focused impedance method (fim)
TRANSCRIPT
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.