MEG stands for magnetoencephalography, or the recording of magnetic activity from the head. MEG is not a type of MRI. Unlike MRI, the patient receives no external energy. The MEG system uses highly sensitive magnetic sensors, called SQUIDS, to detect very small changes in magnetic fields produced by the brain itself. Like electroencephalography (EEG), MEG measures neuronal activity, which provides millisecond temporal resolution. However, the magnetic fields measured by MEG are not smeared like electrical currents, so MEG provides much higher spatial resolution (less than a centimeter).
MSI stands for magnetic source imaging. MEG provides detailed information about brain activity, but it provides no information about brain structure. To see exactly where activity takes place in an individual’s brain, we combine the MEG data with an MRI (or CT) of the person’s head. At the beginning of the MEG session, we create a digital 3-D representation of the shape of the patient’s head, co-register this with the patient’s MRI (or CT), and the activity is aligned to the appropriate places inside the head.
Although the presence of metal presents no danger during a MEG, it may create distortion that prevents the MEG from being able to detect brain activity. Metals containing iron (such as steel) are especially a problem, but many other metals can create distortion. It is best to avoid the presence of metal when possible.
Clinical Uses of MEG
MEG has been approved for the clinical mapping of brain activity in patients who are candidates for neurosurgery. This clinical use falls into one of two categories:
- Presurgical mapping of eloquent cortex (evoked magnetic fields): Brain activity is elicited by certain types of stimulation (somatosensory, auditory, visual) or when engaged in certain tasks (e.g., listening to words, generating language, or performing movements). The stimulation or task is repeated hundreds of times and averaged.
- Mapping of epileptic activity (spontaneous magnetic fields): In some cases, activity spikes related to epilepsy can be measured with MEG (and EEG). These spikes are identified by a neurologist and their source in the brain is identified.
What happens after the MEG?
The MEG activity of interest is identified in the waveform data (i.e., a spike or ongoing activity in the averaged evoked response or epileptiform activity in unaveraged spontaneous activity).
The source of MEG activity that meets certain statistical and anatomical criteria is identified using a single equivalent current dipole model. The dipoles are then displayed on the patient’s MRI (or CT) to produce MSIs. These are typically shown in all three planes (sagittal, coronal, and axial). The technical details and clinical interpretation of the MEG/MSI are included in the report with the MSIs.