Impedance

An interesting controversy has arisen in the development of instruments for imaging by computed tomography based on electricalimpedancemeasurements.The imagingtechniques(1—3) have arisen from the concomitant development of reconstruction from projections and nonimaging impedance measurements, such as impedance plethysmography. By utilizing low frequency alternating current signals (f < 10 MHz,tominimizereactanceeffects),measurementsofelectrical current density may be carried out with relative safety to the subject. Imaging instruments based on these measurements have been developed recently. Reports of an impedance camera (1,2) and electrical impedance computed tomography (ICT) (3) have appeared. Bates, McKinnon, and Seagar (4) evaluated these reports and found that a basic physical limitation exists that usually prevents the described methods from working. Very briefly, the assumption that projections in the line integral sense are available from the described instruments is untrue. The current density measurements available cannot generally be used to develop a two- or three dimensional current density distribution map without a priori knowledge. MICHAELW.VANNIE