Switch Matrix Logic Implementation of Electrical Impedance Tomography Measurement System

Abstract

Electrical Impedance Tomography (EIT) is non-invasive technique that has the potential to be used in clinical environments for lung ventilation monitoring, cardiac and brain imaging applications. EIT has many advantages in terms of simplicity, cost and the absence of ionising radiations compared to the traditional imaging techniques such as magnetic resonance imaging (MRI) and X-rays. EIT systems usually consist of a ring of equally spaced electrodes placed around the part of the body of interest. A sinusoidal current source with a typical frequency of 50 kHz is used to inject current between an adjacent pair of electrodes. Then the imposed voltages on the remaining electrodes are measured sequentially. After that the current source is routed to the next pair and the cycle is repeated until one frame of data is collected. For eight electrodes 8x5 measurements per frame are collected. Mathematical algorithms are used to construct an image of the impedance distribution inside the closed volume. In order to apply the current source and measure the imposed voltages on the electrodes a switch matrix is needed. In this work, a circuit implementation of the switch matrix for eight electrodes is presented. The system consists of logic blocks used to generate eight logic signals (CL1-CL8) needed to route the differential current source to the appropriate electrode pair. For example, when the logic signals CL1 and CL2 are both high, the differential current is routed to electrode pair 1 and 2. During this time other blocks are used to generate 8 logic signals (VL1-VL8) required to measure the imposed voltages on the remaining electrodes in the following sequence, V3-V4, V4-V5, V5-V6, V6-V7 and V7-V8. The logic blocks used in the implementation consists of mono-stable elements in addition to standard logic gates. Simulation results indicate that the implementation of the designed circuit is feasible in both hardware and software form.

Keywords: Electrical Impedance Tomography, Switch Matrix Logic, Simulated Measurement Systems.