simultaneous multi-frequency bio-impedance measurement applying synchronised uniform or non-uniform...

Simultaneous multi-frequency bio-impedance measurement Simultaneous multi-frequency bio-impedance measurement

applying synchronised uniform or non-uniform samplingapplying synchronised uniform or non-uniform sampling––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

by

Ants Ronk, Mart Min, and Toomas Parve

Department of Electronics

Tallinn University of Technology,

Tallinn, Estonia TUTTUT

***

ICEBI ’07ICEBI ’07August 29th – September 2nd 2007 in GRAZ, AUSTRIASimultaneous multi-frequency bio-impedance measurement applying synchronised uniform or non-uniform samplingA. Ronk, M. Min and T. Parve

In this paperthe possibilities to perform multi-frequency bio-impedance measurement simultaneously for several tissue channels are discussed, anda method of synchronous signal sampling by applying uniform or non-uniform sampling, together with digital signal processing is presented.

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TUTTUT

I. INTRODUCTION

Measurement of electrical bio-impedance enables to characterise a state of tissues/organs, to get diagnostic images, to find hemodynamical parameters, etc.

Simultaneous multichannel and multifrequency measurements are needed.

Why multichannel ? Why multifrequency ? Why simultaneous ?

Some examples of electrical bioimpedance measurement from cardiography

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Lung

HeartPacemaker

Bio-ImpedanceMeasurementUnit


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a noninvasive b multielectrode invasive estimation c intracardiac impedance measured plethysmography of the ventricular volume for pacing control


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I. INTRODUCTION

Complex bioimpedance Ż=R+jX is found as Ż = V/ I from the measured voltage response V to the sine wave excitation current I passed through the bio-object, commonly.

Explanation of the EBI. Simple 3-element equivalent. Phasor diagram for a frequency f.

What is so specific in the bioimpedance (EBI)

. V

Iexcit

Ż = R + jX

XC

Ż R

G

V

The phasor diagramme of the static EBI (▬▬), and of its 3-element equivalent (▬▬), and the phasors of the static EBI for 2 frequencies,

low ωl and high ωh.

rint C

rext

.

Simultaneous multifrequency EBI measurement system

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Fig.1 A system for simultaneous two-frequency bio-impedance measurement applying synchronised sampling

II. M E A S U R E M E N T S Y S T E M : SIGNALS AND ALGORITHMS

Oscil lator

Ż

Iexcit

VZ

SamplingControl

Synchronoussampling

Synchro

If1 If2

Analog-to-DigitalConverter

VZ = Iexcit Z = If1 Z(f1) + If2

Z(f2)

Sampling pulses

. . . .

II. MEASUREMENT SYSTEM: SIGNALS AND ALGORITHMS

Multi-site EBI measurement using synchronous sampling

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The real Re and imaginary Im parts of the phasor Ż are determined as

Re = (Re+ – Re–) ∕ 2

and

Im = (Im+ – Im–) ∕ 2

1.0

0.0100 1 2 3 4 5 6 7 8 9

2.0

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

100 1 2 3 4 5 6 7 8 9

Re+

Re–

Im+ Im–

fsampling = 4 · fsignal

Synchronous sampling of a single sine wave response.

Real part samples Re+ are designated as filled red dots ●and Re– as unfilled red ones ○, imaginary part samples Im+ as filled green squares ■, and Im– as unfilled green squares □

Multi-site Multi-frequency

Two typical cases of multifrequency measurement

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a) Two different impedances are measured b) The same impedance is measured at a slightly differing frequency at (two) essentially different frequencies

non-uniform synchronous sampling

Fig. 3. Simultaneous measurement of responses to two excitations Note: Only the Re+ samples are shown for the response signal.


Vf1 + Vf2

Ż G

If1

If2

If1 + If2

Using the non-uniform sampling at simultaneous measurement


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Convergence of results in the case of simultaneous measurement of the responses to two excitations with near (slightly differing) frequencies f1 and f2

f1

f2

fSP,1 = f1

f2

f1

fSP,2 = f2

fSP

6 A1=

ΣAi ⁄ 6

i=1

6

Σ Ai = 0 i=1

5

A2= Σ Ai ⁄ 5

i=1

5

Σ Ai = 0 i=1

non-uniform sampling

An example:

f1 ⁄ f2 = 6 ⁄ 5

tmeas = 6 ⁄ f1 = 5 ⁄ f2


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Timing and weighting of the samples usable for determining the real and the imaginary part of the components of the VZ signal in the case of two-frequency operation

f2 = 2 f1


Simultaneous multifrequency measurement using uniform sampling

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Timing and weighting of the samples usable for determining the real part of the VZ signal in the case of two-frequency operation

f2 = 3 f1


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Timing and weighting of the samples usable for determining the real part of the VZ signal in the case of two-frequency operation

f2 = 6 f1


Example 1 – Decade distances

Choice of frequencies (Examples for some more general cases)

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k = 2i + 1i = 0, 1, 2, ….


2n 1 2 4 8 16 32 64 128

fs 4 8 16 32 64 128 256 512

k 11 5 25 13 63 33 157 79

f 11 10 100 104 1008 1056 10048 10112

Example 2 – Half decade distances2n 1 2 4 8 16 32 64 128

fs 4 8 16 32 64 128 256 512

k 3 5 25 125 625 1 5 25

f 3 10 100 1000 1008 32 320 3200


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Main text


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-4

-2

0

2

4

0 5 10 15 20 25 30

-1

-0.5

0

0.5

1

=>real part

=>imag. part

=>modulus

Processing of one component of a 4-component multi-sine response with 2 pairs of close frequencies.

ν(t)

ReŻ1=R1

ImŻ1=X1

Z1

The signal ν(t) , its four components and 32 samples taken per its period (measurement interval)

0 5 10 15 20 25 30-101

wIm1

wRe1

wIm1

Convergence of results to real and imaginary parts and module for νc1(t)

Weighting patterns for νc1(t) 1.0

0.5

0

–0.5

–1.0

νc1(t)

CONCLUSIONS


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Thank you for your attention !Thank you for your attention !

ACKNOWLEDGMENTEstonian Science Foundation supported this work under the grants 7243 and 7212.

Address of the corresponding author: Ants Ronk, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia Email: [email protected]

The presented measurement signal system(s) and digital signalprocessing method enable to measure bioimpedance and to demodulate bio-modulations of components of a multi-sine measurement signal, which covers a wide frequency range (from kHz up to several MHz) simultaneously.

The same can be done applying Fourier transformation but the proposed approach is significantly simpler andsuits well for microelectronic implementation (e.g. in FPGA).

Thus it looks promising for applications in portable/wearable bioimpedance measurement systems of low power consumption.

TUTTUT

mailto:[email protected]

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