EXTRACTION OF ELECTRICAL CHARACTERISTICS FROM PIXELS OF MULTIFREQUENCY EIT IMAGES

Computer modelling has shown that electrical characteristics of indivi dual pixels may be extracted from within multiple-frequency electrical impedance tomography (MFEIT) images formed using a reference data set obtained from a purely resistive, homogeneous medium. In some applica tions it is desirable to extract the electrical characteristics of ind ividual pixels from images where a purely resistive, homogeneous refer ence data set is not available. One such application of the technique of MFEIT is to allow the acquisition of in vivo images using reference data sets obtained from a non-homogeneous medium with a reactive comp onent. However, the reactive component of the reference data set intro duces difficulties with the extraction of the true electrical characte ristics from the image pixels. This study was a preliminary investigat ion of a technique to extract electrical parameters from multifrequenc y images when the reference data set has a reactive component. Unlike the situation in which a homogenous, resistive data set is available, it is not possible to obtain the impedance and phase information direc tly from the image pixel values of the MFEIT images data set, as the p hase of the reactive reference is not known. The method reported here to extract the electrical characteristics (the Cole-Cole plot) initial ly assumes that this phase angle is zero. With this assumption, an imp edance spectrum can be directly extracted from the image set. To obtai n the true Cole-Cole plot a correction must be applied to account for the inherent rotation of the extracted impedance spectrum about the or igin, which is a result of the assumption. This work shows that the an gle of rotation associated with the reactive component of the referenc e data set may be determined using a priori knowledge of the distribut ion of frequencies of the Cole-Cole plot. Using this angle of rotation , the true Cole-Cole plot can be obtained from the impedance spectrum extracted from the MFEIT image data set. The method was investigated u sing simulated data, both with and without noise, and also for image d ata obtained in vitro. The in vitro studies involved 32 logarithmicall y spaced frequencies from 4 kHz up to 1 MHz and demonstrated that diff erences between the true characteristics and those of the impedance sp ectrum were reduced significantly after application of the correction technique. The differences between the extracted parameters and the tr ue values prior to correction were in the range from 16% to 70%. Follo wing application of the correction technique the differences were redu ced to less than 5%. The parameters obtained from the Cole-Cole plot m ay be useful as a characterization of the nature and health of the ima ged tissues.