Limitations in hardware design in impedance imaging.

The collection of data suitable for impedance imaging is a well defined task. Once the number of electrodes is chosen, it is possible to specify the number of independent measurements which must be made. Having done so, a data collection system can be designed; preferably with the view to both maximising the speed of data collection and minimising the noise on the measurements. The former is desirable to eliminate aliasing when taking measurements on regions in which the conductivity varies with time, the latter to ensure maximum image quality. When designing such a system many practical problems become apparent. Some are a result of the electrical components used. In principle these can be overcome, although in practice they will always be important. Other problems arise from the nature of the measurements and the way in which they must be taken. These problems do not depend on how the hardware is implemented. They impose fundamental constraints on the quality of the measurements. The problems in the design of a data collection system are considered here. The design is analysed at the functional rather than electronic level, so the results are of general use. Factors considered include the number of measurements, speed of data collection, noise, bandwidth, isolation, common mode feedback, dynamic range, and quantisation.