At present, structure is usually assessed with magnetic resonance imaging (MRI), although computed tomography (CT) scans are also used.
Seizures are sometimes caused by abnormal nerve-cell discharges originating in or near a structural abnormality in the brain. Imaging studies therefore are utilized to evaluate the structure and/or function of the brain. Structure is usually assessed with magnetic resonance imaging (MRI) or computerized tomography (CT or CAT) scans. MRI techniques are now beginning to be used to assess brain function as well, using methods such as functional MRI (fMRI). In addition, radiotracer-based methods such as positron emission tomography (PET) and single photon emission computed tomography (SPECT) can be used to look for areas of the brain with altered metabolism, blood flow, or neurotransmitter function. In a patient with new or different seizures, areas of altered activity seen on a scan may be responsible for the problem.
Magnetic Resonance Imaging (MRI)
What Is MRI?
Magnetic resonance imaging (MRI) provides physicians a method of seeing the patient's internal body structures without using X-rays. This technology gives physicians more opportunity to detect reasons for seizures or other symptoms earlier than ever before. MRI uses a magnetic field and radiowaves, like the kind that transmit FM music, to produce very clear images of body structures. Abnormalities in brain structure may infer the presence of seizure-producing areas in or adjacent to the regions of structural change. In addition, new techniques are being developed that can evaluate brain function.
What Kinds Of Machines Are Used To Perform MRI?
The patient's scan will be performed in a room containing "the magnet." The MRI unit looks like a large box with an open-ended tunnel running through the middle. Many different kinds are in use, but they all work in the same way. The patient will be asked to lie on a comfortable, padded table that is gently moved into the opening of the magnet where the scanning is performed.
Sometimes a "coil," which is really just a special radio receiver, will be placed around the body part being scanned (the patient's head, or knee, or stomach, etc.). The data from the scan is fed into computers that compose the very clear pictures.
What Happens The Day Of The Exam?
The patient should allow about an hour for the MRI exam, although most scans require less time.
Unless the patient's doctor says otherwise, the patient can eat normally the day of the exam. The patient should not wear any makeup, because some brands contain metallic components. On arrival at the MRI center, the patient will be asked to put on a gown and to remove all personal possessions such as watch, wallet and car keys, or metallic items such as dentures, pins, etc. It's very important not to take anything that could be affected by a magnet into the examining room. For example, the information on credit cards could be erased during the exam!
While the great majority of people can undergo an MRI exam with no problems, some cannot. The radiologist or the staff at the MRI center will probably ask the patient questions like the following:
- Do you weigh more than 300 pounds?
- Can you lie flat for an extended period of time?
- Do you suffer from claustrophobia?
- Are you pregnant?
- Do you have any implanted devices such as a cardiac pacemaker, a cerebral aneurysm clip, a neurostimulator or a hearing aid?
- Do you have any metal in the body or any metal fragments in the eyes?
- Do you have any kidney problems?
The patient should remember that the questioning process is a two-way street. After providing all of the necessary information, the patient should feel free to air any concerns about the upcoming examination. Radiologists and their technologists expect questions, and part of their job is answering them.
What Happens During The Actual Exam?
The patient will be escorted into the room containing the MRI scanner by a technologist and positioned appropriately on the padded scanner table. During the scan, the patient will be observed by the technologist through a window and will be able to communicate with the technologist by means of an intercom. The scanner emits rather loud thumping and clanking sounds as part of its normal operation, and most MRI centers provide earplugs or stereo headphones tuned to a radio station of choice. In some cases, a radiologist, nurse or technologist may enter the scanning room during a break in the study to inject an intravenous contrast agent.
An intravenous injection may be started before an MRI in case the doctor wants to inject a chemical called gadolinium. Gadolinium lights up certain types of abnormal tissues and may improve the quality of MRI pictures under certain circumstances.
People with kidney disease are at risk for a rare side effect from gadolinium. This side effect is called nephrogenic systemic fibrosis (also called nephrogenic fibrosing dermopathy). It causes swelling and tightness of the skin, followed by skin scarring and thickening. People with kidney disease should always notify the staff about their kidney problem before undergoing an MRI.
Since MRI uses radiowaves and magnetization instead of X-rays, it is considered to be safer than other radiologic techniques that do use X-rays. For people with normal kidney function and people do not have implanted metallic or electrical devices, MRI has no risks or side effects. People with kidney disease may undergo an MRI but should not receive gadolinium because it can cause a rare but serious side effect that leads to swelling and tightness of the skin, followed by skin scarring and thickening.
Positron Emission Tomography (PET Scan)
PET scanning enables changes in brain metabolism and chemistry to be observed, providing information on brain function that can be of particular value in patients with epilepsy.
The PET Scan Procedure
Baseline brain images are often taken prior to the PET scan, either by CT scan or MRI. The patient is then positioned in the PET scanner, and a plastic mask or frame is often used to aid in head positioning in the scanner. The PET scan begins when a small amount of radioactive material is injected intravenously in one arm while a blood sample is taken from the other arm. For some types of PET scans, the patient's eyes will be covered and the patient's ears plugged for thirty minutes after the injection to minimize sensory stimulation. The procedure generally takes less than two hours. If the patient has had a CT scan, the experience of being "inside" the PET scanner is similar.
Risks And Discomforts
There are no serious complications or discomforts with PET scanning. The patient may experience minor discomfort with the placement of the intravenous needle. Some patients find it uncomfortable to lie still in the PET scanning bed for the length of time required for the study. The radioactive tracer used in the PET scan is relatively weak and short-lived, and the associated radiation exposure is slightly more than that received during a chest X-ray but less than that received during other radiological studies, including digestive studies and CT scans. Women who are pregnant would undergo PET scanning only if absolutely necessary.
Last updated February 03, 2010
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