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Experimental validation of a method for removing the capacitive leakage artifact from electrical bioimpedance spectroscopy measurements
KTH, School of Technology and Health (STH), Medical sensors, signals and systems (MSSS).
KTH, School of Technology and Health (STH), Medical sensors, signals and systems (MSSS).ORCID iD: 0000-0002-6995-967X
Department of Theory of the Signal and Communications, University of Alcala, Madrid, Spain. (Applied Signal Processing)
2010 (English)In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 21, no 11Article in journal (Refereed) Published
Abstract [en]

Often when performing electrical bioimpedance (EBI) spectroscopy measurements, the obtained EBI data present a hook-like deviation, which is most noticeable at high frequencies in the impedance plane. The deviation is due to a capacitive leakage effect caused by the presence of stray capacitances. In addition to the data deviation being remarkably noticeable at high frequencies in the phase and the reactance spectra, the measured EBI is also altered in the resistance and the modulus. If this EBI data deviation is not properly removed, it interferes with subsequent data analysis processes, especially with Cole model-based analyses. In other words, to perform any accurate analysis of the EBI spectroscopy data, the hook deviation must be properly removed. Td compensation is a method used to compensate the hook deviation present in EBI data; it consists of multiplying the obtained spectrum, Z meas (ω), by a complex exponential in the form of exp(–jωTd). Although the method is well known and accepted, Td compensation cannot entirely correct the hook-like deviation; moreover, it lacks solid scientific grounds. In this work, the Td compensation method is revisited, and it is shown that it should not be used to correct the effect of a capacitive leakage; furthermore, a more developed approach for correcting the hook deviation caused by the capacitive leakage is proposed. The method includes a novel correcting expression and a process for selecting the proper values of expressions that are complex and frequency dependent. The correctness of the novel method is validated with the experimental data obtained from measurements from three different EBI applications. The obtained results confirm the sufficiency and feasibility of the correcting method.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2010. Vol. 21, no 11
Keyword [en]
electrical bioimpedance spectroscopy; capacitive leakage; artifact removal
National Category
Medical Laboratory and Measurements Technologies
Identifiers
URN: urn:nbn:se:kth:diva-73065DOI: 10.1088/0957-0233/21/11/115802ISI: 000285262000026OAI: oai:DiVA.org:kth-73065DiVA: diva2:488581
Note
QC 20120209Available from: 2012-02-09 Created: 2012-02-01 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Model Based Enhancement of Bioimpedance Spectroscopy Analysis: Towards Textile Enabled Applications
Open this publication in new window or tab >>Model Based Enhancement of Bioimpedance Spectroscopy Analysis: Towards Textile Enabled Applications
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Several signal processing approaches have been developed to overcome the effect of stray capacitances in Electrical Bioimpedance Spectroscopy (EBIS) measurements. EBIS measurements obtained with textile-enabled instrumentation are more vulnerable to stray capacitances. Currently, the most widespread approach for correcting the effect of stray capacitances in EBIS is the time delay (

