Design of high frequency ultrasonic array transducers for medical imaging

228 Pages·2012·13.03 MB·English

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UNIVERSITY OF SOUTHAMPTON

FACULTY OF PHYSICAL AND APPLIED SCIENCE

School of Electronics and Computer Science

Design of High Frequency Ultrasonic Array Transducers
for Medical Imaging

by

Yichen Qian

Thesis for the degree of Doctor of Philosophy

Oct. 2011

UNIVERSITY OF SOUTHAMPTON
Abstract

FACULTY OF PHYSICAL AND APPLIED SCIENCE
School of Electronics and Computer Science

Doctor of Philosophy

DESIGN OF HIGH FREQUENCY ULTRASONIC ARRAY TRANSDUCERS FOR MEDICAL IMAGING

by Yichen Qian

Ultrasonic transducers have been widely used in the medical applications. High frequency array
transducers have recently attracted many research interests, since it provides not only the
electronic beamforming but also the very fine image resolution. The transducers are usually either
annular or linear arrays. This study concentrates on the design of both types of array in high
frequency applications.

The annular array is firstly investigated, and an optimized design of array geometry is developed
including the kerfs influence which is usually neglected in the design of high frequency arrays. The
improved imaging performance is found by this optimized design. More interestingly, the
conventional annular array with element having the same surface area has a relatively large
sidelobe closest to the main lobe. Two methods are applied to suppress the sidelobe. The method
of using high-impedance single matching to enlarge the element bandwidth only shows very
limited lobe suppression, however, a novel matching approach is found. Another method of
changing the array geometry from equal area elements to equal width ones successfully
suppressed the sidelobe. The imaging results suggest that the novel equal width array shows a
good improvement.

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The linear array is then analyzed. The kerfs influence again is found to be significant for the array
especially for high frequency. It is essential to use a high attenuated kerfs material in the linear
array design. More importantly, the limitation of current fabrication techniques forces the linear
array to concede a high grating lobe in the imaging field if it operates in a relatively high frequency.
A novel linear array is developed to suppress the grating lobe without using advanced fabrication
techniques. The imaging results show that the novel linear array has a general reduction of -6dB
for the lobe suppression. The novel design made the linear array operate in a very high frequency
without strong grating lobe, and the array can be fabricated by current techniques.

