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.
A 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 localization 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 localization 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 indispensable 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
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.
What 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 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, 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
After collection, 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.