A phantom for electrical impedance tomography

A novel two-dimensional phantom for electrical impedance tomography using 3D printing

Electrical impedance tomography (EIT) is an imaging method that can be used to image electrical impedance contrasts within various tissues of the body. To support development of EIT measurement systems, a phantom is required that represents the electrical characteristics of the imaging domain. No existing type of EIT phantom combines good performance in all three characteristics of resistivity resolution, spatial resolution, and stability. Here, a novel EIT phantom concept is proposed that uses 3D printed conductive material. Resistivity is controlled using the 3D printing infill percentage parameter, allowing arbitrary resistivity contrasts within the domain to be manufactured automatically. The concept of controlling resistivity through infill percentage is validated, and the manufacturing accuracy is quantified. A method for making electrical connections to the 3D printed material is developed. Finally, a prototype phantom is printed, and a sample EIT analysis is performed. The resulting phantom, printed with an Ultimaker S3, has high reported spatial resolution of 6.9 µm, 6.9 µm, and 2.5 µm for X, Y, and Z axis directions, respectively (X and Y being the horizontal axes, and Z the vertical). The number of resistivity levels that are manufacturable by varying infill percentage is 15 (calculated by dividing the available range of resistivities by two times the standard deviation of the manufacturing accuracy). This phantom construction technique will allow assessment of the performance of EIT devices under realistic physiological scenarios.

FPGA-based voltage and current dual drive system for high frame rate electrical impedance tomography

Electrical impedance tomography (EIT) is used to image the electrical property distribution of a tissue under test. An EIT system comprises complex hardware and software modules, which are typically designed for a specific application. Upgrading these modules is a time-consuming process, and requires rigorous testing to ensure proper functioning of new modules with the existing ones. To this end, we developed a modular and reconfigurable data acquisition (DAQ) system using National Instruments' (NI) hardware and software modules, which offer inherent compatibility over generations of hardware and software revisions. The system can be configured to use up to 32-channels. This EIT system can be used to interchangeably apply current or voltage signal, and measure the tissue response in a semi-parallel fashion. A novel signal averaging algorithm, and 512-point fast Fourier transform (FFT) computation block was implemented on the FPGA. FFT output bins were classified as signal or noise. Signal bins constitute a tissue's response to a pure or mixed tone signal. Signal bins' data can be used for traditional applications, as well as synchronous frequency-difference imaging. Noise bins were used to compute noise power on the FPGA. Noise power represents a metric of signal quality, and can be used to ensure proper tissue-electrode contact. Allocation of these computationally expensive tasks to the FPGA reduced the required bandwidth between PC, and the FPGA for high frame rate EIT. In 16-channel configuration, with a signal-averaging factor of 8, the DAQ frame rate at 100 kHz exceeded 110 frames s (-1), and signal-to-noise ratio exceeded 90 dB across the spectrum. Reciprocity error was found to be for frequencies up to 1 MHz. Static imaging experiments were performed on a high-conductivity inclusion placed in a saline filled tank; the inclusion was clearly localized in the reconstructions obtained for both absolute current and voltage mode data.

Bioelectrical Impedance Methods for Noninvasive Health Monitoring: A Review

Under the alternating electrical excitation, biological tissues produce a complex electrical impedance which depends on tissue composition, structures, health status, and applied signal frequency, and hence the bioelectrical impedance methods can be utilized for noninvasive tissue characterization. As the impedance responses of these tissue parameters vary with frequencies of the applied signal, the impedance analysis conducted over a wide frequency band provides more information about the tissue interiors which help us to better understand the biological tissues anatomy, physiology, and pathology. Over past few decades, a number of impedance based noninvasive tissue characterization techniques such as bioelectrical impedance analysis (BIA), electrical impedance spectroscopy (EIS), electrical impedance plethysmography (IPG), impedance cardiography (ICG), and electrical impedance tomography (EIT) have been proposed and a lot of research works have been conducted on these methods for noninvasive tissue characterization and disease diagnosis. In this paper BIA, EIS, IPG, ICG, and EIT techniques and their applications in different fields have been reviewed and technical perspective of these impedance methods has been presented. The working principles, applications, merits, and demerits of these methods has been discussed in detail along with their other technical issues followed by present status and future trends.

