Comparison of a new integrated current source with the modified Howland circuit for EIT applications

Technical Principles and Clinical Applications of Electrical Impedance Tomography in Pulmonary Monitoring

Pulmonary monitoring is crucial for the diagnosis and management of respiratory conditions, especially after the epidemic of coronavirus disease. Electrical impedance tomography (EIT) is an alternative non-radioactive tomographic imaging tool for monitoring pulmonary conditions. This review proffers the current EIT technical principles and applications on pulmonary monitoring, which gives a comprehensive summary of EIT applied on the chest and encourages its extensive usage to clinical physicians. The technical principles involving EIT instrumentations and image reconstruction algorithms are explained in detail, and the conditional selection is recommended based on clinical application scenarios. For applications, specifically, the monitoring of ventilation/perfusion (V/Q) is one of the most developed EIT applications. The matching correlation of V/Q could indicate many pulmonary diseases, e.g., the acute respiratory distress syndrome, pneumothorax, pulmonary embolism, and pulmonary edema. Several recently emerging applications like lung transplantation are also briefly introduced as supplementary applications that have potential and are about to be developed in the future. In addition, the limitations, disadvantages, and developing trends of EIT are discussed, indicating that EIT will still be in a long-term development stage before large-scale clinical applications.

Robust electrical impedance tomography for biological application: A mini review

Electrical impedance tomography (EIT) has been used by researchers across several areas because of its low-cost and no-radiation properties. Researchers use complex conductivity in bioimpedance experiments to evaluate changes in various indicators within the image target. The diverse volumes and edges of biological tissues and the large impedance range impose dedicated demands on hardware design. The EIT hardware with a high signal-to-noise ratio (SNR), fast scanning and suitable for the impedance range of the image target is a fundamental foundation that EIT research needs to be equipped with. Understanding the characteristics of this technique and state-of-the-art design will accelerate the development of the robust system and provide a guidance for the superior performance of next-generation EIT. This review explores the hardware strategies for EIT proposed in the literature.

Parasitic Effects on Electrical Bioimpedance Systems: Critical Review

Parasitic capacitance represents the main error source in measurement systems based on electrical impedance spectroscopy. The capacitive nature of electrodes' impedance in tetrapolar configuration can give origin to phase errors when electrodes are coupled to parasitic capacitances. Nevertheless, reactive charges in tissue excitation systems are susceptible to instability. Based on such a scenario, mitigating capacitive effects associated with the electrode is a requirement in order to reduce errors in the measurement system. A literature review about the main compensation techniques for parasitic capacitance was carried out. The selected studies were categorized into three groups: (i) compensation in electronic instrumentation; (ii) compensation in measurement processing, and (iii) compensation by negative impedance converters. The three analyzed methods emerged as effective against fixed capacitance. No method seemed capable of mitigating the effects of electrodes' capacitance, that changes in the frequency spectrum. The analysis has revealed the need for a method to compensate varying capacitances, since electrodes' impedance is unknown.

On-Chip Sinusoidal Signal Generators for Electrical Impedance Spectroscopy: Methodological Review

This paper reviews architectures and circuit implementations of on-chip sinusoidal signal generators (SSGs) for electrical impedance spectroscopy (EIS) applications. In recent years, there have been increasing interests in on-chip EIS systems, which measure a target material's impedance spectrum over a frequency range. The on-chip implementation allows EIS systems to have low power and small form factor, enabling various biomedical applications. One of the key building blocks of on-chip EIS systems is on-chip SSG, which determines the frequency range and the analysis precision of the whole EIS system. On-chip SSGs are generally required to have high linearity, wide frequency range, and high power and area efficiency. They are typically composed of three stages in general: waveform generation, linearity enhancement, and current injection. First, a sinusoidal waveform should be generated in SSGs. The generated waveform's frequency should be accurately adjustable over a wide range. The firstly generated waveform may not be perfectly linear, including unwanted harmonics. In the following linearity-enhancement step, these harmonics are attenuated by using filters typically. As the linearity of the waveform is improved, the precision of the EIS system gets ensured. Lastly, the filtered voltage waveform is now converted to a current by a current driver. Then, the current sinusoidal signal is injected into the target impedance. This review discusses the principles, advantages, and disadvantages of various techniques applied to each step in state-of-the-art on-chip SSGs. In addition, state-of-the-art designs are compared and summarized.

