A model of artefacts produced by stray capacitance during whole body or segmental bioimpedance spectroscopy

Determination of resistance at zero and infinite frequencies in bioimpedance spectroscopy for assessment of body composition in babies

Objective. Bioimpedance spectroscopy (BIS) is a popular technique for the assessment of body composition in children and adults but has not found extensive use in babies and infants. This due primarily to technical difficulties of measurement in these groups. Although improvements in data modelling have, in part, mitigated this issue, the problem continues to yield unacceptably high rates of poor quality data. This study investigated an alternative data modelling procedure obviating issues associated with BIS measurements in babies and infants.Approach.BIS data are conventionally analysed according to the Cole model describing the impedance response of body tissues to an appliedACcurrent. This approach is susceptible to errors due to capacitive leakage errors of measurement at high frequency. The alternative is to model BIS data based on the resistance-frequency spectrum rather than the reactance-resistance Cole model thereby avoiding capacitive error impacts upon reactance measurements.Main results.The resistance-frequency approach allowed analysis of 100% of data files obtained from BIS measurements in 72 babies compared to 87% successful analyses with the Cole model. Resistance-frequency modelling error (percentage standard error of the estimate) was half that of the Cole method. Estimated resistances at zero and infinite frequency were used to predict body composition. Resistance-based prediction of fat-free mass (FFM) exhibited a 30% improvement in the two-standard deviation limits of agreement with reference FFM measured by air displacement plethysmography when compared to Cole model-based predictions.Significance.This study has demonstrated improvement in the analysis of BIS data based on the resistance frequency response rather than conventional Cole modelling. This approach is recommended for use where BIS data are compromised by high frequency capacitive leakage errors such as those obtained in babies and infants.

Differences in bioimpedance-derived fluid status between two versions of the Body Composition Monitor

OBJECTIVES

The Body Composition Monitor (BCM) (Fresenius Medical Care) measures body impedances in alternating currents to subsequently calculate fat and lean tissue mass, fluid compartments, and overhydration (OH). The aim of this study was to investigate differences between two versions of the BCM (an older version, 3.2.5, and a newer version, 3.3.3).

METHODS

Between September 2021 and December 2021, 28 hemodialysis patients were included to undergo BCM measurements before each of 14 consecutive dialysis sessions with versions 3.2.5 and 3.3.3 devices. Measurements were performed according to instructions provided by the manufacturer. Differences between BCM devices were tested for statistical significance using paired Wilcoxon tests, neglecting clustering.

RESULTS

A total of 288 measurement pairs of 27 patients were left after exclusion of 43 flawed data points. The mean difference in OH between both BCM devices was 0.548 L (higher for version 3.2.5). Analysis of impedance data revealed differences in the high-frequency spectrum, quantifiable by the intracellular resistance, Ri (median Ri version 3.2.5 = 1750.3 Ω; Ri version 3.3.3 = 1612.45 Ω; P < 0.001), and the time delay, Td (median Td version 3.2.5 = 1.85 ns; Td version 3.3.3 = 8.88 nanoseconds; P < 0.001).

CONCLUSIONS

This study finds that results between the two versions of the BCM differed in a clinically meaningful fashion and that the newer version 3.3.3 device had a bias toward less OH. Circulating BCM devices should be checked for versions and only devices of the same version should be used for each patient to ensure better within-patient consistency.

Characterization and Correction of Low Frequency Artifacts in Segmental Bioimpedance Measurements

Bioimpedance analysis can be used for remote monitoring of volume status for various conditions such as congestive heart failure. The measurement is typically performed with four electrodes, two of them driving an alternating current through the tissue and the other two sensing the resulting voltage. Issues with the measurement setup such as stray capacitance or electrode mismatch can cause artifacts that impact Cole parameters used for volume estimation. While previous research has focused on mitigating high frequency artifacts, little research has been done to understand the cause and impact of low frequency artifacts, nor how to mitigate the impact of these artifacts. These artifacts are most prevalent in wearable segmental bioimpedance systems, especially using textile electrodes, so future research in this area is needed for these systems to be viable. The present study uses simulations to identify the potential sources of low frequency artifacts, and explores techniques to minimize the impact of these artifacts on Cole parameters. Theoretical analysis and simulations show that the mismatch of the voltage electrodes causes artifacts at low frequency. These artifacts are highly dependent on the impedance of the negative current injecting electrode. Averaging measurements of the mismatch of both voltage electrodes and limiting high frequency measurements to 200 kHz can reduce errors due to these artifacts from over 137% to less than 3%. The results of this study suggest the impact of low frequency artifacts can be significantly reduced, enabling future development of wearable bioimpedance systems.Clinical relevance- Reducing the impact of low frequency artifacts on Cole parameter estimation enables wearable segmental bioimpedance systems that can be used for remote monitoring of volume status in home environments.

Noninvasive Monitoring to Detect Dehydration: Are We There Yet?

The need for hydration monitoring is significant, especially for the very young and elderly populations who are more vulnerable to becoming dehydrated and suffering from the effects that dehydration brings. This need has been among the drivers of considerable effort in the academic and commercial sectors to provide a means for monitoring hydration status, with a special interest in doing so outside the hospital or clinical setting. This review of emerging technologies provides an overview of many technology approaches that, on a theoretical basis, have sensitivity to water and are feasible as a routine measurement. We review the evidence of technical validation and of their use in humans. Finally, we highlight the essential need for these technologies to be rigorously evaluated for their diagnostic potential, as a necessary step to meet the need for hydration monitoring outside of the clinical environment.

