Magnetoencephalography: Basic Principles
Magnetoencephalography (MEG) is a non-invasive neurophysiological technique that measures the magnetic fields generated by neuronal activity of the brain. The spatial distributions of the magnetic fields are analyzed to localize the sources of the activity within the brain, and the locations of the sources are superimposed on anatomical images, such as MRI, to provide information about both the structure and function of the brain.
The principle features of MEG are:
MEG is a direct measure of brain function, unlike functional measures such as fMRI, PET and SPECT that are secondary measures of brain function reflecting brain metabolism.
MEG has very high temporal resolution. Events with time scales on the order of milliseconds can be resolved, again differentiating MEG from fMRI, PET and SPECT, which have much longer time scales.
MEG has excellent spatial resolution; sources can be localized with millimeter precision.
MEG is completely non-invasive. It does not require the injection of isotopes or exposure to X-rays or magnetic fields. Children or infants can be studied and repeated tests are possible.
Magnetic fields arise whenever there is a current flow, whether in a wire or a neuronal element.
Unlike electrical signal recorded by EEG, magnetic fields recorded by MEG outside the head pass unaffected through the brain tissue and the skull. Therefore MEG allows much more precise reconstruction of the brain sources of neural activity then EEG does. The magnetic field is extremely weak, but can be detected by sophisticated sensors that are based on superconductivity (SQUIDs). By analyzing the spatial distributions of magnetic fields it is possible to estimate the intracranial localization of the generator source.
The MEG is the only non-invasive method of brain imaging that has both perfect temporal and high spatial resolution. Other functional modalities, except invasive EEG (iEEG), have either poor temporal or spatial resolution. Clearly iEEG has the distinct disadvantage of being invasive. As a whole this means that MEG is a highly practical and a one-of-a-kind resource.
MEG is a unique research tool. MEG is also an effective diagnostic tool for evaluating brain function in a variety of surgical planning applications.
Future development of the MEG method
The functioning of the superconductive MEG sensors (SQUIDs) requires cooling them to 4K. Therefore, the sensors are placed in liquid helium. The need for such low temperature cooling makes the ‘low-temperature-conductive’ (LowTc) MEG device and its maintenance very expensive. Moreover, the necessity to isolate the SQUIDs from room temperature leads to their relatively large distance from the head and the brain, which decreases their sensitivity to brain signals.
Researchers try to overcome these drawbacks of the ‘conventional’ MEG by using alternative techniques, such as those based on high temperature superconductivity (HighTc MEG) or optical technology. However, at present the LowTc-based MEG remains the only reliable and commercially available technique for the full-head measurement of the magnetic fields of the brain.