Recording the Distribution of Cardiac Magnetic Fields in Unshielded Earth’s Field

Magnetocardiography (MCG) is a noninvasive technique that allows to measure the cardiac magnetic fields generated by the electrical activity of the heart. Conventionally, the magnetic field distributions around the body surface are recorded by superconducting quantum interference devices (SQUIDs). The SQUID magnetometer usually requires a big cryogenic Dewar vessel and a magnetically shielded room, which makes the SQUID-based MCG system inflexible and expensive. In this paper, we introduce an unshielded MCG system based on optically-pumped magnetometers (OPMs). By operating the magnetometers in a gradiometric mode, together with magnetic-field stabilization methods, the magnetic field noise from surroundings is suppressed and a noise floor of $\sim 10{\text{fT}}/({\text{cm}} \cdot \sqrt {{\text{Hz}}} )$ is achieved under a bias field of ∼54, 500 nT. With the MCG system, we successfully observe the MCG signal including P-wave, the QRS complex and the T-wave and further measure the MCG signal distribution around the chest in unshielded environment.