Noninvasive Bio-impedance Measurement Using Voltage Current Pulse Technique

7/29/2019 Noninvasive Bio-impedance Measurement Using Voltage Current Pulse Technique

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International Conference on Electrical, Electronics and Biomedical Engineering (ICEEBE'2012) Penang (Malaysia) May 19-20, 2012

AbstractNoninvasive detection is a fundamental prerequisite

for pervasive healthcare system and biosensing. Along with different

noninvasive techniques such as ultrasound, X-ray, magnetic

resonance and optical imaging, electrical impedance spectroscopy is

emerging as more suitable technique for tissue level diagnosis. This

paper presents a noninvasive technique for bioelectrical impedance

estimation by means of impedance spectroscopy applying voltage-

current pulse technique and least square curve fitting method. Simple

circuit for current injection and voltage detection is presented with

common mode noise suppression circuit requires only two electrodes

would reduce problems due to electrode polarization. The currentsource is presented with constant output current as low as 100 A,

about 1M load variation and 10 MHz mid-range bandwidth.

Voltage detection and impedance estimation is done with Data

Acquisition (DAQ) tool box of Matlab R2008a and curve fitting

technique. Successful simulation results in bioelectrical impedance

estimation are found with 1.5% maximum error; suggest impedance

measurement below 100 kHz because of higher response of

permittivity and conductivity of biological tissues in this band than

the high frequency band (MHz band).

Keywordsbioelectrical impedance, impedance spectroscopy,curve fitting, data acquisition tool box, non-invasive biosensing.

I. INTRODUCTIONERVASIVE healthcare system automatically calls for

intermittent, continuous or even contactless (wearable and

wireless) and distant biomedical sensing [1]. Such systems

not involve pathological patients only but also the healthy

people that for the sake of preventive measurement. Therefore,

an invasive sensor is not certainly going to be popular among

the healthy people. Besides, invasive sensing precludes

continuous monitoring and it is sometimes not possible for

specific cases (e.g. blood glucose monitoring) which inevitably

calls for noninvasive sensors. There are different popular

noninvasive techniques for bio-sensing like optical

spectroscopy, ultrasound, X-ray and magnetic resonance

imaging which typically measure the effect of absorption,reflection or transmittance of non-ionizing radiation that

illuminates a portion of the subject. Such techniques offer

basic information on mass or proton density distribution. On

the other hand, one more technique that provides information

Sagar K. Dhar is with the Premier University, 1/A, O.R. Nizam Road,

Prabartak Circle, Panchlaish, Chittagong, Bangladesh (phone:

8801818837084; e-mail: [email protected]).

Quazi D. Hossain is with Chittagong University of Engineering and

Technology, 163 Kaptai Road, Chittagong, Bangladesh (e-mail:

[email protected]).

related to tissue structures and their physiological states and

functions is bioelectrical impedance, has been of significant

interest because of its several advantages such as

noninvasiveness, low cost, ease of application and capability

for online monitoring [2], [3] which measures the conductivity

and permittivity of bio-logical tissues changed directly or

indirectly by the measuring variable.

The original bio-impedance technique was involved with

bioelectrical impedance analysis. Within a decade, this

technique is evolved into Bioelectrical Impedance

Spectroscopy (BIS) which is also called multiple frequencybioelectrical impedance analysis. It enables to have an overall

figure of impedance which is of significant interest in

biomedical sensing for example, the tissue impedance at zero

frequency corresponding to extra-cellular resistance is useful

for the evaluation of mammary and lung cancers and fatty

tissues. BIS applies multiple frequency stimulation to measure

body impedance and study shows its viability for the

applications of body fluid measurement [4], [5] which

estimates extracellular fluid, intracellular fluid, and total body

water; also applicable for tissue volume change and tissue

characterization with impedance plethysmography [6]. It is

well known that the electrical properties of biological tissues

differ significantly depending on their structure andcharacteristics [7]. Depending on such variation it is possible

to detect benign tissues with Electrical Impedance

Tomography (EIT). This has been reported by investigators

that electrical properties of tumors get changed significantly

and according to Fricke and Morse [8], tumor tissues show

higher permeability at 20 kHz compared to that of normal or

non-malignant tissues. Study shows that extracellular

resistance and intracellular resistance of cancer tissues are

significantly higher and plasma membrane capacitance is

significantly lower than normal tissues [9]. Similar results also

found by other investigations based on microwave bio-sensing.

