Tetrapolar Impedance Measurement (TPIM) is a 4-electrode impedance measurement system appropriate for a volume conductor in which current is driven through a pair of electrodes while potential developed across another pair of electrodes is measured to provide a value of transfer impedance. The 8-electrode Focused Impedance Method (FIM-8) consists of two concentric and orthogonal linear TPIM systems with their transfer impedances added for the purpose of localizing the central zone. Detailed 3D sensitivity studies are necessary for proper application of the techniques in specific biomedical applications and most reported work present point sensitivity distributions. The present work mainly focuses on planar average sensitivity in planes parallel to the electrode plane and its variation with depth due to different combinations of electrode separations -both for current drive pair and the potential measuring pair. This was obtained through finite element simulation using COMSOL Multiphysics software for a 40x40x40cm 3 volume. The results give useful information that can be used to design electrode configurations and measurement modalities for various applications.

A noninvasive and radiation free technique for in-vivo measurement of the volume of organs or fluids in the human body is necessary for many clinical applications. Focused Impedance Method (FIM) is a novel technique of electrical impedance measurements which has enhanced sensitivity in a localized region. FIM can sense the change in transfer impedance of an organ within a reasonable depth of the human body using surface electrodes, minimizing contributions from its neighbouring regions. This of course assumes that the impedance properties of the embedded object are different from that of its surrounding tissues. This paper presents a new method for the determination of the volume of an organ within body using dual electrode separations of a concentric 4-electrode FIM configuration. In order to develop this formalism simulated FIM measurements using surface electrodes on a cubic volume conductor with embedded spherical objects were performed using a Finite Element (FE) based simulation software, COMSOL Multiphysics®. For the present methodology, the conductivity of the object with respect to its surroundings and its depth need to be known. The former is obtainable through some primary invasive or in vivo measurements while the latter may be approximated using anatomy. Experimental results on a phantom made up of a cubic tank filled with saline showed that the proposed method can be used to determine the volume of embedded objects to an accuracy of about 5% which is adequate for most physiological measurements. The technique may also find use in geology, oceanography and industry.Bangladesh Journal of Medical Physics Vol.7 No.1 2014 24-33

Focused Impedance Method (FIM) is an innovative and relatively new electrical impedance measurement technique that allows localized measurement of the properties of electrically conducting materials in a volume conductor such as the human body using simple measurement setups. In a previous work, FIM, with dual electrode separation, was used to determine thicknesses of subcutaneous fat layers through experimental study on phantoms and human subjects. The present work, carried out using COMSOL Multiphysics software package, is a finite element simulation study for a similar target object that verifies and extends the results of the previous experimental work. A rectangular box of different heights containing a material of uniform conductivity such as saline together with internally embedded layers of resistive materials of different thicknesses were used for this study. The measured transfer impedance in the FIM method showed marked change with the variation of the thickness of the resistive layer and with electrode separation, showing a point of maximum curvature in the latter. We obtained a calibration curve for the thickness from the electrode separation of this point. This allowed a unique method for the determination of thickness of embedded resistive layers which is more general than what had been done in the previous experimental work. This work will help standardize the application of 4-electrode FIM for determination of the thickness of less-conducting material layer in any finite volume, including determination of fat layer thickness, etc.