Development of an active magnetic screen to allow a biomagnetometer to be used in an unshielded environment

Biomagnetic signals are up to six orders of magnitude smaller than the usual level of urban magnetic interference and noise. SQUID gradiometers and magnetically shielded rooms (MSRs) are therefore commonly used to reduce the disturbances and to record magnetic physiological signals. MSRs are very expensive and are inconvenient for many patients due to their narrow dimensions. We have developed an active magnetic shield with the aim of compensating for the urban magnetic field and its first-order spatial field gradient to allow biomagnetic measurements outside an MSR. Two separate feedback circuits measure the magnetic field and the field gradient and feed two sets of compensation coils. We tested two field gradient compensation configurations which made use of different gradiometer designs and different separations between the reference and measurement sensors. The two types of shield were found to achieve interference suppressions of 43 and 26 dB, respectively. We were able to measure a human magnetocardiogram (MCG) signal in an open laboratory, using the less effective shielding configuration.