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Contents
Abstract ................................................................................................................................................. i
Contents .............................................................................................................................................. iii
List of Figures .................................................................................................................................... viii
List of Tables ..................................................................................................................................... xiii
DECLARATION OF AUTHORSHIP ........................................................................................................ xvi
Acknowledgements .......................................................................................................................... xvii
Definitions and Abbreviations ........................................................................................................ xviii
Chapter 1 Introduction .........................................................................................................................1
1.1 Overview of Medical Ultrasound Imaging ...........................................................................1
1.2 Research Scale......................................................................................................................4
1.3 Published Papers ..................................................................................................................5
1.4 Outline of Thesis ..................................................................................................................5
Chapter 2 Literature Review ................................................................................................................7
2.1 Introduction .........................................................................................................................7
2.2 Transducer components ......................................................................................................8
2.2.1 Piezoelectric Layer ..............................................................................................................8
2.2.2 Matching Layer ...................................................................................................................9
2.2.3 Backing Layer ................................................................................................................... 10
2.3 Heart of the Transducer – Piezoelectric Layer .................................................................. 10
2.3.1 Piezoelectric Effect ........................................................................................................... 10
2.3.2 Piezoelectric Equations .................................................................................................... 12
2.3.3 Vibration Modes .............................................................................................................. 14
2.3.4 Waves produced by transducer element ......................................................................... 18
2.3.5 Piezoelectric materials ..................................................................................................... 19
2.4 Transducer Imaging ........................................................................................................... 22
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2.4.1 Imaging Principle .............................................................................................................. 22
2.4.2 Imaging Properties ........................................................................................................... 25
2.4.3 Focusing Techniques ........................................................................................................ 28
2.5 Design of Transducers by Numerical Models ................................................................... 31
2.5.1 Classic 1-D models ........................................................................................................... 31
2.5.2 Loss factors involved in 1-D model .................................................................................. 34
2.5.3 Sensitivity and Bandwidth ................................................................................................ 35
2.5.4 Electrical and Mechanical Tuning .................................................................................... 36
2.5.5 Model of Transducer Imaging .......................................................................................... 39
2.5.6 Directivity ......................................................................................................................... 41
2.6 Transducers Fabrication and Comparisons ....................................................................... 44
2.6.1 Single-element Transducer .............................................................................................. 44
2.6.2 Linear array ...................................................................................................................... 45
2.6.3 Annular array ................................................................................................................... 47
2.6.4 Comparisons .................................................................................................................... 48
2.7 Summary ........................................................................................................................... 49
Chapter 3 High Frequency Annular Array ......................................................................................... 51
3.1 Introduction ...................................................................................................................... 51
3.2 Methodology ..................................................................................................................... 53
3.3 Optimized Design of Equal Area Annular Array Geometry ............................................... 56
3.3.1 Conventional Design of Annular Element Geometry ....................................................... 57
3.3.2 Optimized Design of Annular Element Geometry ........................................................... 59
3.4 Evaluation of Radiation Pattern by Rayleigh-integral Method ......................................... 64
3.4.1 Direct Calculation ............................................................................................................. 64
3.4.2 Radiation Pattern by Rayleigh-integral Method .............................................................. 67
3.5 Analysis of Sidelobes ......................................................................................................... 69
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3.5.1 Element Vibration ............................................................................................................ 69
3.5.2 Accuracy discussion on FEA and R-I methods .................................................................. 73
3.5.3 Potential Reasons for the Sidelobe .................................................................................. 76
3.6 Summary ........................................................................................................................... 78
Chapter 4 Methods of Sidelobe Suppression .................................................................................... 81
4.1 Introduction ...................................................................................................................... 81
4.2 Sidelobe Suppression by Broadband ................................................................................ 81
4.2.1 Bandwidth Improvement by Tuning ................................................................................ 82
4.2.2 Element Behaviours by FEA ............................................................................................. 84
4.2.3 Imaging Responses ........................................................................................................... 85
4.3 Sidelobe Suppression by Geometric Change .................................................................... 88
4.3.1 Sidelobe Analysis of EA Annular Array ............................................................................. 88
4.3.2 Sidelobe Suppression by Novel Array Geometry ............................................................. 92
4.3.3 Analysis of the R-I Patterns .............................................................................................. 95
4.4 Summary ........................................................................................................................... 97
Chapter 5 High Frequency Equal Width Annular Array ..................................................................... 99
5.1 Introduction ...................................................................................................................... 99
5.2 Performances of Equal Width Annular Array .................................................................. 100
5.2.1 Element Responses ........................................................................................................ 100
5.2.2 Imaging Responses ......................................................................................................... 105
5.3 Discussions on Equal Width Annular Array ..................................................................... 106
5.3.1 Discussion on Pressure Pulses at Focal Point ................................................................. 110
5.3.2 Discussion on Resolution and Contrast .......................................................................... 111
5.3.3 Discussion on Sidelobe and Pedestal Lobe .................................................................... 113
5.3.4 Discussion on Fabrication Difficulties ............................................................................ 115
5.3.5 Overall Discussion .......................................................................................................... 115
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5.4 Summary ......................................................................................................................... 116
Chapter 6 High Frequency Linear Array .......................................................................................... 119
6.1 Introduction .................................................................................................................... 119
6.2 Methodology ................................................................................................................... 120
6.3 Design of a Conventional Linear Array ............................................................................ 123
6.3.1 R-I Evaluation of Array Beam Pattern ............................................................................ 123
6.3.2 Influence of Kerfs Filler on Element Vibration ............................................................... 125
6.4 Imaging Performances of the Linear Array ..................................................................... 130
6.4.1 Influence of the Kerfs Vibration ..................................................................................... 131
6.4.2 Imaging Patterns of Very High Frequency Linear Array ................................................. 133
6.5 Summary ......................................................................................................................... 136
Chapter 7 Novel High Frequency Linear Array ................................................................................ 137
7.1 Introduction .................................................................................................................... 137
7.2 Design of a Novel High Frequency Linear Array .............................................................. 139
7.2.1 Method of Grating Lobe Suppression ............................................................................ 139
7.2.2 Details in the Design of Novel Array by KLM Model ...................................................... 141
7.3 Performances of the Novel Linear Array ......................................................................... 144
7.3.1 Element Responses by FEA ............................................................................................ 145
7.3.2 Imaging Performances ................................................................................................... 150
7.3.3 Compound Beam Patterns ............................................................................................. 157
7.4 Optimization of Grating Lobe Suppression ..................................................................... 159
7.4.1 R-I Evaluation ................................................................................................................. 160
7.4.2 FEA Patterns ................................................................................................................... 163
7.5 Summary ......................................................................................................................... 164
Chapter 8 Conclusions & Future work ............................................................................................ 167
8.1 Conclusions ..................................................................................................................... 167
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8.1.1 High Frequency Annular Array ....................................................................................... 167
8.1.2 High Frequency Linear Array .......................................................................................... 169
8.2 Future Work .................................................................................................................... 170
Appendix A ...................................................................................................................................... 173
Appendix B ...................................................................................................................................... 179
Appendix C ...................................................................................................................................... 181
Appendix D ...................................................................................................................................... 193
Appendix E ...................................................................................................................................... 197
Reference ........................................................................................................................................ 201

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