What is MEG
Why is MEG beneficial?
A guide for patients and their families.Magnetoencephalography
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. Usually abbreviated as MEG. There
are many uses for MEG, including determining the function of various parts of the brain and localizing epileptic activity.
Indications for a MEG scan
Epilepsy is a common chronic neurological disorder that is characterized by recurrent unprovoked seizures. These seizures
occur due to abnormal neuronal activity in the brain. About 3 million Americans have epilepsy. Epilepsy is usually
controlled, but not cured, with medication. Surgery is often the best option in difficult cases.
brain tumor is any intracranial tumor created by abnormal and uncontrolled cell division. It can affect almost any part
of the brain. Many tumors, depending on their location, can be successfully removed surgically. In more difficult
cases, stereotactic radiosurgery, remains a viable option.
MEG is increasingly being used in the preoperative
evaluation of patients with epilepsy and 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, with precise accuracy, the location(s) where the
epileptic activity originates. This information is invaluable in determining if the patient is a good candidate for
surgery and also to plan 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 control).
How MEG works
Modern medical imaging offers a host of options for examining the inner structures
and workings of the human body. For the brain, doctors can now draw on many techniques to plan effective treatments
for a variety of illnesses and injuries. For anatomical information, CT and MRI provide detailed images. For metabolic
activity PET, SPECT and fMRI give useful information on blood flow, oxygenation, etc. These techniques can also give
a measure of function, albeit at the time resolution of blood movement; that is, on the order of seconds. For measurements
of fast phenomena, such as epileptic spiking, EEG is extremely useful, but it suffers from distortions of the electric fields
as they pass through the head, skull and scalp, making accurate localizations difficult. ECoG are invasive recordings
that require implantation 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 localizations accurate to the millimeter level. It
does this noninvasively (without injections or radiation of any kind) by measuring the magnetic fields that naturally emanate
whenever electric current flows within the neurons of the brain. The fields being measured are extremely weak, about
a billion times smaller than the Earth's magnetic field. The MEG technique uses very sophisticated instrumentation,
sensitive enough to detect these weak signals, while simultaneously discriminating against interference from the 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 past 10-15 years.
How to prepare for an MEG
is usually an outpatient procedure. Patient preparation for MEG is relatively minimal and the examination is generally
extremely well tolerated by patients. However, patients younger than about five years of age, if they are anxious or
unable to cooperate, may require general anesthesia to complete the examination successfully. Light sedation, to reduce
anxiety, is sometimes used. There are no needles or physical exams required.
Metal will interfere with MEG measurements,
so upon arrival the patient will be asked to remove any metal objects. This includes jewelry, metal parts of clothing,
some makeups, etc. Most dental work, such as a metal filling, is small enough so as not to cause a problem. The
doctor will have informed the patient beforehand if any other special preparations are required, such as tapering of antiseizure
medication, overnight sleep deprivation, etc. It is important to confirm these with your 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 are also affixed to the head. Next, the position of the coils and the electrodes
are precisely measured with a special wand called a digitizer.
Next 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 itself takes the form of a smooth helmet that completely
covers the head, but open in the front for vision. 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.
to expect during a MEG exam
If the MEG scan is to measure epileptic activity, then the patient will
be measured for about half an hour to an hour. During this time they will have no special tasks to perform and can even
go to sleep. 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 shocks 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 will also localize language areas of the brain with, for example, a reading or picture
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, muscular clenching, etc.
After the 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 general anesthetic was necessary, the patient will be sent to a recovery room, otherwise they are generally
free to go home.
How the MEG data is used
the data will be combined and analyzed by a trained professional, usually a neurologist. From the recorded signals,
it will be determined where in the brain the activity originated from. This applies to both pathological signals (epileptic
spikes) and also healthy signals (for example, those arising from the sensory stimuli). These locations will then be
combined with an MRI, which shows an image of the brain’s structure. The combined images are then included in
a comprehensive report which is prepared. When the report is completed, it is forwarded to the referring physician.
This, when pooled with other information from the patient, forms the basis for determining whether surgery is the best option
for treatment and, if so, how to plan it.