Electrical Impedance Tomography for Biomedical Applications: Circuits and Systems Review

There has been considerable interest in electrical impedance tomography (EIT) to provide low-cost, radiation-free, real-time and wearable means for physiological status monitoring. To be competitive with other well-established imaging modalities, it is important to understand the requirements of the specific application and determine a suitable system design. This paper presents an overview of EIT circuits and systems including architectures, current drivers, analog front-end and demodulation circuits, with emphasis on integrated circuit implementations. Commonly used circuit topologies are detailed, and tradeoffs are discussed to aid in choosing an appropriate design based on the application and system priorities. The paper also describes a number of integrated EIT systems for biomedical applications, as well as discussing current challenges and possible future directions

A comparison of techniques to optimize measurement of voltage changes in electrical impedance tomography by minimizing phase shift errors

In electrical impedance tomography, errors due to stray capacitance may be reduced by optimization of the reference phase of the demodulator. Two possible methods, maximization of the demodulator output and minimization of reciprocity error have been assessed, applied to each electrode combination individually, or to all combinations as a whole. Using an EIT system with a single impedance measuring circuit and multiplexer to address the 16 electrodes, the methods were tested on resistor-capacitor networks, saline-filled tanks and humans during variation of the saline concentration of a constant fluid volume in the stomach. Optimization of each channel individually gave less error, particularly on humans, and maximization of the output of the demodulator was more robust. This method is, therefore, recommended to optimize systems and reduce systematic errors with similar EIT systems

A DC Model for Organic Electrochemical Transistors and Analysis of Their Performance as Voltage Amplifiers

Organic electrochemical transistors (OECTs) have received significant attention especially in biomedical applications. Despite many efforts on modeling these transistors, simulating OECT-based circuits is still a challenge due to the absence of accurate mathematical models. In this paper a DC model for p-type depletion-mode OECTs is proposed that more closely mimics their characteristics compared to the Bernards-Malliaras (B-M) model. Although OECTs are mostly used as transconductance amplifiers, their use as voltage amplifiers is investigated here with measurements at various drain-source voltages. Compared to the B-M model, the proposed model has better matching up to 3.6% between simulations and measurements of the analyzed transistors

Effects of Patient Recumbency Position on Neonatal Chest EIT

This paper investigates the overlooked effects of the patient recumbency positions on one of the key clinically used parameters in chest electrical impedance tomography (EIT) monitoring – the silent spaces. This parameter could impact medical decisions and interventions by indicating how well each lung is being ventilated. Yet it is largely dependent on assumptions of prior model at the reconstruction stage and the closely linked region of interest (ROI) during the final calculations. The potential effect of switching recumbency modes on silent spaces as a results of internal organ movements and consequently changes in initial assumptions, has been studied. The displacement and deformations caused by posture changes from supine to lateral recumbent were evaluated via simulations considering the simultaneous gravity-dependent movement and/or deformations of heart, mediastinum, lungs and the diaphragm. The reliability of simulations was verified against reference radiography images of an 18-month-old infant in supine and decubitus lateral positions. Inspecting a set of 10 patients from age range of 1 to 2 years old revealed improvements of up to 30% in the silent space parameters when applying posture consistent amendments as opposed to fixed model/ROI to each individual. To minimize the influence of image reconstruction technique on the results two different EIT reconstruction algorithms were implemented. The outcome emphasized the importance of including recumbency situation during chest EIT monitoring within the considered age range

Electric Field Characteristics of Bipolar Impedance Sensors

There has been an increased interest in the last decade on Lab-on-a-Chip (LOAC) technologies, which concentrate on the detection and manipulation of biomolecules. Impedance detection plays a vital role in these developments. There are various types of impedance sensors, with four electrode systems being the focus of the work presented here. However, in order to optimize a tetrapolar system, the bipolar needs to be examined first. The electric field properties and characteristics of bipolar systems and in particular, the effect of the electrode width, W, and separation, D, are examined in this paper. This is performed using conformal mapping techniques, which are compared with the Finite Element Method (FEM). An analytical numerical equation for the electric field is derived and examined