A Whole-Head SQUID System in a Superconducting Magnetic Shield

Sensitivity of a SQUID system in a superconducting magnetic shield is studied. The SQUID sensors are made of SNS(superconductor/normal metal/superconductor) junctions. Sensitivities of the SQUID system of SNS junctions in a superconducting magnetic shield are more than 100 times better than those of the same SQUID system in a magnetically shielded room of permalloy. The best sensitivity of the SQUID system of SNS junctions is 0.9 fT/(Hz)
^-1/2
at 100 Hz in a superconducting magnetic shield. We performed a experiment of somatosensory evoked magnetic held of human brains to study the N20 component at 20 ms after stimulation. We estimated locations in the brain of the evoked current dipoles for N20 by a extension of a radar theory named MUSIC(multiple signal classification approach). The locations of the evoked current dipole for N20 estimated by MUSIC is confirmed exactly the same as the locations of the current dipole estimated by the traditional iterative least-squares fitting. This is the best agreement between the MUSIC estimation and the conventional estimation of current dipoles to our knowledge.