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Specification and calibration of a Multi-frequency MEIT
system for stroke.
Alistair McEwan, Rebecca Yerworth, Lior Horesh, Richard Bayford and David Holder.
Department of Medical Physics and Bioengineering, UCL, London.
Abstract
The performance of a modified MEIT system was investigated, using a resistor phantom, for
the imaging of stroke based on specifications revealed in recent modelling (Horesh et al,
2005). Using ideal calibration the system does not meet the specifications as the variation
over the load is 0.9%, variation over frequency is +/-1.5% and variation over electrode
combinations is +/-2% exceeding the better than 1% requirement over 20-500kHz and 1 to 66
Ω. However this system has been useful in imaging gastric emptying (Soulsby et al, 2005);
extending to lower frequencies should aid this application along with further epilepsy trials
(Fabrizi et al, 2005).
Introduction
Time difference imaging is not available in stroke as there is usually no opportunity to take a
reference image. Frequency difference is one method that may be suitable; however, this
relies on spectrally consistent EIT measurements over different electrodes. Unfortunately, the
requirements to image stroke appear to be demanding (Horesh et al, 2005). Using a detailed
anatomical mesh of the head and data from the literature, they estimated that an accuracy of
less than 1% over the frequency range 10 Hz – 500 kHz and load of <1-70 Ω was required. In
our group, we have been developing a MEIT system, based on one module of the Sheffield
Mark 3.5 system (Wilson et al, 2000), but with a cross point switch to permit addressing of up
to 64 electrodes (Yerworth et al, 2003). However, in recent human recordings with this
system, we encountered unexpected difficulties (Romsauerova et al 2005). The purpose of
this work was to revisit the performance of our MEIT system and ascertain if it met the
demanding requirements recently revealed by the precise modelling. Characterization was
achieved by measurement with a resistor phantom; performance was evaluated over different
loads and frequencies.
Methods
+
+
x-point
+
1..32
x-point
DAC
ADC
+
Figure 1: A schematic of the system showing the coupling capacitors and cross point switch
(x-point).
The recent human recordings and modelling work suggested that both the hardware and
software of the system required modifications. The hardware modifications included reducing
the gain of the voltage measurement circuits to ensure the transimpedance of the head, which
is a maximum of 70Ω, could be measured without exceeding the differential input range of
the ADC. The coupling capacitors in the current source and voltage amplifier as shown in
Figure 1 were increased to 100uF to reduce their impedance at the lower frequencies. Each
electrode contact in the cross-point switch requires a dc-blocking (coupling) capacitor for
patient safety. These were increased to 10uF to reduce their impedance at low frequency.
The dc-blocking capacitors increase the discharge time required between measurements. To
ensure this value is similar between electrodes a 1MΩ resistor to ground was included in each
electrode (not shown). This choice reduced the input resistance of the voltage amplifier to
1MΩ and results in a 0.1% error due to the current divider on a 2kΩ load. Reducing this
resistor to say 10kΩ to speed up measurement time would be disastrous, introducing errors of
the order of 10%. The Sheffield Mk3.5 strategy of three epochs of a composite waveform of
10 frequencies was maintained (Wilson et al 2000). One of the epochs was modified to use
frequencies 100 times lower by introducing delays in the DSP code.
The calibration tests used a simple model of skin impedance of a resistor in series with a
parallel combination of resistor and capacitor (Webster 1997). The values of the components
in the skin model were based on a human measurement with the HP4284A (now Agilent
Technologies) impedance bridge between two electrodes on the head of a normal subject.
Standard EEG electrodes were used, the skin abraded so that the two terminal impedance was
less than 1.5kΩ at 20Hz and standard EEG paste was used to improve the electrode-skin
interface. At frequencies below 1kHz it was assumed that the skin impedance remained
constant (Yamamoto et. al. 1976).
800
700
R, Human
Impedance (ohms)
600
X, Human
R, model
500
X, model
400
300
200
100
0
1.00E+03
1.00E+04
1.00E+05
1.00E+06
Frequency, Hz)
D
1
430
R
R
1
22n
F430
R
200
R
R
2
22n
F430
R
200
R
D
2
22n
F430
R
200
R
Gn
d
22n
F430
R
200
R
22n
F
Skin
impedance
200
R
Decade resistor
box
Figure 2: Skin Impedance Model
The modifications were tested in three stages using the skin impedance model and a discrete
resistor as the load:
1. Bandwidth and the effect of the cross point switch.
The frequency response of the system was measured on a load of 50 Ω, with and without
the x-point switch and with and without the skin impedance.
2. Linearity with load of 1 to 66 Ω.
The response of the system at all frequencies was measured over a logarithmically
varying load from 1 to 66 Ω using discrete resistors with skin impedance. The impedance
of the load was validated using the HP4284A. As it is difficult to see non-linearity in the
raw data, the frequency that best matched the HP4282A resistance measurements was
chosen. The deviation from the voltages measured at this frequency was plotted at two
different load ranges to show how the linearity varies with load.
3. Linearity with frequency.
A further step of calibration was to normalise the spectra at each load by the spectra at the
load which varied the least over frequency. The normalised spectra was plotted to show
the best possible performance of the system when calibrated over frequency and load.
4. Discharge time and variation over 96 electrode combinations.
The variation between 96 electrode combinations was tested on a wheel phantom with 16
1% matched resistors of 800Ω without skin impedance and using a polar protocol where
the drive electrodes are diametrically opposed and the receive electrodes are adjacent.
