A method of using a magnetometer system to analyse the magnetic field of a region of a subject's body is disclosed. The method comprises using one or more detectors to detect the time varying magnetic field of a region of a subject's body, filtering a signal or signals from the one or more detectors using a filter or filters, and using the filtered signal or signals to analyse the magnetic field generated by the region of a subject's body. The filter or filters is configured to attenuate noise in the signal or signals that is synchronised with motion of the region of the subject's body such as ballistocardiographic noise.
