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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Sensitivity field calculations #2
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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Sensitivity field calculations #3
• 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
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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
18
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