A MATLAB-Based Boundary Data Simulator for Studying the Resistivity Reconstruction Using Neighbouring Current Pattern

Phantoms are essentially required to generate boundary data for studying the inverse solver performance in electrical impedance tomography (EIT). A MATLAB-based boundary data simulator (BDS) is developed to generate accurate boundary data using neighbouring current pattern for assessing the EIT inverse solvers. Domain diameter, inhomogeneity number, inhomogeneity geometry (shape, size, and position), background conductivity, and inhomogeneity conductivity are all set as BDS input variables. Different sets of boundary data are generated by changing the input variables of the BDS, and resistivity images are reconstructed using electrical impedance tomography and diffuse optical tomography reconstruction software (EIDORS). Results show that the BDS generates accurate boundary data for different types of single or multiple objects which are efficient enough to reconstruct the resistivity images for assessing the inverse solver. It is noticed that for the BDS with 2048 elements, the boundary data for all inhomogeneities with a diameter larger than 13.3% of that of the phantom are accurate enough to reconstruct the resistivity images in EIDORS-2D. By comparing the reconstructed image with an original geometry made in BDS, it would be easier to study the inverse solver performance and the origin of the boundary data error can be identified.

Evaluation of EIT system performance

An electrical impedance tomography (EIT) system images internal conductivity from surface electrical stimulation and measurement. Such systems necessarily comprise multiple design choices from cables and hardware design to calibration and image reconstruction. In order to compare EIT systems and study the consequences of changes in system performance, this paper describes a systematic approach to evaluate the performance of the EIT systems. The system to be tested is connected to a saline phantom in which calibrated contrasting test objects are systematically positioned using a position controller. A set of evaluation parameters are proposed which characterize (i) data and image noise, (ii) data accuracy, (iii) detectability of single contrasts and distinguishability of multiple contrasts, and (iv) accuracy of reconstructed image (amplitude, resolution, position and ringing). Using this approach, we evaluate three different EIT systems and illustrate the use of these tools to evaluate and compare performance. In order to facilitate the use of this approach, all details of the phantom, test objects and position controller design are made publicly available including the source code of the evaluation and reporting software.

A versatile high-permittivity phantom for EIT

Phantoms are frequently used in medical imaging systems to test hardware, reconstruction algorithms, and the interpretation of data. This report describes and characterizes the use of powdered graphite as a means of adding a significant reactive component or permittivity to useful phantom media for electrical impedance imaging. The phantom materials produced have usable complex admittivity at the electrical impedance tomography (EIT) frequencies from a few kilohertz to 1 MHz, as measured by our EIT system (ACT4) and by a commercial bioimpedance analyzer (BIS 4000, Xitron). We have also studied a commercial ultrasound coupling gel, which is highly electrically conductive and semisolid but that permits objects to move within it. The mixture of agar-graphite and gel-graphite, increases in permittivity and conductivity are proportional to the graphite concentration. We also report the use of a porous polymer membrane to simulate skin. A thin layer of this membrane increased resistance and the characteristic frequency of the phantoms, providing a promising candidate to simulate the effect of skin and the layered structure of a breast or other anatomical structure. The graphite also provides a realistic level of "speckle" in ultrasound images of the phantom, which may be useful in developing dual-mode imaging systems with ultrasound and the EIT.

Analysis of resting noise characteristics of three EIT systems in order to compare suitability for time difference imaging with scalp electrodes during epileptic seizures

Electrical impedance tomography measurements in clinical applications are limited by an undesired noise component. We have investigated the noise in three systems suitable for imaging epileptic seizures, the UCH Mark1b, UCH Mark2.5 and KHU Mark1 16 channel, at applied frequencies in three steps from 1 to 100 kHz, by varying load impedance, single terminal or multiplexed measurements, and in test objects of increasing complexity from a resistor to a saline filled tank and human volunteer. The noise was white, and increased from about 0.03% rms on the resistor to 0.08% on the human; it increased with load but was independent of use of the multiplexer. The KHU Mark1 delivered the best performance with noise spectra of about 0.02%, which could be further reduced by averaging to a level where reliable imaging of changes of about 0.1% estimated during epileptic seizures appears plausible.