Evaluating Research Grade Bioimpedance Hardware Using Textile Electrodes for Long-Term Fluid Status Monitoring

Fluid overload is a chronic medical condition that affects over six million Americans with conditions such as congestive heart failure, end-stage renal disease, and lymphedema. Remote management of fluid overload continues to be a leading clinical challenge. Bioimpedance is one technique that can be used to estimate the hydration of tissue and track it over time. However, commercially available bioimpedance measurement systems are bulky, expensive, and rely on Ag/AgCl electrodes that dry out and can irritate the skin. The use of bioimpedance today is therefore limited to clinical and research settings, with measurements performed at daily intervals or over short periods of time rather than continuously and long-term. This paper proposes using wearable calf bioimpedance measurements integrated into a compression sock for long-term fluid overload management. A PCB was developed using standard measurement techniques that measures the calf bioimpedance using a custom analog front-end built around an AD8302 gain-phase detection chip. Data is transmitted wirelessly via Bluetooth Low Energy to an iOS device using a custom iOS app. Bioimpedance data were collected both from the wearable system and a commercial measurement system (ImpediMed SFB7) using RRC networks, Ag/AgCl electrodes, and the textile compression sock. Bioimpedance data collected from the wearable system showed close agreement with data from the SFB7 when using RRC networks and in five healthy human subjects with Ag/AgCl electrodes. However, when using the textile compression sock the wearable system had worse precision than the SFB7 (4% run to run compared to <1% run to run) and there were larger differences between the two systems than when using the RRC networks and the Ag/AgCl electrodes. Wearable system precision and agreement with the SFB7 was improved by pressure or light wetting of the current electrodes on the sock. Future research should focus on reliable elimination of low-frequency artifacts in research grade hardware to enable long-term calf bioimpedance measurements for fluid overload management.

Bioimpedance Spectroscopy of the Breast

Background: Bioimpedance spectroscopy (BIS) measurements of breast lymphedema poses practical and technical challenges, in particular the determination of the resistance at zero frequency (R₀), the index of change in breast lymph content. Conventionally, R₀ is calculated from data analysis by using a procedure eponymously known as Cole modeling, a method that is error-prone in the breast. The aim of this study was to evaluate polynomial curve fitting as an alternative analytic procedure. Methods and Results: A sub-set of breast BIS measurements from 41 women with self-ascribed breast lymphedema obtained as part of the Breast Edema Exercise Trial (BEET) were analyzed by both the Cole and polynomial methods. BIS files for all subjects were able to be analyzed by using the polynomial method but only 73% and 88% of data files were analyzed for the affected and unaffected breasts, respectively, by using the Cole method. For those files that were capable of being analyzed by both methods, R₀ values were highly correlated (r = 0.99) but with a small (1.6%) although statistically significant difference (paired t test, p < 0.001) between methods. Conclusions: Analysis of BIS data using polynomial curve fitting is an acceptable and robust alternative to Cole modeling, particularly where impedance measurements are susceptible to technical sources of error of measurement. The small magnitude of difference observed between methods is unlikely to lead to misclassification of patients with lymphedema based on BIS assessment.

Textile band electrodes as an alternative to spot Ag/AgCl electrodes for calf bioimpedance measurements

OBJECTIVE

To evaluate the performance of five different types of textiles as band electrodes for calf bioimpedance measurements in comparison with conventional spot Ag/AgCl electrodes.

APPROACH

Calf bioimpedance measurements were performed in 10 healthy volunteers with five different textile materials cut into bands and Ag/AgCl spot electrodes as a baseline. Collected bioimpedance data were analyzed in terms of precision, fit error and presence of measurement artifacts. Each textile material was also evaluated for participant comfort.

MAIN RESULTS

Bioimpedance values for spot electrodes were higher at low frequencies as compared with band electrodes but not at high frequencies. This suggests that spot electrodes have frequency dependent current distributions that adversely impact their use for volume measurements and band electrodes are preferable. The SMP130T-B fabric had the highest precision and the lowest best fit error to the Cole model of the tested textile materials. However, it was the least comfortable textile and most expensive. The Stretch material performed slightly worse than the SMP130T-B fabric, but was half the cost and the most comfortable.

SIGNIFICANCE

These results suggest that there are suitable textile materials for use as dry, band electrodes for calf bioimpedance measurements and that these band electrodes enable greater current uniformity. These textiles could be integrated into a compression sock for remote monitoring of diseases such as Congestive Heart Failure.

Textile-Friendly Interconnection between Wearable Measurement Instrumentation and Sensorized Garments-Initial Performance Evaluation for Electrocardiogram Recordings

The interconnection between hard electronics and soft textiles remains a noteworthy challenge in regard to the mass production of textile–electronic integrated products such as sensorized garments. The current solutions for this challenge usually have problems with size, flexibility, cost, or complexity of assembly. In this paper, we present a solution with a stretchable and conductive carbon nanotube (CNT)-based paste for screen printing on a textile substrate to produce interconnectors between electronic instrumentation and a sensorized garment. The prototype connectors were evaluated via electrocardiogram (ECG) recordings using a sensorized textile with integrated textile electrodes. The ECG recordings obtained using the connectors were evaluated for signal quality and heart rate detection performance in comparison to ECG recordings obtained with standard pre-gelled Ag/AgCl electrodes and direct cable connection to the ECG amplifier. The results suggest that the ECG recordings obtained with the CNT paste connector are of equivalent quality to those recorded using a silver paste connector or a direct cable and are suitable for the purpose of heart rate detection.

A Practical Method to Reduce Electrode Mismatch Artefacts during 4-electrode BioImpedance Spectroscopy Measurements

We present a novel and practical method of removing distortions due to electrode impedance mismatch encountered during 4-electrode bioimpedance spectroscopy (BIS) measurements. Recorded Iocalised, or even whole-body, tissue impedances often evidence high frequency artefacts which resemble additional capacitive or inductive behaviours. We show that making two impedance measurements with the same four electrodes, but by connecting them in different arrangements, we can cause either the observed high-frequency capacitive behaviour or the inductive behaviour. Additionally, simply calculating the mean of these two distorted data sets leads to a corrected, "artefact-free" impedance close to that expected. This correction method was validated on R-C networks (simulated as well as measured) and on biological tissue measurements (healthy forearm and oedematous leg). The described method was found valid using an SFB7 Impedimedo over a frequency range of 3 to $1000~\mathrm {k}\mathrm {H}\mathrm {z}$. It is possible that other impedance meters and frequency ranges could also benefit from this simple technique.