Td) compensation method, which also has several drawbacks. In this study, the Td method is revisited and its limitations and its lack of a scientific basis are demonstrated. To determine better ways to overcome the effect of stray capacitances, a simplified measurement model is proposed that is based on previous models of artefacts in EBIS measurements described in the literature. The model consists of a current divider with a parasitic capacitance (Cpar) in parallel with the load. Cpar creates a pathway for the measurement current to leak away from the load, provoking a capacitive leakage effect. In this thesis, three approaches with different limitations are proposed to overcome the capacitive leakage effect. The first approach estimates Cpar and subtracts it from the measurements, thus finding the load. Cpar can be estimated because the susceptance of biological tissue is null at infinite frequency. Therefore, at high frequencies, the susceptance of the tissue can be neglected, and the slope of the susceptance of the measurement is Cpar. The accuracy of Cpar depends on the maximum frequency measured and the value of Cpar. Therefore, it may not be possible to accurately estimate small values of Cpar in the typical frequency ranges used in EBIS. The second and third approaches use the Cole fitting process to estimate the Cole parameters, which form the basis for most EBIS applications. Because the conductance of the measurement is free from the effect of Cpar, performing Cole fitting on the conductance avoids the effect of Cpar in the fitting process. With a poor skin-electrode contact, this approach may not be sufficiently accurate. The third approach would be to perform the Cole fitting on the modulus with a reduced upper frequency limit because the modulus and the low-medium frequencies are very robust against the effect of artefacts. In this approach, a slight capacitive leakage effect is unavoidable. Since it is common to find tainted measurements, especially among those obtained with textile-enabled instrumentation, it is important to find viable methods to avoid their effect. The three methods studied showed that they could reduce the effect of tainted measurements.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. xviii, 42 p.
Series
Trita-STH : report, ISSN 1653-3836 ; 2011:6
Keyword
Physiological measurements, Bioimpedance, modelling, textiles
National Category
Medical Laboratory and Measurements Technologies
Identifiers
urn:nbn:se:kth:diva-90884 (URN)978-91-7501-230-8 (ISBN)
Presentation
2012-02-28, 4X, Alfred Nobels Allé 8, Huddinge, 11:33 (English)
Opponent
Supervisors
Note
QC 20120313Available from: 2012-03-13 Created: 2012-03-02 Last updated: 2012-03-15Bibliographically approved
2. Improvements in Bioimpedance SpectroscopyData Analysis: Artefact Correction, ColeParameters, and Body Fluid Estimation
Open this publication in new window or tab >>Improvements in Bioimpedance SpectroscopyData Analysis: Artefact Correction, ColeParameters, and Body Fluid Estimation
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The estimation of body fluids is a useful and common practice in the status assessment of diseasemechanisms and treatments. Electrical bioimpedance spectroscopy (EBIS) methods are non-invasive,inexpensive, and efficient alternatives for the estimation of body fluids. However, these methods areindirect, and their robustness and validity are unclear.Regarding the recording of measurements, a controversy developed regarding a spectrum deviationin the impedance plane, which is caused by capacitive leakage. This deviation is frequentlycompensated for by the extended Cole model, which lacks a theoretical basis; however, there is nomethod published to estimate the parameters. In this thesis, a simplified model to correct thedeviation was proposed and tested. The model consists of an equivalent capacitance in parallel withthe load.Subsequently, two other measurement artefacts were considered. Both artefacts were frequentlydisregarded with regard to total body and segmental EBIS measurements as their influence isinsignificant with suitable skin-electrode contact. However, this case is not always valid, particularlyfrom a textile-enabled measurement system perspective. In the estimation of body fluids, EBIS dataare fitted to a model to obtain resistances at low and high frequencies. These resistances can berelated to body fluid volumes. In order to minimise the influence of all three artefacts on theestimation of body fluids and improve the robustness and suitability of the model fitting the differentdomains of immittance were used and tested. The conductance in a reduced frequency spectrum wasproposed as the most robust domain against the artefacts considered.The robustness and accuracy of the method did not increase, even though resistances at low and highfrequencies can be robustly estimated against measurement artefacts. Thus, there is likely error in therelation between the resistances and volumes. Based on a theoretical analysis, state of the artmethods were reviewed and their limitations were identified. New methods were also proposed. Allmethods were tested using a clinical database of patients involved in growth hormone replacementtherapy. The results indicated EBIS are accurate methods to estimate body fluids, however they haverobustness limits. It is hypothesized that those limits in extra-cellular fluid are primarily due toanisotropy, in total body fluid they are primarily due to the uncertainty ρi, and errors in intra-cellularfluid are primarily due to the addition of errors in extracellular and total body fluid. Currently, theseerrors cannot be prevented or minimised. Thus, the limitations for robustness must be predicted priorto applying EBIS to estimate body fluids.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. 83 p.
Series
Trita-STH : report, ISSN 1653-3836 ; 2013:7
Keyword
Bioimpedance, Cole, Body Fluid
National Category
Medical Engineering
Identifiers
urn:nbn:se:kth:diva-128529 (URN)978-91-7501-874-4 (ISBN)
Public defence
2013-10-04, Sal 3-264, Alfred Nobels allé 10, Fremingsberg, 13:00 (English)
Opponent
Supervisors
Note

QC 20130917

Available from: 2013-09-17 Created: 2013-09-12 Last updated: 2013-09-17Bibliographically approved

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