Magnetic Source Imaging (MSI) / Magnetoencephalography (MEG)

What is MEG?

  • MEG stands for magnetoencephalography.
  • It is noninvasive and works like a very sensitive microphone that records magnetic fields emitted by brain cells (neurons) instead of sound.
  • The MEG scanner collects information from 275 sensors simultaneously every millisecond.
  • The MEG scanner is silent and fits on your head like a helmet.

How is MEG different from EEG (electroencephalography)?

  • MEG senses the magnetic fields produced by neurons in your brain while EEG senses electrical signals from the brain.
  • Sometimes EEG is collected at the same time as MEG to gather reference information about where the brain activity is coming from.

How is MEG different from brain MRI (magnetic resonance imaging)?

​MRI collects structural information and creates images of the brain while MEG collects magnetic activity from neurons, visualized as brain waves.

  • The MRI machine uses a large magnet to help create images; in MEG there is no magnet.
  • The MEG is used with MRI in magnetic source imaging (see the next section).

​What is MSI?

MSI stands for magnetic source imaging.

  • MSI combines MEG and MRI results to create functional maps of brain activity. These maps can show where areas of normal and abnormal activity are located.
  • Clinically, MSI can provide increased localization of spike activity in patients with seizures. MSI can look at activity deeper in the brain than EEG alone.
  • Clinically, MSI can also map areas of function (language, hearing, touch, movement, etc.) in presurgical patients.
  • Both spike localization and functional mapping information are typically used prior to surgery for planning purposes and also during surgery using surgical navigation systems.

What should I know before coming to my appointment?

Because the test is very sensitive to metal, we ask that you please let us know immediately if any of the following conditions apply to you. A technologist will then let you know whether you can have the exam or not.

  • Have an implanted electrical device such as a pacemaker, vagus nerve stimulator (VNS), or deep brain stimulator (DBS).
  • Have an implanted metal device or prosthesis such as a metal plate, aneurysm clip, rod, pin, screw, etc.
  • Have nonremovable dental work such as braces, permanent retainer, bridge, etc.

How to prepare for your appointment

  • Wear clothes free of metal zippers or snaps above the waist (pants with zippers are ok).
  • Avoid make-up, especially eye make-up (make-up can contain metallic ingredients that can affect the scan).
  • Please wear your hair down and without hair products.
  • Please let us know if you need interpreting services, this can be arranged for you.
  • Please bring a list of your current medications

What can I expect before my MSI exam?

​There is little preparation for an MSI exam. You can eat normally and take your daily medications as you normally would, unless instructed otherwise by your physician.

  • You will be asked to remove all metallic items from your body such as belts, eyeglasses, cell phones, credit cards, keys, etc.
  • If you have metal on your clothing, you will be asked to change into a hospital gown.
  • A technologist will verify your identification and record a brief interim history including your current symptoms.
  • If you are having an EEG exam done at the same time, we will place electrodes on your scalp with a little electrode paste prior to your exam.

What can I expect during my MSI exam?

  • You will be asked to lie on a bed and the technologist will place you in the scanner. The scanner fits on your head like a helmet and is open.
  • You will be asked to remain still while the scan is running. Usually, each scan lasts about 4 to 10 minutes, and in between the scans you will be able stretch and readjust your position. Most appointments involve multiple scans and last between 1-2 hours but can be longer or shorter depending on what your doctor has ordered.
  • At times you will be asked to relax with your eyes closed and other times you may be asked to actively participate in the scan such as listening to tones or responding to pictures.
  • There is a two-way intercom and video monitoring system in the room so that the technologist will be able to monitor and communicate with you at all times.

What can I expect after my MSI exam?

  • The technologist will remove the electrodes if you had an EEG performed and escort you out of the room.
  • The clinical team will analyze and review your scan results and send the results to your health care professionals.

​Will any part of the testing hurt?

The scan is completely noninvasive and should not hurt at all.

Does the scanner use radiation?

No, the scanner does not emit radiation or magnetic fields. It is like a giant microphone that detects brain activity.

Will you inject anything into me?

No, we do not use any intravenous (IV) equipment, contrast material, or sedation.

Who can I ask any additional questions I may have about my appointment?

Please feel free to call the Biomagnetic Imaging Lab at (415) 476-6888 if you have any additional questions or concerns.