Analysis, Simulation, and Development of a Low-Cost Fully Active-Electrode Bioimpedance Measurement Module

A low-cost 1 kHz–400 kHz operating frequency fully-active electrode bioimpedance measurement module, based on Howland current source, is presented in this paper. It includes a buffered positive feedback Howland current source, implemented with operational amplifiers, as well as an AD8421 instrumentation amplifier, for the differential voltage measurements. Each active electrode module can be connected to others, assembling a wearable active electrode module array. From this array, 2 electrodes can be selected to be driven from a THS413 fully differential amplifier, activating a mirrored Howland current source. This work performs a complete circuit analysis, verified with MATLAB and SPICE simulations of the current source’s transconductance and output impedance over the frequency range between 1 kHz and 1 MHz. Resistors’ tolerances, possible mismatches, and the operational amplifiers’ non-idealities are considered in both the analysis and simulations. A comparison study between four selected operational amplifiers (ADA4622, OPA2210, AD8034, and AD8672) is additionally performed. The module is also hardware-implemented and tested in the lab for all four operational amplifiers and the transconductance is measured for load resistors of 150 Ω, 660 Ω, and 1200 Ω. Measurements showed that, using the AD8034 operational amplifier, the current source’s transconductance remains constant for frequencies up to 400 KHz for a 150 Ω load and 250 kHz for a 1200 Ω load, while lower performance is achieved with the other 3 operational amplifiers. Finally, transient simulations and measurements are performed at the AD8421 output for bipolar measurements on the 3 aforementioned load resistor values.

A Low-Power Stable Wideband Current Source for Acupuncture Point Skin Impedance Measurements

A low-power stable wideband current source for acupuncture point skin impedance measurements has been designed employing a differential architecture and negative feedback. The circuits extend bandwidth to 1 MHz, reducing harmonic distortion to 0.24% at 1 MHz. The output impedance is 37 MΩ at 100 kHz and 11 MΩ at 1 MHz. The stability of the output current of the current source when connected to different loads is below 0.1% at frequencies up to 500 kHz and increases to 0.74% at 1 MHz. The circuit was manufactured in a 0.13-μm CMOS technology and measured results are presented. The area of the current source is 0.09 mm² and its consumption is 1.2 mW. It is intended for low-power acupuncture point skin impedance measurements.

Bandwidth and Common Mode Optimization for Current and Voltage Sources in Bioimpedance Spectroscopy

Bioimpedance measurements use current or voltage sources to inject an excitation signal into the body. These sources require a high bandwidth, typically from 1 kHz to 1 MHz. Besides a low common mode, current limitation is necessary for patient safety. In this paper, we compare a symmetric enhanced Howland current source (EHCS) and a symmetric voltage source (VS) based on a non-inverting amplifier between 1 kHz and 1 MHz. A common mode reduction circuit has been implemented in both sources. The bandwidth of each source was optimized in simulations and achieved a stable output impedance over the whole frequency range. In laboratory measurements, the output impedance of the EHCS had its -3 dB point at 400 kHz. In contrast, the VS reached the +3 dB point at 600 kHz. On average over the observed frequency range, the active common mode compensation achieved a common mode rejection of -57.7 dB and -71.8 dB for the EHCS and VS, respectively. Our modifications to classical EHCS and VS circuits achieved a low common mode signal between 1 kHz and 1 MHz without the addition of complex circuitry, like general impedance converters. As a conclusion we found VSs to be superior to EHCSs for bioimpedance spectroscopy due to the higher bandwidth performance. However, this only applies if the injected current of the VS can be measured.

