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Design and development of ultrasonic array transducers for specialized applications

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Design and development of ultrasonic array transducers for specialized applications

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Content i

Design and Development of Ultrasonic Array
Transducers for Specialized Applications
by
Chi Tat Chiu

A dissertation presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In partial fulfillment of the
Requirements for the degree
DOCTOR OF PHILOSOPHY
(BIOMEDICAL ENGINEERING)

December 2016

i

Dedication

I would like to dedicate this dissertation to my beloved wife, Winnie, and my
mother, Ada, with my gratitude for their support and love throughout all these years.

ii

Acknowledgements
Time flies, and it has been five years already since I started pursuing my Ph.D
degree. All the knowledge that I have acquired, all the respectable people that I have
met, all the highs and lows that I have gone through – I can’t say enough how grateful
I am for all of these. No doubt I will always treasure these memories within my heart.
First and foremost, I would like to express my sincere gratitude to my
supervisor, Prof. K. Kirk Shung, for his support and insightful advices. His
mentorship really guides me through my years working in this lab. Also I would like
to thank you Dr. Qifa Zhou, Dr. Jesse Yen and Dr. Mike Chen for all their helps and
suggestions in my dissertation projects.
Second I would also like to thank my colleagues at the Resource Center for
Medical Ultrasonic Transducer Technology. Especially I am thankful to Dr.
Hao-Chung Yang, Dr. Ruimin Chen, Dr. Fan Zheng and Dr. Bong Jin Kang for all
their guidance when I first started working in the center. Our previous lab managers
Dr. Hyung-Ham Kim and My Jay Williams also helped me a lot with my projects, of
which I am also very grateful. And of course I have to thank my friends, including Dr.
Teng Ma, Dr. Chunlong Fei, Mr. Nestor Cabrera-Munoz, Dr. Yang Li, Mr. Zeyu Chen,
Mr. Xuejun Qian, Mr. Xiaoyang Chen and Miss Mingyue Yu – I always enjoy
working and brainstorming with you all.
Last by not least, I would like to express my deepest gratitude to my family,
especially to my beloved wife and mother. I won’t be able to make it this far without
your support.

iii
Abstract of dissertation entitled
Design and Development of Ultrasonic Array
Transducers for Specialized Applications
Submitted by
Chi Tat Chiu

Background:
For more than half century, ultrasound imaging has been widely employed as a
valuable diagnostic tool. Although it has often been viewed as a mature imaging
modality, its emergence into new areas has been continuing. Active researches are
going on and new applications using ultrasound imaging are being explored. However,
despite the availability of a variety of commercial ultrasonic transducer arrays,
researchers often would come across certain specific requirements that cannot be
accommodated by the existing transducers. Although some may try to use the standard
transducer arrays, the resulting images might not be ideal for research purposes.
In this dissertation study, the goal is to design and develop ultrasonic transducers that
can fully meet the specifications for some specific projects. It is expected that by
employing the newly-developed transducer array, the research projects can be
advanced and the possibilities of using ultrasound in various aspects can be further
explored.
Interventional Procedure Guidance:
The first project focused on the use of ultrasound imaging in guiding interventional

iv
procedures such as biopsy. Currently the ultrasound probes are often placed at the
body surface of the patients, leading to several drawbacks including the limitation of
penetration and image quality. A miniature phased array transducer that can be placed
adjacent to the intervention device has been developed. The transducer comprised 48
elements housed in a 3-mm-diameter needle. The center frequency and the bandwidth
were around 21 MHz and 42% respectively. The imaging capability of the transducer
was evaluated by acquiring the B-mode images of a needle in a cow liver. The
performance of the proposed phased array transducer demonstrates the feasibility of
such an approach for interventional guidance.
Early Detection of Rheumatoid Arthritis:
The second project aimed at the early detection of rheumatoid arthritis using
photoacoustic tomography (PAT). As it is known that angiogenesis correlates with the
early stage of rheumatoid arthritis, using PAT for screening purpose seems promising
due to its high sensitivity for blood. A ring-shaped transducer array would be
fabricated to accommodate a human finger so as to acquire the PAT image at the
finger joint. The one-way pulse measurement of the prototype showed that the array
has a center frequency and bandwidth of 9.35 MHz and 74.6% respectively,
suggesting the array has promising acoustic performance for this application.

v

Table of Contents
Dedication ................................................................................................................. i
Acknowledgements ...................................................................................................... ii
Abstract .............................................................................................................. iii
List of Figures ............................................................................................................. vii
List of Tables .............................................................................................................. xii
Chapter 1. Introduction to Medical Ultrasound Imaging ................................... 1
1.1. Background .................................................................................................. 1
1.2. Outline of Thesis Study ............................................................................... 2
1.2.1. Research Motivation ......................................................................... 2
1.2.2. Goals and Objectives ........................................................................ 2
1.2.3. Chapter Overview ............................................................................. 3
1.3. Overview of Medical Ultrasound Imaging .................................................. 3
1.3.1. General Concepts .............................................................................. 3
1.3.2. Gray-scale Ultrasound Imaging ........................................................ 4
1.3.3. Applications of Medical Ultrasound Imaging .................................. 7
1.4. Overviews of Ultrasonic Transducer Array ............................................... 12
1.4.1. General Concepts ............................................................................ 12
1.4.2. Working Principles ......................................................................... 12
1.4.3. Operations of Ultrasonic Transducer Array .................................... 14
1.4.4. Advantages and Drawbacks ............................................................ 18
1.4.5. Current Development of Transducer Array .................................... 19
1.5. Concluding Remarks ................................................................................. 20
Chapter 2. Fabrication and Characterization of Miniature Phased Array for
Intervention Guidance ............................................................................................... 22
2.1. Chapter Overview ...................................................................................... 22
2.2. Background: Guiding Interventional Procedures ...................................... 23
2.2.1. Anatomic Landmark Approach ...................................................... 23
2.2.2. CT and MRI .................................................................................... 23
2.2.3. Ultrasound ...................................................................................... 25

