ECG: body surface mapping

Principle

Body surface mapping of the electric activity of the heart refers to the use of many recording sites (>64) arranged on the body so that isopotential surfaces could be computed and analyzed over time. Extensive computer algorithms, applied in experimental cardiologic research, calculate these isopotential surfaces.

Fig. 1 Front view of a torso phantom with 138 electrodes (63 on the left anterior chest wall).

The main objective of body surface potential maps (BSPMs) is to calculate with complicated mathematics the underlying electric sources approximated by equivalent current dipoles (ECDs). There are many solutions of this calculation, in terms of number of dipoles, their time-varying positions, directions and magnitudes. This problem is called the backward problem. In contrast, the forward problem (calculating the potential distribution on the surface of the torso) has an unique solution. The theory of the forward and backward problem holds for any electrical source in the body and is, in addition to the heart, most advanced developed for brain activity (see Magnetoenceplalography (MEG)).

The backward problem is piece wise solved. For instance, by focusing to a specific temporal window and restricting the area of location, one of the dipoles can be found. The solution can suppose electric isotropy of the heart and the surrounding torso tissues. A refinement is reached by supposing anisotropic heart models (Fig. 2). The myocardial anisotropy can be determined by a heterogeneous 3D matrix from segmented magnetic resonance images.

The most relevant dipoles are located in the septum, apical area, left ventricular wall or right ventricular wall. The solution of the apical source is most dependent on the anisotropy of the torso tissues.

Fig. 2 Left Isocontour lines (0.5 mV resolution). Solid lines positivity, dashed lines negativity, zero line is dotted. The arrow indicates the ECD calculated with a BEM model. Right Contour plot of the BSPM in frontal view for a z-oriented (vertical) apical ECD (FEM). Potentials in mV and distances in cm.

More Info

The current and potential distributions in the torso can be computed using ECDs decomposed in the orthogonal x, y, z direction, or with a 3D algorithm. Solution can be based on the finite element method (FEM) or boundary element method (BEM). FEM is a volume-discretisation method of numeric calculus applied to a set of equations in a 3D-matrix (the elements). BEM is a numerical method of solving linear equations formulated in the boundary integral form. Conceptually, it works by constructing a "mesh" over the surface (with a small surface/volume ratio to be effective). The mesh is often a set non-uniform triangles (triangular segmentation), depending on the local curvature of the surface.

Often BSPM is combined with magnetocardiography. Together they give better solutions of the ECDs.