Brief background
Bio-impedance technique is a non-invasive imaging and monitoring technique, which is based on the electrical properties of biological tissues. Physiological or pathological conditions that cause changes in tissue conductivity, or changes in conductivity distribution, are good candidates for bio-impedance monitoring. In impedance cardiography, the cardiac functionality is determined from measurements of the electrical impedance of the thorax. During the cardiac cycle, blood perfusion causes great changes in the thorax conductivity distribution, most prominently because of the heart conductivity changes. While at the time of systole, the relatively high conductive blood is ejected from the ventricles towards peripheral organs, increasing the total resistivity of the heart, an opposite process occurs during diastole. Bio-impedance technique is therefore proposed as a monitoring tool for cardiac perfusion functions. There are two main current applying approaches used for impedance technique - injecting the current directly via surface electrodes, and inducing it using a surrounding coil, with the former being much more practiced. There are benefits and limitations for each approach, and a comprehensive comparison between them can be found in the relevant literature, yet the induced-current approach is not widely used in practice, and is not familiar to most research groups.
Research progress
(a) A comparison between two current applying approaches, injected vs. induced, was performed in a two-dimensional model of the thorax, for the medical application of impedance cardiography. It has been shown, that although the resulted current sources and potential maps are very different, the sensitivity to volume changes of the LV is very similar: we can expect a 5%-7% change due to the changes of blood content and geometrical shape of the LV during the cardiac cycle, in both approaches.
Apparently, for this medical application, there is no advantage using the induced current approach, while in brain medical applications we found a great difference in the sensitivity, in favor of the induced current. This can be explained due to the anatomical difference between the thorax and the head. The brain is protected by the low-conducting skull that prevents penetration of current lines into the brain when using injecting electrodes. Using the induced currents causes internal current sources, thus providing information from deeper regions inside the head. In trying to apply impedance technique to the thorax, there is no such masking effect by low-conductive tissues; therefore, there is no advantage to inner current sources.
Nevertheless, when examining the behavior of the surface potentials, it appears as if the induced current approach does suggest an advantage: since the spatial slope is more moderate, the measurements will be less prone to errors due to the measuring electrodes movements. While in the injected current approach, a potential change measurement may occur not from the volume change of the LV, but by a diminutive movement of the electrodes, the measured potential changes in the induced approach (at a single electrode) are more likely to result from the real measured phenomenon.
(b) A reconstruction algorithm, based on the modified Newton-Raphson algorithm, was developed for induced current EIT and studied in theoretical 2-D geometry representing a human thorax. The finite-volume method was applied for the discretization of the physical domain, resulting in a symbolic representation of the Jacobian matrix, which is accurate and fast to construct. Several system configurations, differing in the number of excitation coils and electrodes, were simulated, and the performance in thoracic imaging was studied. It was found that a 6-coil system shows a significant 40% improvement of conductivity values reconstruction over the 3-coil system (an error of 2.06 Ω -1 compared to 3.44 Ω -1). A number of 32 electrodes was found to be sufficient, being the smallest number of electrodes to still provide a reasonable performance (only 4.2% degradation in average conductivity error compared to the maximum possible 106-electrode system).
(c) A preliminary 3D forward problem solver was developed. For imaging purposes, it appears that due to computer resource limitations; only parameterized imaging is feasible, i.e., using limited number of parameters to describe the geometry and conductivity distribution of the thorax cavity, and using the Newton-Raphson algorithm for their optimization.
