Simple electrode configurations for probing deep organs using Electrical Bio-Impedance techniques

Abstract

Tetrapolar Impedance Method (TPIM) and Focused Impedance Method (FIM) are two simple modalities of electrical bio-impedance measurement that could be employed to give useful physiological and diagnostic information of the human body. FIM is based on TPIM but uses a combination of two sets of TPIM, producing a focusing effect, which is useful to localize specific target organs. Most non-invasive electrical bio-impedance measurements based on TPIM and FIM use electrodes on one side of the body, outside the skin surface, which gives a shallow depth sensitivity. The sensitivity decreases with depth so that deep organs like lungs, heart, liver, stomach and bladder are not fully assessed through such measurements. Based on a long experience of studying electrical impedance methods, several qualitative ideas are presented in this article for probing deep organs using a few modified TPIM and FIM configurations. The suggestions are based on visualisations of both equipotentials and a popular sensitivity equation for transfer impedance, but not based on any quantitative analysis. Simulation and phantom studies based on these ideas may produce some practically useful electrode configurations for real life bio-impedance measurements.
 Bangladesh Journal of Medical Physics Vol.11 No.1 2018 P 1-15

Probing for stomach using the Focused Impedance Method (FIM)

“…For the geometry, the origin (0,0,0) has been taken at the lower left corner of the trunk model as shown in Figure 2, which is on the right side of the back of the trunk, in physical perspective. The co-ordinate (x, y, z) of the center of the circular cylinder used to make the stomach model is (19,14,19) in cm, as can be seen from Figures 2 and 3. The zcoordinate for all electrodes were also chosen at 19 cm, the vertical central plane of the stomach.…”

“…Small electrodes, as used in the above work have wide diverging current patterns within the volume conductor. Since the stomach has a vertical height, in order to get more contribution from this 3D stomach, long strip electrodes, as suggested by Rabbani [ 14 ] was also used for the electrode configuration giving the maximum contribution of the stomach using the small cylindrical electrodes (2 cm diameter). The comparison would give valuable insight.…”

“…However, most of the applications attempted so far using FIM targeted measurement from one side of the human body in which contribution of the deep organs were limited, only partial contribution were coming from the regions at shallow depths with respect to the electrodes. In order to probe deeper regions of these organs, some new ideas on the placements of electrodes were suggested [ 14 ] using visualizations based on the concepts of equipotential surfaces [ 15 ] and of sensitivity equations involving current density vectors due to two separate pairs of electrodes in TPIM [ 16 ]. Finite element simulation and phantom measurements using one of these suggested methods targeting the lungs produced promising results [ 17 ].…”

“…Finite element simulation and phantom measurements using one of these suggested methods targeting the lungs produced promising results [ 17 ]. The present work was taken up using another of the novel ideas on electrode placement suggested in the above work [ 14 ] that has the possibility of targeting the stomach. This idea was originally suggested for an asymmetrically placed deep object as shown in a cross sectional diagram in Figure 1 .…”

“…Two pairs of electrodes indicated by I 1 -I 1 ’ and V 1 -V 1 ’ form the first TPIM configuration while I 2 -I 2 ’ and V 2 -V 2 ’ indicate the second TPIM. The average of the two TPIM values make the FIM [ 14 ]. The dashed lines in the left hand figure show the maximum current lines between respective pairs of electrodes, which also includes the potential measuring electrodes, while the dotted lines on the right hand figure show approximate equipotential lines through the potential measuring electrodes.…”