vi

2.2.4. Research Goal ................................................................................. 26
2.3. Miniature Phased Array for Intervention Guidance .................................. 27
2.3.1. Transducer Specifications ............................................................... 28
2.3.2. Fabrication Process ......................................................................... 30
2.3.3. Performance Evaluation ................................................................. 35
2.4. Summary and Future Works ...................................................................... 42
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic
Tomography .............................................................................................................. 44
3.1. Chapter Overview ...................................................................................... 44
3.2. Background ................................................................................................ 45
3.2.1. Overview of Rheumatoid Arthritis ................................................. 45
3.2.2. Diagnosis and Treatment ................................................................ 46
3.2.3. Early Detection of Rheumatoid Arthritis: Benefits ........................ 47
3.2.4. Proposed Solution: Photoacoustic Tomography ............................. 48
3.2.5. Research Goal ................................................................................. 50
3.3. Ring-shaped Transducer Array for PAT ..................................................... 51
3.3.1. Transducer Design .......................................................................... 52
3.3.2. Fabrication of the Ring-shaped Array ............................................ 58
3.3.3. Evaluation of the Ring-shaped Array ............................................. 66
3.4. Summary and Ongoing Works ................................................................... 73
Chapter 4. Proposal Summary and Future Directions ...................................... 75
4.1. Chapter Overview ...................................................................................... 75
4.2. Summary of Thesis Study .......................................................................... 75
4.2.1. Overall Scope ................................................................................. 75
4.2.2. Specific Developments ................................................................... 76
4.3. Future Research Directions ....................................................................... 77
4.3.1. Miniature Phased Array .................................................................. 77
4.3.2. Ring-shaped Array .......................................................................... 78
References .............................................................................................................. 80

vii

List of Figures
Figure 1-1 Systemic illustration of a B-mode imaging setting ................................ 5
Figure 1-2 a) shows a typical B-mode image of a fetus, and b) shows a sampling
catheter (indicated by the white arrow) placed within the placenta (courtesy of Philips
Medical Bothell, WA). ............................................................................................. 8
Figure 1-3 shows a typical cross-sectional image of an artery obtained by IVUS. An
atherosclerotic plaque was presented in the artery. The right image illustrates the
standard measurements of the inside lumen (inner circle) and the external arterial
membrane (outer circle). [22] ................................................................................ 10
Figure 1-4 a) shows a simple cyst inside the breast, featuring the posterior echo
enhancement and the well-defined border, and b) shows a suspicious mass, of which
the characteristics include the posterior shadowing and the irregular margin [29].11
Figure 1-5 a) shows the structure of a typical pressed-focused single-element
transducer, and b) shows the actual device [8]. ..................................................... 13
Figure 1-6 a) illustrates different types of transducer array (elements shown in yellow),
and b) shows some of the commercial transducer arrays (taken from Philips’ webpage:
www.usa.philips.com). ........................................................................................... 15
Figure 1-7 a) illustrates a simple sequential linear scan in which adjacent groups of
elements are excited in sequence to generate the acoustic beams, and b) shows a
cross-shaped aperture used in realizing 3D imaging. ............................................ 16
Figure 1-8 illustrates a simple time delay beamforming scheme, showing a) focusing
and b) steering operations. ..................................................................................... 17

viii

Figure 2-1 a) illustrates the current ultrasound intervention guidance, in which the
transducer is placed at the body surface; b) illustrates the proposed device for
intervention guidance, in which the transducer is inserted into the body alongside with
the needle. (background image taken from www.llu.edu/images)......................... 24
Figure 2-2 shows the schematic section drawing of the designed phased array
transducer. Please note that the housing has certain length for the intervention
procedure. .............................................................................................................. 27
Figure 2-3 shows simulated pulse-echo results from PiezoCAD. ......................... 29
Figure 2-4 shows the process flow of fabrication process for the acoustic stacks. 30
Figure 2-5 a) shows the dicing saw used for fabricating the composite, and b) shows the
finished composite with the electrodes on individual elements. ........................... 31
Figure 2-6 a) shows the schematic drawing of the flexible circuit (units are in mm), and
b) shows the actual fabricated flexible circuit. ...................................................... 32
Figure 2-7 a) shows the pre-fabricated backed block made from Epotek 301, and b)
shows the bonding process of the acoustic to the backing block. .......................... 33
Figure 2-8 a) and b) shows the finished prototype transducer accommodated in 3mm
and 4mm needle housing respectively, and c) shows the corresponding pulse-echo
experiment result of a representative element. ...................................................... 34
Figure 2-9 a) shows the center frequencies and bandwidths of all the elements in the
transducer array (element #48 is not working), and b) shows the crosstalk
measurements of the prototype transducer. The maximum crosstalk value is -28 dB
over the frequency range tested. ............................................................................ 37
Figure 2-10 illustrates the signal routing scheme from the phased array transducer to
the back-end imaging system. ................................................................................ 38

ix

Figure 2-11 a) shows the image of the wire phantom with a 30 dB dynamic range. b)
and c) shows the axial and lateral line spread functions at the focal point respectively.
............................................................................................................................... 39
Figure 2-12 a) shows the experimental setup of the ex vivo imaging experiment, and b)
shows the image of the same needle inserted in a cow liver. ................................. 41
Figure 3-1 illustrates the basic concept of rheumatoid arthritis. In the normal joint (left
hand side), the joint capsule is lined with synovium. In the diseased joint (right hand
side), the synovium becomes inflamed and the joint is eroded eventually. ........... 45
Figure 3-2 a) shows a linear-array PAT setup for noninvasive in vivo functional
imaging of Methylene Blue [101], and b) shows a circular-array PAT setup for
recording the cerebral hemodynamic changes in a rat [102]. ................................ 49
Figure 3-3 illustrates the imaging setup with the ring-shaped transducer array. ... 51
Figure 3-4 a) and b) show the beam profile of the ring-shaped array, while c) and d)
show the pressure intensities at the focal point along the X-direction and Z-direction
respectively. ........................................................................................................... 53
Figure 3-5 a) shows the wire phantom images obtained by using the 256-element array
and b) shows the cyst phantom image. .................................................................. 54
Figure 3-6 Pulse-echo simulation results of PZT-5H/Epo-Tek 301 piezocomposites for
different volume fractions (50%, 60% and 70%): (a) time-domain echo and b)
frequency-domain spectrum .................................................................................. 58
Figure 3-7 a) and b) shows the composited bonded to the matching layers with
electrodes aligned. ................................................................................................. 59