Again, impedance measurement of bodily fluid such as saliva

and blood provides much fundamental information onphysiological state like glucose level and electrolyte

concentration. Study shows significant change in blood

conductivity and permittivity due to different level of glucose

[10]. Besides, any significant change in tissue characteristics

would be detectable with electrical impedance spectroscopy

and study shows its viability for visceral fat measurement [11],

prostate and lung cancer detection and bone fracture [12].

Biomedical sensing which directly or indirectly related to

change in biological tissues, diagnosis based on bioelectrical

Non-invasive Bio-impedance Measurement

Using Voltage-Current Pulse Technique

Sagar K. Dhar and Quazi D. Hossain

P

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mailto:[email protected]:[email protected]:[email protected]:[email protected]


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impedance spectroscopy is emerging with greater prospects for

noninvasive bio-sensing than any other noninvasive

techniques. This paper presents bioelectrical impedance

measurement by means of voltage current pulse technique.

Bio-impedance parameters Re, Ri and Cm of electrical

equivalent circuit shown in Fig. 1 are estimated with least

square curve fitting method in Matlab 2008a. A current source

is designed with the option of setting any constant current

value as less as 100 A, mid-range bandwidth of 10 MHz andload variation up to 1 M . Voltage detection is designed with

an instrumentation amplifier to provide good noise

performance and necessary signal gain along with parallel port

interfacing through Data Acquisition Toolbox of Matlab

R2008a.

II.METHODS

The electrical equivalent circuit of biological cells is usually

represented with similar circuit shown in Fig. 1 which is

comprised of extra-cellular medium resistance, intra-cellular

medium resistance and cell membrane capacitance [13]. In

every case, subjected electrical current will flow through

biological cells or extra-cellular medium. Again, the currentthrough the cells may be classified as the current across the

trans-membrane ionic channel (shown as the path with Rm) or

across the plasma membrane. The cell membrane shows

dielectric property where the intra-cellular medium and

extracellular medium shows resistive behavior represented

with Re and Ri which consequently provides the capacitance

effect to plasma membrane represented with Cm. Due to the

very high trans-membrane channel resistance, it may be

ignored and the circuit is simplified with single extra-cellular

medium resistance Re, intra-cellular medium resistance Ri and

cell membrane capacitance Cm.

Fig 1. Electrical equivalent circuit of biological cells.

Bioelectrical impedance in this paper is measured with

electrical impedance spectroscopy by means of voltage-current

pulse technique. Since, a discrete pulse contains all frequency

components, injection of a short duration current pulse would

provide complete frequency response of biological tissues. The

frequency behavior is detected by the distortion in voltage

pulse which is solely defined by the electrical behavior of

biological tissues and it is important to last the current pulse

duration until the resulting voltage is stable which practicallymeans that Cm has to be completely charged. The transfer

function of bioelectrical equivalent circuit is provided by (1)

and the change in pulse shape is shown in Fig. 2.

(1)

When a current pulse is injected, the voltage across the

tissue would be V(t) = I(t)*h(t) where I(t)*h(t) represents the

convolution sum of time domain current, I(t) and the impulse

response of the bioelectrical equivalent circuit, h(t).

Consequently, the Fourier transform of V(t) will provide the

voltage transform V(s). So, once the time domain voltage V(t)

and current I(t) are acquired and are transformed to V(s) and

I(s), the impedance spectroscopy Z(s) is recovered by (2)

which represents both magnitude and phase of impedance

represented by (3) and (4).

( ) ( ) ( )Z s V s I s= (2)

(3)

(4)

( ) ( )( )0 max eZ w Z w R= = = (5)

Once impedance data is retrieved from frequency transform of

voltage and current pulse, it is enough either to work with

magnitude or phase to estimate Re, Ri and Cm. In this paper,

magnitude data of impedance is fitted with (2) by means of

least square method with iterated value of Ri and Cm where the

value of Re is directly calculated with (5).

Fig 2. Pulse response of bioelectrical circuit.

2 2 2 2 2 2( ( ) ) ( )

( )2 2 2

1 ( )

R R R R R w C w C R Re e i e i m m e iZ w

R R w Ce i m

+ + +=

+ +

( )( )

( )

( )

2 221

2 22 2 2 2tan

1 1

e e i e im e

e i m e i m

R R R R R w CwC Rw

R R w C R R w C

+ + = + + + +

( ) ( ) ( )( )R R R C 1 R R Ce e i m e i mH s jw s s= = + + +

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III. BIOIMPEDANCE MEASUREMENT SYSTEM

Impedance measurement is accomplished in two sections

comprising of current injection & voltage detection and

impedance estimation as shown in Fig. 3. The first section

involves two major hardware parts of impedance measurement

system: 1) current source and 2) voltage sensor. The second

section involves the software to transform the voltage and

current in frequency domain to estimate impedance

spectroscopy and impedance parameters Re, Ri and Cmsubsequently.

