bioimpedance basics

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BioimpedanceBasics

FYS4180 – høsten 2012Ørjan G. Martinsen

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BioimpedanceBasics

Impedance:• is a measure of the impediment or opposition to the flow of alternating current

• is a complex number.

• has an “ohmic” part (free ions) and a capacitive part (bound ions).

• is the inverse of admittance (conductance and susceptance)

It:• changes with tissue type, physiology and anatomy

• can hence be used as a transducer mechanism for physiological parameters

• can be measured with two, or more electrodes

• can be measured invasively or non‐invasively

• depends on the measurement frequency

• does normally not require any expensive components

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Material propertiesSimple parallel plate:

AdC

AdG

r

0 :tyPermittivi

:tyConductivi

dAR :yResistivit

Complex values

= ’ + j ’’

ρ = ρ’ + j ρ’’

= ’ + j ’’

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Two‐electrode system

sinIm Reactance

cosRe Resistance

Impedance

iv

ivX

iv

ivR

ivZ

sinIm eSusceptanc

cosRe eConductanc

1 Admittance

vi

viB

vi

viG

ZviY

sin1sin90 ifonly 1 and

cos1cos0 ifonly 1

BX

GR

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Four‐electrode system

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Transfer impedance

i v i v

• Input: i , output: v , transfer function:

• This is transfer impedance, not impedance

• Cannot use for calculation of , or

• Except for homogenous, uniform material

A B

Ziv

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Sensitivity field calculations

• Simple and very powerful tool, but not used very much

• Example: Direct current resistance measurement with a four‐electrode system on a homogeneous medium

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Sensitivity field calculations #1

Imagine that you inject a current I between the two current electrodes, and compute the current density J1 in each small volume element in the material as a result of this current

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Imagine that you instead inject the same current between the voltage pick-up electrodes, and again compute the resulting current density J2 in each small volume element.

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• The sensitivity S and total measured resistance R are then (reciprocal system):

• Positive S: Increased resistivity in this area higher total measured resistance• Negative S: Increased resistivity in this area lower total measured resistance• High absolute value of S: Very sensitive to changes in resistivity

dvI

RI

SV

2

212

21 and JJJJ

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Four‐electrode system

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Closer look …

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Adding a low‐conducting slice

A slice with only 20% of the conductivity of the surrounding medium is added

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Volume impedance density

The plot now shows the sensitivity divided by the conductivity (same as multiplying with the resistivity, since this is DC).

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Two‐electrode system

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Dispersions

Counter-ions

Charging of cell membranes

Relaxation ofwater molecules

E.g. makromolecules

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Data analysis• Measured with Solartron 1260 +

1294• Four electrodes on lower leg• Exported as text file

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Bode plot of impedance

-1.00E+01

0.00E+00

1.00E+01

2.00E+01

3.00E+01

4.00E+01

5.00E+01

1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06

Z'Z''

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Bode plot of admittance

-2.00E-02

-1.00E-02

0.00E+00

1.00E-02

2.00E-02

3.00E-02

4.00E-02

5.00E-02

6.00E-02

1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06

Y'Y''

2222 XRXB

XRRG

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Bode plot of |Z| and

-20

-10

0

10

20

30

40

50

1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06

|Z|f

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Bode plot of |Y| and

0

0.01

0.02

0.03

0.04

0.05

0.06

1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06-1.50E+01

-1.00E+01

-5.00E+00

0.00E+00

5.00E+00

1.00E+01

1.50E+01

|Y|f

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Wessel, Nyquist, Argand, Cole, …

-6.00E+00

-4.00E+00

-2.00E+00

0.00E+00

2.00E+00

4.00E+00

1.5E+01 2.0E+01 2.5E+01 3.0E+01 3.5E+01 4.0E+01 4.5E+01

Impedance

-1.00E-02

-5.00E-03

0.00E+00

5.00E-03

1.00E-02

2.0E-02 2.5E-02 3.0E-02 3.5E-02 4.0E-02 4.5E-02 5.0E-02 5.5E-02

Admittance

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Measuring sweat activity

24http://www.fys.uio.no/elg/bioimp

Elektrodermal response EDR

• Psychiatry• Neurological

diseases• Pain assessment• Lie detection• Anesthetic depth

0

2

4

6

8

10

12

14

16

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0 10 20 30 40 50 60 70

Time [sec]

Con

duct

ance

G o

r sus

cept

ance

B S

/cm

2 ] G righ

G left

B right

B left

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Measurements under stress

0

2

4

6

8

10

12

14

16

18

20

13:33 13:34 13:35 13:36 13:36 13:37 13:38 13:39 13:40 13:40 13:41 13:42 13:43 13:44 13:44 13:45 13:46 13:47

Skin

AC

con

duct

ance

[µS/

cm2 ]

Left axilla

Right axilla

Hypothenar

Abdomen

Relaxation Relaxation

Stress‐period withcalculations

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Measurements during physical excercise

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Needle positioning –where is the needle tip?

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• Insulated shafts

• Commonly used for EMG

• Apply a small electric current between the needle and a counter‐electrode

• Measurement sensitivity is J 2

• Can also use hollow needle

Needle positioning

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Collecting tissue data

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Classification – arteries and veinsone outlier removed

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Classification – muscle and fat

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Classification – subdermis and fat

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Needle arrayTo find blood vessels (artery and vein) during Emergency Cardiopulmonary Bypass (ECPB) for hypothermia and controlled reperfusion. The objective is to develop an automatic tool that finds blood vessels and

guides the cannulae into the vessels.

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Invasive 2D imaging?

• Ongoing data analysis• No classification yet• Simple plot of phase angle

at 100 kHz

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First invasive impedance tomography image

• No multivariate analysis• No classification only continuous color

• Simple plot of phase angle at 100 kHz

• Multivariate classification will most likely increase the image quality

• Step in vs. step out

bioimpedance basics

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