like PET and fMRI, MEG “lights up” brain areas activated by a task. In epilepsy, MEG can show the propagation of activity from one brain region over a few milliseconds or during the onset of a seizure; in fact, ictal MEGs constitute approximately 15 percent of MEG scans performed at Cleveland Clinic Epilepsy Center. MEG results are coregistered with anatomic images from MRI and are reconstructed three-dimensionally to show the exact areas of activity.

For patients with epilepsy, MEG helps pinpoint the origin of their epileptic discharges without intracranial insertion of electrodes. When implantation of intracranial electrodes is necessary, MEG can help to better plan the exact implantation site.

For patients undergoing neurosurgery, MEG technology provides valuable information for presurgical mapping in a noninvasive way. MEG technology allows for the combination of structural and functional information, achieving both high spatial resolution and high temporal resolution — a combination that no other modality for studying the brain currently offers.

Referring physicians can take advantage of MEG technology to aid in the diagnosis and treatment of many conditions their patients face. MEG readings provide more accurate information than ever before, allowing for more informed decisions to be made.