Editing Magnetoencephalography (section)

== The basis of the MEG signal ==
[[Neural synchronization|Synchronized neuronal currents]] induce weak magnetic fields. The brain's magnetic field, measuring at 10 [[femto]][[tesla (unit)|tesla]] (fT) for [[cerebral cortex|cortical]] activity and 10<sup>3</sup> fT for the human [[alpha rhythm]], is considerably smaller than the ambient magnetic noise in an urban environment, which is on the order of 10<sup>8</sup> fT or 0.1 μT. The essential problem of biomagnetism is, thus, the weakness of the signal relative to the sensitivity of the detectors, and to the competing environmental noise.

[[Image:Magnetoencephalography.svg|thumbnail|250px|right|Origin of the brain's magnetic field. The electric current also produces the EEG signal.]]The MEG (and EEG) signals derive from the net effect of ionic currents flowing in the [[dendrite]]s of neurons during [[synapse|synaptic]] transmission. In accordance with [[Maxwell's equations]], any electrical current will produce a magnetic field, and it is this field that is measured. The net currents can be thought of as [[dipole|current dipoles]],<ref name="HämäläinenHari1993" /> i.e. currents with a position, orientation, and magnitude, but no spatial extent.{{dubious|reason=No such thing, currents are at least one-dimensional and run in a loop (Kirchoff's current law) or end at a charge sink (capacitor) which doesn't apply here.|date=March 2018}} According to the [[right-hand rule]], a current dipole gives rise to a magnetic field that points around the axis of its vector component.

To generate a signal that is detectable, approximately 50,000 active neurons are needed.<ref>{{cite book | vauthors = Okada Y | year =  1983 | chapter = Neurogenesis of evoked magnetic fields | chapter-url = https://books.google.com/books?id=7x3aBwAAQBAJ&pg=PA399 | veditors = Williamson SH, Romani GL, Kaufman L, Modena I | title = Biomagnetism: an Interdisciplinary Approach | location = New York | publisher = Plenum Press | pages = 399–408 | isbn = 978-1-4757-1785-3 }}</ref> Since current dipoles must have similar orientations to generate magnetic fields that reinforce each other, it is often the layer of [[pyramidal cell]]s, which are situated perpendicular to the cortical surface, that gives rise to measurable magnetic fields. Bundles of these neurons that are orientated tangentially to the scalp surface project measurable portions of their magnetic fields outside of the head, and these bundles are typically located in the [[sulcus (neuroanatomy)|sulci]]. Researchers are experimenting with various [[signal processing]] methods in the search for methods that detect deep brain (i.e., non-cortical) signal, but no clinically useful method is currently 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 nerve system.