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.

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

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 ${\rm s} ^{-1}$ , 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.

FPGA-Based Voltage and Current Dual Drive System for High Frame Rate Electrical Impedance Tomography

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

Performance evaluation of wideband bio-impedance spectroscopy using constant voltage source and constant current source

An electrical impedance tomography (EIT) system based on frequency division multiplexing (FDM) is proposed for real-time lung physiological imaging. The FDM technique allows the integration of 13 dedicated voltage sensing channels by combining data on-chip and sharing of ADC to alleviate area penalty caused by multi-channel. The EIT system-on-chip (SoC) is of the following features. 1) Early I/Q demodulation to relax the bandwidth requirement of analog front end and minimize the impact of motion artifacts and dc electrode offset. 2) Eliminates the need of adaptive gain control with constant inverted “U-shape” gain configuration to compensate amplitude variations across all channels. 3) FDM to combine 13 pairs of I/Q signals into two data streams for quantization using only two ΔΣ modulators. 4) Batch data recovery by Blackman window corrected fast Fourier transform without any digital filtering involved. 5) Lowest power consumption and smallest area occupation per channel reported to date. The EIT SoC occupies an area of 11.28 mm2 in 130-nm CMOS technology with a total power consumption of 1.53 mW under 1-V power supply. As a result, it generates lung EIT images at up to five frames per second.

A 13-Channel 1.53-mW 11.28-mm 2 Electrical Impedance Tomography SoC Based on Frequency Division Multiplexing for Lung Physiological Imaging

Electrical impedance tomography (EIT) is a promising medical imaging technique which could aid differentiation of haemorrhagic from ischaemic stroke in an ambulance. One challenge in EIT is the ill-posed nature of the image reconstruction, i.e., that small measurement or modelling errors can result in large image artefacts. It is therefore important that reconstruction algorithms are improved with regard to stability to modelling errors. We identify that wrongly modelled electrode positions constitute one of the biggest sources of image artefacts in head EIT. Therefore, the use of the Frechet derivative on the electrode boundaries in a realistic three-dimensional head model is investigated, in order to reconstruct electrode movements simultaneously to conductivity changes. We show a fast implementation and analyse the performance of electrode position reconstructions in time-difference and absolute imaging for simulated and experimental voltages. Reconstructing the electrode positions and conductivities simultaneously increased the image quality significantly in the presence of electrode movement.

https://iopscience.iop.org/article/10.1088/0967-3334/36/12/2423/pdf

Correcting electrode modelling errors in EIT on realistic 3D head models

Most bio-impedance spectroscopy (BIS) systems inject sinusoidal current with a variable frequency into a sample with a known geometry through a pair of electrodes. Adopting the so-called tetra-polar configuration, it measures induced voltage data on a separate pair of electrodes. Impedance spectra are plotted in a certain range of frequency. We found that its accuracy decreases at high frequencies primarily due to the deteriorated performance of the constant current source at high frequencies. Using a previous BIS system we developed, we found that the overall performance can be kept high up to several hundred kHz. In this study, we propose a design of a wideband BIS system using a constant voltage source. It is based on the simple voltage division between an internal resistor and an external sample or load. We switch the value of the internal resistor (Rs) so that the source voltage is divided more or less equally. Two pairs of electrodes are attached to the sample. Two independent voltmeters are used to separately measure two voltages across the chosen internal resistor and the sample. The voltage measurement across the sample is done between the second electrode pair only. This enables us to adopt the tetra-polar configuration to avoid the problem related with contact impedances. We describe the design, construction and performance of the new BIS system with 468 Hz to 2.2 MHz bandwidth. We will compare the results with those using the impedance analyzer.

https://iopscience.iop.org/article/10.1088/1742-6596/224/1/012160/pdf

Wideband bio-impedance spectroscopy using voltage source and tetra-polar electrode configuration

The present invention relates to a sleep apnea monitoring system for monitoring a sleep state of a user by using impedance tomographic image information on a bio-signal and a characteristic of a tissue with respect to a measured part of a user The present invention can be simply manufactured so as to measure the body during natural sleep at home, and it can be used for the diagnosis and treatment plan of sleep apnea by using information of the sleep state acquired through the same

System for sleep apnea monitoring

We have developed a multi-channel, multi-frequency EIT system with operating frequency of 10Hz to 500KHz. The number of digital voltmeters using phase-sensitive demodulation can be varied from 8 to 64 and we found that 16 and 32-channels are most practical. This paper describes the design, implementation, and construction of 16 and 32-channel systems. The performance of the system was thoroughly tested and we found that CMRR of the developed voltmeter is about 85dB with unbalancing series resistor. The SNR is greater than 99.6dB and the output impedance of the constant current source is at least for all frequencies. Imaging experiments using a banana with frequency-dependent conductivity and permittivity show that frequency-difference imaging is possible using the developed system. Future works of animal and human experiments are discussed.

/pdf/multi-frequency-electrical-impedance-tomography-system-2zrnti3rg7.pdf

Multi-Frequency Electrical Impedance Tomography System

In MREIT conductivity imaging experiments of animal and human subjects, we should minimize the noise level in measured magnetic flux density data induced by injection currents with low amplitude. Since noise and artifact from an MREIT current source directly affect the quality of the data, a low-noise current source is desirable. In order to be compatible with various MREIT pulse sequences, it should be also programmable. We have developed a new MREIT current source, which is controlled by a PC program for flexibility. We designed it in such a way that it is located inside the shield room of an MRI system. To minimize noise and artifact, we adopted an optical link for the connection to the PC outside the shield room. The enclosure of the new current source provides a magnetic as well as electric shielding to prevent high frequency switching noise of the current source from interfering with the scanner. It is powered by a rechargeable battery so that the entire current source is isolated from the ground. Equipped with automatic lead switching capability, it simplifies and automates MREIT imaging experiments. Our experimental results show that its performance is superior to the previous version, which is located outside the shield room.

https://iopscience.iop.org/article/10.1088/1742-6596/224/1/012150/pdf

Development of a low noise MREIT current source

The present invention relates to an apparatus for imaging and diagnosing the aspect of upper airway obstruction, which can image a state of change in an upper airway of an obstructive sleep apnea patient, occurring during natural sleep, by using conductivity tomography, and extract information necessary for diagnosis. The apparatus for imaging and diagnosing the aspect of upper airway obstruction using conductivity tomography enables a repetitive inspection with low costs and without danger of radiation exposure. The apparatus enables the inspection for a longer period of time in a real natural sleep state, thereby accurately diagnosing the aspect of the upper airway obstruction.

Tong In Oh

Papers

Wideband bio-impedance spectroscopy using voltage source and tetra-polar electrode configuration

System for sleep apnea monitoring

Multi-Frequency Electrical Impedance Tomography System

Development of a low noise MREIT current source

Upper airway imaging and diagnostic apparatus using electrical impedance tomography