A. Current Pulse Generation

Current source is the most important part of bioelectrical

impedance measurement system and should have very good

stability in providing current irrespective to the load variation

and current value should be in 700 A to 1mA [2]. Moreover,

the current source should have tuning option of both amplitude

and pulse width and invariable operation in low and high

frequencies simultaneously.

The constant current pulse in this paper is developed by

means of conversion of a voltage pulse which is generated

with the mono-stable operation of a 555 timer. Since, thevoltage pulse width is easily tunable in such operation; it in

other words provides the tuning of current pulse width. The

voltage pulse is then converted to constant current pulse

according to the circuit shown in Fig. 4. The current to load is

solely depended on RS, I = VIN/RS, and tunable as low as 100

A. To meet up the stability as a current source against a wide

load variation, operational amplifier AD380JH is selected with

very high differential and common mode input impedance of

100 G which makes the circuit applicable for constant

current source up to 1 M as shown in Fig. 5. The frequency

response of the source in Fig. 6 reveals its mid-range

bandwidth as large as 10 MHz.

Fig 3. Block diagram of bio-impedance measurement system.

Fig 4. Current pulse generation circuit.

Fig 5. Variation of source current with Load.

Fig 6. Frequency response of current source.

B. Voltage Detection

Pulse based impedance measurement systems generally use

four electrodes [14]: two for current injection and two for

voltage detection. This paper presents only two electrode

arrangement for both current injection and voltage detection

shown in Fig. 7 which would reduce tissue irritation due to

polarization. Here, current is injected by the electrodes B and

A where the voltage is detected across electrode A and ground

which would be equivalent to voltage across A and B

according to the virtual ground phenomenon and makes

possible two electrode setup for voltage detection.

Fig 7. Voltage detection with two electrodes.

U1

4

3

5

10

9

82

11

RS

Rlimit

AD380JH

AB

Re

RiCm

Voltage

Pulse1

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Biomedical sensors usually suffer from common mode noise

and to implement and manipulate bio-medical electrical signal

information, it is very important to deploy the front amplifier

with high common mode rejection ratio (CMRR) and high

differential gain. This paper presents the instrumentation

amplifier setup shown in Fig. 8 with the common mode

rejection performance shown in Fig. 9 where the gain ACL is

set at desired value according to (6). Finally, the voltage is

acquired with an ADC and parallel port interfacing by theDAQ of Matlab R2008a for processing and estimation of

bioelectrical impedance parameters Re, Ri and Cm.

1 2CL GA R R= + (6)

C. Impedance Estimation

Once the potential across bio-logical tissues is fetched,

voltage spectrum is found using FFT in Matlab. By the same

procedure current spectrum is also found from known pulse

information and together with voltage and current spectrum,

impedance spectroscopy is found by (2). At this final point, the

magnitude of impedance spectrum data are calculated and

fitted to (3) to find impedance parameters Ri and Cm applying

least square method as shown in Fig. 10 and Re is directly

found by (5).

Fig 8. Common mode noise suppression circuit.

Fig 9. Noise rejection with suppression circuit.

Fig 10. Curve fitting method.

IV. RESULT AND DISCUSSION

Electrical impedance is successfully estimated with around

1.5% maximum tolerance. The program of the impedance

parameter estimation is run in Matlab R2008a according to the

flow chart shown in Fig. 10. To test the accuracy of the curve

fitting algorithm, some predetermined value of Re, Ri and Cmare set to create known impedance data and the outcome of the

program for Re, R i and Cm are checked with predefined value.

Table I shows some instances of impedance parameter output

by the curve fitting program with error percentage. It is found

only 1.5% error in curve fitting method.

TABLE IESTIMATED IMPEDANCE PARAMETERS

Re ( ) Err.

% Re

Ri ( ) Err. %

Ri

Cm (F) Err. %

Cm

3.0000e+3 0 1000 0 8.005e-6 0.0625

5.0000e+3 0 2000 0 7.005e-6 0.0714

6.9999e+3 1.43 3000 0 5.985e-6 0.2500

8.9999e+3 0.01 4000 0 5.005e-6 0.1000

1.1000e+4 0 5010 0 4.005e-6 0.1250

1.3000e+4 0 6000 0 3.005e-6 0.1667

1.5000e+5 0.050 7000 0 2.007e-6 0.3500

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101

102

103

104

105

250

300

350

400

450

500Variation of Cm

Frequency

(b)

Impedance

Fig 11. Variation of impdance (a) with Re and (b) with Cm.