x

Figure 3-8 a) shows the milling process of the TPX lens. The ball-end mill was used to
mill the lens surface in a back-and-forth manner, with a small increment in the milling
depth for each pass. The finished lens surface is shown in b). .............................. 60
Figure 3-9 a) shows the assembly process of the acoustic stack, and b) shows the actual
conformation of the acoustic stack using the fixture. ............................................ 61
Figure 3-10 a) and b) shows the front and back side of the PCB. Noted that the signal
traces are not visible as they are layered between the ground plates. c) shows the
pattern of the signal traces running from the array connector pads to the Samtec QSE
connector. ............................................................................................................... 63
Figure 3-11 a) shows a finished quarter-ring array connected to the PCB and b) shows
the cable assembly used to connect the array to the imaging system. ................... 64
Figure 3-12 a) shows the completed ring array transducer comprising four individual
quarter-ring array transducer, and b) shows the housing of the transducer, with the
bowl-like configuration at the top of the array for optical fibers placement. ........ 65
Figure 3-13 a) shows the pulse-echo response of a representative element of the array
transducer, and b) shows the frequency distribution of all the array elements. ..... 67
Figure 3-14 shows the experiment setup for the one-way pulse measurements using
the hydrophone. ..................................................................................................... 68
Figure 3-15 a) and b) shows the pressure profile in the azimuth and the elevation
direction respectively. The symmetric shape of the profiles suggests that the
hydrophone is aligned with the array prototype. c) shows the pressure time response at
the focus, with a center frequency and bandwidth of 9.35 MHz and 74.6% respectively.
............................................................................................................................... 69

xi

Figure 3-16 a) illustrates the signal flow of the imaging system (with a Verasonics V1
back-end) and b) shows the actually connection of the prototype to the system. .. 71
Figure 3-17 The echo data received using the Verasonics system....................... 72

xii

List of Tables
Table 2-1. Comparisons between different intervention guidance approaches ..... 25
Table 2-2. Specifications for the phased array ....................................................... 28
Table 2-3. Properties of the transducer materials................................................... 29
Table 2-4. The materials of the phased array ......................................................... 29
Table 3-1. The specifications of the ring-shaped array .......................................... 52
Table 3-2. Properties of the transducer materials................................................... 56
Table 3-3. Results of the finite element modeling ................................................. 57

Chapter 1. Introduction to Medical Ultrasound Imaging
1

Chapter 1. Introduction to Medical Ultrasound
Imaging
1.1. Background
The non-invasive visualization of living tissues with the use of ultrasound can be
dated back to the late 1940s, and since then it has been used for diagnostic purposes in the
medical field. Nowadays, ultrasound has long become a widely accepted diagnostic tool.
Comparing to other imaging modalities such as computed tomography (CT), magnetic
resonance imaging (MRI), etc., ultrasound has its own unique advantages, including
radiation-free, portable and capable in real-time imaging, which makes it an attractive tool
in various areas.
A typical ultrasound scanner comprises two major components: the ultrasonic
transducer and the backend imaging system. Over the past several decades, the
performance of the ultrasound scanners and hence the quality of ultrasound images have
improved greatly. The imaging system has more powerful data acquisition and processing
modules, and equipped with advanced algorithms for various applications. On the other
hand, a series of technical breakthrough has also been undergone in the transducer
fabrication process. For example, the advance in piezoelectric material has improved the
transducer sensitivity and the image resolution, and the new technique in managing signal
routing has made feasible the realization of high element-count array transducer.
Other than that, in order to cater for different medical or research applications,
various types of ultrasonic transducers, especially array transducers, are commercially
available nowadays. These mature transducers, including linear switched array, phased
Chapter 1. Introduction to Medical Ultrasound Imaging
2

array, and even two-dimensional array, are employed in medical disciplines such as
cardiology, obstetrics, pediatrics, gynecology, and many others based on each of its own
characteristics.
1.2. Outline of Thesis Study
1.2.1. Research Motivation
However, besides the areas mentioned above in which ultrasound has been long
adapted as a major diagnostic tool, more and more researchers nowadays are exploring into
applications that require ultrasound imaging, while at the same time having certain specific
requirements that cannot be accommodated by the current existing transducers in the
market. Although some may try to use the standard transducer arrays, the resulting images
might not be ideal for research purposes.
Therefore to facilitate the research in these areas, it would be important to design and
develop ultrasonic transducers that can fully meet the specifications for the projects. By
developing the specific transducers, it is expected that the research projects can be
advanced and the possibilities of using ultrasound in various aspects can be further
explored.
1.2.2. Goals and Objectives
In this thesis study, our goal is to design and develop different ultrasonic transducers
for some specific applications. Specifically, two different applications were investigated,
and will be introduced in the following chapters. In order to achieve this goal, each project
has been broken into the following objectives:
 Design the corresponding transducer array in accordance with the specific
Chapter 1. Introduction to Medical Ultrasound Imaging
3

requirements;
 Identify the fabrication challenges, and develop a production protocol;
 Fabricate the transducer array prototypes;
 Evaluate the performance of the developed prototype and perform preliminary
testing experiments to demonstrate the feasibility of such approach
1.2.3. Chapter Overview
The rest of this introductory chapter will provide a basic overview of ultrasound
imaging, the working principles and the importance of ultrasonic transducer array,
together with the challenges encountered its design and development. In particular, the
chapter contents have been organized as follows:
 Section 1.3 gives an overview on medical ultrasound imaging, including its basic
principles and some application examples.
 Section 1.4 discusses the working principles of ultrasonic transducer array, together
with its current development, challenges encountered these days, and the future
direction.
 Section 1.5 summarizes the research problem and outlines the organization of the
subsequent chapters in this thesis.
1.3. Overview of Medical Ultrasound Imaging
1.3.1. General Concepts
Ultrasound imaging generally refers to making use of high frequency sound wave
for imaging purpose (usually >1 MHz in the medical field). Researchers have started
using ultrasound for flaws detection in metals since 1930s [1, 2], and in 1942 Dussik [3]
Chapter 1. Introduction to Medical Ultrasound Imaging
4