Design of an electrical impedance tomography phantom using active elements

The architecture of a novel phantom for electrical impedance tomography (EIT) is proposed. The design employs active elements, which include multiplying digital to analogue converters (MDAC), so that the impedance distribution in the phantom may be varied dynamically using computer control. The phantom is designed to assist in the validation of an EIT system under test. A number of published layouts for passive phantoms are analysed, and the requirements for an active element are specified for the most applicable of these. The use of active elements throughout a phantom imposes significant costs because of the need for each active element to operate independently. This proposal limits the cost and complexity by employing active elements in a restricted region of the phantom. Currently available technology, principally due to the limited analogue bandwidth of the MDAC, precludes the construction of a fully capable phantom from active elements. However, a design is specified that would enable its future development to cover the frequency range from 10 kHz to 1 MHz.

A simple method to check the dynamic performance of electrical impedance tomography systems

The test concept as well as the design of a simple resistor phantom suitable for the evaluation of the properties of electrical impedance tomographic (EIT) systems is presented. Input and transfer impedance of the phantom are matched with those of the human thorax. Amplitude of the local impedance variations similar to in vivo conditions (ventilation) can be intentionally set to perform measurements on different states. The theoretical potential differences between the electrodes are calculated. The evaluation procedure is performed in terms of the local amplitude of the relative impedance change as well as the local distribution of noise. The whole procedure can be applied either to compare quantitatively the performance of different EIT data acquisition systems or to determine the amount of measurement disturbance caused by the external electrical environment in clinical settings.

Use of a priori information in estimating tissue resistivities--application to measured data

A statistically constrained minimum mean squares error estimator (MiMSEE) has been shown to be useful in estimating internal resistivity distribution by the use of simulated data. In this study, the performance of the MiMSEE algorithm is tested by using measured data from resistor phantoms. The MiMSEE uses a priori information on body geometry, electrode position, statistical properties of tissue resistivities, instrumentation noise and linearization error to calculate the optimum inverse matrix which maps the surface potentials to unknown regional resistivities. In this study, the MiMSEE is also constrained with the variance-covariance of the modelling error to improve the estimation accuracy. The data are obtained from two different phantom geometries, namely five-region and thorax. Using the measured data, the estimations are realized and errors are calculated. Then, the results are compared with the results obtained by using a conventional least squares error estimator (LSEE). The five-region model results show similarity with the simulation study results of Baysal and Eyüboğlu. On the thorax model, the total estimation error is 34.2% with the MiMSEE compared with 856% with the LSEE. It is concluded that the MiMSEE is more robust than the LSEE and applicable to measured data.

A Cole phantom for EIT

A phantom was designed for testing and comparing multifrequency EIT data collection systems. The phantom simulates a cylinder of homogeneous conductor with 16 drive and 16 receive electrodes interleaved. Combinations of resistors and capacitors were used to simulate the complex impedance, Z*, of a typical tissue in the frequency range 8-2048 kHz obeying the Cole equation Z* = Z infinity + (Z0 - Z infinity)/[1 + (if/fc)(1- alpha )] where Z* is the complex impedance at frequency f, Z0 and Z infinity are the limiting values of impedance at low and high frequencies, fc is the characteristic frequency and alpha is a constant. A practical phantom was then constructed on which four different sets of spectroscopic parameters could be selected: (i) alpha = 0.20, fc = 150 kHz, Z0/Z infinity = 3.31; (ii) alpha = 0, fc = 273 kHz, Z0/Z infinity = 2.64; (iii) alpha = 0, fc = 71.1 kHz, Z0/Z infinity = 1.36; and (iv) Z0/Z infinity = 1.00 with no dispersion.

