Tangential cardiomagnetic field measurement system based on double relaxation oscillation SQUID planar gradiometers

We present a 62-channel double relaxation oscillation SQUID (DROS) planar gradiometer system to measure magnetocardiogram (MCG) signals tangential to the chest surface. The DROSs were fabricated using Nb/Al-oxide/Nb junctions, and provided flux-to-voltage transfers of larger than 1 mV/Φ₀ typically, which is large enough for direct readout by room-temperature flux-locked loop (FLL) circuits. The analog FLL circuits and digital control circuits are made in a single printed circuit board. The planar gradiometers are integrated, series-connected first-order pickup coils with a baseline of 4 cm. White noise level of the planar gradiometer is about 1.5 (fT/cm)/√Hz or 6 fT/√Hz, operated inside a magnetically shielded room. The planar gradiometers were arranged to measure dB_x/dz and dB_y/dz simultaneously. The covering area of the 62 channel system is 162 mm × 162 mm, which is smaller than the sensor coverage area of vertical measurement systems, but large enough to measure the essential MCG field distribution in a single position measurement.