Some practical biological phantoms for calibrating multifrequency electrical impedance tomography.

Three groups of materials have been assessed with a Hewlett-Packard 4284A impedance analyser and Sheffield Mark 1 electrical impedance tomography (EIT) system for suitability for calibration of multifrequency EIT systems. They were required to be easy to use, stable over several hours, and have complex impedance similar to biological tissue. The groups were: (i) inorganic materials including barium titanate, polystyrene microspheres and fumed silica, all in aqueous suspension; these had phase angles below 1 degrees and so were unsuitable. (ii) Cucumber in KCl solution. Cucumber cortex had a phase angle of 40 degrees at a centre frequency of 50 kHz. Contrast between the cucumber and bathing solution could be selected by varying the KCl concentration. (iii) Polyurethane sponge immersed in packed red cells. The phase angle of packed cells was about 25 degrees at 1 MHz. Sponge resistivities and permittivities when immersed in packed cells were 5-20% higher than the bathing solution itself, for densities of 2-6.2% w/v. Both the biological materials appear suitable for the intended purpose; system (iii) is inherently more stable, and has capacitance in both bathing medium and test object. If an initial accuracy of greater than about +/- 15% is required, direct measurement with an impedance analyser is recommended prior to imaging studies.