Multi-frequency Electrical Impedance Tomography System Research Articles

Exploratory study of a multifrequency EIT-based method for detecting intracranial abnormalities.

The purpose of this paper is to compare the differences in the features of multifrequency electrical impedance tomography (MFEIT) images of human heads between healthy subjects and patients with brain diseases and to explore the possibility of applying MFEIT to intracranial abnormality detection. Sixteen healthy volunteers and 8 patients with brain diseases were recruited as subjects, and the cerebral MFEIT data of 9 frequencies in the range of 21 kHz - 100 kHz of all subjects were acquired with an MFEIT system. MFEIT image sequences were obtained according to certain imaging algorithms, and the area ratio of the ROI (AR_ROI) and the mean value of the reconstructed resistivity change of the ROI (MVRRC_ROI) on both the left and right sides of these images were extracted. The geometric asymmetry index (GAI) and intensity asymmetry index (IAI) were further proposed to characterize the symmetry of MFEIT images based on the extracted indices and to statistically compare and analyze the differences between the two groups of subjects on MFEIT images. There were no significant differences in either the AR_ROI or the MVRRC_ROI between the two sides of the brains of healthy volunteers (p > 0.05); some of the MFEIT images mainly in the range of 30 kHz - 60 kHz of patients with brain diseases showed stronger resistivity distributions (larger area or stronger signal) that were approximately symmetric with the location of the lesions. However, statistical analysis showed that the AR_ROI and the MVRRC_ROI on the healthy sides of MFEIT images of patients with unilateral brain disease were not significantly different from those on the affected side (p > 0.05). The GAI and IAI were higher in all patients with brain diseases than in healthy volunteers except for 80 kHz (p < 0.05). There were significant differences in the geometric symmetry and the signal intensity symmetry of the reconstructed targets in the MFEIT images between healthy volunteers and patients with brain diseases, and the above findings provide a reference for the rapid detection of intracranial abnormalities using MFEIT images and may provide a basis for further exploration of MFEIT for the detection of brain diseases.

Design and analysis of a multifrequency electrical impedance tomography-based pressure sensor

Electrical impedance tomography (EIT) is a nondestructive imaging technique for estimating the electrical impedance distribution within a conductive process. EIT is employed in many fields, including medical, geology, and industrial. The pressure sensor design is one of the most recent applications of this method. In this application, a thin and flexible conductor uses as the pressure sensor to apply physical pressure on a specific region and cause local variations in the electrical impedance of that region. We present the principles of designing and implementing a multifrequency EIT system for pressure sensing and imaging. Considering this, the theoretical topics and the hardware components of an EIT system are described. Subsequently, with the aim of examining the designed system, the conductive fabric was employed as the pressure sensor. In numerous experiments, the reconstructed electrical impedance images of the applied pressure demonstrated that there is a relatively linear relation between the pressure and the impedance variations at each frequency. The experimental results reveal that the designed system successfully detects the pressure area and determines its magnitude in the applied pressure region.

Design of multi-frequency electrical impedance tomography system based on multisine excitation and integer-period sampling

Starting from the principle that the integer-period sampling (IPS) of periodic signals is free of spectrum leakage, in this paper we propose the multisine-IPS theory, deduce theoretically the sampling rate setting formula of multisine-IPS condition for the first time, and build its realization method based on field-programmable gate array (FPGA) plus digital-to-analog converter (DAC) plus analog-to-digital converter (ADC). A new multi-frequency electrical impedance tomography (mfEIT) system based on multisine excitation and its IPS theory is developed, and a dual-target imaging model including a carrot stick and a cucumber stick is designed. The experiments of multi-frequency time-difference imaging and frequency-difference imaging are carried out on the mfEIT system. The experimental results show that the newly-designed mfEIT system can achieve full-band impedance measurements on multiple objective tissue boundary at 20 frequency points (2–997 kHz) within one fundamental period (1 ms), and the structure and position of biological tissues with different electrical properties can also be distinguished from the resulting images. The proposed multisine-IPS theory and its implementation method can complete a full-band impedance measurement within one multisine fundamental period, which lays a theoretical and technical foundation for developing high-speed mfEIT system.

