Electrode modelling: The effect of contact impedance

The most realistic model for current-to-voltage measurements of electrical impedance tomography is the complete electrode model which takes into account electrode shapes and contact impedances at the electrode/object interfaces. When contact impedances are small, numerical instability can be avoided by replacing the complete model with the shunt model in which perfect contacts, that is zero contact impedances, are assumed. In the present work we show that using the shunt model causes only a (almost) linear error with respect to the contact impedances in modelling absolute current-to-voltage measurements. Moreover, we note that the electric potentials predicted by the two models exhibit different Sobolev regularity properties. This causes, in particular, different convergence rates for a widely used finite element approximation of the potentials. The theoretical results are backed up by two-dimensional numerical experiments.