Design considerations and performance of a prototype system for imaging neuronal depolarization in the brain using 'direct current' electrical resistance tomography

The ability to image the impedance changes that accompany neuronal depolarization in the brain would constitute a major advance in neuroscience technology. Unfortunately, these changes are likely to be small and rapid and so difficult to measure. The impedance change at frequencies above 10 kHz, as used by conventional EIT systems, may be estimated to be about 0.1%. Modelling indicates that a much larger impedance change of about 7% may occur with DC or very-low-frequency excitation. Difficulties with this approach include a low permissive current level and high electrode impedance. We constructed a prototype system employing square wave excitation at 5 Hz to evaluate such problems. It was tested in a saline-filled tank, recording 4000 frames s-1 at a current level of 50 microA. After averaging 100 sets of frames, the signal to noise ratio was 40-50 dB, and reciprocity errors were mostly 10-20%. Images of discrete resistivity changes of less than 10% could be obtained, but with significant systematic errors. While our prototype would not be suitable for neurophysiological imaging as it stands, it has enabled us to determine the modifications that would be required to construct a system for this application.

Single-plane multifrequency electrical impedance instrumentation

When tissue interacts with electromagnetic radiation it exhibits resistivity and permittivity changes, which decrease with frequency. Above 100 kHz it is expected that dielectric changes in tissue (permittivity) will allow one to distinguish damaged and necrotic tissue. Furthermore, tissue impedance at medium frequencies (100 kHz-1 MHz) have not been well characterized. The aim of this work was to design instrumentation for an impedance tomographic spectrometer to cover the minimum band 10 kHz-1 MHz. In order to produce images sensitive to small changes in resistivity, voltage measurement must be accurate to at least 0.1%. Using commercially available operational amplifiers, PSPICE simulations demonstrated 0.1% accuracy up to 800 kHz, falling off to 0.5% at 1 MHz. Implementation achieved a reasonably flat amplitude (+/- 0.5 dB) and a phase shift of 50 degrees from 10 kHz to 3 MHz and a receive response of 0.13 dB to 5 MHz and phase shift of -40 degrees at 3 MHz. With channel correction this design will provide useful readings up to 3 MHz.

Experimental acquisition system for impedance tomography with active electrode approach

An experimental system for impedance tomography has been constructed. The acquisition system uses 16 multifunctional active electrodes, each including a current source and a voltage buffer. Images of active and reactive parts of different target impedances in a phantom filled with liquid have been obtained. The system performance has been compared with those of other systems using either a mesh phantom or rods as point sources used for the determination of the modulation transfer function.

Imaging the complex impedance of the thorax

Respiration-related changes in the complex impedance were obtained on the thorax in three volunteers. The real part of the image clearly showed the lungs as regions of increased conductivity on expiration. The imaginary part of the image, reflecting changes in the ratio of permittivity to conductivity, showed a central negative region surrounded by a positive region extending to the periphery of the lungs. These features may be due to movement of the diaphragm and liver within the sensitive volume during respiration.

Raw data interchange format for electrical impedance tomography

A data interchange format is described to allow groups working on electrical impedance tomography (EIT) with disparate algorithms and instruments to compare results. The procedure has been tested by exchanging data by e-mail. The format is defined in the appendix.

A dual-frequency electrical impedance tomography system

An electrical impedance tomography (EIT) system has been constructed, operating at two frequencies, 40.96 and 81.92 kHz, for investigating the practicability of the dual-frequency imaging method discussed theoretically in a previous paper. For testing the system, a phantom with a frequency-dependent electrical conductivity was designed. The properties of the phantom can be adjusted to match the frequency dependence observed in a given type of tissue. Dual-frequency images were obtained from a phantom simulating liver and also from 200 g of porcine liver in a saline tank. Prior to image reconstruction, it was necessary to apply a correction to the data to cancel the effects of stray capacitance within the electronics.