28-CHANNEL HYBRID SYSTEM FOR NEUROMAGNETIC MEASUREMENTS

This paper describes progress toward the development of a 28-multichannel system for neuromagnetic measurements. A novel "hybrid" design consisting of 16 first-order axial gradiometers and 12 first-order planar gradiometers was chosen, which optimize the use of the available cylindrical volume of the dewar tail. This configuration mantains the symmetry of the detected pattern with respect to rotation of a biomagnetic source located under the center of the array and features a localization power considerably better than an array of all first-order planar gradiometers. The detecting array permits simultaneous magnetic measurements over a circular scalp region of 16 cm diameter. The magnetic sensors used are Nb/PbAuIn dc-SQUIDs fabricated at the IBM. The devices incorporate resonant damping resistors shunting the inductance, resulting in smooth flux-voltage characteristics and, consequently, very low noise figures in a flux-locked loop configuration. A simple and low cost electronic system has been designed and fabricated for the dc-SQUID sensors, consisting of a compact head mounted on top of the cryogenic probe and a control unit, where all the dc remote controls are performed. The dynamic range of 10(7) square-root-Hz and the bandwidth of 50KHz are specifically designed to work in multichannel dc-SQUID instrumentation.