proposed that ultrasound might be used for diagnostic purposes. The pulse-echo signals
were displayed in an oscilloscope (namely A-mode as Amplitude mode) and investigated
for detecting various pathologies such as brain tumors [4], gall stones [5], and breast
cancer [6].
A major breakthrough was made in 1952 by Wild and Reid, who developed an
ultrasound imaging system capable of producing 2D cross-sectional images [7]. This
B-mode (as Brightness mode) visualization of the pulse-echo data allowed the underlying
anatomy to be much more readily interpreted and hence led to a gradual acceptance of
using ultrasound imaging in various medical fields.
Nowadays, ultrasound has already become a widely accepted imaging modality.
Besides the mentioned A-mode and B-mode, different imaging modes have been
developed for various applications, including M-mode (as Motion mode) and Doppler
mode. Areas that have employed the use of ultrasound extensively include cardiology,
pediatrics, obstetrics, and many more. In this thesis study, we will mainly focus on
B-mode imaging. Therefore in the following section, a more detailed discussion of
B-mode imaging will be given, followed by some of its applications.
1.3.2. Gray-scale Ultrasound Imaging
As discussed, A-mode imaging is the first imaging mode developed in the history of
ultrasound imaging. The idea is simple: first a single-element ultrasonic transducer is
excited by a high-energy pulse, then the echoes coming back are captured using the same
transducer and displayed as voltage traces (namely A-line). It is simple and useful in
detecting objects/flaws having a high acoustic impedance mismatch with the surrounding
environments, however with its drawbacks: 1) not very successful in low contrast
Chapter 1. Introduction to Medical Ultrasound Imaging
5

situation, and 2) not very intuitive when talking about visualizing the underlying
structure.
B-mode imaging provides the solutions when trying to overcome the drawback of
A-mode. The setting of B-mode is very similar to that of A-mode, except that the
transducer is steered or linearly translated mechanically to cover a finite field of view. Fig.
1-1 illustrates the setting of a B-mode imaging setup. Like in A-mode, the transducer is
excited by high-energy pulses continually, but with the motor running, the A-lines
received will now correspond to a different location. With the position feedback from the
motor, the received A-lines are mapped and a 2D image is then obtained. The amplitudes
of the echo signals are represented by the brightness at each pixel, i.e. bright pixel
represents a high amplitude echo, vice versa. Given the position of the echoes on a 2D
plane, B-mode imaging provides a much more intuitive way to interpret the image.

Figure 1-1 Systemic illustration of a B-mode imaging setting
Chapter 1. Introduction to Medical Ultrasound Imaging
6

Spatial Resolution
Like many other imaging modalities, the spatial resolution of the image is one of the
most important concerns. The axial resolution R
axial
of ultrasound images is given by [8,
9]:
𝑅 𝑎 𝑥𝑖𝑎 𝑙 =
𝑐 0
𝑁 𝑐𝑦𝑐 𝑙𝑒 2 𝑓 0
, (1-1)
where N
cycle
is the number of cycles (which in general is inversely proportional to the
bandwidth), c
0
is the speed of sound and f
0
is the center frequency of the transmitted
wave. On the other hand, the azimuth resolution R
azimuth
is given by:
𝑅 𝑎 𝑧 𝑖𝑚 𝑢𝑡 ℎ
=
𝐹 # × 𝑐 0
𝑓 0
, (1-2)
where F# is the F-number of the transducer, i.e. focal length divided by the transducer
aperture. Typically for the commercial scanners available in the market nowadays, the
image resolutions are in the range of 100 μm to 1000 μm.
From equations 1-1 and 1-2, there are two points to note. First, both the axial and the
azimuth resolutions are governed by the center frequency of the ultrasound wave, i.e.
higher the frequency, finer the resolution. This even leads to the motivation to increase
the center frequency for visualizing micro-structures such as the capillary bed. In general
when the frequency goes up to 20 MHz (and beyond), it can be called micro-ultrasound
imaging, and has its own set of new applications which will be briefly discuss later.
Second would be the relationship between the azimuth resolution and the F-number.
As the geometry of the transducer is fixed and hence the F-number cannot be changed, it
is expected that the azimuth resolution would be the best at the focal point but worsen at
pixels away from the focus. This can be greatly improved by employing array transducer
Chapter 1. Introduction to Medical Ultrasound Imaging
7

and appropriate beamforming techniques, of which more will be covered in Section 1.4.
Penetration Depth
Besides resolution, penetration depth is another important factor to take into
considerations. It can be used to assess how deep it can see through a certain structure,
which is important in some applications. For ultrasound wave in tissue, the signal
intensity I
x
after travelling a distance of x is given by [8, 9]:
𝐼 𝑥 = 𝐼 0
𝑒 − 2𝛼𝑥
(1-3)
where I
0
is the incident signal intensity and α is the attenuation coefficient of the tissue of
interest, ranging from 1 to 2. Typically for the medical ultrasound scanners nowadays, the
imaging depth can go up to 20 cm, which is sufficient in most clinical situations.
However, as seen from equation 1-3, the penetration depth decreases with increasing
frequency, and it is one of the major drawbacks of micro-ultrasound imaging. According
to some of the latest report on micro-ultrasound scanning systems [10, 11], the
penetration depth is only around 1 to 4 cm. Therefore often compromise has to be made
between the imaging resolution and penetration depth, depending on the target
applications.
1.3.3. Applications of Medical Ultrasound Imaging
As discussed in section 1.3.1, ultrasound imaging has been employed for a number
of clinical applications and biological researches. In this section, several well-developed
applications will be discussed.
Chapter 1. Introduction to Medical Ultrasound Imaging
8

Obstetric Ultrasound
Obstetric ultrasound perhaps is one of the most famous and widely-accepted
applications of ultrasound imaging. Being known as a non-radioactive and safe imaging
modality, ultrasound is particularly attractive in monitoring the fetus without adversely
affecting its growth. Starting from the late 1950s, it has soon become an important
diagnostic tool in obstetric.
Despite of the development of various imaging modes, the major use of obstetric
ultrasound still relies on 2D B-mode imaging in real-time, as shown in Fig. 1-2 a). This
allows the clinicians to visualize and locate any structural abnormalities in a
cost-effective manner. Examples include the screening for fetal chromosomal aneuploidy
[12] and the assessment of tricuspid valvular insufficiency [13] .The examination can be
carried out transvaginally or transabdominally. Although the former is less convenient
and may cause patient discomfort, it allows the transducer to be placed closer to the fetus
and hence a higher frequency of ultrasound can be used. It is particularly useful during

