A magnetoencephalogram (MEG) is a test that identifies brain activity by measuring small electrical currents arising from the neurons of the brain. These currents produce magnetic fields. The MEG generates an accurate location of the magnetic fields produced by the neurons. The MEG test is somewhat related to the electroencephalography (EEG) test except that it is using magnetic fields as the primary information for determining where seizures might be originating.
The MEG test takes approximately two hours. An EEG is done at the same time as the MEG. Electrodes will be placed on the patient’s scalp using paste. All electronic items or items with a magnetic strip (cell phones, credit cards) will be removed. Also, all metal above the waist should be removed, such as under wire bra, shirts with zippers or snaps, jewelry and wrist watches. The patient will be in a specially shielded room during the test, and will lay in a bed with the head resting in the helmet-like MEG scanner. The MEG scanner does not emit radiation or magnetic fields. It only detects and amplifies magnetic signals produced by the brain.
There are video and intercom systems in the room. The technician can see, hear, and communicate with the patient at all times. During the scan, the patient must keep his/her head still. The head is immobilized by using cushions that are placed on both sides of the head.
The patient may be asked to arrive sleep-deprived in the hopes of recording brain activity in the awake, drowsy, and sleep states.
The MEG test causes no discomfort. The scanner does not make noise. The patient will be able to have someone in the room with you during the test, but they must also remove electronic and metallic objects.
What is Magnetic Resonance Imaging (MRI)?
An MRI (magnetic resonance imaging) scan is a non-invasive procedure that uses powerful magnets to construct pictures of the body. Unlike conventional radiography, which uses potentially harmful radiation (X-rays), MRI imaging is based on the magnetic properties of atoms. A powerful magnet in the machine generates an intense magnetic field around the body, and some hydrogen atoms within the human tissue will align with this field. When radio wave pulses are directed at this tissue, the hydrogen atoms that have been affected by the magnet will return a signal. These signals are used to construct pictures of the part of the body being examined. In the case of epilepsy, the brain is the subject of interest.
The MRI scanner must be located within a specially shielded room to avoid outside interference. The patient will be asked to lie on a narrow table which slides into a large tunnel-like tube. Once inside the machine, small devices may be placed around the head, arm, or leg, or adjacent to other areas to be studied. These are special coils which are designed to improve the quality of the images.
If contrast is to be administered, an IV will be placed, usually in a small vein of the hand or forearm. A technologist will operate the machine and observe you during the entire study from an adjacent room.
Several sets of images are usually obtained, each taking from 2 to 15 minutes. A complete scan may take up to one hour or more.
No preparatory tests, diets, or medications are usually needed before having an MRI performed. An MRI can be completed immediately after other imaging studies. Because of the strong magnets, certain metallic objects are not allowed into the room. Items such as jewelry, watches, credit cards, and hearing aids can be damaged, and must be removed prior to taking the test. Removable dental work should be taken out just prior to the scan. Pens, pocketknives, and eyeglasses can become dangerous projectiles when the magnet is activated and should not accompany the patient into the scanner area.
Because the strong magnetic fields can displace or disrupt the action of implanted metallic objects, people with cardiac pacemakers cannot be scanned and should not enter the MRI area. MRI also should not be used for people with metallic objects in their bodies, such as: inner ear (cochlear) implants, brain aneurysm clips, some artificial heart valves, older vascular stents, and recently placed artificial joints. Sheet metal workers, or persons with similar potential exposure to small metal fragments, will first be screened for metal shards within the eyes with X-rays of the skull.
The patient will be asked to sign a consent form confirming that none of the above issues apply before the study will be performed. A hospital gown may be recommended, or the patient may be allowed to wear "sweats" or similar clothing without metal fasteners.
There is no pain experienced during the scanning procedure. However, some people do have a claustrophobic. The table may be hard or cold, but you can request a blanket or pillow. The machine produces loud thumping and humming noises during normal operation. Ear plugs are given to the patient to reduce the noise.
A technologist observes the patient during the entire procedure and may speak through an intercom in the scanner. Some MRI scanners are equipped with televisions and special headphones to help the examination time pass.
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About Pre-surgical Evaluation for Epilepsy
Many patients have similar questions about having seizure surgery and what tests may be needed before surgery. The following general information is intended to provide some answers. However, each epilepsy patient is unique and his or her care is always individualized. All patients should discuss their unique situation with their physician.
