ELECTRONICS FOR REAL-TIME AND THREE-DIMENSIONAL ELECTRICAL IMPEDANCE TOMOGRAPHS

CHRISTOPHER WILLIAM LAWRENCE DENYER BSc MSc

PhD Thesis Oxford Brookes University

January 1996


Abstract

The subject of this thesis is the design of the data acquisition electronics for use in a medical imaging system based on Electrical Impedance Tomography (EIT). The system is required to operate at one of three applied frequencies and to collect current and voltage data at a rate sufficient to generate images at a frame rate of 25 images per second. The data is required to be sufficiently accurate to allow the successful imaging of absolute conductivity. The research work also concerns the design of a 3D EIT system for use in imaging in vitro from a 3D phantom.

To achieve these objectives, the existing design work on a previous system (OXBACT 2) was developed further to increase the data acquisition speed, to extend the operation to three frequencies, to use a current driver based current application method to eliminate the effects of variable electrode impedance as occurs with in vivo measurements, and to allow accurate phase and amplitude data to be collected. To improve the data accuracy, a calibration sub-system is incorporated into the design.

The research work includes some original contributions. The available voltage controlled current sources were reviewed and a novel supply-current-sensing current source was developed and tested. A novel DC feedback system was designed to allow AC coupling to the load without a shunt resistor at the output and the design was modified to ensure stability. A calibration system was developed which determines output current by modelling the performance of each current source under variable load impedance conditions. The system also compensates for inter-channel variations in complex gain.

Imaging studies have been carried out in vitro and the system has demonstrated its ability to image clearly conductive and insulating targets at each of the three operating frequencies. Imaging studies have been carried out in vivo at near real-time rates which clearly show lung filling and some cardiac-related changes near the centre of the image.


Acknowledgements

I would also like to thank my supervisor, Chris McLeod for invaluable help and suggestions in the design of both OXBACT 3 and OXBACT 4 and in the production of the thesis.

Thanks are also due to John Lidgey for his suggestions for current-driver designs, his invaluable suggestions and criticisms in the production of the papers and for presenting the paper on the calibration system at Toulouse on my behalf.

I would like to thank Yu Shi, with whom I have worked closely in the past year and have spent many long and sometimes frustrating hours perfecting the calibration system.

Thanks also to the other members of the team, Bill Lionheart, Tieying Duan and Mike Pidcock, for their help and encouragement, to Kevin Paulson in particular for his help and useful comments on the first reconstructed images and to Matthew Rose for his work on the 3D phantom.

I would like to thank Q. Zhu for laying the foundations for OXBACT 3, and whose work on OXBACT 2 was a vital link in the chain.

I would also like to thank my wife Kristina, who has encouraged and supported my efforts during the past three years on the research and on this thesis.


CONTENTS

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Abstract, Acknowledgements and Contents

Chapter 1 EIT review

1.0 Introduction 1
1.1 History of EIT 3
1.1.1 Back projection-based systems 5
1.1.2 Non-back projection systems 7
1.2 Summary 8

Chapter 2 EIT reconstruction

2.0 Introduction 9
2.1 Mathematical basis 9
2.2 Reconstruction methods 10
2.2.1 Backprojection method 11
2.2.2 Perterbation method 12
2.2.3 Newton's method 13
2.2.4 Layer stripping method 14
2.3 Optimal currents 14
2.4 Resolution 15
2.5 Hardware requirements 16
2.6 Summary 16

Chapter 3 Electrodes and tissue impedance

3.0 Introduction 18
3.1 Frequency variation of tissue impedance 18
3.2 Factors affecting tissue impedance 20
3.2.1 Blood velocity 20
3.2.2 Tissue temperature 21
3.2.3 Organ type 21
3.2.4 Physiological state of tissue 21
3.2.5 Tissue structure 22
3.3 The electrode 22
3.4 Biological limits 23
3.5 Practical limits 23
3.6 Electrode utilisation 23
3.7 Electrode location 24
3.8 Summary 24

Chapter 4 EIT at Oxford Brookes University

4.0 Introduction 25

4.1 OXBACT 1 25
4.2 OXBACT 2 26
4.2.1 Investigation into current sources 26
4.2.2 Development of wideband, high CMRR instrum. amplifiers 27
4.2.3 Signal demodulation in EIT 27
4.2.4 A method of setting up voltage driven current patterns 28
4.2.5 Design and development of OXBACT 2 system 28
4.2.6 Characterisation of OXBACT 2 system performance 28
4.2.7 Distinguishability study and image reconstruction 29
4.3 OXBACT 3 29
4.4 OXBACT 4 30
4.5 Summary 31

Chapter 5 Demodulation

5.0 Introduction 32
5.1 Matched filter demodulation 34
5.1.1 Noise performance 36
5.1.2 Distortion effects 37
5.2 Squaring demodulator 39
5.3 Switching demodulator 40
5.3.1 Distortion effects 42
5.4 Rectifying demodulator 42
5.5 Digital demodulation 43
5.6 Finite sampling 44
5.7 Reduction of quantisation noise 46
5.8 Non-uniform sampling 49
5.9 Summary 50

