Use of superconducting plates and shells to deflect magnetic noise fields: application to MEG

Insertion of a superconducting plate or shell into a magnetic field causes a surface current distribution which opposes the applied field. Resultant fields near plate edges are higher while those near centers are much lower than applied fields. These principles are used in magnetoencephalography by placing suitably oriented gradiometers at points where noise fields are smallest. Signals from nearby brain wave sources are enhanced because the net signal is a combination of that from the sources and that from its image. The principles of noise deflection and source imaging can be used to provide new approaches to gradiometry