Figure 1-2 a) shows a typical B-mode image of a fetus, and b) shows a sampling catheter (indicated by
the white arrow) placed within the placenta (courtesy of Philips Medical Bothell, WA).
a) b)
Chapter 1. Introduction to Medical Ultrasound Imaging
9

the early stage of pregnancy as high resolution is crucial for visualization of the small
fetus.
Obstetric ultrasound is also used to assess the well-being of the fetus. By looking at
different parameters including the fetal movement, fetal tone, fetal breathing and
amniotic fluid volume, a well-being score can be assigned quantitatively [14, 15].
Another important application would be providing guidance for interventional operations,
including sampling or biopsy which requires the insertion of needles and/or catheters, as
shown in Fig. 1-2 b).
Intravascular Ultrasound (IVUS)
Unlike normal ultrasound imaging, IVUS is an invasive application involving the
insertion of the ultrasound transducer inside the vessel lumen. The transducer is placed on
a catheter which is advanced to the region of interest inside the cardiovascular system.
This allows the detailed visualization of vasculatures which are deep in the body and well
beyond the imaging depth of non-invasive ultrasound imaging. To achieve a fine
resolution so as to give precise measurements on the vessel geometry, micro-ultrasound is
employed in IVUS.
IVUS is used extensively in getting the cross-sectional geometric information of the
coronary artery. Fig. 1-3 shows an IVUS image (40 MHz center frequency) of an artery
with an eccentric plaque. The advantage of using ultrasound is that it enables the
visualization of the full circumference of the vessel. Also, the tissue beyond the vessel
wall can be imaged as shown in Fig. 1-3. Therefore through IVUS valuable insights can
be obtained, like the lumen area, plaque size and composition, etc. These kinds of
information are hard to get using only traditional diagnostic tools, like angiography
Chapter 1. Introduction to Medical Ultrasound Imaging
10

which only gives a 2D image of the lumen. Besides the evaluation of coronary diseases,
common applications of IVUS include guiding and checking stent implementation
[16-18], measuring volume flow through the lumen [19, 20], guiding angioplasty [21],
assessing atherosclerotic plaque [22, 23] etc.
However, IVUS also has its limitations. Although images with fine resolution can be
obtained, it is difficult to distinguish between different types of tissues as the
echogenicity of different tissue may be similar. This has been assessed recently by the
development of IVUS elastrography [24, 25]. By measuring the local elasticity of the
tissue, characterization of different tissues can be realized in a quantitatively manner.
Breast Ultrasound
Due to its good image contrast with soft tissue, ultrasound has been showing
excellent potential in breast disease diagnosis. Over the past two decades, studies have
shown that by incorporating the use of ultrasound, the detection rate of breast pathologies

Figure 1-3 shows a typical cross-sectional image of an artery obtained by IVUS. An atherosclerotic
plaque was presented in the artery. The right image illustrates the standard measurements of the
inside lumen (inner circle) and the external arterial membrane (outer circle) [22].
Chapter 1. Introduction to Medical Ultrasound Imaging
11

can be improved [26-28]. Since then, ultrasound has been gaining importance in this field.
Nowadays, ultrasound is often used as a complement to X-ray mammography, which is
usually the front-line screening and diagnostic tool for breast cancer and/or other breast
lesions.
Fig. 1-4 shows the ultrasonograms of two different types of breast lesions, i.e. a) a
simple cyst, and b) a suspicious solid lesion. By identifying the different characteristics
on the images, the sonographers would be able to differentiate between various types of
lesions [30, 31]. Besides, ultrasound can also be used to evaluate palpable masses which
are not clearly visible mammographically [26, 32] and to determine the status of lymph
nodes [33, 34]. To further improve the diagnostic accuracy of breast ultrasound, various
computer-aided diagnosis algorithms have also been developed to minimize the
dependency on the sonographers and to give a more quantitative assessment of the
targeted feature [35, 36].

Figure 1-4 a) shows a simple cyst inside the breast, featuring the posterior echo enhancement and the
well-defined border, and b) shows a suspicious mass, of which the characteristics include the posterior
shadowing and the irregular margin [29].
a) b)
Chapter 1. Introduction to Medical Ultrasound Imaging
12

Upon the identification of suspicious lesions, very often a biopsy operation will be
performed, in which ultrasound also takes an important role [37, 38]. By tracking the
position of the interventional device in real-time, the efficiency and accuracy of the
operation can be improved. Biopsy guidance is closely related to one of the applications
focused in this study, and a more detailed discussion would be given in Chapter 2.
1.4. Overviews of Ultrasonic Transducer Array
In the previous section, the basics of medical ultrasound imaging were reviewed. It
can be seen that a series of major breakthroughs have been undergone to make ultrasound
imaging flourish in various area. One of the most important steps forward was the
introduction of ultrasonic transducer array, in replacement of the single-element
transducer. In this section, the working principles of ultrasonic transducer array will be
discussed, followed by its technical advantages over single-element transducers.
1.4.1. General Concepts
Ultrasonic transducer is the component responsible for the conversion between
electrical energy and acoustic energy. The fabrication of suitable transducers has long
been one of the major topics in the development of all ultrasound imaging systems as the
properties of the transducer will be one of the primary factors in determining the overall
system performance. In general, ultrasonic transducers can be classified into
single-element transducer and transducer array.
1.4.2. Working Principles
Probably being the most simple formed of ultrasonic transducer, single-element
Chapter 1. Introduction to Medical Ultrasound Imaging
13

transducer is a good starting point for illustrating the basic structure and working
principles of ultrasound transducer. Fig. 1-5 a) illustrates the structure of a typical
press-focused single-element transducer, and b) shows the actual transducer. Put aside the
housing and connector, it can be seen that an ultrasonic transducer comprises three major
components: 1) piezoelectric element; 2) matching layers; and 3) backing layer.
Piezoelectric element is the heart of an ultrasonic transducer, as it is the primary
component that converts the electrical energy into acoustic energy, and vice versa.
Specifically, it works based on the piezoelectric effect – a phenomenon in which a
deformation in the material results in an electrical field, and vice versa [8, 9]. By
applying a varying electrical field, e.g. a sinusoid, the material will vibrate with the
corresponding frequency, generating a sound wave. Common piezoelectric materials used
in the fabrication of ultrasonic transducers include lead zirconate titanate (PZT) [39, 40],
single crystal lithium niobate (LiNbO
3
) [41], polyvinylidene fluoride (PVDF) [42], single
crystal lead magnesium niobate-lead titanate (PMN-PT) [43, 44], etc.

