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:
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.