A wearable EIT system for detection of muscular activity in the extremities

Electrical Impedance Tomography (EIT) is a method for measuring physiological states and processes that can be used as an imaging method for muscular activities. In addition to the medical evaluation of the EIT data of the lung, this technology can be used to make a statement about muscular activity in the extremities. This paper presents a developed, mobile EIT system that can be used with an electrode bracelet on the arm. In a rst study, the EIT data for different hand gestures were evaluated.

AN EXPERIMENTAL ANALYSIS AND VALIDATION OF ELECTRICAL IMPEDANCE TOMOGRAPHY TECHNIQUE FOR MEDICAL OR INDUSTRIAL APPLICATION

Electrical impedance tomography is a recently established technique by which impedance of an object (medical or nonmedical applications) is measured data from the surface of the object, and a numerically simulated reconstruction of the object internal shape of the image can be obtained. This imaging technique based on boundary or surface voltage is measured when the different current pattern is injected into it. For current pulse, we are creating a voltage controlled current source, which is based on the different RC circuits, according to current amplitude and frequency values. The current source used in inject the current pulse of the various phantoms. The current position and measuring voltage is controlled by the created control unit or programmable system on chip (PSOC) of the proposed EIT system. After that image reconstruction of the cross-sectional image of resistivity requires sufficient data collection from used phantoms, which is based on finite element method (FEM) method and Tikhonov regularization method with helps of graphical user interface (GUI) on MatLab. The objective of the GUI was to produce an image (2D/3D), impedance distribution graph, and the FEM mesh model according to used electrode combinations from the various phantoms. EIT system has a great potential for imaging modality, is non-invasive, radiation-free, and inexpensive for medical applications.

Analog Integrated Current Drivers for Bioimpedance Applications: A Review

An important component in bioimpedance measurements is the current driver, which can operate over a wide range of impedance and frequency. This paper provides a review of integrated circuit analog current drivers which have been developed in the last 10 years. Important features for current drivers are high output impedance, low phase delay, and low harmonic distortion. In this paper, the analog current drivers are grouped into two categories based on open loop or closed loop designs. The characteristics of each design are identified.

Mirrored enhanced Howland current source with feedback control

An impedance spectrum is calculated by the ratio between an injecting current and a resulting measured voltage, which allows the extraction of electrical properties from the material under study. The current source is considered an essential block to deliver a controlled current to a wide range of working loads and large bandwidth. To comply with such requirements, the current source output impedance must be much higher than the load impedance at each discrete frequency within the range. However, stray capacitance from cables and circuitry reduce the output impedance, especially at higher frequencies. We proposed a modified mirrored enhanced Howland current source (MEHCS) by using the feedback technique for a wide frequency range applications on electrical bioimpedance. We implemented four MEHCS circuits [with/without multiplexer (MUX) and with/without feedback], and then the output current and impedance were measured up to 20 MHz. The proposed current source showed an improvement in the frequency response at lower and higher frequencies when compared to the standard circuit. The measured output impedance was 10 times higher in the proposed circuit than in the standard MEHCS. The use of a feedback also increased the bandwidth in almost one decade in low and high frequencies when loaded with a resistor of about 1 kΩ.

Design and Evaluation of an Electrical Bioimpedance Device Based on DIBS for Myography during Isotonic Exercises

Electrical Bioimpedance Spectroscopy (EIS) is a technique used to assess passive electrical properties of biological materials. EIS detects physiological and pathological conditions in animal tissues. Recently, the introduction of broadband excitation signals has reduced the measuring time for application techniques such as Electrical Bioimpedance Myography. Therefore, this work is aimed at proposing a prototype by using discrete interval binary sequences (DIBS), which is based on a system that holds a current source, impedance acquisition system, microcontroller and graphical user interface. Measurements between 5 Ω to 5 kΩ had impedance acquisition and phase angle errors of aproximately 2% and were lower than 3 degrees, respectively. Based on a proposed circuit, bioimpedance of the chest muscle (Pectoralis Major) was measured during isotonic exercise (push-up). As a result, our analyses have detected tiredness and fatigue. We have explored and proposed new parameters which assess such conditions, as both the maximum magnitude and tiredness coefficient. These parameters decrease exponentially with consecutive push-ups and were convergent in the majority of the sixteen days of measurement.