Designing a bench for testing medically and technically an information and measuring system for multi-frequency electrical impedance tomography of human lungs

This paper solves the problem of developing and creating multifunctional tools for conducting research into systems of multi-frequency electrical impedance tomography of human lungs. A test bench consisting of modern medical equipment, simulating pre-and postoperative environments to create conditions that are most similar to clinical ones, under which it is planned to operate multi-frequency electrical impedance tomography systems, was proposed and manufactured. This makes it possible to reduce significantly the time for approbation, testing, and elimination of practical inaccuracies and problems of clinical application of the developed medical and technical facilities. A positive result is achieved due to the possibility of forming new test plans with specified conditions and different levels of complexity. This enables enhancing the effectiveness of subsequent clinical tests on patients who are treated in a resuscitation unit or an intensive care unit. The operability of the bench is proved by the repeatability of obtained results of monitoring ventilation and perfusion for each examined person, the continuity of dynamic visualization of the breathing process, as well as a high degree of correlation of obtained values of differences of potentials with the readings of a bedside monitor of a patient. An information and measuring system of multi-frequency electric impedance tomography of human lungs, developed by the author earlier, was used as the EIT device. The EIT tests were performed for the frequency range of 50 kHz – 400 kHz at a current of 5 mA. All experimental studies involved volunteers who gave written information consent to participate in the tests. The results of the research show that the proposed bench can be used in practice to solve a wide range of scientific and applied problems in the field of electrical impedance tomography

Design of electrical impedance tomography for biomedicine

Electrical Impedance Tomography (EIT) has an advantage than another imaging systems in the medical field, they are safe, fast, and simple. The role of EIT is promising, so that it is necessary to develop electrical impedance tomography devices, especially in the biomedical field. we have successfully constructed a multi-frequency EIT system for biomedicine consisting of an oscillator, buffer, VCCS (voltage controlled current source), mux-demux, differential amplifiers based on Arduino Mega. The system succesfully has been tested to produce an image of conductors, insulators, and animal organs so that suitable for biomedical field.

An FPGA-Based Multifrequency EIT System With Reference Signal Measurement

Multifrequency electrical impedance tomography (MFEIT) has been rapidly developed and has shown great potential for biomedical analysis and imaging. This article presents the design and evaluation of a wideband and high-precision MFEIT system which incorporates novel features to fit biomedical application. First, a high-accuracy multifrequency current source with adjustable output frequency and amplitude as well as high-output impedance is designed based on the combination of the classic current mirror and a differential topology, which demonstrates stable performance in high frequencies. Second, a reference measurement circuit is developed for the calibration of phase measurement data through which precise measurements of both amplitude data and phase data can be obtained. Furthermore, the low-pass filters utilized in the system have been designed based on the Butterworth filter theory to realize the flattest frequency response in the passband. The MFEIT system is systematically tested and evaluated in terms of filter performance, signal to noise ratio (SNR), channel consistency and measurement repeatability, flatness analysis and load dependence as well as imaging experiments. Experimental results showed that the maximum amplitude SNR and phase SNR of the boundary measurements are 71.55 and 64.12 dB, respectively. The evaluation results indicate that the system has good performance over a wide frequency band (100 Hz–1 MHz) and achieves a relatively high SNR.