Figure 1-5 a) shows the structure of a typical pressed-focused single-element transducer, and b) shows
the actual device [8].
a)
b)
Chapter 1. Introduction to Medical Ultrasound Imaging
14

The second component is the matching layer, which couples the piezoelectric
element to the front loading medium (usually water or biological tissues). Due to the
acoustic impedance mismatch between the piezoelectric element and the loading medium,
a significant portion of acoustic energy will be reflected back instead of transmitted to the
front, resulting in a low efficiency. By incorporating layers of material with
well-designed acoustic impedance, the energy transmission can be greatly improved [45,
46].
The last component is the backing layer. First it can provide a good mechanical
support to the acoustic stack. Second, it can absorb the acoustic energy transmitted to the
back, preventing them from reflected back into the piezoelectric element causing ringing.
Although this might reduce the transducer sensitivity, it can improve the device
bandwidth, which in turn improves the image resolution.
1.4.3. Operations of Ultrasonic Transducer Array
When comparing to single-element transducer, the working principles of transducer
array are the same, and it also contains the three major components discussed. The main
difference between the two, as the names imply, is that transducer array consists of more
than one element. The elements are usually arranged in certain pattern, and by controlling
all or groups of the elements in a synchronized manner, tasks that cannot be done
normally with single-element transducer can be accomplished.
The most common type of transducer array probably is the linear array, or
one-dimensional array. All the elements of a linear array are arranged in a line, as shown
in Fig. 1-6 a). Based on the geometry and the transmission mode, it can be further
classified into subtypes such as linear-switched array, curved linear array and phased
Chapter 1. Introduction to Medical Ultrasound Imaging
15

array. Another type of array is the two-dimensional (2D) array. It consists of elements
arranged in a 2D plane, leading to a very high element count in general. Although it has
great potential in providing better image quality and more information than 1D array, it
was not realized until 1990s [47, 48] due to the technical difficulties in fabrication.
Annular array comprises rings of element, and usually serve as a bridge between
single-element and array transducer. Although there exists many different types of
transducer array, their operations share some basic concepts, which will be discussed
briefly in this section.
Aperture Control
One of the major advantages of transducer array is the fact that it has multiple
elements which can be controlled individually. Each element is connected to separate
circuitry for independent transmission and reception. This leads to a high flexibility in
controlling the transducer aperture, which has several major applications.
First, it allows the switching of aperture across the field of view without having to

Figure 1-6 a) illustrates different types of transducer array (elements shown in yellow), and b) shows
some of the commercial transducer arrays (taken from Philips’ webpage: www.usa.philips.com).
a)
b)
Chapter 1. Introduction to Medical Ultrasound Imaging
16

mechanically translate the transducer as shown in Fig. 1-7 a). Specifically, groups of
elements of the array are excited in succession, and the sound beam generated each time
would be at a different lateral position to form a complete B-mode image. Going one step
further would be the configuration of any arbitrary shape of aperture for more advanced
imaging scheme. One example is shown in Fig. 1-7 b), in which 3D imaging was realized
by employing the cross-shaped aperture [49].
Second would be the capability to configure the aperture apodization, i.e. controlling
the signal strength at individual elements. Any arbitrary apodization function can be
employed, and some common functions include the Guassian window and Hamming
window. This is useful in side lobes suppression [50, 51], increasing image contrast [52,
53], etc.
Beamforming
Beamforming is an important component in imaging with ultrasonic transducer array.

Figure 1-7 a) illustrates a simple sequential linear scan in which adjacent groups of elements are excited
in sequence to generate the acoustic beams, and b) shows a cross-shaped aperture used in realizing 3D
imaging.
b)
a)
Chapter 1. Introduction to Medical Ultrasound Imaging
17

Specifically, it is a technique that allows focusing and/or steering of the acoustic beam
via signal processing, and can be employed to both transmission and reception.
Fig. 1-8 shows the basic concept of beamforming using time delays. By breaking up
the whole aperture into numerous elements, the excitation timing at specific locations can
be controlled. The focusing is illustrated in Fig. 1-8 a), in which the acoustic beam can be
focused at the desired point. By adjusting the delay values, the beam can be focused at
different locations. Another application would be beam steering, as shown in Fig. 1-8 b).
Again, the beam propagation direction can be manipulated by configurating the delay
settings. Focusing and steering can also be applied simultaneously for a steering focused
beam.
Besides applying time delays, there are other more advanced beamforming schemes,
a)

b)

Figure 1-8 illustrates a simple time delay beamforming scheme, showing a) focusing and b) steering
operations.
Chapter 1. Introduction to Medical Ultrasound Imaging
18

such as the minimum variance beamforming [54, 55] and synthetic aperture beamforming
[56], and they all share a common goal – enhancing the image contrast and resolution. By
employing the appropriate beamforming scheme, the image quality can be greatly
improved.
1.4.4. Advantages and Drawbacks
Advantages
Transducer array has a number of advantages over single-element transducer, and
many of which has been discussed briefly in the previous sections. Here is a summary of
the advantages:
 Increased frame rate:
By electronically sweeping the beam across the field of interest without mechanical
translation, a higher frame rate can be achieved. This is particularly important for
real-time monitoring.
 Improved image quality:
In general images obtained using transducer array has better contrast and resolution as
beamforming allows beam focusing across the whole image, while for single-element
transducer the focal point is fixed.
 Realization of advanced algorithms:
Transducer array allows the implementation of various imaging schemes which might
not be practical with single-element transducer. One example is the measurement of
flow velocity vectors by the transverse oscillation method [57].
Chapter 1. Introduction to Medical Ultrasound Imaging
19

Drawbacks
Nevertheless, transducer array also has its own drawbacks, and among which the
main concern would be the challenges encountered in the development process.
 Difficulties in acoustic stack fabrication:
In order to suppress the grating lobes, the separation between array elements (pitch)
should not exceed λ/2 so as to prevent grating lobe, where λ is the wavelength of the
transmitted ultrasound wave [8, 9]. When the center frequency goes up, the pitch
would have to be as low as 10 – 20 μm, which is very challenging [58, 59].
 Electrical connections:
In a transducer array, each element has to be routed to the back-end via individual
connections for separate control. This would be particularly challenging when the
element count is high, like in 2D arrays [60, 61].
 Development cost:
Set aside the fabrication difficulties, the development cost of a transducer array is
usually much higher than a single-element transducer. Though the cost would be
lower for commercialized arrays with optimized fabrication protocols, building an
imaging system with transducer array can still be expensive.
1.4.5. Current Development of Transducer Array
Nowadays ultrasonic transducer arrays have been widely employed in the field of
medical imaging, with a wide variety of commercial products in the market. The most
common types of commercial arrays include linear-switched array, phased array, curved
linear array, and 2D array. All of them come with various shapes and a wide range of
frequencies, accommodating the demands for ultrasound scanners for almost all common
Chapter 1. Introduction to Medical Ultrasound Imaging
20

