A Whole-head squid system for detecting vector components

We have developed a whole-head magneto-encephalogram system having 65 detection sites for detecting vector components of the magnetic field of the brain. Each site contains three first-order gradiometers, and the three detection coils of the gradiometers are perpendicular to each other. This configuration enables the radial and tangential components at each measuring site to be calculated. The helmet shaped dewar is held on a stand fixed to the floor inside the MSR (magnetically shielded room), and the tilt angle of the dewar is adjusted to fit the head. A square-double washer DC-SQUID having a large transfer function dV/dΦ (of the order of mV/Φ0) is used in the gradiometer. The large dV/dΦ enables the low-noise SQUID to be coupled directly to the pre-amplifier without additional positive feedback (APF). The typical noise of the system in the MSR is less than 10 fT/√ Hz. Linearized output from the flux locked loop circuit is fed into the amplifier-filter unit, and digitized by an A/D converter. The brain function data from 195 channels are stored and analyzed by the workstation.