Editing Magnetoencephalography (section)

== Magnetic shielding ==
Since the magnetic signals emitted by the brain are on the order of a few femtoteslas, shielding from external magnetic signals, including the [[Earth's magnetic field]], is necessary. Appropriate [[magnetic shielding]] can be obtained by constructing rooms made of [[aluminium]] and [[mu-metal]] for reducing high-frequency and low-frequency [[signal noise|noise]], respectively.
[[Image:MSR layered door.jpg|thumbnail|250px|left|Entrance to MSR, showing the separate shielding layers]]

=== Magnetically shielded room (MSR) ===
A magnetically shielded room (MSR) model consists of three nested main layers. Each of these layers is made of a pure aluminium layer plus a high-permeability [[Ferromagnetism|ferromagnetic]] layer, similar in composition to molybdenum [[permalloy]]. The ferromagnetic layer is supplied as 1&nbsp;mm sheets, while the innermost layer is composed of four sheets in close contact, and the outer two layers are composed of three sheets each. Magnetic continuity is maintained by overlay strips. Insulating washers are used in the screw assemblies to ensure that each main layer is electrically isolated. This helps eliminate [[radio frequency]] radiation, which would degrade SQUID performance. Electrical continuity of the aluminium is also maintained by aluminium overlay strips to ensure [[alternating current|AC]] [[eddy current]] shielding, which is important at frequencies greater than 1&nbsp;Hz. The junctions of the inner layer are often electroplated with silver or gold to improve conductivity of the aluminium layers.<ref>{{cite journal | vauthors = Cohen D, Schläpfer U, Ahlfors S, Hämäläinen M, Halgren E | title = New Six-Layer Magnetically-Shielded Room for MEG | journal = Proceedings of the 13th International Conference on Biomagnetism | date = August 2002 | volume = 10 | pages = 919–921 | location = Jena, Germany | publisher = VDE Verlag | s2cid = 27016664 | url = https://pdfs.semanticscholar.org/92c5/d54611a8c35f262f82bd81b1c6223d71d29f.pdf | archive-url = https://web.archive.org/web/20200803165556/https://pdfs.semanticscholar.org/92c5/d54611a8c35f262f82bd81b1c6223d71d29f.pdf | url-status = dead | archive-date = 2020-08-03 }}</ref>

=== Active shielding system ===
Active systems are designed for three-dimensional noise cancellation. To implement an active system, low-noise fluxgate [[magnetometer]]s are mounted at the center of each surface and oriented orthogonally to it. This negatively feeds a [[direct current|DC]] amplifier through a low-pass network with a slow falloff to minimize positive feedback and oscillation. Built into the system are shaking and [[degaussing]] wires. Shaking wires increase the magnetic permeability, while the permanent degaussing wires are applied to all surfaces of the inner main layer to degauss the surfaces.<ref name="cohen1" /> Moreover, noise cancellation algorithms can reduce both low-frequency and high-frequency noise. Modern systems have a [[noise floor]] of around 2–3&nbsp;fT/Hz<sup>0.5</sup> above 1&nbsp;Hz.