is the solid angle subtended by the surface element dS at r, as viewed from the observation point r0 (Barnard et al. 1967a; Barnard et al. 1967b; Geselowitz 1967). This equation shows how $ at each point r0 in V depends upon the integral of $ over each tissue boundary surface S, and that the surface contributions are of the same form as a surface dipole layer.

In numerical implementations of (5.4), the basic approach is to discretize the surface with a set of triangular elements, and evaluate the surface integral as a discrete sum. Figure 3 shows surface meshes for this purpose. In setting up the sum, the potential on the surface may be expressed in terms of either the potentials at the corners, or the potentials on the faces. The former is faster computationally because the number of corners is approximately half the number of faces (Barr et al. 1977). It also allows an improvement in which

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