A NOVEL FIELD FOR POSSIBLE USE IN MEDICAL IMPEDANCE TOMOGRAPHY

A single-phase electrical supply is normally used to energize the elec trodes of the belt worn by the subject for medical impedance tomograph y. In the process, the supply is switched from one adjacent pair of el ectrodes to the next, so that all such pairs are energized in turn. Ho wever, ifa three-phase voltage of constant amplitude were to be employ ed, with electrodes uninterruptedly connected to the lines of the supp ly in phase sequence around the periphery, and the current flowing thr ough each electrode were to be simultaneously measured, it will be sho wn that, in principle, data of higher utility would result. This paper examines that possibility, and considers the field spatial harmonics arising in consequence of the electrode geometry. Expressions are obta ined for the magnitude of those harmonics in close proximity to the su rface of the belt, from which the spatial harmonic components of the a pplied field at deeper levels can be calculated. It is shown that if b ar electrodes are employed, the spatial harmonic composition of the fi eld is critically dependent on the ratio of the width of the electrode s to the width of the gaps between them.