A Human-Machine Interface Using Electrical Impedance Tomography for Hand Prosthesis Control

This paper presents a human-machine interface that establishes a link between the user and a hand prosthesis. It successfully uses electrical impedance tomography, a conventional bio-impedance imaging technique, using an array of electrodes contained in a wristband on the user's forearm. Using a high-performance analog front-end application specific integrated circuit (ASIC), the user's forearm inner bio-impedance redistribution is accurately assessed. These bio-signatures are strongly related to hand motions and using artificial neural networks, they can be learned so as to recognize the user's intention in real time for prosthesis operation. In this work, eleven hand motions are designed for prosthesis operation with a gesture switching enabled sub-grouping method. Experiments with five subjects show that the system can achieve 98.5% accuracy with a grouping of three gestures and an accuracy of 94.4% with two sets of five gestures. The ASIC comprises a current driver with common-mode reduction capability and a current feedback instrumentation amplifier (that occupy an area of 0.07 mm²). The ASIC operates from ±1.65 V power supplies and has a minimum bio-impedance sensitivity of 12.7 mΩ_p-p.

Howland current source for high impedance load applications

For Electrical Impedance Spectroscopy (EIS) applications, the Enhanced Howland Current Source (EHCS) is a popular choice as an excitation circuit due to its simplicity, reliability, and safety. However, its output impedance degradation at high frequency leads to errors that are unacceptable for high load impedance applications, such as the ones which use dry or microelectrodes. Based on a proposed mathematical model, this work designed an EHCS circuit which includes an output current buffer and frequency compensation. PSpice simulations were performed as proof of concept, and then the measured data were collected for comparison. For the proposed circuit, called here Load-in-the-Loop Compensated Enhanced Howland Source (LLC-EHCS), the results showed that the output current errors are lower than 1% up to 3.7 MHz over the load range of 560-2200 Ω and 1.2 MHz with 5.6 kΩ. On the other hand, for the case of the standard EHCS circuit, these frequencies are 170 and 80 kHz, respectively. Also, the output linear swing was found to be 3 times higher than the EHCS. It can be concluded that the proposed LLC-EHCS may be widely used as an excitation circuit for high load and wide bandwidth EIS applications.

A Sinusoidal Current Driver With an Extended Frequency Range and Multifrequency Operation for Bioimpedance Applications

This paper describes an alternative sinusoidal current driver suitable for bioimpedance applications where high frequency operation is required. The circuit is based on a transconductor and provides current outputs with low phase error for frequencies around its pole frequency. This extends the upper frequency operational limit of the current driver. Multifrequency currents can be generated where each individual frequency is phase corrected. Analysis of the circuit is presented together with simulation and experimental results which demonstrate the proof of concept for both single and dual frequency current drivers. Measurements on a discrete test version of the circuit demonstrate a phase reduction from 25° to 4° at 3 MHz for 2 mAp-p output current. The output impedance of the current driver is essentially constant at about 1.1 M Ω over a frequency range of 100 kHz to 5 MHz due to the introduction of the phase compensation. The compensation provides a bandwidth increase of a factor of about six for a residual phase delay of 4°.

Design of Bioimpedance Spectroscopy Instrument With Compensation Techniques for Soft Tissue Characterization

