Experimental assessment of phase magnitude imaging in multifrequency EIT by simulation and saline tank studies

Multifrequency EIT (MFEIT) data can be used to form dual-frequency images that display the change of complex resistivity with frequency relative to a selected reference frequency. It has only been possible to calculate the complex resistivity at the image frequency from MFEIT images by assuming some a priori information, such as a homogeneous, resistive reference or a resistivity described by a single dispersion Cole model. The purpose of this paper is to introduce an alternative method of image formation, referred to as "phase magnitude imaging", which enables the complex resistivity of tissue to be obtained independently of the tissue model and without the need for a homogeneous reference. It was expected that the major obstacle to practical application of this technique would be instrumentation errors that affect the measurement of phase, To assess these effects, a cucumber cortex was imaged in a saline-filled tank using an HP4284A impedance analyzer. Cole parameters determined from the phase magnitude images differed by up to 300% from results of the simulation, These errors were reduced to less than 15% by using reciprocity data to partially correct for the influence of stray capacitance on the voltage measurement.