Extracting Parasite Effects of Electrical Bioimpedance Measurements

The objective of this work is to develop a technique for filtering parasitic effects from the impedance spectra (IS) measured in biological material phantoms. IS data are contaminated with unexpected capacitive and inductive effects from cable, input/output amplifiers capacitances, electrode polarization, temperature and contact pressure when collecting data. It is proposed a model which contains an RLC-network in series with the Cole model (RSC), then called RLC-Cole. It was built four circuits composed by resistors, capacitors and inductors. An impedance analyzer (HF2IS) was used to perform the measurements in the frequency range of 1 to 3000 kHz. Data were fitted into the model and comparisons to the nominal values were made. In order to validate the proposed model, a gelatin phantom and a chicken breast muscle impedance spectra were also collected and analyzed. After filtering, Cole fitting was performed. Results showed a maximum root-mean-square error of 1% for the circuits, 2.63% for the gelatin phantom, whereas 2.01% for the chicken breast. The RLC-Cole model could significantly remove parasitic effects out of a tissue impedance spectrum measured by a 4-point electrode probe. This may be highly important in EIS systems whose objective is to discriminate a normal tissue from a cancerous one.

Sensitivity distribution simulations of surface electrode configurations for electrical impedance myography

INTRODUCTION

Surface-based electrical impedance myography (EIM) is sensitive to muscle condition in neuromuscular disorders. However, the specific contribution of muscle to the obtained EIM values is unknown.

METHODS

We combined theory and the finite element method to calculate the electrical current distribution in a 3-dimensional model using different electrode array designs and subcutaneous fat thicknesses (SFTs). Through a sensitivity analysis, we decoupled the contribution of muscle from other surrounding tissues in the measured surface impedance values.

RESULTS

The contribution of muscle to surface EIM values varied greatly depending on the electrode array size and the SFT. For example, the contribution of muscle with 6-mm SFT was 8% for a small array compared with 32% for a large array.

CONCLUSIONS

The approach presented can be employed to inform the design of robust EIM electrode configurations that maximize the contribution of muscle across the disease and injury spectrum. Muscle Nerve 56: 887-895, 2017.

Electrical Impedance Myography and Its Applications in Neuromuscular Disorders

Electrical impedance myography (EIM) refers to the specific application of electrical bioimpedance techniques for the assessment of neuromuscular disorders. In EIM, a weak, high-frequency electrical current is applied to a muscle or muscle group of interest and the resulting voltages measured. Among its advantages, the technique can be used noninvasively across a variety of disorders and requires limited subject cooperation and evaluator training to obtain accurate and repeatable data. Studies in both animals and human subjects support its potential utility as a primary diagnostic tool, as well as a biomarker for clinical trial or individual patient use. This review begins by providing an overview of the current state and technological advances in electrical impedance myography and its specific application to the study of muscle. We then provide a summary of the clinical and preclinical applications of EIM for neuromuscular conditions, and conclude with an evaluation of ongoing research efforts and future developments.

A Handheld and Textile-Enabled Bioimpedance System for Ubiquitous Body Composition Analysis. An Initial Functional Validation

In recent years, many efforts have been made to promote a healthcare paradigm shift from the traditional reactive hospital-centered healthcare approach towards a proactive, patient-oriented, and self-managed approach that could improve service quality and help reduce costs while contributing to sustainability. Managing and caring for patients with chronic diseases accounts over 75% of healthcare costs in developed countries. One of the most resource demanding diseases is chronic kidney disease (CKD), which often leads to a gradual and irreparable loss of renal function, with up to 12% of the population showing signs of different stages of this disease. Peritoneal dialysis and home haemodialysis are life-saving home-based renal replacement treatments that, compared to conventional in-center hemodialysis, provide similar long-term patient survival, less restrictions of life-style, such as a more flexible diet, and better flexibility in terms of treatment options and locations. Bioimpedance has been largely used clinically for decades in nutrition for assessing body fluid distributions. Moreover, bioimpedance methods are used to assess the overhydratation state of CKD patients, allowing clinicians to estimate the amount of fluid that should be removed by ultrafiltration. In this work, the initial validation of a handheld bioimpedance system for the assessment of body fluid status that could be used to assist the patient in home-based CKD treatments is presented. The body fluid monitoring system comprises a custom-made handheld tetrapolar bioimpedance spectrometer and a textile-based electrode garment for total body fluid assessment. The system performance was evaluated against the same measurements acquired using a commercial bioimpedance spectrometer for medical use on several voluntary subjects. The analysis of the measurement results and the comparison of the fluid estimations indicated that both devices are equivalent from a measurement performance perspective, allowing for its use on ubiquitous e-healthcare dialysis solutions.

Guidelines to electrode positioning for human and animal electrical impedance myography research

The positioning of electrodes in electrical impedance myography (EIM) is critical for accurately assessing disease progression and effectiveness of treatment. In human and animal trials for neuromuscular disorders, inconsistent electrode positioning adds errors to the muscle impedance. Despite its importance, how the reproducibility of resistance and reactance, the two parameters that define EIM, are affected by changes in electrode positioning remains unknown. In this paper, we present a novel approach founded on biophysical principles to study the reproducibility of resistance and reactance to electrode misplacements. The analytical framework presented allows the user to quantify a priori the effect on the muscle resistance and reactance using only one parameter: the uncertainty placing the electrodes. We also provide quantitative data on the precision needed to position the electrodes and the minimum muscle length needed to achieve a pre-specified EIM reproducibility. The results reported here are confirmed with finite element model simulations and measurements on five healthy subjects. Ultimately, our data can serve as normative values to enhance the reliability of EIM as a biomarker and facilitate comparability of future human and animal studies.

