An electronic second-order gradiometer for biomagnetic applications in clinical shielded rooms

A modular 55-channel magnetometer for biomagnetic studies in clinical shielded rooms is described. Seventy-six superconducting quantum interference device (SQUID) magnetometers in three levels are used to electronically form first-order or second-order gradiometer configurations. The magnetometers are completely integrated on 7.2-mm*7.2-mm chips. Additional positive feedback is used to increase the gradient of the V- Phi (voltage-flux) characteristics to dV/d Phi approximately=1 mV/ Phi /sub 0/, and a novel scheme called bias current feedback reduces the bias current sensitivity to theta Phi / theta I/sub b/ approximately=0.02 Phi /sub 0// mu A. This allows one to couple the SQUIDs directly to simplified feedback electronics without helium-temperature impedance-matching circuits or flux modulation techniques. The magnetometer noise level is typically 1.4 fT/ square root Hz at 1 kHz and 3.2 fT/ square root Hz at 1 Hz, measured inside a Pb/Cryopern shield. A seven-channel prototype module has been tested successfully inside a clinical shielded room. A typical first-order gradiometer noise level of 2.2 fT/ square root THz at 1 kHz and 7 fT/ square root Hz at 2 Hz has been achieved.<>