Magnetoencephalography (MEG) is an 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. There are many uses for the MEG, including assisting surgeons in localizing a pathology, assisting researchers in determining the function of various parts of the brain, neurofeedback, and others.
The Basis of the MEG Signal
Synchronized neuronal currents induce very weak magnetic fields that can be measured on MEG. However, the magnetic field of the brain is considerably smaller at 10 fT (femtotesla) for cortical activity and 103 fT for the human alpha rhythm than the ambient magnetic noise in an urban environment, which is on the order of 108 fT. Two essential problems of biomagnetism arise: weakness of the signal and strength of the competing environmental noise. The development of extremely sensitive measurement devices, SQUIDs, facilitates analysis of the brain’s magnetic field and confronts the aforementioned problems.
The MEG (and EEG) signals derive from the net effect of ionic currents flowing in the dendrites of neurons during synaptic transmission. In accordance with Maxwell’s equations, 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 which are currents defined to have 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. Furthermore, it is often bundles of these neurons located in the sulci of the cortex with orientations parallel to the surface of the head that project measurable portions of their magnetic fields outside of the head. Researchers are experimenting with various signal processing methods to try to find methods that will allow deep brain (i.e., non-cortical) signal to be detected, but as of yet there is no clinically useful method available.
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
In research, MEG’s primary use is the measurement of time courses of activity, as such time courses cannot be measured using functional magnetic resonance imaging (fMRI). MEG also accurately pinpoints sources in primary auditory, somatosensory and motor areas, whereas its use in creating functional maps of human cortex during more complex cognitive tasks is more limited; in those cases MEG should preferably be used in combination with fMRI. It should be noted, however, 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).
Magnetoencephalography Publication References
Schwartz ES, Dlugos DJ, Storm PB, Dell J, Magee R, Flynn TP, Zarnow DM, Zimmerman RA, Roberts TP. Magnetoencephalography for pediatric epilepsy: how we do it. AJNR. 2008;29(5):832-837.
Rowley HA, Roberts TP. Functional localization by rnagnetoencephalography.Neuroimaging Clin North Am. 1995;5:695-710.
Gallen CC, Hirschkoff EC, Buchanan DS. Magnetoencephalograpby and magnetic source imaging. Capabilities and limitations. Neuroimaging Clin North Am. 1995;5:22749.
Sutherling WW, Crandall PH, Darcey TM, Becker DP, Levesque MF, Barth DS. The magnetic and electric fields agree with intracranial localizations of somatosensory cortex. Neurology. 1988;38:1705-14.
Shibasaki H, Ikeda A, Nagamine T. Use of Magnetoencephalography in the presurgical evaluation of epilepsy patients. Clinical Neurophysiology. 118(7): 1438-1448.
Ref: Okada Y (1983): Neurogenesis of evoked magnetic fields. In: Williamson SH, Romani GL, Kaufman L, Modena I, editors. Biomagnetism: an Interdisciplinary Approach. New York: Plenum Press, pp 399-408.