Detection and Classification of Measurement Errors in Bioimpedance Spectroscopy

Bioimpedance spectroscopy (BIS) measurement errors may be caused by parasitic stray capacitance, impedance mismatch, cross-talking or their very likely combination. An accurate detection and identification is of extreme importance for further analysis because in some cases and for some applications, certain measurement artifacts can be corrected, minimized or even avoided. In this paper we present a robust method to detect the presence of measurement artifacts and identify what kind of measurement error is present in BIS measurements. The method is based on supervised machine learning and uses a novel set of generalist features for measurement characterization in different immittance planes. Experimental validation has been carried out using a database of complex spectra BIS measurements obtained from different BIS applications and containing six different types of errors, as well as error-free measurements. The method obtained a low classification error (0.33%) and has shown good generalization. Since both the features and the classification schema are relatively simple, the implementation of this pre-processing task in the current hardware of bioimpedance spectrometers is possible.

A Bioimpedance Analysis Platform for Amputee Residual Limb Assessment

OBJECTIVE

The objective of this research was to develop a bioimpedance platform for monitoring fluid volume in residual limbs of people with trans-tibial limb loss using prostheses.

METHODS

A customized multifrequency current stimulus profile was sent to thin flat electrodes positioned on the thigh and distal residual limb. The applied current signal and sensed voltage signals from four pairs of electrodes located on the anterior and posterior surfaces were demodulated into resistive and reactive components. An established electrical model (Cole) and segmental limb geometry model were used to convert results to extracellular and intracellular fluid volumes. Bench tests and testing on amputee participants were conducted to optimize the stimulus profile and electrode design and layout.

RESULTS

The proximal current injection electrode needed to be at least 25 cm from the proximal voltage sensing electrode. A thin layer of hydrogel needed to be present during testing to ensure good electrical coupling. Using a burst duration of 2.0 ms, intermission interval of 100 μs, and sampling delay of 10 μs at each of 24 frequencies except 5 kHz, which required a 200-μs sampling delay, the system achieved a sampling rate of 19.7 Hz.

CONCLUSION

The designed bioimpedance platform allowed system settings and electrode layouts and positions to be optimized for amputee limb fluid volume measurement.

SIGNIFICANCE

The system will be useful toward identifying and ranking prosthetic design features and participant characteristics that impact residual limb fluid volume.

Mean Expected Error in Prediction of Total Body Water: A True Accuracy Comparison between Bioimpedance Spectroscopy and Single Frequency Regression Equations

For several decades electrical bioimpedance (EBI) has been used to assess body fluid distribution and body composition. Despite the development of several different approaches for assessing total body water (TBW), it remains uncertain whether bioimpedance spectroscopic (BIS) approaches are more accurate than single frequency regression equations. The main objective of this study was to answer this question by calculating the expected accuracy of a single measurement for different EBI methods. The results of this study showed that all methods produced similarly high correlation and concordance coefficients, indicating good accuracy as a method. Even the limits of agreement produced from the Bland-Altman analysis indicated that the performance of single frequency, Sun's prediction equations, at population level was close to the performance of both BIS methods; however, when comparing the Mean Absolute Percentage Error value between the single frequency prediction equations and the BIS methods, a significant difference was obtained, indicating slightly better accuracy for the BIS methods. Despite the higher accuracy of BIS methods over 50 kHz prediction equations at both population and individual level, the magnitude of the improvement was small. Such slight improvement in accuracy of BIS methods is suggested insufficient to warrant their clinical use where the most accurate predictions of TBW are required, for example, when assessing over-fluidic status on dialysis. To reach expected errors below 4-5%, novel and individualized approaches must be developed to improve the accuracy of bioimpedance-based methods for the advent of innovative personalized health monitoring applications.

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.

Multifrequency right-side, localized and segmental BIA obtained with different bioimpedance analysers

The aim of this study is to compare two commercial bioimpedance analysers, BioparHom Z-Métrix and Impedimed SFB7, measuring the impedance of three different body segments. The segments measured were 'right-side' (or 'whole-body'), 'segmental right-lower limb' and 'localized longitudinal right-quadriceps'. The comparison was made on electrical models of each segment, including electrode-skin impedance, and in vivo on nine healthy volunteers. Both devices are designed to measure right-side impedances and, in the present study, as the length of the segment investigated decreased, the accuracy of the impedance measured was found to decrease. The accuracy of the devices was calculated via measurements performed on RC networks of known values. It was found that adding electrode-skin contact impedances in the electrical model affected the accuracy by both devices.

The theory and fundamentals of bioimpedance analysis in clinical status monitoring and diagnosis of diseases

Bioimpedance analysis is a noninvasive, low cost and a commonly used approach for body composition measurements and assessment of clinical condition. There are a variety of methods applied for interpretation of measured bioimpedance data and a wide range of utilizations of bioimpedance in body composition estimation and evaluation of clinical status. This paper reviews the main concepts of bioimpedance measurement techniques including the frequency based, the allocation based, bioimpedance vector analysis and the real time bioimpedance analysis systems. Commonly used prediction equations for body composition assessment and influence of anthropometric measurements, gender, ethnic groups, postures, measurements protocols and electrode artifacts in estimated values are also discussed. In addition, this paper also contributes to the deliberations of bioimpedance analysis assessment of abnormal loss in lean body mass and unbalanced shift in body fluids and to the summary of diagnostic usage in different kinds of conditions such as cardiac, pulmonary, renal, and neural and infection diseases.

