Magnetoencephalography (MEG) is a non-invasive diagnostic tool that can help neurologists and neurosurgeons evaluate and map brain activity before surgery. MEG offers a way to localize brain activity with a high spatial and temporal resolution, and represents a significant advance in the care of patients with brain tumors or severe epilepsy. A MEG scan is performed as an outpatient procedure. For some patients, MEG can replace invasive testing done in the ICU or in the operating room.
Mapping the Brain with MEG
MEG brings anatomical and physiological data together to yield images of brain physiology registered to the patient's brain structures.
MEG uses extremely sensitive and sophisticated sensors to record magnetic fields over the surface of the head. These fields are generated by the tiny electrical currents that flow between neurons (nerve cells) inside the brain. MEG scans these magnetic fields in the brain millisecond by millisecond. MEG is totally non-invasive.
Images are generated without the use of radioactive isotopes (used in nuclear medicine) or exposure to X-rays or external magnetic fields. MEG moves beyond existing technology in its ability to pinpoint in space and time normal and abnormal brain activity. By measuring the magnetic fields, the cell assemblies that produce each field can be located with an unprecedented millisecond temporal resolution.
For patients with brain tumors or for epilepsy patients who require surgery, MEG can help precisely identify the areas to be removed. This allows more complete and selective removal of the involved sections, while identifying brain tissue that must be spared to preserve important body functions. This more precise measurement also enhances the possibility of surgical success for patients whose tumor or epilepsy is considered inoperable.
Learn how to prepare for MEG
Magnetic Source Imaging
After the MEG exam, the MEG recordings are combined with a magnetic resonance imaging (MRI) scan, which shows the actual structure of the brain. The combined scan, called magnetic source imaging or MSI, combines the structural image from an MRI and the time-resolved functional images provided by MEG. MSI is very helpful for showing areas of the brain that may be generating seizures or for locating a tumor, as well as for localizing areas of normal brain function with precise timing. MSI can provide neurosurgeons a detailed “map” that allows them to remove only damaged brain tissue while preserving healthy tissue.
Differences Between MEG and EEG
MEG measures magnetic fields, and EEG (electroencephalography) measures electrical potentials generated by the electrical activity of the brain. An important difference is that the skull and tissue surrounding the brain don’t affect the magnetic fields measured by MEG, while they strongly affect the electrical potentials measured by EEG.
When electrical signals from the brain pass through the skull and scalp, they are distorted and severely weakened. These same tissues, however, are transparent to the magnetic fields generated in the brain. Therefore, MEG offers a more accurate spatial estimate of brain activity than EEG. Compared to EEG, MEG allows for more usable and reliable localization of brain function. MEG can localize brain activity with a much higher spatial resolution than EEG, with the same millisecond scale time-resolution as EEG.
Learn More at mcw.edu
Learn more about MEG at the Web site for the Medical College of Wisconsin, mcw.edu. The college's site provides detail about the program and its participation in important research.