After successful estimation of impedance components Re, Riand Cm from current and voltage pulse, different impedance

spectroscopy curves shown in Fig. 11 are plotted for the

variation of Re and Cm. Result shows that the variation of

electrical impedance for Re is more in lower frequencies

(below 1 kHz) although has good variation in higher

frequencies too. On the other hand, impedance variation for

Cm is strictly within 10 kHz and almost no change thereafter.

But it is also found that the bio-impedance change due to Ri(intracellular medium change) is reflected in high frequencies

(MHz band). Research shows much more close relation of bio-

information with Re and Cm than Ri and according to the result

found, it is suggested to design bio-medical sensors below 100

kHz rather than MHz band for better sensitivity.

V.CONCLUSION

In the present work, stable current source and voltage

detection circuit with common mode noise suppression for

bioelectrical impedance measurement is presented.

Appropriate signal processing for impedance estimation from

impedance spectroscopy is modeled based on least square

curve fitting technique. Moreover, it is investigated that the

variation of bioelectrical impedance is higher in frequencies

less than 100 kHz for extra-cellular medium and plasmamembrane capacitance variation. Such findings along with

presented impedance measurement system would be applicable

for noninvasive bio-sensing based on bioelectrical impedance

spectroscopy.

ACKNOWLEDGMENT

The authors wish to acknowledge the invaluable

suggestions, support and resources by Dr. Sabuj Baran Dhar,

Assistant professor, Coxs Bazar Medical College Hospital

and Mr. Tanzilur Rahman, Research Assistant, University of

Tokyo.

REFERENCES

[1] U. Varshney, Pervasive healthcare, Computer vol. 36, Issue 12, pp.138 140, Dec. 2003.

[2] S. Grimnes and O. G. Marinsen, Bioimpdance and Bioelectricity Basics,2nd Ed. San Diego, CA: Academic, 2000.

[3] M. E. Valentinuzzi, Bioelectrical impdeance techniques in medicine.Part I: Bioimpedance measurement. First section: General concepts,Crit. Rev. Biomed. Eng., vol. 24, no. 4-6, pp. 223-255, 1996.

[4] S. F. Siconolfi, R. J. Gretebeck, W. W. Wong, R. A. Pietrzyk, and S. S.Suire, Assessing total body water from bioelectrical responsespectroscopy, J. Appl. Physiol., vol. 82, pp. 704-710, Feb. 1997.

[5] B. J. Thomas, B. H. Cornish, and L. C. Ward, Bioelectrical impedanceanalysis for measurement of body fluid volumes: A review, J. ClinEng., vol. 17, no. 6, pp. 505-510, No./Dec. 1992.

[6] J. Nyboer, Electrical Impedance Plethymography, 2nd ed. Springfield,IL: Charles C. Thomas, 1970.

[7] H. P. Schwann, Electrical properties of tissue and cell suspensions:mechanisms and models, Proc. IEEE Adv. Biol. Med. Soc., vol. 1, pp.A70-A71, Aug. 2002.

[8] H. Fricke, S. Morse, The electric capacity of tumors of the breast, J.Cancer Res. 16: 310-376 (1926).

[9] K. Okazaki, A. Tnagoku, T Marimoto, Keigo Hattori, R. Kotani, E.Yasuno, and Y. Kinouchi, Basic study of a new diagnostic modalityby noninvasive measurement of the electrical impedance tomography onlocalized tissues, ICBPE, pp. 1-5, Dec. 2009.

[10] B. Freer and J Venkataraman, Feasibility study for noninvasive bloodglucose monitoring, Proc. IEEE Symp. APSURSI 2010, pp. 1-4, Sept.2010.

[11] K. Kim, M. Lee, J. Kim, S. M. Jung, S. Lee and S. Yoo, Performance

evaluation of the electrode configuration for visceral fat measurement,Proc. IEEE Symp. EMBS, pp. 892-895, 2009.

[12] I. Schneider, Broadband signals for electrical impedance measurementsfor long bone fracture, Proc. IEEE Symp. EMBS, vol. 5, pp. 134-1935, Nov. 1996.

[13] L. I. Kalakutskiy and S. A. Akulov, Modeling of the bioelectricalimpedance of blood by synthesis of the equivalent electrical circuits,Proc. IFMBE 25/VII, pp. 575-577, 2009.

[14] T. Dai and A. Adler, In vivo blood characterisation frombioimpedance spectroscopy of blood pooling, IEEE Trans. on Instr. andMeas., vol. 58, no. 11, pp. 3831-3838, Nov. 2009.

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Noninvasive Bio-impedance Measurement Using Voltage Current Pulse Technique