Adverse effects of near current-electrode placement in non-invasive bio-impedance measurements

A major problem confronting application of impedance techniques to studies of disease is the extraction of intrinsic properties of the tissue from the measured impedances, which unavoidably involve geometric factors as well. Amongst the foremost are the sizes and locations of the measuring electrode arrays, and this paper addresses one of these, the location of current injecting electrodes. Tetrapolar impedance measurements on a 17.5 cm segment of the thigh gave R and X values three to four times larger when the current injecting electrodes were placed 2.5 cm from the sensing electrodes than when very distant placement was used. The frequency dependences of R and X were affected as well, though the X versus R plots still showed virtually perfect depressed-center semicircles, as in the Cole model. R(f) and X(f) for the set of contiguous 2.5 cm wide sub-segments show that these behaviors can be explained by a combination of the transverse orientation of current flow lines near the injecting electrodes and the anisotropy of the resistivity associated with the bundled fiber structure of muscle tissue. The measured impedance was found to be a separable function of geometric and intrinsic tissue variables, but far more complicated than is implicit in the usual cylindrical models. The results also suggest that many full and segmental body composition studies in the literature may be prone to substantial errors due to too close placement of the current injecting electrodes.

Stroke damage detection using classification trees on electrical bioimpedance cerebral spectroscopy measurements

After cancer and cardio-vascular disease, stroke is the third greatest cause of death worldwide. Given the limitations of the current imaging technologies used for stroke diagnosis, the need for portable non-invasive and less expensive diagnostic tools is crucial. Previous studies have suggested that electrical bioimpedance (EBI) measurements from the head might contain useful clinical information related to changes produced in the cerebral tissue after the onset of stroke. In this study, we recorded 720 EBI Spectroscopy (EBIS) measurements from two different head regions of 18 hemispheres of nine subjects. Three of these subjects had suffered a unilateral haemorrhagic stroke. A number of features based on structural and intrinsic frequency-dependent properties of the cerebral tissue were extracted. These features were then fed into a classification tree. The results show that a full classification of damaged and undamaged cerebral tissue was achieved after three hierarchical classification steps. Lastly, the performance of the classification tree was assessed using Leave-One-Out Cross Validation (LOO-CV). Despite the fact that the results of this study are limited to a small database, and the observations obtained must be verified further with a larger cohort of patients, these findings confirm that EBI measurements contain useful information for   assessing on the health of brain tissue after stroke and supports the hypothesis that classification features based on Cole parameters, spectral information and the geometry of EBIS measurements are useful to differentiate between healthy and stroke damaged brain tissue.

Cole function and conductance-based parasitic capacitance compensation for cerebral electrical bioimpedance measurements

One of the most common measurement artifacts present in Electrical Bioimpedance Spectroscopy measurements (EBIS) comes from the capacitive leakage effect resulting from parasitic stray capacitances. This artifact produces a deviation in the measured impedance spectrum that is most noticeable at higher frequencies. The artifact taints the spectroscopy measurement increasing the difficulty of producing reliable EBIS measurements at high frequencies. In this work, an approach for removing such capacitive influence from the spectral measurement is presented making use of a novel method to estimate the value of the parasitic capacitance equivalent that causes the measurement artifact. The proposed method has been tested and validated theoretically and experimentally and it gives a more accurate estimation of the value of the parasitic capacitance than the previous methods. Once a reliable value of parasitic capacitance has been estimated the capacitive influence can be easily compensated in the EBIS measured data. Thus enabling analysis of EBIS data at higher frequencies, i.e. in the range of 300-500 kHz like measurements intended for cerebral monitoring, where the characteristic frequency is remarkably higher than EBIS measurements i.e. within the range 30 to 50 kHz, intended for body composition assessment.

On the effect of body capacitance to ground in tetrapolar bioimpedance measurements

Tetrapolar bioimpedance measurements on subjects have long been suspected of being affected by stray capacitance between the subjects' body and ground. This paper provides a circuit model to analyze that effect in the frequency range from 100 Hz to 1 MHz in order to identify the relevant parameters when impedance is measured by applying a voltage and measuring both the resulting current and the potential difference between two points on the surface of the volume conductor. The proposed model includes the impedance of each electrode and the input impedance of the differential voltage amplifier. When common values for the circuit parameters are assumed, the simplified model predicts: 1) a frequency-independent gain (scale factor) error; 2) inductive artifacts, that is, the measured impedance increases with increasing frequency and may include positive angle phases; and 3) resonance that can affect well below 1 MHz. In addition to the stray capacitance to ground, relevant parameters that determine those errors are the capacitance of the "low-current" electrode and the input capacitance of the differential voltage amplifier. Experimental results confirm those theoretical predictions and show effects from several additional resonances above 1 MHz that also depend on body capacitance to ground.

Influence of electrode mismatch on Cole parameter estimation from total right side electrical bioimpedance spectroscopy measurements

Applications based on measurements of Electrical Bioimpedance (EBI) spectroscopy analysis, like assessment of body composition, have proliferated in the past years. Currently Body Composition Assessment (BCA) based in Bioimpedance Spectroscopy (BIS) analysis relays on an accurate estimation of the Cole parameters R(0) and R(∞). A recent study by Bogonez-Franco et al. has proposed electrode mismatch as source of remarkable artefacts in BIS measurements. Using Total Right Side BIS measurements from the aforementioned study, this work has focused on the influence of electrode mismatch on the estimation of R(0) and R(∞) using the Non-Linear Least Square curve fitting technique on the modulus of the impedance. The results show that electrode mismatch on the voltage sensing electrodes produces an overestimation of the impedance spectrum leading to a wrong estimation of the parameters R(0) and R(∞), and consequently obtaining values around 4% larger that the values obtained from BIS without electrode mismatch. The specific key factors behind electrode mismatch or its influence on the analysis of single and spectroscopy measurements have not been investigated yet, no compensation or correction technique is available to overcome the deviation produced on the EBI measurement. Since textile-enabled EBI applications using dry textrodes, i.e. textile electrodes with dry skin-electrode interfaces and potentially large values of electrode polarization impedance are more prone to produce electrode mismatch, the lack of a correction or compensation technique might hinder the proliferation of textile-enabled EBI applications for personalized healthcare monitoring.

