Probing deep tissue organ of the human body by electrical impedance technique

Abstract

Electrical Impedance measurements of biological tissue has potential applications in many physiological investigations. Two existing methods, Tetrapolar Impedance Measurement (TPIM) and Focused Impedance Method (FIM) are typically applied from one side of the body. So they have a limited depth sensitivity which may be used to study objects at shallow depths. However, for studying objects at deeper regions of the body, particularly of the human thorax, a different approach needs to be taken. The present work used a novel six electrode configuration in which two current electrodes, in the form of vertical strips, were fixed at right and left extremes of the thorax, below the armpits. Two potential electrodes were placed on the front in a horizontal plane at mid-level of the strip electrodes with a certain separation between them. Two other electrodes were placed at the back in the same horizontal plane, and at corresponding symmetrical positions. Each of the two symmetrical electrodes in the front and back were shorted together or connected through different resistors. The two connected points were taken to the potential measuring terminals of the impedance measuring instrument. In the present work the effects of the different configurations of electrodes were studied first using COMSOL Multiphysics Finite Element software package. 2D and 3D point sensitivities were evaluated using this software package assuming the thorax as an elliptical cylinder which showed that the new 6 electrode configuration with the corresponding front and back electrodes shorted gave the highest sensitivity in the deep regions, although the sensitivities were the highest near the potential electrodes on both sides. Again the value deep inside increases with increasing separation of the potential electrodes. The minimum sensitivity at the deepest point was about 25% of that near the surface for a potential electrode separation of 10cm. On the other hand for one sided TPIM or FIM, the sensitivity keeps on decreasing from the surface and goes down to about 25% at the centre for the same potential electrode separation, and further down on the opposite side. Phantom measurements were then made using a circular shaped made of plastic containing saline and fixing electrodes appropriately on the surface. For the same conductivity of the saline, the sensitivity values were almost identical with the software simulation result. The change in impedance within human thorax model is found smaller for novel six electrode compared to TPIM with software simulation. Therefore, the novel six-electrode configuration may be useful for electrical impedance study of deep organs of the human thorax.