Elizabeth M. Davenport, PhD; Amy L. Proskovec, PhD; Bhavya R. Shah, MD; Christopher T. Whitlow, MD, PhD; Joseph A. Maldjian, MD
Magnetoencephalography (MEG) is a noninvasive and passive imaging technique used to measure the magnetic fields produced by electrical activity in the brain via extremely sensitive devices such as superconducting quantum interference devices (SQUIDs). These measurements are commonly used in both research and clinical settings. Given the excellent temporal (< 1 ms) and good spatial (2-3 mm) precision of MEG, there are many uses for the modality, including clinical applications assisting surgeons in localizing seizure activity and preoperative assessment of eloquent cortex, as well as assisting researchers in determining the function of various parts of the brain.
Basis of the MEG Signal
Synchronized neuronal currents induce very weak magnetic fields that can be measured by MEG. The magnetic fields emitted bythe brain are considerably smaller than the ambient magnetic noise in an urban environment, with cortical activity at 10 fT (femtotesla; 10-15 T), the human alpha rhythm at 10-12 T, and environmental noise on the order of 10-6T. Two essential problems of biomagnetism arise: weakness of the signal and strength of the competing environmental noise. The development of extremely sensitive measurement devices (e.g., SQUIDs) facilitates analysis of the brain’s magnetic field Almost all MEG devices are situated in a magnetically shielded room (MSR) in order to reduce the interference from environmental noise sources.
The MEG signals, similar to EEG, derive from the net effect of ionic currents flowing in the dendrites of neurons during synaptic transmission. In accordance with Maxwell’s equations and Ampere’s law, any electrical current will produce an orthogonally oriented magnetic field. It is this field which is measured with MEG. The net currents can be thought of as current dipoles with an associated position, orientation, and magnitude, but no spatial extent. According to the right-hand rule, a current dipole gives rise to a magnetic field that flows around the axis of its vector component.
In order to generate a signal that is detectable, approximately 50,000 active neurons are needed. Since current dipoles must have similar orientations to generate magnetic fields that reinforce each other, it is often the layer of pyramidal cells in the cortex, which are generally perpendicular to its surface, that give rise to measurable magnetic fields. Bundles of these neurons located in the sulci of the cortex with orientations parallel to the surface of the head are most easily detected by MEG. MEG is also capable of detecting cerebellar and deep brain sources like the amygdala and hippocampus.
It is worth noting that action potentials do not usually produce an observable field, mainly because the currents associated with action potentials flow in opposite directions and the magnetic fields cancel out. However, action fields have been measured from peripheral nerves.
MEG Use in the Field
MEG’s primary uses are the direct measurement of time courses of neural activity (evoked activity) and quantification of neural oscillatory activity (induced activity), as such metrics cannot be measured using functional magnetic resonance imaging (fMRI). It should be noted that neuronal (MEG) and hemodynamic (fMRI) data do not necessarily agree and the methods complement each other. However, the two signals may have a common source: it is known that there is a tight relationship between LFP (local field potentials) and BOLD (blood oxygenation level dependent) signals. Since the LFP is the source signal of MEG/EEG, MEG and BOLD signals may derive from the same source (though the BOLD signals are filtered through the hemodynamic response).
MEG is clinically approved to identify eloquent cortex for pre-surgical mapping including sources in primary auditory, somatosensory, motor, visual, and language areas. In addition, it is used to localize abnormal activity, such as interictal epileptiform discharges, within the brain.
MEG is often used in research to create dynamic functional maps of the human brain and assess functional connectivity during complex cognitive tasks, such as working memory, attention, perception, and language, in both healthy and clinical populations. In addition, MEG has shown promise as a potential diagnostic tool for autism, concussion, and Alzheimer’s disease.
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