Construction of tangential vectors from normal cardiac magnetic field components

A tangential vector construction method is proposed that reduces the number of SQUID sensors needed to obtain tangential cardiac magnetic field components compared to measurement of the actual components. Tangential vectors calculated from measured normal components have a spatial distribution that is qualitatively similar to the distribution of the actual vectors. In a feasibility study, we compared these calculated and actual tangential components through a simple simulation and measured data. In the simulation, a horizontally layered conductor in the half space was used to model the torso, and equivalent current dipoles were used to model the electrophysiological sources of the heart. We used two different SQUID systems to measure the normal component (Bz) of an cardiac magnetic field, and the tangential components (Bx and By). Each component was measured at 64 points with an 8×8 array from a 175×175 mm area of the chest. Both our simulated and measured results showed a qualitative similarity between the calculated and actual tangential components. Thus, from a conventional measurement of the normal component, we can extract equivalent information about the heart that enables three-dimensional vector magnetocardiogram measurement