We report the development of a new multi-frequency electrical impedance tomography (EIT) system called the KHU Mark2. It is descended from the KHU Mark1 in terms of technical details such as digital waveform generation, Howland current source with multiple generalized impedance converters and digital phase-sensitive demodulators. New features include flexible electrode configurations to accommodate application-specific requirements, multiple independent current sources and voltmeters for fully parallel operations, improved data acquisition speeds for faster frame rates and compact mechanical design. Given an electrode configuration, we can design an analog backplane in such a way that both current injections and voltage measurements can be done without using any switch. The KHU Mark2 is based on an impedance measurement module (IMM) comprising a current source and a voltmeter. Using multiple IMMs, we can construct a multi-channel system with 16, 32 or 64 channels, for example. Adopting a pipeline structure, it has the maximum data acquisition speed of 100 scans s(-1) with the potential to detect fast physiological changes during respiration and cardiac activity. Measuring both in-phase and quadrature components of trans-impedances at multiple frequencies simultaneously, the KHU Mark2 is apt at spectroscopic EIT imaging. In this paper, we describe its design, construction, calibration and performance evaluation. It has about 84 dB signal-to-noise ratio and 0.5% reciprocity error. Time-difference images of an admittivity phantom are presented showing spectroscopic admittivity images. Future application studies using the KHU Mark2 are briefly discussed.
Electrical Impedance Tomography (EIT) is a safe medical imaging technology, requiring no ionizing or heating radiation, as opposed to most other imaging modalities. This has led to a clinical interest in its use for long-term monitoring, possibly at the bedside, for ventilation monitoring, bleeding detection, gastric emptying and epilepsy foci diagnosis. These long-term applications demand auto-calibration and high stability over long time periods. To address this need we have developed a new multi-frequency EIT system called the KHU Mark2.5 with automatic self-calibration and cooperation with other devices via a timing signal for synchronization with other medical instruments. The impedance measurement module (IMM) for flexible configuration as a key component includes an independent constant current source, an independent differential voltmeter, and a current source calibrator, which allows automatic self-calibration of the current source within each IMM. We installed a resistor phantom inside the KHU Mark2.5 EIT system for intra-channel and inter-channel calibrations of all voltmeters in multiple IMMs. We show the deterioration of performance of an EIT system over time and the improvement due to automatic self-calibration. The system is able to maintain SNR of 80 dB for frequencies up to 250 kHz and below 0.5% reciprocity error over continuous operation for 24 hours. Automatic calibration at least every 3 days is shown to maintain SNR above 75 dB and reciprocity error below 0.7% over 7 days at 1 kHz. A clear degradation in performance results with increasing time between automatic calibrations allowing the tailoring of calibration to suit the performance requirements of each application.
Current sources are widely used in bio-impedance spectroscopy (BIS) measurement systems to maximize current injection for increased signal to noise while keeping within medical safety specifications. High-performance current sources based on the Howland current pump with optimized impedance converters are able to minimize stray capacitance of the cables and setup. This approach is limited at high frequencies primarily due to the deteriorated output impedance of the constant current source when situated in a real measurement system. For this reason, voltage sources have been suggested, but they require a current sensing resistor, and the SNR reduces at low impedance loads due to the lower current required to maintain constant voltage. In this paper, we compare the performance of a current source-based BIS and a voltage source-based BIS, which use common components. The current source BIS is based on a Howland current pump and generalized impedance converters to maintain a high output impedance of more than 1 MΩ at 2 MHz. The voltage source BIS is based on voltage division between an internal current sensing resistor (Rs) and an external sample. To maintain high SNR, Rs is varied so that the source voltage is divided more or less equally. In order to calibrate the systems, we measured the transfer function of the BIS systems with several known resistor and capacitor loads. From this we may estimate the resistance and capacitance of biological tissues using the least-squares method to minimize error between the measured transimpedance excluding the system transfer function and that from an impedance model. When tested on realistic loads including discrete resistors and capacitors, and saline and agar phantoms, the voltage source-based BIS system had a wider bandwidth of 10 Hz to 2.2 MHz with less than 1% deviation from the expected spectra compared to more than 10% with the current source. The voltage source also showed an SNR of at least 60 dB up to 2.2 MHz in comparison to the current source-based BIS system where the SNR drops below 40 dB for frequencies greater than 1 MHz.
Electrode properties are key to the quality of measured biopotential signals. Ubiquitous health care systems require long-term monitoring of biopotential signals from normal volunteers and patients in home or hospital environments. In these settings it is appropriate to use dry textile electrode networks for monitoring purposes, rather than the gel or saline-sponge skin interfaces used with Ag/AgCl electrodes. In this study, we report performance test results of two different electrospun conductive nanofiber webs, and three metal plated fabrics. We evaluated contact impedance, step response, noise and signal fidelity performance indices for all five dry electrodes, and compared them to those of conventional Ag/AgCl electrodes. Overall, we found nanofiber web electrodes matched Ag/AgCl electrode performance more closely than metal plated fabric electrodes, with the contact resistance and capacitance of Ag plated PVDF nanofiber web electrodes being most similar to Ag/AgCl over the 10 Hz to 500 kHz frequency range. We also observed that step responses of all three metal-plated fabrics were poorer than those for nanofiber web electrodes and Ag/AgCl. Further, noise standard deviation and noise power spectral densities were generally lower in nanofiber web electrodes than metal plated fabrics; and waveform fidelity of ECG-like traces recorded from nanofiber web electrodes was higher than for metal plated fabrics. We recommend textile nanofiber web electrodes in applications where flexibility, comfort and durability are required in addition to good electrical characteristics.
We demonstrated the feasibility of time difference and weighted frequency difference conductivity imaging for real-time monitoring of temperature distribution and ablation region estimation during radiofrequency (RF) ablation. The electrical conductivity spectrum of biological tissue reflects mobility of ions in intra- and extra-cellular fluids and changes in cellular morphology induced by heating. The time series conductivity spectra were measured in an ex vivo bovine liver by a high-speed electrical impedance tomography (EIT) system. The EIT system was protected by filters to suppress RF energy and allow interleaved real-time imaging. We recorded time and weighted frequency-difference conductivity images and direct temperature variations at the ablation region and control region during 8 min ablation and for the following 66 min of cooling. Conductivity variation in regions of interest was compared with temperature recordings. Contours of conductivity change were visualized and compared to estimate the ablation area. EIT images confirmed increase of conductivity at all frequencies and loss of frequency conductivity change associated with loss of cellular structure. Time difference conductivity images showed changes due to both heating during ablation and heat dissipation following ablation together with tissue property changes. Weighted frequency-difference images presented persistent changes following heating due to the morphological change in the ablation zone.
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