Cole parameter estimation from electrical bioconductance spectroscopy measurements

Several applications of Electrical Bioimpedance (EBI) make use of Cole parameters as base of their analysis, therefore Cole parameters estimation has become a very common practice within Multifrequency- and EBI spectroscopy. EBI measurements are very often contaminated with the influence of parasitic capacitances, which contributes to cause a hook-alike measurement artifact at high frequencies in the EBI obtained data. Such measurement artifacts might cause wrong estimations of the Cole parameters, contaminating the whole analysis process and leading to wrong conclusions. In this work, a new approach to estimate the Cole parameters from the real part of the admittance, i.e. the conductance, is presented and its performance is compared with the results produced with the traditional fitting of complex impedance to a depressed semi-circle. The obtained results prove that is feasible to obtain the full Cole equation from only the conductance data and also that the estimation process is safe from the influence capacitive leakage.

Hook effect correction & resistance-based cole fitting prior cole model-based analysis: experimental validation

The analysis of measurements of Electrical Bioimpedance (EBI) is on the increase for performing non-invasive assessment of health status and monitoring of pathophysiological mechanisms. EBI measurements might contain measurements artefacts that must be carefully removed prior to any further analysis. Cole model-based analysis is often selected when analysing EBI data and might lead to miss-conclusion if it is applied on data contaminated with measurement artefacts. The recently proposed Correction Function to eliminate the influence of the Hook Effect from EBI data and the fitting to the real part of the Cole model to extract the Cole parameters have been validated on experimental measurements. The obtained results confirm the feasible experimental use of these promising pre-processing tools that might improve the outcome of EBI applications using Cole model-based analysis.

Textile electrodes in Electrical Bioimpedance measurements - a comparison with conventional Ag/AgCl electrodes

Work has been intensified around the integration of textile and measurement technology for physiological measurements in the last years. As a result nowadays it is possible to find available commercial products for cardiovascular personal healthcare monitoring. Most of the efforts have been focused in the acquisition of EKG for cardiovascular monitoring where textile electrodes have shown satisfactory performance. Electrical Bioimpedance is another type of physiological measurement that can be used for personal healthcare monitoring where the integration and the performance of the textile electrodes has not been investigated that thoroughly. In this work, the influence of the textile electrodes on the measurements and on the estimation of the Cole (R(0), R(infinity), f(C) and alpha) and body composition (TBW, ICW, ECW and FFM) parameters has been especially addressed. Complex Spectroscopy 4-electrode wrist-to-ankle electrical bioimpedance measurements taken with conventional Ag/AgCl and textile-electrodes on customized bracelets have been compared and analyzed in the frequency range 3 to 500 kHz. The obtained results suggest that the use of textile electrodes do not influence remarkably on the complex spectral measurements neither in the estimation of Cole nor body composition parameter. In any case any possible effect introduced by the use of textile is smaller than the effect of preparing the skin by the using abrasive conductive paste.

Electrical impedance myography at frequencies up to 2 MHz

Extension of the frequency range of electrical impedance myography (EIM) to 2 MHz discloses a major rise in the reactance of muscle above 3-500 kHz, together with a slow decrease in the resistance consonant with the Kramers-Kronig relations. This 'upturn' phenomenon is found when the distant current electrode configuration of EIM is employed, but not when the current electrodes are placed close to the voltage measuring area. In that case the impedance qualitatively mimics the commonly used 3-element resistor-capacitor model. The possibility that the upturn is an artifact of the measuring system rather than a true property of the tissue is examined in detail. In particular, experiments are reported which argue against the transmission line mechanism proposed to explain similar increases in reactance in some high frequency whole-body BIA studies. Also, scaling of X versus R plots for muscle segments of different lengths strongly suggests that the upturn is as much a property of the underlying tissue as is the low frequency maximum in reactance.

A review of errors in multi-frequency EIT instrumentation

Multi-frequency electrical impedance tomography (MFEIT) was proposed over 10 years ago as a potential spectroscopic impedance imaging method. At least seven systems have been developed for imaging the lung, heart, breast and brain, yet none has yet achieved clinical acceptance. While the absolute impedance varies considerably between different tissues, the changes in the spectrum due to physiological changes are expected to be quite small, especially when measured through a volume. This places substantial requirements on the MFEIT instrumentation to maintain a flat system frequency response over a broad frequency range (dc-MHz). In this work, the EIT measurement problem is described from a multi-frequency perspective. Solutions to the common problems are considered from recent MFEIT systems, and the debate over four-terminal or two-terminal (multiple source) architecture is revisited. An analysis of the sources of MFEIT errors identifies the major sources of error as stray capacitance and common-mode voltages which lead to a load dependence in the frequency response of MFEIT systems. A system that employs active electrodes appears to be the most able to cope with these errors (Li et al 1996). A distributed system with digitization at the electrode is suggested as a next step in MFEIT system development.

Design and preliminary evaluation of a portable device for the measurement of bioimpedance spectroscopy

Portable bioimpedance spectroscopy (BIS) devices are of great value for monitoring the pathological status of biological tissues in clinical and home environments. The two traditional techniques for measuring complex bioimpedance, the bridge method and quadrature demodulation method, are either time-consuming or often associated with high cost, high power consumption, and large board space, and therefore are not ideally suitable for designing a portable device for BIS measurement. This paper describes a novel design of a portable BIS device based on the magnitude-ratio and phase-difference detection method and its implementation using the newest generation of analog electronic products which greatly decrease the complexity of both hardware and software. In order to improve the accuracy of the device, a three-reference calibration algorithm was applied. Experimental sweep-frequency measurements on RC circuits were carried out to preliminarily evaluate the performances of the device. The results obtained by the device were found to be in good agreement with the results measured by a commercial impedance analyzer HP4194, with an overall mean error of 0.014% in magnitude and 0.136 degrees in phase over a frequency range of 20 kHz to 1 MHz.

