Magnetoencephalography (MEG)

What is magnetoencephalography (MEG)?

Magnetoencephalography (MEG) is a test that measures the magnetic fields your brain’s electrical currents produce. Healthcare providers use this test to map brain function and identify the exact location of the source of epilepsy-related seizures.

MEG is noninvasive, which means the test happens on the outside of your body and it doesn’t hurt. There aren’t any needles or incisions involved, and you’re not exposed to radiation (X-rays).

MEG is currently the most advanced method of recording and evaluating your brain activity.

What is magnetoencephalography used for?

Neurologists and neurosurgeons use MEG for two purposes:

• To evaluate your brain’s spontaneous activity. This helps find the exact location of the source of epilepsy-related seizures to plan for epilepsy surgery.
• To check your brain’s response to specific external stimuli. This helps map motor and sensory areas, language, vision and other functions. Providers use this to help plan surgeries for people who have brain tumors.

MEG is also an important part of neurological and psychiatric research to help scientists better understand human brain function.

How does magnetoencephalography work?

Brain cells (neurons) interact with each other by generating tiny electrical voltages. The flow of electrical current produces a magnetic field. MEG detects, records and analyzes these magnetic fields using sensitive magnetic sensors. The strength of the magnetic field your brain produces is very small, so providers need highly specialized systems to pick up the signal.

These sensing systems consist of small, high-resolution coils, coupled to devices called SQUIDs (superconducting quantum interference devices). There are more than 300 of these sensors inside a helmet that fits over your head during this test.

The sensors in the helmet and specialized computer software work together to detect and record the activity of neurons as you lie still or complete a series of tasks, such as listening to sounds or looking at pictures. MEG records both normal and abnormal brain signals every millisecond. The magnetic fields show up on an anatomical image of your brain on a computer screen to “map out” where specific brain activities originate and when they activate.

How do I prepare for a MEG test?

Your healthcare provider will let you know if you need to stop eating or drinking before the test. They’ll also let you know if you need to make any changes to your medications or routine.

Don’t wear makeup or use any hair products on the day of the test. They can interfere with the testing.

You also won’t be able to wear any metal items in the exam room, such as jewelry, eyeglasses, hair pins, underwire bras and clothing with metal fixtures. A provider will give you a medical gown to wear if necessary.

If you have any medical devices in or on your body, you’ll need to let your provider know before the test. If they contain metal or anything that could interfere with the magnetic fields and you can’t remove them, you might not be able to have the test. These devices may include:

• Artificial heart valves.
• Artificial limbs or metallic prosthetic joints.
• Cardiac defibrillators and pacemakers.
• Certain brain aneurysms.
• Certain metal coils placed within blood vessels.
• Cochlear implants.
• Implanted drug infusion ports.
• Implanted nerve stimulators.
• Metal teeth braces.
• Metal pins, screws, plates, stents or surgical staples.
• Programmable shunts.

What should I expect during a MEG test?

The process of a MEG test can vary depending on the reason for it. In general, you can expect:

• You’ll remove all metal accessories from your body and change into a medical gown.
• Infants and young children may need sedation (medication that helps you relax) or general anesthesia to complete a MEG exam without moving. If this is the case, the provider will insert an IV for the medication.
• A provider may attach three to four positioning coils to your head with temporary tape to help determine the location of your head in relation to the sensors in the helmet.
• The provider will measure the positions of the coils using a special wand-like device.
• The provider will use a round circular device (called a “degausser”) to remove tiny magnetic signals on you that you may catch in a natural environment.
• You’ll then lie on a moveable exam table or sit in a special chair attached to the MEG machine.
• The provider will position the helmet that contains the MEG sensors on your head. Feelings of claustrophobia from wearing the helmet are very rare. It’s important to put your head as deep as possible into the helmet so your brain is close to the sensors in the MEG machine.
• Depending on the reason for the test, you may lie still or even go to sleep. It’s important to try to keep your head still during the testing. If you’re having the test to map sensory areas of your brain, you’ll do certain activities, like reading words on a screen, answering questions or repeatedly pushing a button.
• A provider may use a small electrical current to stimulate your finger to measure how your brain responds to it. You’ll feel a tingling sensation, but it won’t hurt.
• Once the provider has all the information they need, the test will be over and a specialist will analyze the recordings.

You won’t feel any sensations from the MEG machine during the testing and it won’t hurt. There’s a two-way intercom and video monitoring system in the room, so you’ll be able to communicate with the provider performing the test at all times and vice versa.

Some providers perform an electroencephalogram (EEG) or a magnetic resonance imaging (MRI) scan alongside a MEG test. If this is the case, there will be additional steps to the process.

The MEG test takes about one to two hours to complete.

What happens after a MEG test?

If you or your child had sedation or anesthesia for the test, a healthcare provider will observe you for 30 minutes to two hours after the exam to make sure you recover well. You’ll need someone else to drive you home.

If you didn’t have sedation, there’s no recovery period. You can return to your usual activities.

What are the risks of a MEG test?

There aren’t any known risks of a MEG test.

When should I know the results of the MEG test?

It may take several days or even weeks for specialists to review the recordings of your MEG test. This is because providers use MEG tests to plan complex brain surgeries, and several specialists may work together to do so.

Once they’ve established a plan, your healthcare team will go over the results with you and discuss the next steps.

How is a MEG different from an MRI?

Brain MRI (magnetic resonance imaging) collects structural information and creates images of your brain using a large magnet and a computer. MEG records magnetic fields from neurons in your brain using hundreds of special sensors and a computer. The fields appear as brain waves.

Providers typically combine MRI with MEG to get what’s called magnetic source imaging (MSI). MSI creates functional maps of brain activity.

MEG is also different from functional MRI (fMRI). Functional MRI evaluates subtle regional blood flow changes in your brain that happen when you perform certain tasks. While both MEG and fMRI evaluate the function of your brain, MEG pinpoints more specific areas of activity, whereas fMRI shows general areas of activity. In addition, MEG is better at showing time-related (temporal) characteristics of brain activation than fMRI.

What is the difference between MEG and EEG?

The first difference between these two diagnostic tests is that EEG (electroencephalography) records the electrical activity of your brain, and MEG records the magnetic fields of your brain (which come from the electrical activity).

Another difference is that MEG is much more sensitive and precise at recording brain activity than EEG. Your skull and cerebrospinal fluid (CSF) can distort the recording of electrical activity during an EEG. These tissues don’t distort magnetic fields. Because of this, MEG provides better “images,” or spatial resolution, of your brain and can specifically pinpoint areas of abnormal activity.

Healthcare providers sometimes perform an EEG at the same time as MEG to gather more information about where your brain activity is coming from.

A note from Cleveland Clinic

Magnetoencephalography (MEG) is a very useful, noninvasive and safe test that healthcare providers use to map the sensory areas of your brain and to pinpoint the source of seizures. If you need a MEG test and are worried about the exam or have questions about it, don’t hesitate to ask your healthcare provider. They’re available to answer your questions and support you.