Focused Impedance Method (FIM)

Innovative medical diagnostics and imaging for health stations in developing countries

Professor K Siddique-e Rabbani

Dept of Biomedical Physics & Technology, University of Dhaka,

Bangladesh

Body is a fluid

both positive & negative ions move to carry electrical

current

I

I

v

Unlike metals, where atoms do not move, only electrons flow

Biological Electrical Resistivity

Current bends around the cells

à High resistance

Current pattern in biological tissues

I

I

at dc and at low frequency ac:

With an insulating membrane, a CELL

acts like an insulated object

current enters the cells

à Low resistance

Current pattern in biological tissues

at high frequency ac:

cell membrane acts as a capacitor - sandwitched between conducting fluids, inside and out

I

I Capacitive reactance α 1/ freq

Impedance (Z) in biological tissues

0 2 4 6 8

10 12 14 16 18 20

-3 -2 -1 0 1 2 3 Log normalised frequency

Impe

danc

e Low

frequency

High frequency

Resistence is frequency independent Reactance: frequency dependent Impedance: Resistance + Reactance

Capacitive reactance

α 1/ freq

Tissue (Ω-m)

CSF 0.65 Blood 1.46-1.76 Skeletal muscle (longitudinal) 1.25-3.45 Skeletal muscle (transverse) 6.75-18.0 Lung – full inspiration 17.0 Lung – full expiration 8.0 Brain – grey matter 2.8 Brian – white matter 6.8 Fat 20 Bone >40

Electrical resistivity of body tissues (at low freq)

Potential in identifying different types of tissues

Advantage: Simple instrumentation and measurement Disadvantage: Wide zone in volume conductor (not focused)

Tetra-Polar Electrode Impedance Measurement (TPIM)

Alternating current (I) is passed through outer pair of electrodes, Potential (V) is measured across the inner pair. Impedance of shaded zone, Z = V / I If I is kept constant, then Z is proportional to V. Main advantage: Since voltmeter takes no current, effect of electrode contact impedance is eliminated. Z is that of the bulk region.

v

I

I

Z

Focused Impedance Method (FIM) (6 electrode)

(Z 1 + Z 2) has more contribution from central region (green), hence, ‘Focused’

- a new idea from Biomedical Physics Lab Dhaka University

Old TPIM

v

I

I

Current, I1 and I2 in two perpendicular directions (in phase, but isolated). Potential, V, measured

across two diagonally placed electrodes at centre. V = Z1+Z2

I 2 Z 1

Z 2

I 1

I 1

I 2 V

4-Electrode FIM

Z1 = V1 / I1 Horizontal sensitive zone

Z2 = V2 / I2 Vertical sensitive zone

Z1 + Z2 gives focused impedance (central green zone is focused)

v

v

I1

I2

I1

I2

V1

V2

3D sensitivity allows deeper organ study

Sensitivity Map of FIM (Phantom study)

current electrode

potential electrode

current electrode

Focusing

evident

GASTRIC EMPTYING USING FIM

EIT Gold standard

FIM

TEST: GASTRIC EMPTYING AFTER A DRINK OF SALINE

-2.5

-2

-1.5

-1

-0.5

0

0 5 10 15 20 25 30 Time, minutes

Cha

nge

in p

ixel

sum

-60

-50

-40

-30

-20

-10

0

0 5 10 15 20 25 30 Time,minutes

% C

hang

e in

impe

danc

e

Could eliminate effect of neighbouring dudenum

Lungs ventilation study

Trial-I

0 0.05 0.1

0.15 0.2

0.25 0.3

0 0.5 1 1.5 2 2.5 3 3.5 4 Change in Air Volume(L)

Cha

nge

in V

olta

ge(V

)

0 0.05 0.1

0.15 0.2

0.25 0.3

0 0.5 1 1.5 2 2.5 3 3.5 4 Change in Air Volume (L)

Cha

nge

in Im

peda

nce

Comparison with Spirometric study:

First breathe in, then breathe out a little & hold, volume of air measured using spirometer.

FIM is linearly related to air volume

Ø  Study lung ventilation, perfusion and disorders

Ø  Monitor respiration (very useful in artificial respiration)

Ø  Study gastric emptying

Ø  Measure gastric acid secretion

Ø  Measure localized edema

Ø  Measure abdominal fat thickness

Ø  Detection of cervical cancer

Ø  Characterisation of breast tumours – benign or malignant ?

Ø  Monitor tissue ablation in cancer therapy; irreversible electroporation using same electrodes

Ø  Do we require high quality imaging to achieve similar diagnoses?

FIM : potential in Diagnosis and Physiological study

Recommendations

Section 1

Conclusions and Recommendations

•  Drastic need for improvcd communication

with health stations. •  Telemedicine to play a vital role in

diagnostics and training. •  Focused impedance method could become

an important low cost diagnostic method for Health Stations.