Segment-specific resistivity improves body fluid volume estimates from bioimpedance spectroscopy in hemodialysis patients

Discrepancies in body fluid estimates between segmental bioimpedance spectroscopy (SBIS) and gold-standard methods may be due to the use of a uniform value of tissue resistivity to compute extracellular fluid volume (ECV) and intracellular fluid volume (ICV). Discrepancies may also arise from the exclusion of fluid volumes of hands, feet, neck, and head from measurements due to electrode positions. The aim of this study was to define the specific resistivity of various body segments and to use those values for computation of ECV and ICV along with a correction for unmeasured fluid volumes. Twenty-nine maintenance hemodialysis patients (16 men) underwent body composition analysis including whole body MRI, whole body potassium (40K) content, deuterium, and sodium bromide dilution, and segmental and wrist-to-ankle bioimpedance spectroscopy, all performed on the same day before a hemodialysis. Segment-specific resistivity was determined from segmental fat-free mass (FFM; by MRI), hydration status of FFM (by deuterium and sodium bromide), tissue resistance (by SBIS), and segment length. Segmental FFM was higher and extracellular hydration of FFM was lower in men compared with women. Segment-specific resistivity values for arm, trunk, and leg all differed from the uniform resistivity used in traditional SBIS algorithms. Estimates for whole body ECV, ICV, and total body water from SBIS using segmental instead of uniform resistivity values and after adjustment for unmeasured fluid volumes of the body did not differ significantly from gold-standard measures. The uniform tissue resistivity values used in traditional SBIS algorithms result in underestimation of ECV, ICV, and total body water. Use of segmental resistivity values combined with adjustment for body volumes that are neglected by traditional SBIS technique significantly improves estimations of body fluid volume in hemodialysis patients.

Body composition modeling in the calf using an equivalent circuit model of multi-frequency bioimpedance analysis

An equivalent electrical circuit model is used to describe the response of different tissue components in the calf to multi-frequency current. This model includes seven electrical components: skin resistance, contact capacitance, fat resistance, fat capacitance, extracellular resistance, intracellular resistance and cell membrane capacitance. Calf bioimpedance was measured on 30 pts using a multi-frequency bioimpedance device (Xitron 4200) with a range of frequency from 5 kHz to 1000 kHz. MRI was performed on each measured calf to provide body composition components: fat, muscle mass and bone. An equivalent circuit containing seven parameters (P1, P2, P3, P4, Q1, Q2, Q3) was constructed to represent the model. To identify the effect of different body compositions on their parameters, subjects were subgrouped according to (1) their range of fat mass: F1>0.4 kg, F2>0.4 & F2<0.25 kg and F3<0.25 kg; (2) their range of muscle mass: M1>1.2 kg, M2<1.2 & M2>1.0 kg and M3<0.25 kg. Curve fitting and simulation programs (Matlab Toolbox) were used to obtain the solution of the electrical equations. The results show a decrease in impedance with an increase in excitation frequency that differed among subjects with different fat contents. Simulation results show a high correlation (R2>0.98) between the bioimpedance measurements and the value calculated from the model. There are significant differences in parameters P1 (32.5+/-5.9 versus 26+/-4.4, p<0.05), P3 (-15,330+/-3352 versus -10,973+/-3448, p<0.05) and P4 (42,640 versus 24,191, p<0.05) between groups F1 and F3. P2 is significantly different (1045+/-442 versus 1407+/-349, p<0.05) between groups M1 and M2. The parameters that characterize the bioimpedance data depend upon many more tissue characteristics of electrical properties than those incorporated in current models and they are affected by aspects of body composition that are not considered in the fitting of bioimpedance data. This study shows a new model and methodology to analyze bioimpedance data and further work is likely to lead to much better understanding of electrical properties of body tissue.

Methode zur Verbesserung der Meßgenauigkeit bei der Multikanal-Impedanz-Spektroskopie (MIS) / A method for Improving Measuring Accuracy in Multi-channel Impedance Spectroscopy (MIS)

The use of impedance spectroscopy as a diagnostic tool for the investigation of biological objects involves the consideration of numerous parameters impacting on measuring accuracy. This paper describes a calibration method for multichannel instruments that reduces the non-inconsiderable influence of frequency response variations between the channels, thus significantly increasing measuring accuracy. The method is tested in a recently developed, high-resolution, multi-channel bio-impedance analyser. Reduction of the measuring error is demonstrated, and the magnitude and phase resolution is quantified. The advantage of this method lies in its applicability to existing systems. Furthermore, an additional calibration impedance is not needed.

Accuracy of an optically isolated tetra-polar impedance measurement system

Accurate electrical transfer impedance measurement at the high frequencies (> 1 MHz) required to characterise blood and intracellular structures is very difficult, owing to stray capacitances between lead wires. To solve this problem, an optically isolated measurement system has been developed using a phase-locked-loop technique for synchronisation between current injection (drive) and voltage measurement (receive) circuits. The synchronisation error between drive and receive circuits was less than 1 ns. The accuracy and reproducibility of the developed system was examined using a tissue equivalent Cole model consisting of two resistors and one capacitor. The absolute value Z and phase shift theta in impedance of the Cole model was measured at 1.25 MHz by both an LCR meter and the isolated measurement system. The difference between the values measured by the isolated measurement system and those measured by the LCR meter was less than 0.27omega (2.9%) in Z and 0.79 degree in theta. The standard deviation was less than 0.09 omega in Z and 0.60 degree in theta.

