The Effects of Torso Geometry on Magnetocardiographic Lead Fields

A study of the effects of the torso on the sensitivity of magnetocardiograms (MCG´s) is performed; comparisons with the effects on electrocardiograms (ECG´s) are made. The effects are determined by comparing MCG or ECG lead fields (LF´s) in a realistically shaped computer model of the human torso with the LF´s that the same MCG or ECG measurements have when made on a semi-infinite volume conductor. Detailed data concerning the effects of the external torso boundary and lung regions on MCG and ECG LF´s at various points in the ventricular region are presented. It is found that the boundary has effects on MCG LF´s that are equal to or greater than the effects on ECG LF´s. The effects of the lung regions on MCG LF´s are found to be smaller than the effects of the boundary and to be equal to or greater than the effects of the lung regions on ECG LF´s. On the basis of these results, it is concluded that MCG´s have no advantage over ECG´s in being able to simply and´accurately determine the nature and location of sources in the heart. In addition, the MCG LF´s are compared with magnetic heart vector (MHV) LF´s, i.e., the LF´s that measure only the magnetic dipolar components of the sources in the heart. The MCG LF´s are found to differ significantly from the MHV LF´s; this indicates that accurate measurements of the MIHV in a real torso would be difficult to make.