Abstract
Monitoring of anatomical structures and physiological processes by electrical impedance has attracted scientists as it is noninvasive, nonionizing and the instrumentation is relatively simple. Focused Impedance Method (FIM) is attractive in this context, as it has enhanced sensitivity at the central region directly beneath the electrode configuration minimizing contribution from neighboring regions. FIM essentially adds or averages two concentric and orthogonal combinations of conventional Tetrapolar Impedance Measurements (TPIM) and has three versions with 4, 6, and 8 electrodes. This paper describes the design and testing of a multi-frequency FIM (MFFIM) system capable of measuring all three versions of FIM at 8 frequencies in the range 10 kHz—1 MHz. A microcontroller based multi-frequency signal generator and a balanced Howland current source with high output impedance (476 kΩ at 10 kHz and 58.3 kΩ at 1 MHz) were implemented for driving currents into biological tissues with an error <1%. The measurements were carried out at each frequency sequentially. The peak values of the amplified voltage signals were measured using a novel analogue synchronous peak detection technique from which the transfer impedances were obtained. The developed system was tested using TPIM measurements on a passive RC Cole network placed between two RC networks, the latter representing skin-electrode contact impedances. Overall accuracy of the measurement was very good (error <4% at all frequencies except 1 MHz, with error 6%) and the resolution was 0.1 Ω. The designed MFFIM system had a sampling rate of >45 frames per second which was deemed adequate for noninvasive real-time impedance measurements on biological tissues.
Highlights
Bioimpedance is an emerging technique for monitoring anatomical structures and physiological processes
Most bioimpedance measurement schemes use a Tetrapolar Impedance measurement (TPIM) technique where separate pairs of electrodes are used for current drive and potential measurement
Tetrapolar Impedance Measurements (TPIM) essentially eliminates the tissue-electrode impedances and is the basis of more sophisticated measurement techniques including the Focused Impedance Method (FIM), instrumentation for which is the subject of this paper, and electrical impedance tomography (EIT)
Summary
Bioimpedance is an emerging technique for monitoring anatomical structures and physiological processes. Conventional electrical impedance measurements use a bipolar configuration in which potential developed across two terminals in a network are measured for a specified driven current. This scheme is not suitable for bioimpedance measurements since tissue-electrode contact impedances are included in the measurement. Most bioimpedance measurement schemes use a Tetrapolar Impedance measurement (TPIM) technique where separate pairs of electrodes are used for current drive and potential measurement. The ratio of the measured voltage to the current is known as the transfer impedance and is a property of the bulk without any significant contribution from tissue-electrode contact impedances (Brown et al, 2000a; Martinsen and Grimnes, 2009). Use of ‘impedance’ in the bioimpedance literature essentially should be taken to mean “Transfer Impedance.” The above implies that contact resistances of any switching circuitry at the electrodes will not affect the transfer impedance measurement
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