Fig. 2.

Schematics of MEG instrumentation. (a) A single-channel axial gradiometer and associated SQUID inside a dewar filled with liquid helium. Bottom depicts the sensor array of a 306-channel MEG helmet where each sensor unit contains two orthogonal planar gradiometers and one magnetometer. (b) Flux transformer and SQUID. The external magnetic field generates in the pickup coil (a part of the flux transformer that can take a shape of a magnetometer, or an axial or planar gradiometer) a current that flows in the superconducting loop where one part (input coil) then couples by means of a magnetic field into the SQUID. The electronics monitors the state of the SQUID. Modified from Hari and Puce (2017). (c) Axial and planar gradiometers. An axial gradiometer detects the largest signal a couple of centimeters away from the site of the local source (arrow), whereas the planar gradiometer detects the maximum signal just above the source. Note, however, that the signal in the planar gradiometer depends strongly on its orientation; be it rotated by 90 degrees, the obtained signal would in this case vanish. Thus, devices using planar gradiometers have two orthogonal planar gradiometers at the same sensor unit (see the bottom left insert in (a)). Modified from Hari and Puce (2017) with the permission of Oxford University Press.