Conference Paper

A Simple Instrumentation Calibration Technique for Electrical Impedance Tomography (EIT) Using A 16-Electrode Phantom

Dept. of Instrum., Indian Inst. of Sci.-Bangalore, Bangalore, India
DOI: 10.1109/COASE.2009.5234117 Conference: IEEE Conference on Automation Science and Engineering, CASE 2009, Bangalore, India, 22-25 August, 2011
Source: DBLP

ABSTRACT

A simple analog instrumentation for electrical impedance tomography is developed and calibrated using the practical phantoms. A constant current injector consisting of a modified Howland voltage controlled current source fed by a voltage controlled oscillator is developed to inject a constant current to the phantom boundary. An instrumentation amplifier, 50 Hz notch filter and a narrow band pass filter are developed and used for signal conditioning. Practical biological phantoms are developed and the forward problem is studied to calibrate the EIT-instrumentation. An array of sixteen stainless steel electrodes is developed and placed inside the phantom tank filled with KCl solution. 1 mA, 50 kHz sinusoidal current is injected at the phantom boundary using adjacent current injection protocol. The differential potentials developed at the voltage electrodes are measured for sixteen current injections. Differential voltage signal is passed through an instrumentation amplifier and a filtering block and measured by a digital multimeter. A forward solver is developed using finite element method in MATLAB 7.0 for solving the EIT governing equation. Differential potentials are numerically calculated using the forward solver with a simulated current and bathing solution conductivity. Measured potential data is compared with the differential potentials calculated for calibrating the instrumentation to acquire the voltage data suitable for better image reconstruction.

