SQUID-Based MCG Measurement Using a Full-Tensor Compensation Technique in an Urban Hospital Environment

Magnetocardiography (MCG) within a magnetically shielded room is now widely used for clinical research studies, but its high cost and immobility impose major obstacles toward its market penetration. As an alternative method, the unshielded MCG measurements are traditionally equipped with the axial second-order gradiometers and three-axial reference magnetometers. In this paper, we evaluate the effectiveness of a full-tensor compensation technique for MCG measurements under a magnetically unshielded urban hospital environment. The full-tensor module with eight low-T _c superconducting quantum interference device magnetometers is used to record three uniform components and five independent first-order gradient tensor components of the environmental field simultaneously. The field and gradient fluctuation are measured to be about 1000 nT and 10 nT/m, respectively. After a full-tensor compensation, the equivalent output noise fluctuation of the axial second-order gradiometer is suppressed from about ±4 nT to ±4 pT under a bandwidth of 0.1-95 Hz. The MCG signal with a signal-to-noise ratio of about 27 dB is achieved. It must be indicated that the noise cancelation performance can be also achieved by adding just one outstanding first-order gradient component Bzz. This method enables the routine use of a magnetically unshielded MCG system in a clinical environment.