Discharge time was tested experimentally when driving with the high or low frequency
epoch first.
Results
1. Bandwidth and the effect of the cross point switch.
The bandwidth of the system extended to 40Hz - 1.6MHz. At 20Hz the signal is attenuated
by 0.9dB. The inclusion of skin impedance with the x-point switch attenuates the signal above
512kHz (frequency number 25) and at 1.6MHz the signal is attenuated by 0.87dB.
2. Linearity with load of 1 to 66 Ω.
The frequency that deviated least from the HP4284 measurements was 101.6kHz. The
deviation from the voltage measured at 101.6kHz was calculated for the other frequencies.
For frequencies between 40Hz and 512kHz the error over a load of 1 to 66 Ω varied from
1.5% to 0.25%. Over the same frequency range the error over a load of 5 to 66 Ω varied from
0.9% to 0.2%.
3. Linearity with Frequency
1.1
1.4 Ohms
1.7 Ohms
4.65 Ohms
8.21 Ohms
14.9 Ohms
32.5 Ohms
66.6 Ohms
1.08
1.06
Voltage(V)
1.04
1.02
1
0.98
0.96
0.94
0.92
1
10
2
10
3
10
4
10
Frequency (Hz)
5
10
6
10
7
10
Figure 3: Spectra at each load normalised to the spectra at a load of 66 Ω.
The load that varied the least over all frequencies was 66 Ω. The two smallest loads of 1.4 Ω
and 1.7 Ω varied by 5% at over 40Hz to 512kHz. Over the same frequency range the
variation with load is 3.6% for loads greater than 1.7Ω. Ignoring the first 3 frequencies of the
high frequency epochs (2kHz, 4kHz, 8kHz), the variation is +/-1.5%.
5. Discharge time and variation over 16 electrode combinations.
1.3
1.2
Normalised Voltage (to Frequency 5)
1.1
1
0.9
0.8
0.7
0.6
0.5
1
10
2
10
3
10
4
10
Frequency (Hz)
5
10
6
10
7
10
Figure 4: Variation between 96 electrode combinations at each frequency.
The variation was less when the low frequency epoch was driven first. The figure shows that
the variation between 16 electrodes is less than +/- 2% over most frequencies. The
frequencies in the low frequency epoch are the dots. The high frequency epochs are
represented by the circles and crosses. In both epoch sets the three highest frequencies show
the largest variation.
Discussion
The coupling capacitors chosen for patient safety limit the low frequency to 40Hz and
capacitance in the cross point switches limits higher frequencies to 512kHz. Over this
frequency range, after calibration, the variation over a load of 1-66 Ω was 1.5% over load and
5% over frequency. However these results are dominated by poor measurements at the lowest
two loads of 1.4 and 1.7 Ω where the systematic noise was as high as 5%. Ignoring these two
measurements and ignoring the three highest frequencies in the low frequency epoch
improved the performance to 0.9% over load, +/-1.5% over frequency and +/-2% over
electrode combinations. Comparing these results to the recent specification for imaging stroke
suggests that this system may not be suitable for this application without significant
improvements. Possible future work includes introducing these errors into the model to test
how they will affect the reconstructed images and discovering the origin of the noise that
limits the dynamic range and bandwidth of the system. However this system has been useful
in imaging gastric emptying (Soulsby et al, 2005); extending to lower frequencies should aid
this application along with further epilepsy trials (Fabrizi et al, 2005).
References
“Stroke type detection by Multi-Frequency Electrical Impedance Tomography MFEIT - a feasibility study” L Horesh , O Gilad,
A Romsauerova, A Tizzard, S R Arridge, R Bayford and D S Holder, Conference on Biomedical Applications of Electrical
Impedance Tomography, University College London, June 22 – 24th, 2005.
“Characterization of an EIT system for acute stroke imaging in patients with brain tumours, arteriovenous malformations, and
chronic stroke”. Andrea Romsauerova, Rebecca Yerworth, David Holder., Conference on Biomedical Applications of Electrical
Impedance Tomography, University College London, June 22 – 24th, 2005.
“Mk3.5: a modular, multi-frequency successor to the Mk3a EIS/EIT system”, A.J. Wilson, P. Milnes, A.R. Waterworth, R.H.
Smallwood and B.H. Brown., Physiological Measurement 22 pp49-54, 2001.
“Medical Instrumentation: Application and Design, 3rd Edition” Webster, J.G. (ed) y John Wiley ,1997.
“Electrical impedance tomography spectroscopy (EITS) for human head imaging”., R J Yerworth, RH Bayford, B Brown, P
Milnes, M Conway and D S Holder., Physiological Measurement 24, pp 477-489, 2003.
“Electrical properties of the epidermal stratum corneum”, Yamamoto, T. and Yamamoto, Y., Medical and Biological
Engineering, pp151-158, March 1976.
“EIT during epileptic seizures: not an easy task” L Fabrizi, M Sparkes, J F Perez Juste-Abascal, L Horesh, D Holder, Conference
on Biomedical Applications of Electrical Impedance Tomography, University College London, June 22 – 24th, 2005.
“Extending the range of test meals for EIT of gastric emptying by optimisation of the applied Frequency” Soulsby C,
Romsauerova A, Yerworth R, Horesh L, Evans D, Holder D, Conference on Biomedical Applications of Electrical Impedance
Tomography, University College London, June 22 – 24th, 2005.