Frequency marked electrodes in electrical impedance tomography

Abstract

In Electrical Impedance Tomography current is injected into a body and voltage is measured to estimate the impedance or conductivity distribution inside the body. In many implementations, the current is injected through a pair of electrodes while the other electrodes are measuring the voltage. Then, the system continues to the next pair of electrodes and injects current, while measuring the voltage with the remaining electrodes. In some systems, two sets of electrodes are used, one for voltage measurements and the other set for current injection. We propose a method of injecting current through all of the current injecting electrodes simultaneously, and measuring all of the resulting voltages at once. Each current injecting electrode is marked by injecting a unique frequency for that electrode. At the voltage measurement electrodes, the voltage related to each current injecting electrode is isolated by using a Fourier transform. This method is beneficial in instances where tissue impedance is changing fast. The underlying assumption is that the impedance is changing from one measurement to another but slowly enough to be considered constant during a single measurement. To ensure that the electrical properties of the tissue are similar across the frequency band, the injected frequencies are chosen in a range between the alpha and beta dispersions for the examined tissue. We analyze a case of a rapidly changing tissue and compare simulation results between simultaneous and sequential current injections. We also show results from a proof-of-concept system applying this technique. © Springer-Verlag 2007.