Magnetic resonance imaging: an essential and safe exam
The care of people with epilepsy has been revolutionized by developments in brain imaging techniques. The most important technique for the diagnosis of epilepsy is called magnetic resonance imaging (MRI), a safe and non-invasive test used to create images of brain anatomy. The first purpose of the MRI in a patient with seizures is to help confirm the diagnosis of epilepsy and to identify its cause.
MRI is the most powerful tool for detecting brain abnormalities that can cause seizures: the most frequent causes that can be seen on MRI scans are hippocampal sclerosis, cortical dysplasias, scars, among other causes. Therefore, MRI should be performed early in all persons with epilepsy because people with these abnormalities on MRI scans often have seizures that cannot be controlled by anti-seizure drugs. In these cases, surgery is the only solution to stop seizures. Importantly, the MRI should be done in specialized centers and be analyzed by a neurologist or radiologist with expertise in epilepsy.
In some people, the brain anomalies that cause seizures cannot be seen by the naked eye on regular (routine) MRI scans. In specialized epilepsy centers, doctors can perform computerized analyses (also called post-processing) on the MRI images after they have been collected. These tests are more precise than the human eye. Doctors can discover very small lesions by comparing a large number of brain measurements (such as volume, shape and thickness) between people with epilepsy to those without epilepsy. These analyses help doctors better identify patients that may benefit from epilepsy surgery.
Other imaging techniques
Several other brain imaging exams may be used to better pinpoint the area of the brain that causes seizures. These tests are mainly done in specialized centers that investigate patients for epilepsy surgery.
Diffusion MRI identifies nerve bundles connecting different brain areas to one another. Functional MRI (fMRI) measures brain activity by detecting changes in blood flow. These methods aid in surgical planning by reducing the risk of damage to brain areas responsible for important functions, such as vision, language and memory.
Combining the recording of EEG with an fMRI test may provide additional information on the location where seizures may originate.
Two techniques that use radioactive substances are called single photon emission computed tomography (SPECT) and positron emission tomography (PET). SPECT examines the change in the amount of blood flow that occur during seizures as a means of identifying where in the brain the seizures may be coming from. With PET, the metabolism of the brain in-between seizures is measured and sometimes the part of the brain generating seizures has reduced metabolism compared to the rest of the brain. This, too, can help doctors identify where in the brain seizures may originate for some patients.
Magnetic resonance spectroscopy (MRS) examines changes in brain chemistry caused by epilepsy, and is used as another method of identifying where seizures come from in a given patient. This is more commonly used in people with suspected genetic or metabolic epilepsies.
Magnetoencephalography (MEG) is another neuroimaging method used for localization of epileptic foci, although it is less widely available in Canada. MEG measures the magnetic currents produced by neuronal activity in the brain. The spatial distribution of these magnetic currents can be used to identify the source of the epileptiform discharges. MEG has a higher spatial resolution than EEG, while still providing high temporal resolution, allowing for localization of deeper epileptogenic foci by non-invasive means.