A novel 16-electrode impedance imaging system

We describe here a 16-electrode biomedical impedance imaging system, incorporating some novel techniques aimed at improving the quality of the acquired data, as well as providing facilities for multi-frequency studies. One aim of the design was real-time scanning; a complete frame of data is produced every 1/16 second. Actual display of images at this rate is not possible with our present computing facilities, but simple back-projection reconstructions can be displayed at 8 frames per second. Instead of CW excitation, current drive waveforms are stored in EPROM, allowing different frequencies and composites of several frequencies to be generated. A novel method of overcoming electrode offset potentials is incorporated. This is achieved by (a) ensuring that the current drive is zero at the instant of switching the electrode allocation, and (b) using a “clamp” in the receiver circuit immediately after switching the electrode allocation. Current drive is zero during the clamp interval. Data is acquired directly from a high-speed ADC, i.e. no demodulation is attempted. Any demodulation into in-phase or quadrature components is carried out in software if required. The unit is interfaced to an Archimedes series computer, and has been shown to operate satisfactorily with a variety of models from the original A310 up to the recent 33 MHz A5000