The influence of inhomogeneous volume conductor models on the ECG and the MCG.

The authors investigated the influence of human body inhomogeneities such as the lungs, blood masses and the skeletal muscle layer on the electrical body surface potential and the magnetic field. The surface potentials and magnetic fields are calculated using a boundary element method. As a rule the blood masses have a large influence on both potential and magnetic field amplitude as well as on the potential and magnetic field map orientation, but the influence on the topology of the map is less in the electric case than in the magnetic case. The single-dipole reconstruction was applied to estimate the error caused by neglecting inner inhomogeneities in source localization. The neglect of lungs and blood masses results in a localization error of less than 1 cm in the electric case but more than 1 cm for deep sources at the posterior side of the heart in the magnetic case. The authors tried to assess the influence of the skeletal muscle layer by both an analytical two-layered anisotropic half-space model and the torso extension method. The skeletal muscle layer causes a smoothing effect on the electrical surface potential and to a lesser extent on the magnetic field, leading to an overestimation of the actual source depth of about 1-2 cm. In principle this can be reduced by taking data from all over the thoracic surface. The authors designed experiments for simultaneous measurement of body surface potential and extracorporeal magnetic field from the same subject. The evaluation of data from two patients showing Wolff-Parkinson-White syndrome has shown that localization results from electric potential data and magnetocardiographic data are consistent.