clinical applications.
Nevertheless, when looking at emerging fields, very often researchers are not able to
find suitable transducer arrays that can fully accommodate their research need due to the
lack of demands. For example, it is still quite difficult to find arrays specifically designed
for photoacoustic imaging [62], which is a relatively new imaging modality. Therefore it
raises the need for custom-designed probe, which is expected to be useful in advancing
the researches on various fields and opening up new markets.
1.5. Concluding Remarks
Medical ultrasound imaging is a mutual diagnostic tool for that has been adapted in
a various clinical fields. With a mutual market, the current commercial products are able
to cater for most of the common applications. However, ultrasound imaging is still
growing and technical advances are still being made. For researchers working on
development of new applications, very often their needs cannot be accommodated just
using commercial ultrasonic transducer alone. This raises the need for specially-designed
transducers.
In this thesis study, two research projects were investigated and will be presented in
the remaining of this thesis. In particular, the remaining chapters have been organized as
follow:
 Chapter 2 discusses the use of ultrasound imaging for interventional operation
guidance and the development of a miniature phased array for a novel way of biopsy
guidance.
 Chapter 3 reviews the role of photoacoustic imaging (PAI) and its potential
Chapter 1. Introduction to Medical Ultrasound Imaging
21

applications, followed by the development of a ring-shaped transducer array for
arthritis detection using PAI.
 Chapter 4 summarizes the major contributions of this thesis study and discusses
some of the possible future developments.

Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
22

Chapter 2.
Fabrication and Characterization of Miniature Phased
Array for Intervention Guidance
2.1. Chapter Overview
In the previous introductory chapter we have briefly discussed about the current
status of medical ultrasound imaging. One of the important applications discussed is
biopsy guidance, or in general, guidance of interventional operations. Being one of the
commonly employed methods in interventional guidance, ultrasound has been successful
in improving the operation precision and accuracy, while also having it limitations.
In an attempt to further enhance the performance of ultrasound intervention
guidance, researches have been carried out, of which the key is making use of a miniature
phased array together with the intervention device. In this chapter, the development of the
miniature phased array will be presented. The rest of this chapter has been organized as
follow:
 Section 2.2 surveys the current approaches used in interventional operation guidance
and evaluates their pros and cons.
 Section 2.3 presents the design and development of the miniature phased array.
 Section 2.4 evaluates the performance of the developed array and demonstrates its
potentials for interventional operation guidance.
 Section 2.5 summarizes the main features of the developed approach and proposed
some future research directions.
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
23

2.2. Background: Guiding Interventional Procedures
Access to diseased tissue has been important in clinics for both diagnosis (e.g. biopsy)
and therapy delivery (e.g. surgical excision, draining excess fluid). The procedure would
be relatively simple and safe for superficial pathology; however, it is much more
complicated when dealing with internal organs and structures. One of the major challenges
for the interventional procedures in access internal structures would be guiding the devices.
Despite the fact that various techniques have been employed for this purpose, including
surface anatomic landmark approach, CT, MRI, etc., each of them has their own
limitations.
2.2.1. Anatomic Landmark Approach
As its name suggests, the operator using anatomic landmark approach would try to
locate the underlying structures under investigation based on their relative positions to the
anatomy on the body surface [63, 64]. This approach is easy to use and requires no
additional equipment. Nevertheless, in some pathologic states, the internal anatomy might
be altered significantly thus decreasing the reliability of this method. Besides, more
importantly the operator would not be able to keep track of the interventional device in the
body, leading to a high likelihood of collateral damage.
2.2.2. CT and MRI
In order to increase the procedure reliability and to prevent damaging surrounding
critical structures, it would be beneficial if the operator can keep track of the
interventional device in real-time during the operation. CT can provide a good image
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
24

contrast when tracking metallic device, however the major drawback would be the
exposure to substantial amount of ionizing radiation.
MRI is another good candidate for device guiding. It has superior soft tissue contrast
and spatial resolution, doesn’t involve any use of ionizing radiation and with the latest
technology it is even capable of real-time tracking [65, 66]. Despite of all the above
advantages, it can’t be perform on patients with metallic implants due to the presence of a
strong magnetic field. Also, for both CT and MRI, the transferring of patients to the
imaging room would be necessary and the whole process could be time-consuming. It
might also pose additional problems for patients requiring resuscitation of any kind.

Figure 2-1 a) illustrates the current ultrasound intervention guidance, in which the transducer is placed at
the body surface; b) illustrates the proposed device for intervention guidance, in which the transducer is
inserted into the body alongside with the needle. (background image taken from www.llu.edu/images)
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
25

2.2.3. Ultrasound
As discussed in the previous chapter, ultrasound has been commonly used for
intervention guidance. It is biologically safe and can provide anatomical images with
reasonable quality in real-time. Besides, most of the ultrasound scanners are portable;
allowing them to be used bedside and the patients would not need to be transferred.
Previous studies have shown that ultrasound guidance can improve the reliability and
safety of the interventional procedures, including liver biopsies [67, 68] and
pericardiocentesis [69, 70].
The drawbacks of ultrasound rise from the compromise between image resolution and
penetration depth. Currently, during the guidance procedure, the ultrasound probes are
Table 2-1. Comparisons between different intervention guidance approaches
Image
Quality
Operation
Time
Portable Remarks
Anatomical
Landmark
N/A N/A Yes  Convenient
 Unable to keep track of the device in
real-time
CT Good Moderate No  Ionizing radiation involved
 Preparation of patients required
 Inexpensive
MRI Good Moderate No  Preparation of patients required
 Special equipments needed to work in
magnetic field
 Expensive
Ultrasound Moderate Short Yes  Bed-side monitoring
 Ease for real-time imaging
 Highly dependent on the operator
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
26