Design of a multi-frequency electrical impedance tomography system based on multisine excitation and integer-period sampling

本文从周期信号的整周期采样无频谱泄露这一原理出发，提出基于multisine信号的整周期采样理论，首次从理论上推导出满足multisine整周期采样的采样率设置条件，构建了基于FPGA+数模转换器+模数转换器的整周期采样实现方法，研制了一种基于multisine激励和整周期采样的新型多频电阻抗成像(EIT)系统；设计了胡萝卜棒+黄瓜棒的双目标成像模型，并进行了多频时差成像和频差成像实验。实验表明，本多频电阻抗成像系统能够一个基波周期（1 ms）内实现20个频率点（2 kHz -997 kHz）多目标组织边界的全频阻抗测量，成像结果可区分具有不同电特性生物组织的结构与位置。本文提出的基于multisine信号的整周期采样理论及其实现方法，只需一个multisine基波周期即可完成一次全频阻抗测量，为研制高速多频EIT系统奠定了理论和技术基础。

BAND-PASS FILTER CIRCUIT DESIGN FOR MULTI-FREQUENCY ELECTRICAL IMPEDANCE TOMOGRAPHY SYSTEM

BAND-PASS FILTER CIRCUIT DESIGN FOR MULTI-FREQUENCY ELECTRICAL IMPEDACE TOMOGRAPHY SYSTEM . Electrical Impedance Tomography (EIT) is one of the emerging imaging technologies recently. The principle of EIT system is inject electric current into impedance object and measure the voltage based on the electrodes array. The current source that precise, stable, and wide-bandwidth is required to obtain voltage measurements accurately. The current source used for this EIT system is Voltage-Controlled Current Source (VCCS). The VCCS block circuit consists of several circuits, one of them is a filter circuit. The suitable filter for multi-frequency EIT system is band-pass filter. This research has been designed and fabricated a band-pass filter with cut-off frequency 1 and 250 kHz, which is using Butterworth coefficient, unity gain, and Sallen-Key topology.

A Low-Cost Portable Wireless Multi-frequency Electrical Impedance Tomography System

Electrical impedance tomography (EIT) is an approach to reconstruct electrical resistivity images of the body noninvasively. Here we present a novel wireless high-speed frequency division multiplexing and flexible switching-based EIT system with increased temporal resolution using raspberry pi. Proposed multi-frequency electrical impedance system has good performance, portable, energy and cost-efficient. Current is injected to all the electrodes simultaneously having different frequencies which increase the speed for capturing the fast change in impedances. Proposed system can reconstruct real-time 2D images for continuously monitoring applications where impedance changes rapidly. It can capture 70 frames/sec, and its power consumption is 10 mW. The system is designed for a bandwidth of 1 kHz–1 MHz, which covers most of the medical impedance investigations. Performance of the system was measured at 25 KHz by calculating parameter of blur radius (PBR) and percentage of position error (PPE). Average PBR and PPE obtained were 0.985 and 7.24%, respectively. SNR, linearity and stability were calculated after proper calibration by applying a small current of $$500\,\upmu \hbox {A}$$ . SNR of the system is more than 70 dB. Amplitude and phase measurement repeatability were 0.7% and $$1^{\circ }$$ , respectively. CMRR of both wired and wireless systems were measured to find the effect of noise on the wireless system. Result shows that the proposed WMFEIT system has comparable performance with the reference systems and has good scope to be used for clinical imaging applications.

Design of current source for multi-frequency simultaneous electrical impedance tomography.

Multi-frequency electrical impedance tomography has been evolving from the frequency-sweep approach to the multi-frequency simultaneous measurement technique which can reduce measuring time and will be increasingly attractive for time-varying biological applications. The accuracy and stability of the current source are the key factors determining the quality of the image reconstruction. This article presents a field programmable gate array-based current source for a multi-frequency simultaneous electrical impedance tomography system. A novel current source circuit was realized by combining the classic current mirror based on the feedback amplifier AD844 with a differential topology. The optimal phase offsets of harmonic sinusoids were obtained through the crest factor analysis. The output characteristics of this current source were evaluated by simulation and actual measurement. The results include the following: (1) the output impedance was compared with one of the Howland pump circuit in simulation, showing comparable performance at low frequencies. However, the proposed current source makes lower demands for resistor tolerance but performs even better at high frequencies. (2) The output impedance in actual measurement below 200 kHz is above 1.3 MΩ and can reach 250 KΩ up to 1 MHz. (3) An experiment based on a biological RC model has been implemented. The mean error for the demodulated impedance amplitude and phase are 0.192% and 0.139°, respectively. Therefore, the proposed current source is wideband, biocompatible, and high precision, which demonstrates great potential to work as a sub-system in the multi-frequency electrical impedance tomography system.

