Bio-impedance active electrode for in vivo measurement

A multifrequency serial EIT system

A multifrequency (1 kHz-1 MHz) serial electrical impedance tomography (EIT) system has been developed. It is based on 16 active electrodes and can be extended up to 32. Each active electrode can be programmed for current driving and for measuring either the injected current or the voltage difference between adjacent electrodes, and includes calibration facilities. Real and imaginary parts of the impedance are obtained by applying a parametric identification method (extended Prony), but other techniques are easily adaptable. Image reconstruction is carried out using the Sheffield filtered back-projection algorithm. Characteristic frequency images are under development and should be of great interest to distinguish between normal and tumorous tissues.

Broadband quasi-differential multifrequency electrical impedance imaging system

Dynamic and multifrequency imaging methods have been demonstrated both theoretically and experimentally. Multifrequency methods are able to produce images of static structures inside the measured object. Data collection systems, however, are affected by errors due to their non-ideal frequency behaviour. If the frequencies used in the measurement were close enough, the system would behave in almost the same way. In this case, however, the impedance change displayed by biological tissues is small, so the situation is similar to dynamic imaging. We call this method the quasi-differential imaging method. We have designed and built an instrument able to apply signals from 1 kHz to 1 MHz, with frequency increments of 10 Hz. Patient interface circuits and demodulators were designed to display a flat response in the full frequency range of operation. Signals are digitized with 16 bit resolution and sent to the host computer using a high-speed serial interface. This allows a maximum measurement speed of about 8 images/s. All the system parts were full characterized out of the system and the results of these measurements are given as an indication of the limits of its use as a quasi-static imaging or quasi-differential imaging data collection system.