SQUID imaging—reconstructing bioelectric current distributions from measurements of their magnetic fields

With the development of superconducting quantum interference device (SQUID) detectors for measuring very weak magnetic fields, it has become possible to map those fields generated by electrical activity in humans. New medical diagnostic tools are now emerging using this technology. We describe the historical development of these diagnostic tools. We begin with some experiments which predate the SQUID. Next we give a brief intuitive explanation of SQUID operating principles. We continue the historical descriptions with developments in magnetoencephalography and magnetocardiography. Finally, we move to a newly developing area: SQUID imaging. In SQUID imaging, SQUID measurement and computer processing are combined to provide images of the currents' sources. Analogies to optical imaging systems are shown, and modifications of common characterization tools (PSF and OTF) are presented.