A multi-frequency electrical impedance tomography system for real-time 2D and 3D imaging.

This paper presents the design and evaluation of a configurable, fast multi-frequency Electrical Impedance Tomography (mfEIT) system for real-time 2D and 3D imaging, particularly for biomedical imaging. The system integrates 32 electrode interfaces and the current frequency ranges from 10 kHz to 1 MHz. The system incorporates the following novel features. First, a fully adjustable multi-frequency current source with current monitoring function is designed. Second, a flexible switching scheme is developed for arbitrary sensing configuration and a semi-parallel data acquisition architecture is implemented for high-frame-rate data acquisition. Furthermore, multi-frequency digital quadrature demodulation is accomplished in a high-capacity Field Programmable Gate Array. At last, a 3D imaging software, visual tomography, is developed for real-time 2D and 3D image reconstruction, data analysis, and visualization. The mfEIT system is systematically tested and evaluated from the aspects of signal to noise ratio (SNR), frame rate, and 2D and 3D multi-frequency phantom imaging. The highest SNR is 82.82 dB on a 16-electrode sensor. The frame rate is up to 546 fps at serial mode and 1014 fps at semi-parallel mode. The evaluation results indicate that the presented mfEIT system is a powerful tool for real-time 2D and 3D imaging.

System Description and First Application of an FPGA-Based Simultaneous Multi-Frequency Electrical Impedance Tomography.

A new prototype of a multi-frequency electrical impedance tomography system is presented. The system uses a field-programmable gate array as a main controller and is configured to measure at different frequencies simultaneously through a composite waveform. Both real and imaginary components of the data are computed for each frequency and sent to the personal computer over an ethernet connection, where both time-difference imaging and frequency-difference imaging are reconstructed and visualized. The system has been tested for both time-difference and frequency-difference imaging for diverse sets of frequency pairs in a resistive/capacitive test unit and in self-experiments. To our knowledge, this is the first work that shows preliminary frequency-difference images of in-vivo experiments. Results of time-difference imaging were compared with simulation results and shown that the new prototype performs well at all frequencies in the tested range of 60 kHz–960 kHz. For frequency-difference images, further development of algorithms and an improved normalization process is required to correctly reconstruct and interpreted the resulting images.

Electrical impedance spectroscopy (EIS)-based evaluation of biological tissue phantoms to study multifrequency electrical impedance tomography (Mf-EIT) systems

Electrical impedance tomography (EIT) phantoms are essential for the calibration, comparison and evaluation of the EIT systems. In EIT, the practical phantoms are typically developed based on inhomogeneities surrounded by a homogeneous background to simulate a suitable conductivity contrast. In multifrequency EIT (Mf-EIT) evaluation, the phantoms must be developed with the materials which have recognizable or distinguishable impedance variations over a wide range of frequencies. In this direction the impedance responses of the saline solution (background) and a number vegetable and fruit tissues (inhomogeneities) are studied with electrical impedance spectroscopy (EIS) and the frequency responses of bioelectrical impedance and conductivity are analyzed. A number of practical phantoms with different tissue inhomogeneities and different inhomogeneity configurations are developed and the multifrequency impedance imaging is studied with the Mf-EIT system to evaluate the phantoms. The conductivity of the vegetable inhomogeneities reconstructed from the EIT imaging is compared with the conductivity values obtained from the EIS studies. Experimental results obtained from multifrequency EIT reconstruction demonstrate that the electrical impedance of all the biological tissues inhomogenity decreases with frequency. The potato tissue phantom produces better impedance image in high frequency ranges compared to the cucumber phantom, because the cucumber impedance at high frequency becomes lesser than that of the potato at the same frequency range.

