Using voltage sources as current drivers for electrical impedance tomography

In multi-frequency electrical impedance tomography (EIT) systems it is much easier to design voltage sources than current sources. It is also at times desirable to operate a voltage driving system as if it were a current driving system. This can be done by adjusting the individual voltage sources until a desired current pattern is obtained. Questions remain regarding the form of the adjustment algorithm and the circumstances under which it will converge. Through simulation and experimentation we have developed a simple algorithm which functions satisfactorily in most practical situations. We have also investigated its theoretical limits for convergence. Simulations showed that convergence, is reached in all cases with little (0.1%) or no measurement noise. With moderate noise (0.5%) our algorithm failed to converge in 17 out of 6000 runs, while more significant measurement noise (1%) resulted in convergence in only 7 out of 6000 runs. Experiments with a 16-channel EIT system converged in all cases on volunteer arms, but failed some of the time in saline, with the number of successful runs decreasing with frequency (73.3% at 10 kHz, 71.1% at 125 kHz and 15.5% at 750 kHz), suggesting a possible link to measurement error.