Seizure surgery is an operation aimed at removing the area of your brain that is causing the seizures. The goal of surgery is to improve seizure control and when possible, to stop seizures completely. There are several types of seizure surgery, and every patient is approached as a unique individual.
After several medications have failed to control a patient’s seizures that individual’s epilepsy is called refractory. Studies have shown that in cases of refractory epilepsy, medications are less likely to work. Surgery may help some of these patients, and seizures may come under control with the addition of surgery. For patients who are open to this option, a surgical evaluation may be initiated.
Results from the video EEG telemetry session give the doctors a better understanding of what part(s) of your brain the seizures may be arising from. If most of the seizures are coming from one specific area, it may be possible to safely remove this area (seizure focus) in an effort to control the seizures. However, there are additional tests that will be needed in order to help the epilepsy team determine exactly where the seizures arise from and whether surgery is a safe option.
The full epilepsy team consists of epileptologists (doctors specializing in epilepsy), brain surgeons (neurosurgeons), radiologists, neuropsychologists, an epilepsy nurse, a neurosurgery nurse practitioner, the EEG technologists, and the scheduling staff.
There are several types of additional tests that may be needed before being considered for surgery. Some of the tests include: Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET), Magnetoencephalography (MEG), Neuropsychological testing, and the Wada test.
The information gathered from the tests above is aimed at better defining and localizing your specific seizure focus and to help determine if that focus can be safely removed.
In some cases, however, the results of the tests may give the epilepsy team an idea as to where the seizures start but not enough information to be conclusive. If the initial scalp EEG or radiological testing doesn’t provide enough localizing information, the patient may need further evaluation using intracranial monitoring (Invasive Monitoring). Intracranial monitoring records brain wave activity using electrodes placed surgically in or on the surface of the brain.
If surgery is found to be an option, the last two tests are the neuropsychological evaluations and the Wada test. The neuropsychological evaluation will be done by a neuropsychologist. Extensive testing may determine the site of the seizures, as well as appraise the possible effects of the surgery on cognitive functioning.
The Wada test evaluates the location of speech and memory functions in the patient’s brain. This test is done in an angiography suite with the radiologist, neurologist and anesthesiologist. Sodium amytal, a short acting medicine, will be used to sedate one side of the brain while the other side is being tested by a neuropsychologist. The results of this test determine which side of the brain is the dominant area for crucial functions, such as language. It may determine whether the patient will be awake during part of the surgery. This test and its results should be discussed with the patient’s physician in detail.
Once the Epilepsy Team has obtained and reviewed all the necessary tests, they will present the results to the patient and together, the patient and physician team will make a decision about surgery.
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What is Invasive Monitoring?
Invasive monitoring is a term that refers to recording brain waves using electrodes placed in or on the surface of the brain. In rare cases when scalp recordings do not localize your seizures, intracranial electrodes may be necessary to acquire further information. This procedure involves undergoing surgery to place the electrode at various places on the surface of the brain. These electrodes allow your physicians to get much higher quality information that what normal scalp-based electrodes can detect.
Ultimately, your physicians will determine if you are a candidate for having intracranial electrodes. In almost all cases, this is an advanced test that is only performed for patients who are being considered for epilepsy respective surgery.
If this decision is made, the epilepsy team will determine the number, the type and the exact placement of electrodes needed to further evaluate the seizure focus. There are different types of intracranial electrodes, including depth electrodes, subdural strips, and subdural grids all of which are placed by the neurosurgeon. You will get details about your particular situation if this intervention becomes necessary.
If you have this procedure, you will be admitted to the hospital at least a day before the placement of the intracrainal electrodes. You will need to wash your hair the night before the procedure. Part of your head will likely be shaved. This is done to prevent infection. Plan to stay in the hospital for at least 5 to 7 days after the electrodes are placed. After the electrodes are placed, you will be in a private monitored room such as the one you were in for video-EEG recordings. The overall test is very similar to routine video EEG monitoring, except that the electrodes are invasive.
You may experience scalp pain and headaches after the procedure. You may also experience some scalp or face swelling, especially with a grid set of electrodes. Your nurses and doctors will do all they can to make you comfortable.
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