64-Channel Second-Order Planar Gradiometer System for Magnetocardiograms

A 64-channel planar gradiometer system was developed to measure magnetocardiogram (MCG) signals. The planar gradiometers are second-order planar gradiometers with a baseline of 50 mm, fabricated from Nb thin films on Si wafers using thin film processes. The SQUIDs were fabricated from Nb/Al-oxide/Nb junctions, and a double relaxation oscillation SQUID (DROS) scheme was used to have a large flux-to-voltage transfer, typically about 1 mV/Phi₀. The planar gradiometers were arranged to measure magnetic field components tangential to the chest surface, which has a field distribution peak just above the current dipole. The diameter of the sensor coverage area is 190 mm, which was proved to be large enough to provide MCG field distribution in a single measurement. Inside a thin-wall shield room, the white noise level of the planar gradiometer system is about 8 fTradicHz. The 64-channel system was operated in a thin-wall shield room, and MCG signals were measured.