Chapter 6 EIT system design

6.0 Introduction 51
6.1 System design 51
6.2 ITC 51
6.3 Reconstruction 52
6.4 Data acquisition system 52
6.4.1 Demodulator design 52
6.4.2 Absolute or differential measurement? 56
6.4.3 Current application system 60
6.4.4 Calibration
63 6.4.5 Safety 63
6.4.5.1 Patient auxiliary current 64
6.4.5.2 Mains supply isolation 65
6.5 Summary 67

Chapter 7 Current generator design

7.0 Introduction 68
7.1 Current-driven versus voltage-driven approach 68
7.2 Deterministic current source 69
7.3 Voltage-controlled current source (VCCS) design 71
7.3.1 Current-mirror 71
7.3.2 Buffered current mirror 71
7.3.3 Floating load circuit 78
7.3.4 Positive feedback VCCS 79
7.3.5 Closed loop current controlling system 81
7.3.6 Supply-current sensing VCCS 83
7.3.6.1 Choice of current mirror 84
7.3.6.2 Current mirror types 84
7.3.6.3 DC limitation 86
7.3.6.4 Stability 86
7.3.6.5 Frequency response 87
7.3.6.6 Tests 89
7.4 Output admittance cancellation 90
7.5 Summary 91

Chapter 8 Calibration system

8.0 Introduction 93
8.1 Calibration of voltage measurement system 94
8.2 Current error due to finite output impedance 95
8.3 Conventional current measurement method 95
8.4 New method 96
8.4.1 Measurement of output admittance YS 99
8.4.2 Measurement of Is 101
8.4.3 Measurement of load current 102
8.5 Implementation 102
8.5.1 Calibration table 104
8.5.2 Calibration resistor values 104
8.5.3 Calibration sequence. 107
8.5.3.1 Voltage and current channel inverse gains KV & KC 107
8.5.3.2 PGA inverse gain KG 108
8.5.3.3 Current generator output impedance 109
8.5.3.4 Current generator transconductance 112
8.5.3.5 Current measurement 113
8.5.3.6 Current setting 113
8.5.4 Calibration system tests 113
8.6 Possible improvements to the calibration system 113
8.6.1 Calibration resistor capacitance. 114
8.6.2 Correction for output phase. 114
8.6.3 Improved method for determining output impedance 115
8.7 Summary 116

Chapter 9 Operation of the System

9.0 Introduction 117
9.1 DBEX bus 117
9.2 Multiplexer control bus 118
9.3 Reference signal 119
9.4 Current output 120
9.5 PGA 120 9.6 Calibration board. 120
9.7 Communication to ITC 121
9.8 External timing control 121
9.9 Operational sequence 121
9.10 Summary 123

Chapter 10 OXBACT 3 circuit description

10.0 Introduction 124
10.1 The bus system 124
10.2 Signal generator board 127
10.2.1 Cosine reference generation 127
10.2.2 Multiplexer control 130
10.3 DAC board 132
10.4 Current generator board 132
10.5 Voltage multiplexer board 136
10.6 Calibration board 139
10.7 Summary 139

Chapter 11 Test results of data acquisition system

11.0 Introduction 141
11.1 Voltage channel calibration tests 141
11.2 Output current calibration tests 143
11.3 Noise 151
11.4 Input impedance 151
11.5 Crosstalk 152
11.6 Current linearity 153
11.7 Phantom imaging studies 154
11.8 In vivo images 155
11.9 Summary 156

Chapter 12 OXBACT 4

12.0 Introduction 165
12.1 OXBACT 4 166
12.2 Phantom design 166
12.3 OXBACT 4 system design 168
12.4 Detailed circuit design 169
12.4.1 Data acquisition board control. 170
12.4.2 Data acquisition board design. 173
12.4.3 Signal generator board 174
12.4.4 Analogue interface board. 175
12.4.5 Digital interface board 175
12.5 Summary 175

Chapter 13 Conclusions

13.0 Introduction 178
13.1 Summary of major achievements 178
13.1.1 Literature review 178
13.1.2 Development of novel current generator circuit 178
13.1.3 Development of calibration system 178
13.1.4 Design of OXBACT 3 hardware 179
13.1.5 Testing of OXBACT 3 system 179
13.1.6 Design of OXBACT 4 system 179
13.2 Discussion 180
13.2.1 Aims of OXBACT 3 180
13.2.2 Current sources 180
13.2.3 Calibration system 181
13.2.4 Reconstructed images 181
13.3 Future work 182
13.3.1 Isolation 182
13.3.2 Software 182
13.3.3 Adaptive imaging studies 182
13.3.4 Multifrequency imaging studies 183
13.3.5 Determination of boundary shape 183
13.3.6 Clinical experiments 183
13.4 Summary 184

References 185

Apendices:

Appendix A Formula derivations 193

Appendix B Published papers 200


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