Development of SQUID magnetometer with direct-feedback noise cancellation for magnetocardiogram without magnetically shielded room

Abstract

Superconducting quantum interference device (SQUID) magnetometers have been widely used to perform biomagnetic measurements. In biomagnetic measurements such as magnetocardiograms (MCGs) and magnetoencephalograms, a magnetically shielded room (MSR) is used for noise reduction. However, because the MSR is expensive and heavy, the environments in which it can be used are restricted. In addition, MCG measurements obtained outside the MSR have a poor signal-to-noise ratio. We developed a wide dynamic range SQUID magnetometer that uses the flux-quanta counting (FQC) method with direct-feedback noise cancellation for MCG measurements and does not require the MSR. The FQC method is used to expand the dynamic range of the SQUID magnetometers to enable MCG measurements outside the MSR in the presence of large magnetic noise. The noise cancellation system uses two SQUID sensors; one for sensing and another one for reference. Both the sensing and reference signals are fed back to the sensing SQUID. This makes it possible to reduce the magnetic noise. We demonstrated the dynamic range, noise spectrum of the developed SQUID system, and MCG waveforms measured outside the MSR when the noise cancellation was set to ON or OFF. The noise cancellation factor of the developed SQUID system ranged from 10 to 20 dB. In MCG waveforms with noise cancellation, the QRS complex and T wave were clearly observed. The results show that the developed SQUID system clearly measures MCG waveforms in a condition involving large environmental magnetic noise.