A 31-CHANNEL SQUID SYSTEM FOR BIOMAGNETIC IMAGING

A modular multichannel SQUID-system, in which every single channel can be optimized or replaced individually, is presented. The DC-SQUIDs ba sed on the materials NbN/MgO are prepared by thin film technology and show noise values below 10muPHI0/square-root Hz. A simplified way of c oupling the modulation and feedback current directly to the coupling c oil is realized. The complete SQUID module including the superconducti ng shield was miniaturized down to a diameter of 5mm. The gradiometers are wire wound and an as made balancing better than 10(-3) is achieve d. The cryogenic system was optimized with respect to low vibrations a nd low helium boil off rate. Simple conductive paint with precisely ad justed surface resistivity is used for RF-shielding. The complete SQUI D-electronic of one channel has been realized on one single board and uses a new bias modulation scheme to completely suppress intrinsic 1/f noise. The noise level of the complete system is below 10fT/square-ro ot Hz. Biomagnetic measurements of the human heart and brain are prese nted Single current dipole reconstructions and current density imaging techniques can be used to find the underlying sources. Using a specia l coil positioning system an overlay of the functional current images with morphological MR-images can be carried out.