Nonideal Volumes, Reciprocal Lead Field

Initially, the lead field is often unknown, but may be found by finite element method (FEM) modeling (Section 6.5) to the extent that the volume dimensions and resistivity distribution are known. The lead field illustrates the sensitivity distribution of the PU electrodes for CC electrodes or biological processes somewhere in the finite volume. 

Because J = aE (isotropic case), we may equally well express u as a function of the two electrical fields  

A lead vector H is determined by the geometry and conductivity distribution of the signal PU system, and is valid for one tissue volume and one set-up with all electrode and source positions and distributions defined and constant. Changing one PU electrode or source position, leaving all other factors constant, results iu another lead vector H. 

Swapping the PU and CC pairs does not change the transfer impedance as long as the system is electrically linear (reciprocity). This is valid for both tripolar and tetrapolar electrode systems. 

---