Resolving multiple simultaneous sources of visual evoked magnetic fields

Due to their extremely weak intensity, biomagnetic fields are usually detected by means of superconducting interference devices (SQUID) which are currently the most sensitive magnetic sensors available. To reduce construction and running costs, extensive efforts have been dedicated to the development of High-Tc SQUIDs working at liquid nitrogen temperature and suitable for biomagnetic applications, but the results up to now are only partially satisfactory. Optically pumped magnetometer (OPM) technology, although known for many years, has recently emerged as a promising alternative to SQUIDs, at least for applications with more moderate signal-to-noise requirements. Since OPMs were presented to the biomedical community, a controversy has surrounded their proposed usefulness for different biomagnetic applications. This contribution compares SQUID and OPM methods based on their physical principles: for example by the fact that a SQUID measure the flux through the pick-up-coil area, while the OPM is sensitive to a volume averaged magnetic field component. At the implementation level, the most significant differences between SQUID and OPM are, 1) the absence in OPMs of cryogenic and vacuum parts reducing the gap between source and sensor, and 2) the need for more complex read-out electronics of the OPM. The intrinsic insensitivity against RF-interference of the OPM reduces the shielding requirements and may ease the introduction of biomagnetic systems into noisy environments. Full technological maturity is by far not yet achieved, thus leaving a large improvement potential, which may eventually allow the OPM to compete with the SQUID even in high-resolution applications.