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    • "Till date, Electrical Impedance Tomography (EIT) [19] [20] [21] [22] has been researched extensively in the medical field [23] [24] [25] as well as in other areas like industrial process control [18], chemical engineering [26], geotechnical research [27] and biotechnology [28] due to its several advantages [29] [30] [31] over other computed tomographic techniques [32]. Practical phantoms [20, 33–35] with surface electrodes [36] [37] [38] are used to assess the performance of EIT systems for their validation, calibration and comparison purposes. In EIT, the impedance images are reconstructed from the surface potentials developed by a constant current signal injected to the boundary (δΩ) of the domain (Ω) to be imaged (figure 1). "
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    ABSTRACT: It is very essential to visualise the internal condition of human body not only for studying the anatomy and physiology, but also for diagnosing a disease. Physicians always try to analyze an organ or body part in order to study its physiological and anatomical status for understanding and/or treating its illness. Thus, it is always requisite to introduce the diagnostic tool called medical imaging. The period of medical imaging started in 1895, when Roentgen discovered the powerful invisible rays called X-rays. Gradually the medical imaging introduced X-Ray CT, Gamma Camera, Positron emission tomography (PET), Single-Photon Emission Computed Tomography (SPECT), Magnetic Resonance Imaging (MRI), and Ultra SonoGraphy (USG). Recently, medical imaging field is more advanced with comparatively newer tomographic imaging modalities like Electrical Impedance Tomography (EIT), Diffuse Optical Tomography (DOT), Optical Coherence Tomography (OCT), and Photoacaustic Tomography (PAT). The EIT has been extensively researched in different fields of science and engineering due to its several advantages. In correlation with the application of the EIT in the medical field, thoracic electrical impedance tomography (EIT) is used to diagnose patients suffering from the acute respiratory distress syndrome (ARDS) for monitoring their conditions ranging from dynamic shifting of body fluids to lung aeration, right at the bedside. Moreover, EIT-derived numeric parameters would help the physician to evaluate the state of the lung of a patient under observation. Thus, here we have performed a Finite Element Method based simulation study for monitoring the condition of lungs and heart of ARDS patients. Therefore, a finite element method (FEM) model of human thorax in three dimensional platform with FEM Multiphysics software is created and tested with new ventilation indices regarding their ability to quantitatively describe structural changes in the lung due to the gravitationally dependent lung collapse. Additionally, analysis is made to find the electrode pairs capable of separating the lung and heart activity when a particular amount of constant current is injected through them, are also carried out. Finally, a real time EIT system using 16 Ag-AgCl electrodes are developed to get the image of human thorax. The data are collected using the adjacent current injection technique and are plotted using FEM Multiphysics software. The reconstructed FEM images using the forward solver of EIT method shows the approximate area of the thorax (lungs, heart etc.) under observation. This chapter will present a brief overview on application of EIT for monitoring of the lung fluid movement and estimation of lung area in a human being alongwith physical and mathematical aspect with a goal to achieve a system having higher potential to cater medical challenges in lung oriented diseases.
    Full-text · Chapter · Jul 2015
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    ABSTRACT: A Radio Frequency (RF) based digital data transmission scheme with 8 channel encoder/decoder ICs is proposed for surface electrode switching of a 16-electrode wireless Electrical Impedance Tomography (EIT) system. A RF based wireless digital data transmission module (WDDTM) is developed and the electrode switching of a EIT system is studied by analyzing the boundary data collected and the resistivity images of practical phantoms. An analog multiplexers based electrode switching module (ESM) is developed with analog multiplexers and switched with parallel digital data transmitted by a wireless transmitter/receiver (T-x/R-x) module working with radio frequency technology. Parallel digital bits are generated using NI USB 6251 card working in LabVIEW platform and sent to transmission module to transmit the digital data to the receiver end. The transmitter/receiver module developed is properly interfaced with the personal computer (PC) and practical phantoms through the ESM and USB based DAQ system respectively. It is observed that the digital bits required for multiplexer operation are sequentially generated by the digital output (D/O) ports of the DAQ card. Parallel to serial and serial to parallel conversion of digital data are suitably done by encoder and decoder ICs. Wireless digital data transmission module successfully transmitted and received the parallel data required for switching the current and voltage electrodes wirelessly. 1 mA, 50 kHz sinusoidal constant current is injected at the phantom boundary using common ground current injection protocol and the boundary potentials developed at the voltage electrodes are measured. Resistivity images of the practical phantoms are reconstructed from boundary data using EIDORS. Boundary data and the resistivity images reconstructed from the surface potentials are studied to assess the wireless digital data transmission system. Boundary data profiles of the practical phantom with different configurations show that the multiplexers are operating in the required sequence for common ground current injection protocol. The voltage peaks obtained at the proper positions in the boundary data profiles proved the sequential operation of multiplexers and successful wireless transmission of digital bits. Reconstructed images and their image parameters proved that the boundary data are successfully acquired by the DAQ system which in turn again indicates a sequential and proper operation of multiplexers as well as the successful wireless transmission of digital bits. Hence the developed RF based wireless digital data transmission module (WDDTM) is found suitable for transmitting digital bits required for electrode switching in wireless EIT data acquisition system.
    Full-text · Article · Jan 2011 · Measurement
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    ABSTRACT: Surface electrodes are essentially required to be switched for boundary data collection in electrical impedance tomography (EIT). Parallel digital data bits are required to operate the multiplexers used, generally, for electrode switching in EIT. More the electrodes in an EIT system more the digital data bits are needed. For a sixteen electrode system, 16 parallel digital data bits are required to operate the multiplexers in opposite or neighbouring current injection method. In this paper a common ground current injection is proposed for EIT and the resistivity imaging is studied. Common ground method needs only two analog multiplexers each of which need only 4 digital data bits and hence only 8 digital bits are required to switch the 16 surface electrodes. Results show that the USB based data acquisition system sequentially generate digital data required for multiplexers operating in common ground current injection method. The profile of the boundary data collected from practical phantom show that the multiplexers are operating in the required sequence in common ground current injection protocol. The voltage peaks obtained for all the inhomogeneity configurations are found at the accurate positions in the boundary data matrix which proved the sequential operation of multiplexers. Resistivity images reconstructed from the boundary data collected from the practical phantom with different configurations also show that the entire digital data generation module is functioning properly. Reconstructed images and their image parameters proved that the boundary data are successfully acquired by the DAQ system which in turn indicates a sequential and proper operation of multiplexers.
    No preview · Conference Paper · Jan 2011
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