Diagnostic methods for imaging the brain generally divide between two categories: anatomic and functional. CT and MRI are most common for anatomic imaging, while PET and fMRI are examples of functional imaging. Like EEG, MEG records the electrophysiological effect of neuronal activity over time; however, with its higher sensor count and simpler modeling physics, MEG has a higher source resolution.
Additionally, recordings with MEG are reference-free; its signals are not attenuated by bone and multichannel, whole-head, high spatial-density recordings are easily obtained. By its very nature, MEG shows areas of function: It localizes the signals generated by neurons as they are activated, as they communicate and as activity spreads through them.
MEG is sometimes called a functional imaging test, but it differs in significant ways from other such tests:
- The functional tests available at most centers are indirect measurements, dependent on changes in oxygen consumption (fMRI), glucose uptake (PET) and blood flow (SPECT). Conversely, MEG measures neuronal activity directly.
- While PET and fMRI measure changes in metabolism and blood flow, respectively, over many seconds, MEG measures electrical activity millisecond by millisecond.
Localizing the entire sequence of activation as it evolves over time is what MEG does superbly. Hence, the activity of the whole chorus of neurons required for everyday actions (pressing on the accelerator) or abnormal episodes (an epileptic aura) — not just the maximally involved area — can be mapped in space chronologically as it changes.
Like PET and fMRI, MEG “lights up” brain areas activated by a task. In epilepsy, MEG can show the propagation of activity from one brain region over a few milliseconds or during the onset of a seizure; in fact, ictal MEGs constitute approximately 15 percent of MEG scans performed at Cleveland Clinic Epilepsy Center. MEG results are coregistered with anatomic images from MRI and are reconstructed three-dimensionally to show the exact areas of activity.