Bioimpedance spectroscopy (BIS) has shown significant potential in many areas of medicine to provide new physiologic markers. Several acute and chronic diseases are accompanied by changes in intra- and extracellular fluid within various areas of the human body. The estimation of fluid in various body compartments is therefore a simple and convenient method to monitor certain disease states. In this work, the design and evaluation of a BIS instrument are presented and three key areas of the development process investigated facilitating the BIS measurement of tissue hydration state. First, the benefit of incorporating DC-stabilizing circuitry to the standard modified Howland current pump (MHCP) is investigated to minimize the effect of DC offsets limiting the dynamic range of the system. Second, the influence of the distance between the bioimpedance probe and a high impedance material is investigated using finite element analysis (FEA). Third, an analytic compensation technique is presented to minimize the influence of parasitic capacitance. Finally, the overall experimental setup is evaluated through ex vivo BIS measurements of porcine spleen tissue and compared to published results. The DC-stabilizing circuit demonstrated its ability to maintain DC offsets at less than 650 μV through 100 kHz while maintaining an output impedance of 1 MΩ from 100 Hz to 100 kHz. The proximity of a bioimpedance probe to a high impedance material such as acrylic was shown to increase measured impedance readings by a factor of 4x as the ratio of the distance between the sensing electrodes to the distance between the bioimpedance probe and acrylic reached 1:3. The average parasitic capacitance for the circuit presented was found to be 712 ± 128 pF, and the analytic compensation method was shown to be able to minimize this effect on the BIS measurements. Measurements of porcine spleen tissue showed close correlation with experimental results reported in published articles. This research presents the successful design and evaluation of a BIS instrument. Specifically, robust measurements were obtained by implementing a DC-stabilized current source, investigating probe-material proximity issues and compensating for parasitic capacitance. These strategies were shown to provide tissue measurements comparable with published literature.

On the application of frequency selective common mode feedback for multifrequency EIT

Common mode voltages are frequently a problem in electrical impedance tomography (EIT) and other bioimpedance applications. To reduce their amplitude common mode feedback is employed. Formalised analyses of both current and voltage feedback is presented in this paper for current drives. Common mode effects due to imbalances caused by the current drives, the electrode connections to the body load and the introduction of the body impedance to ground are considered. Frequency selective narrowband common mode feedback previously proposed to provide feedback stability is examined. As a step towards multifrequency applications the use of narrowband feedback is experimentally demonstrated for two simultaneous current drives. Measured results using standard available components show a reduction of 62 dB for current feedback and 31 dB for voltage feedback. Frequencies ranged from 50 kHz to 1 MHz.

Electrical impedance imaging system using FPGAs for flexibility and interoperability

BACKGROUND

Modern EIT systems require simultaneously operating multiple functions for flexibility, interoperability, and clinical applicability. To implement versatile functions, expandable design and implementation tools are needed. On the other hand, it is necessary to develop an ASIC-based EIT system to maximize its performance. Since the ASIC design is expensive and unchangeable, we can use FPGAs as a prior step to the digital ASIC design and carefully classify which functions should be included in the ASIC. In this paper, we describe the details of the FPGA design adopted in the KHU Mark2.5 EIT system.

METHODS

We classified all functions of the KHU Mark2.5 EIT system into two categories. One is the control and processing of current injection and voltage measurement. The other includes the collection and management of the multi-channel data with timing controls for internal and external interconnections. We describe the implementation of these functions in two kinds of FPGAs called the impedance measurement module (IMM) FPGA and the intra-network controller FPGA.

RESULTS

We present functional and timing simulations of the key functions in the FPGAs. From phantom and animal imaging experiments, we show that multiple functions of the system are successfully implemented in the FPGAs. As examples, we demonstrate fast multi-frequency imaging and ECG-gated imaging.

CONCLUSION

Given an analog design of a parallel EIT system, it is important to optimize its digital design to minimize systematic artifacts and maximize performance. This paper described technical details of the FPGA-based fully parallel EIT system called the KHU Mark2.5 with numerous functions needed for clinical applications. Two kinds of FPGAs described in this paper can be used as a basis for future EIT digital ASIC designs for better application-specific human interface as well as hardware performance.

