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Research Papers

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

[+] Author and Article Information
Robert E. Dodde

Stryker Corporation,
4100 E. Milham Avenue,
Kalamazoo, MI 49001
e-mail: robert.dodde@stryker.com

Grant H. Kruger

Mechanical Engineering,
University of Michigan,
1031 H.H. Dow Building,
2350 Hayward Street,
Ann Arbor, MI 48109
e-mail: ghkruger@umich.edu

Albert J. Shih

Mechanical Engineering,
University of Michigan,
3001E EECS,
1301 Beal,
Ann Arbor, MI 48109
e-mail: shiha@umich.edu

1Corresponding author.

Manuscript received February 22, 2014; final manuscript received January 22, 2015; published online April 24, 2015. Assoc. Editor: Carl Nelson.

J. Med. Devices 9(2), 021001 (Jun 01, 2015) (8 pages) Paper No: MED-14-1133; doi: 10.1115/1.4029706 History: Received February 22, 2014; Revised January 22, 2015; Online April 24, 2015

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.

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Figures

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Fig. 1

BIS probe prototype showing (a) actual probe and electrodes and (b) illustration of probe measuring tissue sample

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Fig. 2

Expected impedance magnitudes to be driven by the current source based on literature values for platinum electrode contact impedance and the measured bioimpedance of spleen tissue

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Fig. 3

Circuit diagram with experimental setup for the measurement system. The function generator supplies the reference voltage for the current source whose output is measured by the current-to-voltage converter. The differential amplifier uses a front end (AD8065) with very high input impedance to sense the voltage difference between the inner two electrodes and DC couples the readings. The output from the differential amplifier and the current-to-voltage converter are sent to an oscilloscope for measurement.

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Fig. 4

Schematic of the FEA used to analyze potential distribution within a cylindrical column of saline. A normal current density is defined at the HC electrode and a ground condition was defined at the LC electrode (see Fig. 3). All other external boundaries were given an insulation boundary condition, and all internal boundaries were given a continuity boundary condition.

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Fig. 5

Theoretical, simulated, and measured output impedance for the DC-stabilized MHCP

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Fig. 6

Comparison of DC offset voltage measured in MultiSim for a standard and DC stabilized VCCS as a function of load

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Fig. 7

Measured RMS current output for low and high impedance loads for the DC-stabilized MHCP

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Fig. 8

Susceptance plot showing a linear increase as frequency is increased. The slope of this line is used to compensate for the parasitic capacitance in the system.

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Fig. 9

Results from the numerical and experimental effect of probe proximity to the bottom of the experimental tissue chamber, h. (a) Top view of the potential distribution (shading) and current density (streamlines) for h = 11 mm at the probe surface (left) and for h = 1 mm at the probe surface (right). Potential drop between the middle two electrodes is seen to increase in the case on the right. (b) Plot of impedance normalized to the measured impedance at 11 mm (|Z|/|Z|*) versus h. For the finite element model, impedance was calculated by dividing the potential drop by 60 μA. For the experimental measurements, impedance was calculated by dividing the measured potential drop by the measured current. The plot shows a dramatic increase in measured impedance as the probe moves closer to the bottom of the acrylic tissue chamber.

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Fig. 10

Raw (Zraw), corrected (Zcor), and fitted data (Zfit) for an example bioimpedance measurement on porcine spleen tissue. The order of points from left to right in the figure each represents an individual measurement performed at frequencies of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10.0, 20.0, and 50.0 kHz, respectively.

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Fig. 11

Comparison of electrical conductivity (σ) and electrical permittivity (ε) experimental results for porcine spleen with the literature (note literature based on a range of animal spleen values at various temperatures). Errors bars are provided, but are not clearly legible due to the log axis scale. The measurement error for conductivity values were fairly consistent, ranging from about ±21% at 0.1 kHz down to ±18% at 100 kHz. Permittivity measurement errors below 20 kHz were ±36%, decreasing to about ±20% over the 1–20 kHz range, and again increasing to ±36% by 0.5 kHz, and ±49% by 0.2 kHz.

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