A FPGA-Based Broadband EIT System for Complex Bioimpedance Measurements—Design and Performance Estimation

Electrical impedance tomography (EIT) is an imaging method that is able to estimate the electrical conductivity distribution of living tissue. This work presents a field programmable gate array (FPGA)-based multi-frequency EIT system for complex, time-resolved bioimpedance measurements. The system has the capability to work with measurement setups with up to 16 current electrodes and 16 voltage electrodes. The excitation current has a range of about 10 µA to 5 mA, whereas the sinusoidal signal used for excitation can have a frequency of up to 500 kHz. Additionally, the usage of a chirp or rectangular signal excitation is possible. Furthermore, the described system has a sample rate of up to 3480 impedance spectra per second (ISPS). The performance of the EIT system is demonstrated with a resistor-based phantom and tank phantoms. Additionally, first measurements taken from the human thorax during a breathing cycle are presented.

Modular architecture of a multi-frequency electrical impedance tomography system: design and implementation.

Electrical impedance tomography (EIT) provides means of imaging the electrical properties distribution of biological tissues and fluids while impedance spectroscopy (IS) allows measuring their frequency response in a more global way. Both require precise and well-integrated instrumentation. In this work, we propose a modular architecture of a multi-frequency EIT (MfEIT) system which has capabilities in implementing both IS and MfEIT. First, IS performance is assessed in vivo using a cuff electrode implanted around the rodent cervical vagus nerve. Second, MfEIT performance is evaluated in vitro based on saline phantom experiments. Overall system allows addressing a wide range of applications and proves effective both in vitro and in vivo.

Multi-Frequency Electrical Impedance Tomography System With Automatic Self-Calibration for Long-Term Monitoring

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.

Flexible electrode belt for EIT using nanofiber web dry electrodes

Efficient connection of multiple electrodes to the body for impedance measurement and voltage monitoring applications is of critical importance to measurement quality and practicality. Electrical impedance tomography (EIT) experiments have generally required a cumbersome procedure to attach the multiple electrodes needed in EIT. Once placed, these electrodes must then maintain good contact with the skin during measurements that may last several hours. There is usually also the need to manage the wires that run between the electrodes and the EIT system. These problems become more severe as the number of electrodes increases, and may limit the practicality and portability of this imaging method. There have been several trials describing human–electrode interfaces using configurations such as electrode belts, helmets or rings. In this paper, we describe an electrode belt we developed for long-term EIT monitoring of human lung ventilation. The belt included 16 embossed electrodes that were designed to make good contact with the skin. The electrodes were fabricated using an Ag-plated PVDF nanofiber web and metallic threads. A large contact area and padding were used behind each electrode to improve subject comfort and reduce contact impedances. The electrodes were incorporated, equally spaced, into an elasticated fabric belt. We tested the electrode belt in conjunction with the KHU Mark1 multi-frequency EIT system, and demonstrate time-difference images of phantoms and human subjects during normal breathing and running. We found that the Ag-plated PVDF nanofiber web electrodes were suitable for long-term measurement because of their flexibility and durability. Moreover, the contact impedance and stability of the Ag-plated PVDF nanofiber web electrodes were found to be comparable to similarly tested Ag/AgCl electrodes.