High-power CMOS current driver with accurate transconductance for electrical impedance tomography

Current drivers are fundamental circuits in bioimpedance measurements including electrical impedance tomography (EIT). In the case of EIT, the current driver is required to have a large output impedance to guarantee high current accuracy over a wide range of load impedance values. This paper presents an integrated current driver which meets these requirements and is capable of delivering large sinusoidal currents to the load. The current driver employs a differential architecture and negative feedback, the latter allowing the output current to be accurately set by the ratio of the input voltage to a resistor value. The circuit was fabricated in a 0.6- μm high-voltage CMOS process technology and its core occupies a silicon area of 0.64 mm (2) . It operates from a ± 9 V power supply and can deliver output currents up to 5 mA p-p. The accuracy of the maximum output current is within 0.41% up to 500 kHz, reducing to 0.47% at 1 MHz with a total harmonic distortion of 0.69%. The output impedance is 665 k Ω at 100 kHz and 372 k Ω at 500 kHz.

The differential Howland current source with high signal to noise ratio for bioimpedance measurement system

The stability and signal to noise ratio (SNR) of the current source circuit are the important factors contributing to enhance the accuracy and sensitivity in bioimpedance measurement system. In this paper we propose a new differential Howland topology current source and evaluate its output characters by simulation and actual measurement. The results include (1) the output current and impedance in high frequencies are stabilized after compensation methods. And the stability of output current in the differential current source circuit (DCSC) is 0.2%. (2) The output impedance of two current circuits below the frequency of 200 KHz is above 1 MΩ, and below 1 MHz the output impedance can arrive to 200 KΩ. Then in total the output impedance of the DCSC is higher than that of the Howland current source circuit (HCSC). (3) The SNR of the DCSC are 85.64 dB and 65 dB in the simulation and actual measurement with 10 KHz, which illustrates that the DCSC effectively eliminates the common mode interference. (4) The maximum load in the DCSC is twice as much as that of the HCSC. Lastly a two-dimensional phantom electrical impedance tomography is well reconstructed with the proposed HCSC. Therefore, the measured performance shows that the DCSC can significantly improve the output impedance, the stability, the maximum load, and the SNR of the measurement system.

Simulation of a current source with a Cole-Cole load for multi-frequency electrical impedance tomography

An accurate current source is one of the keys in the hardware of Electrical impedance Tomography systems. Limitations appear mainly at higher frequencies and for non-simple resistive loads. In this paper, we simulate an improved Howland current source with a Cole-Cole load. Simulations comparing two different op-amps (THS4021 and OPA843) were performed at 1 kHz to 1 MHz. Results show that the THS4021 performed better than the OPA843. The current source with THS4021 reaches an output impedance of 20 MΩ at 1 kHz and above 320 kΩ at 1 MHz, it provides a constant and stable output current up to 4 mA, in the complete range of frequencies, and for Cole-Cole (resistive and capacitive) load.

Biocompatible, high precision, wideband, improved Howland current source with lead-lag compensation

The Howland current pump is a popular bioelectrical circuit, useful for delivering precise electrical currents. In applications requiring high precision delivery of alternating current to biological loads, the output impedance of the Howland is a critical figure of merit that limits the precision of the delivered current when the load changes. We explain the minimum operational amplifier requirements to meet a target precision over a wide bandwidth. We also discuss effective compensation strategies for achieving stability without sacrificing high frequency output impedance. A current source suitable for Electrical Impedance Tomography (EIT) was simulated using a SPICE model, and built to verify stable operation. This current source design had stable output impedance of 3.3 MΩ up to 200 kHz, which provides 80 dB precision for our EIT application. We conclude by noting the difficulty in measuring the output impedance, and advise verifying the plausibility of measurements against theoretical limitations.

Comparison of three current sources for single-electrode capacitance measurement

The capacitance of a single electrode is usually measured by injecting a current to the electrode and measuring the resultant voltage on the electrode. In this case, a voltage-controlled current source with a high bandwidth is needed because the impedance is inversely proportional to the excitation frequency. In this design note, three different current sources are discussed: (1) the Howland current source, (2) a modified Howland current source, and (3) a dual op-amp current source. The principle and dynamic performances are presented and compared. Simulation and experimental results show that although the Howland current source has the lowest (i.e., worst) output impedance, its output is the most stable among the three current sources when the frequency changes. Therefore, it is suitable for single-electrode capacitance measurement. Initial tests have proven the feasibility of single-electrode capacitance sensor with the Howland current source.