In-vivo measurement of swine myocardial resistivity

We used a four-terminal plunge probe to measure myocardial resistivity in two directions at three sites from the epicardial surface of eight open-chest pigs in-vivo at eight frequencies ranging from 1 Hz to 1 MHz. We calibrated the plunge probe to minimize the error due to stray capacitance between the measured subject and ground. We calibrated the probe in saline solutions contained in a metal cup situated near the heart that had an electrical connection to the pig's heart. The mean of the measured myocardial resistivity was 319 ohm x cm at 1 Hz down to 166 ohm x cm at 1 MHz. Statistical analysis showed the measured myocardial resistivity of two out of eight pigs was significantly different from that of other pigs. The myocardial resistivity measured with the resistivity probe oriented along and across the epicardial fiber direction was significantly different at only one out of the eight frequencies. There was no significant difference in the myocardial resistivity measured at different sites.

Error analysis of tissue resistivity measurement

We identified the error sources in a system for measuring tissue resistivity at eight frequencies from 1 Hz to 1 MHz using the four-terminal method. We expressed the measured resistivity with an analytical formula containing all error terms. We conducted practical error measurements with in-vivo and bench-top experiments. We averaged errors at all frequencies for all measurements. The standard deviations of error of the quantization error of the 8-bit digital oscilloscope with voltage averaging, the nonideality of the circuit, the in-vivo motion artifact and electrical interference combined to yield an error of +/- 1.19%. The dimension error in measuring the syringe tube for measuring the reference saline resistivity added +/- 1.32% error. The estimation of the working probe constant by interpolating a set of probe constants measured in reference saline solutions added +/- 0.48% error. The difference in the current magnitudes used during the probe calibration and that during the tissue resistivity measurement caused +/- 0.14% error. Variation of the electrode spacing, alignment, and electrode surface property due to the insertion of electrodes into the tissue caused +/- 0.61% error. We combined the above errors to yield an overall standard deviation error of the measured tissue resistivity of +/- 1.96%.

Assessing abdominal fatness with local bioimpedance analysis: basics and experimental findings

OBJECTIVE

Abdominal fat is of major importance in terms of body fat distribution but is poorly reflected in conventional body impedance measurements. We developed a new technique for assessing the abdominal subcutaneous fat layer thickness (SFL) with single-frequency determination of the electrical impedance across the waist (SAI).

SUBJECTS AND MEASUREMENTS

The method uses a tetrapolar arrangement of surface electrodes which are placed symmetrically to the umbilicus in a plane perpendicular to the body axis. Twenty-four test subjects (12 male, 12 female) underwent SAI and abdominal magnetic resonance imaging (MRI). The SFL below the sensing electrodes was determined from MRI and correlated with the SAI data at four different frequencies (5, 20, 50 and 204 kHz).

RESULTS

A highly significant linear correlation (r2=0.99) between SFL and SAI over a wide range of the abdominal SFL was found. Separate regression models for female and male subjects did not differ significantly, except at 50 kHz.

CONCLUSION

SAI represents a good predictor of the SFL and provides an excellent tool for the assessment of central obesity.

Impedance analyser module for EIT and spectroscopy using undersampling

In this paper we present the concept, the design and the test procedure for a DSP-based high-precision and high-performance wide-band (up to 10 MHz) bioimpedance analyser module for application in EIT or bioimpedance spectroscopy. The module implements a digital concept with appropriate signal conditioning hardware for voltage and current measurement, early signal digitization and subsequent digital signal processing in order to calculate the components of impedance (or admittance). At low frequencies, the module utilizes the conventional direct conversion method, whereas at high frequencies the undersampling technique is used. The advantages of the described system are the following: (a) the frequency range is extended to higher frequencies, (b) the number of data sampled per time interval is significantly reduced, and (c) the current consumption and the costs of the ADCs can be significantly reduced. The validation procedure is performed by comparing the measured and theoretical values of the magnitude and the phase of the impedance for a commonly used tissue model. The module offers an accuracy of better than 0.012% for the magnitude of impedance and better than 0.02 degrees for the phase.

A method for bio-electrical impedance analysis based on a step-voltage response

Bioimpedance analysis (BIA) has been researched broadly, since it is simple, it presents good results and the analysers are portable, allowing it to be used in field studies. This paper presents a new technique of BIA based on a step-voltage current response and bipolar electrode array. A prototype of this new kind of analyser was developed and constructed to test the technique. Bench tests were performed to calibrate the prototype and the obtained results were comparable to those of commercial analysers. Body composition tests were conducted on 67 subjects of both sexes. Besides the bioimpedance analysis, anthropometric measures, consisting of weight, height, circumference and skinfold thickness, were also obtained from the subjects to allow an estimation of the body composition from anthropometric equations established in the literature. The results point to a good correlation (Pearson coefficient, r = 0.9645) between the anthropometric estimated fat-free mass (FFM) and its analogue estimated by the new bioimpedance technique.

Inductively coupled wideband transceiver for bioimpedance spectroscopy (IBIS)

Most measurement devices for bioimpedance spectroscopy are coupled to the measured object (tissue) via electrodes. At frequencies > 500 kHz, they suffer from artifacts due to stray capacitances between electrode leads as well as between the ground and object. The noninvasive measurement of the brain conductivity is hardly possible with surface electrodes. These disadvantages can be obviated by inductive coupling. The aim of this work was the development of a wideband transceiver for inductive impedance spectroscopy. In order to define its specifications, a feasibility study has been carried out with a simulation model for three different coil systems above a homogeneous conducting plate. According to simulation results, all systems render it possible to resolve conductivity changes down to 10(-3) (omega m)-1 at frequencies > 50 kHz. The transceiver electronics must then provide a resolution of > or = 1 microV and an excitation current of up to 1 A. The realized receiver matches these specifications with an S/N ratio of 22 dB at 1 microV in the frequency range of 50 kHz to 5 MHz.