Comparison of frequency difference reconstruction algorithms for the detection of acute stroke using EIT in a realistic head-shaped tank

Imaging of acute stroke might be possible using multi-frequency electrical impedance tomography (MFEIT) but requires absolute or frequency difference imaging. Simple linear frequency difference reconstruction has been shown to be ineffective in imaging with a frequency-dependant background conductivity; this has been overcome with a weighted frequency difference approach with correction for the background but this has only been validated for a cylindrical and hemispherical tank. The feasibility of MFEIT for imaging of acute stroke in a realistic head geometry was examined by imaging a potato perturbation against a saline background and a carrot-saline frequency-dependant background conductivity, in a head-shaped tank with the UCLH Mk2.5 MFEIT system. Reconstruction was performed with time difference (TD), frequency difference (FD), FD adjacent (FDA), weighted FD (WFD) and weighted FDA (WFDA) linear algorithms. The perturbation in reconstructed images corresponded to the true position to <9.5% of image diameter with an image SNR of >5.4 for all algorithms in saline but only for TD, WFDA and WFD in the carrot-saline background. No reliable imaging was possible with FD and FDA. This indicates that the WFD approach is also effective for a realistic head geometry and supports its use for human imaging in the future.

A Chicken Tissue Phantom for Studying an Electrical Impedance Tomography (EIT) System Suitable for Clinical Imaging

The study of practical phantoms is essential for assessing the reconstruction algorithms and instrumentation used in Electrical Impedance Tomography (EIT). Responses of saline phantoms with insulator inhomogeneities differ from the real tissue phantoms in several aspects. Also, it is difficult to reconstruct the actual resistivity of the insulator inhomogeneity in a saline background because of their large resistivity difference. A practical biological phantom consisting of two different materials with low resistivity difference is more suitable for impedance imaging studies. In order to demonstrate this, a chicken tissue phantom was developed to study the resistivity imaging in EIT. A 16-electrode array was placed inside the phantom tank filled with chicken muscle tissue paste and chicken tissue. A 1 mA, 50 kHz sinusoidal current was injected at the phantom boundary and the boundary potentials are measured using opposite current injection protocol. Resistivity images were reconstructed from the boundary data using Electrical Impedance and Diffuse Optical Reconstruction Software (EIDORS) and the reconstruction was evaluated by calculating the contrast parameters of the images. Results show that the resistivity of the chicken fat is successfully reconstructed with a proper background resistivity. Impedance spectroscopic studies show that the chicken tissue phantom can be suitably used to evaluate a multifrequency EIT system.

Electrical Impedance Spectroscopic Studies on Broiler Chicken Tissue Suitable for the Development of Practical Phantoms in Multifrequency EIT

Abstract Phantoms are essential for assessing the system performance in Electrical Impedance Tomography (EIT). Saline phantoms with insulator inhomogeneity fail to mimic the physiological structure of real body tissue in several aspects. Saline or any other salt solution is purely resistive and hence studying multifrequency EIT systems cannot be assessed with saline phantoms because the response of the purely resistive materials do not change over frequency. Animal tissues show a variable response over a wide band of signal frequency due to their complex physiological and physiochemical structures and hence they can be suitably used as bathing medium and inhomogeneity in the phantoms of multifrequency EIT systems. An efficient assessment of a multifrequency EIT system with a real tissue phantom needs a prior knowledge of the impedance profile of the bathing medium as well as the inhomogeneity. In this direction Electrical Impedance Spectroscopy (EIS) studies on broiler chicken muscle tissue paste, muscle tissue blocks and fat tissue blocks are conducted over a wide range of signal frequency using impedance analyzers, and their impedance profiles are analyzed. Results show that the chicken muscle tissue paste is less resistive than the fat tissue and hence it can be used successfully as the bathing medium of the phantoms for impedance imaging in multifrequency EIT. Fat tissue is found more resistive than the muscle tissue which makes it more suitable for the inhomogeneity in phantoms of impedance imaging study. Moreover, as there is a large difference between the resistivities of muscle tissue and fat tissue they can be used as either inhomogeneity or background medium. EIS studies also show that the variations in the impedance parameters of a muscle tissue block are greater than in the tissue paste as the cell membrane structures are destroyed in tissue paste. Results also show that the α and β dispersions are visible in all the parameters of both the tissue samples, but both the dispersions are larger in the muscle tissue block. The Nyquist plot obtained for the muscle tissue block demonstrates that the equivalent electric model of the tissue sample contains Warburg impedance and a constant phase element.