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The following diagnostic procedures determine if surgical treatment for epileptic seizures is necessary. These procedures are used to assist a physician in evaluating the cause of a patient’s seizures and locate the origin of the seizures within the brain. After a procedure, a patient remains within the Epilepsy Monitoring Unit (EMU) for observation.
Imaging studies are used to evaluate the structure of the brain or its function. There are five different radiologic tests which are most often ordered for preoperative/epilepsy evaluations in seizure patients. These are MRI, PET, Wada, Ictal SPECT and MRS.
Magnetic Resonance Imaging (MRI) allows a physician to see a patient's internal body structures without using x-rays. With this technology, a physician can detect developing diseases or abnormalities earlier than ever before. MRI involves a powerful but harmless magnetic field and radio waves like the kind that transmit FM music. The combination of radio waves and magnetic field produces very clear images of body structures like the brain, the spine and other vital organs. By looking for abnormalities in structure, our team can determine if there may be seizure producing areas in or adjacent to the regions of structural change. At Johns Hopkins, we order an epilepsy protocol MRI that contains a 3-D portion and special coronal cuts so the epileptologist can evaluate the temporal lobes for signs of mesial temporal sclerosis or hippocampal malformation.
What kinds of machines are used to perform an 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, knee or stomach, etc.). The data from the scan is fed into computers which compose very clear pictures that the physician will use to make a diagnosis.
What happens on the day of the exam?
The patient should allow about two hours for the MRI exam, although most scans take an hour or less.
Unless the patient's doctor indicates otherwise, the patient can eat normally the day of the exam. The patient should not wear any makeup, since some brands contain metallic components. If the patient has had any recent tattoos, he or she should let the physician know as some tattoo dyes contain metal. On arrival at the MRI center, the patient will be asked to put on a gown and remove all personal possessions such as jewelry, wallet, 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 if the patient has them in a pocket during the exam.
While the vast 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:
- Does the patient weigh more than 300 pounds?
- Can the patient lie flat for an extended period of time?
- Does the patient suffer from claustrophobia?
- Is the patient pregnant?
Since the MRI scan involves the use of a powerful magnet, the patient will also be asked the following:
- Does the patient have any implanted devices such as a cardiac pacemaker, a cerebral aneurysm clip, a neurostimulator or a hearing aid?
- Does the patient have any metal shrapnel in the body or any metal fragments in the eyes?
- Does the patient have any kidney problems?
After providing all of the necessary information, the patient should feel free to air any concerns about the upcoming examination.
What Happens During the Actual Exam?
The patient will be escorted into the room containing the "magnet" by a technologist and will be asked to lie down on the padded table. The technologist will position the patient inside the magnet so that the appropriate part of the patient's body is ready to be scanned. During the exam, the patient will be able to talk with the people conducting the exam by means of an intercom. The people conducting the exam will be able to watch the patient through a glass window.
The machine makes loud clanking and thumping sounds as the technologists adjust the radio frequencies and other controls. These sounds are completely normal. Some centers supply ear plugs for patients who may be bothered by the sound.
Sometimes a radiologist, nurse, or technologist may come into the scanning room to inject an enhancement agent to help the radiologist interpret the pictures taken during the exam.
Side Effects: Since MRI uses harmless radio waves and magnetization instead of x-rays, it is considered to be safer than other radiologic techniques. There are no known side effects.
With a PET scan, changes in brain metabolism and chemistry can be observed. Although a PET scan can help to evaluate many medical conditions, it can be of particular value in evaluating patients with epilepsy.
The PET scan procedure
To begin the procedure, a plastic mask is fitted onto the patient's face. This aids in positioning the patient's head in the scanner. Baseline x-rays are often taken, either by CT scan or MRI at another location. The PET scan begins when a small amount of radioactive material is injected into a vein in the arm and, at the same time, 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 PET scan generally takes less than two hours, during which time the patient must lie quietly with the plastic mask in place. If the patient has had a CT scan, the experience of being "inside" the PET scan 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.
As with all radioactive materials, including x-rays, there is a slight risk associated with the radioactive material that is injected. The dose of radiation received in a PET scan is slightly more than that received in a chest x-ray, but less than that received from most digestive studies. Women who are pregnant, or suspect that they are pregnant, should not have a PET scan.
Patients should not eat or drink anything, except water, for four hours prior to the PET scan. Additionally, patients should remain as inactive as possible the night before and the day of the PET scan and refrain from all types of physical exercise or activity.
Single Photon Emission Computed Tomography (SPECT) can be used to look for areas of the brain with altered metabolism, cerebral blood flow, or neurotransmitter function. The idea is that these areas of altered activity are more likely to be areas producing seizures in a given patient.
This test is done to evaluate localization of onset for patients whose EEG might not show an exact focus of onset. It can be used to guide the placement of grids. This specialized testing is done in two phases, Ictally (during a seizure) and Interictally (in-between seizures). Patients will be admitted to the EMU for the Ictal portion of the test, the Neurolite is injected at the bedside, at the beginning of a seizure, by a specially trained nurse. The patient is then transported to Nuclear medicine for a specialized scan. This is used to assess cerebral blood flow at the seizure onset. The patient will have an interictal injection by nuclear medicine personnel as an outpatient and will be scanned afterwards as well.
The scans from the interictal and ictal injections are compared. The area of seizure focus is associated with an area of increased blood flow in an area which showed decreased blood flow after the interictal injection.
Along with the MRI, a MRS might be ordered. This is specialized imaging which is used to evaluate and compare the metabolic/chemical function of different parts of the brain. It can be used to compare temporal lobes size and metabolic function. It cannot be used evaluate the whole brain at once. It is used to assess areas of focused interest. This procedure is performed in the MRI Suite and the preparation would be the same as for an MRI.
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