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What is MEG and why is it beneficial?
A guide for patients with epilepsy or brain tumor
Magnetoencephalography (mag NEE-TO en-SEFF uh-LAW graphy):
A completely noninvasive procedure that uses an array of highly sensitive sensors to detect and record the magnetic fields associated with electrical activity in the brain. This procedure usually is abbreviated as MEG. There are many clinical and research applications for which MEG may be useful, including determining the function of various parts of the brain and localizing epileptic activity.
MEG is increasingly being used in the preoperative evaluation of patients with epilepsy and for those who will undergo tumor resection surgery. In either case, the MEG can localize the precise areas that are, despite the pathology, still healthy and functioning. This helps the surgeon to determine a successful surgical approach and also how aggressively to resect a given area. With a “roadmap” of which areas to avoid, the surgeon has a better chance of performing the procedure without affecting critical functions such as the senses, language and motor control. These functions are controlled from so-called “eloquent cortex”. For epilepsy surgery, MEG has the added benefit of being able to localize where the epileptic activity(s) originates. This information is invaluable in determining if the patient is a good candidate for surgery and also in planning the operation itself.
The ability to localize pathological areas and their relationship to eloquent cortex allows the medical team to more accurately assess the likelihood of a successful surgery. This is defined as one where the patient is left free from the disturbance (for example, the tumor or the uncontrolled seizures from epilepsy), while suffering minimal functional deficits (for example, loss of senses or motor control).
Beyond epilepsy and surgical planning, researchers are continually uncovering new applications for the unique capabilities of MEG in a variety of Functional Mapping investigations. This includes a range of neurological and neuropsychiatric disorders such as stroke, traumatic brain injury, Alzheimer’s disease, autism and schizophrenia.
How MEG Works
For measurements of fast phenomena in the brain, such as epileptic spiking, electroencephalography (EEG) is extremely useful, but it suffers from distortions of the electric fields due to the skull and scalp, making accurate localizations difficult. ECoG are invasive recordings that require implantation of electrode grids or depth electrodes. This necessitates additional surgeries, which result in increased hospital stays along with the risks of intracerebral hemorrhage, infection and other complications.
Only MEG can measure fast, millisecond phenomena and also perform accurate localizations, often as precise as a few millimeters. It does this noninvasively (without injections or radiation) by measuring the magnetic fields that naturally emanate whenever electric current flows within groups of neurons in the brain. The fields measured are extremely weak, about a billion times smaller than the Earth's magnetic field. MEG uses very sophisticated instrumentation, sensitive enough to detect these weak signals, while simultaneously discriminating against interference from much stronger magnetic background noise.
MEG is rapidly becoming an indispensible brain imaging technology. It has been demonstrated to improve the surgical outcome of epilepsy patients based on the evaluation of several thousands of patients over the last 10 to 15 years.
How to prepare for an MEG scan
IMEG usually is an outpatient procedure. Patient preparation for MEG is relatively minimal and generally patients tolerate the examination extremely well. However, patients younger than about five years of age may be too anxious or unable to cooperate, and therefore may require general anesthesia to complete the examination successfully. Light sedation, to reduce anxiety, also is sometimes used. No needles or physical exams are required.
Metal can interfere with MEG measurements, so on arrival the patient may be asked to remove any metal objects. This includes jewelry, metal parts of clothing, and some cosmetics. Most dental work, such as a metal filling, is small enough that it won’t cause a problem. The doctor will have informed the patient in advance if any other special preparations are required, such as tapering of anti-seizure medication or overnight sleep deprivation. It is important that patients confirm these recommendations with their referring/prescribing physician before the scan.
Before the exam, the patient will be fitted with three or more head positioning coils. These are small and are painlessly affixed to the head with tape. Their purpose is to determine the precise position of the head relative to the MEG detectors during the scan itself. The doctor may also want to measure EEG simultaneously with the MEG. In that case, electrodes (either individually or configured in an array much like a bathing cap) also are affixed to the head. Next, the position of the coils and electrodes are precisely measured with a special wand called a digitizer.
Then the patient will be brought to the MEG system itself. All MEG studies are performed inside a magnetic shield, which is a large metal-walled room that helps keep interference from the environment out. Inside the room, the MEG device itself takes the form of a smooth helmet that completely covers the head, but is open in the front to enable the patient to see. The system can rotate, so the patient can either lie down on a bed or sit up in a chair during the scan. The doctor or technician performing the measurement will ensure that the head is completely inserted in the helmet and that the patient is comfortable.
What to expect during a scan
If the MEG scan is to measure epileptic activity, then the patient will be measured for about 30 minutes to an hour. During this time, they will usually have no special tasks to perform and are even allowed to fall asleep. Patients can be given breaks, if needed.
If the scan is to localize sensory areas of the brain, then the patient will be presented with some stimuli. This could be tones to localize the auditory area of the brain; images on a screen to localize visual areas; or mild electric pulses to localize somatosensory areas. Likewise, motor areas can be localized by, for example, asking the patient to push a button every few seconds. In any case, for accuracy the measurement will be repeated about one hundred times in rapid succession. Such exams take about ten minutes total for each sense. Some MEG centers also will localize language areas of the brain with, for instance, a reading or picture naming task.
No matter what measurement is made with the MEG, the patient will be asked to hold relatively still during the recording and to minimize eye-movement and muscular clenching.
After MEG data collection is complete, the doctor or technician will assist the patient out of the shielded room. Electrodes and head position indicator coils will be removed. If anesthesia was necessary, the patient will be sent to a recovery room, otherwise they are generally free to go home.
How the MEG data are used
After collection, the data will be combined and analyzed by a trained professional, usually a neurologist. From the recorded signals, clinicians will determine from where in the brain the activity originated. This applies to both pathological signals (epileptic spikes) and also healthy signals (e.g., those arising from the sensory stimuli). These locations will then be combined with an MRI, which shows images of the brain’s structure. The combined images are then included in a comprehensive report. When completed, the report is forwarded to the referring physician. This, when pooled with other information, forms the basis for determining whether surgery is the best option for treatment and, if so, how to plan it.
Where to find more information
MEG is covered by a growing list of insurance providers. Measurement capability is also available at an increasing number of centers around the world. Please see [link to MEG centers page].
Ask your doctor if a MEG scan would provide useful information to assist your treatment.