In the analogous three-dimensional version, one calculates the solid angles about the centroids of each tetrahedron, which will sum to for interior points and 0 for exterior points. We can exploit this idea further to complete one last bit of preprocessing before solving the system. Since our solution domain, the head or thorax, is not homogeneous, we need to assign conductivity values to the various tissue regions.
In the FE formulation, we may need to assign a conductivity tensor to each element of the model. This is usually done by using a table look-up scheme in which each element is given a number according to the type of tissue it contains. During processing, the corresponding value from a table of conductivity tensors is used to calculate the global stiffness matrix, making it a trivial matter to change the various conductivities without changing the geometry or any other parameters of the model.