As well as generating electrical activity, the action potentials originating in myocardial cells also create magnetic fields which are perpendicular to the electrical current. [Yamada & Yamaguchi 2005]. Changes in perfusion affect the action potential leading to commensurate changes in the magnetic field. Mapping the cardiac magnetic field therefore creates a signal that reflects the extent of change in action potential within the heart. Since ECG and MCG are detecting signals generated by the same electrical activity of the heart, MCG can be considered the magnetic equivalent of an ECG, but with a number of key advantages [Kwong et al. 2013].
Although the magnetic fields are extremely small, a magnetic field map is capable of differentiating between normal and abnormal cardiac behaviour and has been shown in clinical trials to provide useful and different information than ECG over a range of cardiac conditions [Hailer et al. 2005; Park et al. 2005; Steinberg et al. 2005; Tolstrup et al. 2006; Park et al. 2008a; Park et al. 2008b; Kandori et al. 2010]. The reason for this is that a small disruption to the magnetic field, caused by necrosis or ischaemia, impacts and distorts the entire magnetic field and thus has a significant effect on the MCG scan. Once familiar with the outputs, healthcare professionals will be able to identify different cardiac conditions by studying the unique magnetic maps, waveforms and numerical data produced.