placed at the body surface of the patients, as illustrated in Fig. 2-1 a). As discussed in
section 1.3.2, in order to visualize organs or structures which are deep inside the body, the
center frequency would have to be lowered to increase the penetration depth. This leads to
a worsened image resolution, thus decreasing the guidance reliability. Besides, by using a
separate ultrasound probe, very often the operator would be manipulating the intervention
device and the probe at the same time, or having another sonographer for assistance [71,
72]. This heavy skill dependency also poses instability in this approach.
2.2.4. Research Goal
When compared to the other approaches, it has the advantages of being able to
perform real-time guidance at the bed-side with minimal patient disturbance, as
summarized in Table 2-1. However, currently it is highly skill-dependent and the image
quality still has rooms for improvement. In order to overcome the above limitations, the
proposed solution would be placing the ultrasound probe adjacent to the intervention
device. Specifically, a multi-lumen device that comprises an imaging channel right next
to an interventional channel would be inserted into the body, as illustrated in Fig. 2-1 b).
This approach has several advantages. First, as the transducer is directly inserted
into the body, the required imaging depth would be significantly reduced. Therefore a
higher center frequency can be used for a better image quality. Second, the relative
position of the intervention device to the transducer is fixed, hence eliminating the need
for the operator to manually adjust the probe location for a good visualization of the
device.
Therefore in order to do so, a miniature ultrasonic transducer must be developed to
be fitted into the imaging channel. To demonstrate the feasibility of this approach,
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
27

imaging experiments were carried out to evaluate the performance of the developed
transducer.
2.3. Miniature Phased Array for Intervention Guidance
The essence of the required transducer would be its ability to cover a large field of
view while being small enough to be fitted into the imaging channel. Therefore the
phased array configuration was chosen during to its small footprint. By employing the
beam steering technique, a large imaging area can be obtained. In this section, the design
and development of the miniature phased array transducers would be presented.

Figure 2-2 shows the schematic section drawing of the designed phased array transducer. Please note
that the housing has certain length for the intervention procedure.
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
28

2.3.1. Transducer Specifications
Transducer Geometry
A 48-element, 20 MHz phased array transducer was designed with the specifications
listed in Table 2-2. The pitch of the array was designed to be λ/2 for grating lobe
suppression. Given the pitch, an element count of 48 was chosen as a compromise
between the image quality and the transducer footprint.
Other than the small aperture size, a long needle housing would be required for the
phased array due to the fact that the transducer has to be inserted into the body alongside
with the intervention device. In order to facilitate this setting, a specially designed
flexible circuit was employed, of which the details would be described in the next section.
The schematic section drawing of the phased array showing all major components are
given in Fig. 2-2.
Transducer Materials
The materials used in building the phased array and there corresponding properties
are listed in Table 2-3 and Table 2-4. PZT-5H ceramic (C-92H, Fuji Ceramics, Japan) was
chosen as the piezoelectric material due to its good balance between performance and
Table 2-2. Specifications for the phased array
Center frequency 20 MHz
Number of elements 48
Element size 25 µ m × 800 µ m
Element pitch 37 µ m
Element kerf 12 µ m
Housing diameter 3 mm
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
29

Table 2-3. Properties of the transducer materials
Materials c [m/s] Z [MRayl] k
t
k
33
d
33
[pC/N] Κ
ε
PZT-5H (C92-H) 4300 35 0.51 0.73 770

1895
Parylene C 2350 2.6 N/A N/A N/A N/A
Epo-Tek 301 2650 3.0 N/A N/A N/A N/A
Silver-loaded epoxy 1900 7.3 N/A N/A N/A N/A

Table 2-4. The materials of the phased array
Layer Material Thickness (µ m)
Piezoelectric PZT-5H (C-92H) 90
Backing Epotek 301 N/A
1
st
matching 2-3 µ m silver epoxy 22
2
nd
matching Parylene C 28

Figure 2-3 shows simulated pulse-echo results from PiezoCAD.
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
30

fabrication cost-effectiveness. Its fine grain nature also can help alleviate the damage
from dicing, which would be necessary for composite fabrication (details will be given in
the next section). Then the materials used for each layers and their corresponding
thicknesses were computed and optimized using the commercial software PiezoCAD
(Sonic Concepts Inc., Woodinville, WA), which is based on the KLM model. The
simulated pulse-echo result is given in Fig. 2-3. The simulated transducer bandwidth is
53%, which is reasonable for imaging purpose.

2.3.2. Fabrication Process
2.3.2.1. Acoustic Stack Fabrication
The fabrication process of the acoustic stack is illustrated in Fig. 2-4. First, a 2-2
composite of PZT-5H and Epotek 301 was fabricated. The composite was fabricated

Figure 2-4 shows the process flow of fabrication process for the acoustic stacks.
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
31

using the conventional dice-and-fill approach. The PZT-5H ceramic was first diced with a
double pitch (i.e. 74 µ m) by a dicing saw (Tcar864-1 dicing saw, Thermocarbon Inc.,
Casselberry, FL, shown in Fig. 2-5 a)), and the cuts were then filled with Epotek 301
epoxy. After curing, the piece was diced the second time to yield the final composite with
37 µ m pitch. The cuts are again filled with epoxy and the composite was lapped down to
the final thickness (90 µ m). The reason for dicing the composite two times was to reduce
the chance of pillar breaking. Although C-92H has an average grain size of 3.5 µm,
dicing it directly into pillars which are 25 µm wide and 250 µm tall (i.e. aspect ratio of 10)
still has a high failure rate. Fig. 2-5 b) shows the finished composite under microscope.
The top and bottom surfaces of the composite was then cleaned and sputtered
(NSC-3000, Nano-Master Inc., Austin, TX) with a total of 500/1000 Å Cr/Au as a
conduction layer. The first matching layer, which comprises 2-3 µ m silver particles
(Adrich Chem. Co., Milwaukee, WI) mixed with Insulcast 501 (American Safety Tech.
Roseland, NJ) epoxy, was then casted on the top surface of the composite. In order to

Figure 2-5 a) shows the dicing saw used for fabricating the composite, and b) shows the finished
composite with the electrodes on individual elements.
b) a)
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
32

achieve the desired acoustic impedance of 7.3 MRayls [73], the stack was centrifuged
with 3000 rpm. It was then lapped down to 22 µ m after curing. On the bottom surface of
the composite, individual element electrodes are separated by removing the gold layer on
the epoxy by acetone based on the fact that the electrodes tend to adhere better on the
ceramics than on the epoxy.
2.3.2.2. Flexible Circuits Fabrication
The interconnection between the array elements and the imaging system was
facilitated by an intermediate flexible circuit. As discussed, the flexible circuit was
specially designed to accommodate for the long needle configuration. The schematic

Figure 2-6 a) shows the schematic drawing of the flexible circuit (units are in mm), and b) shows the
actual fabricated flexible circuit.
a)
b)
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
33

drawing of the flexible circuit is shown in Fig. 2-6 a). The array-end of the circuit
consists of a thin and long strip (left part of the circuit), so that it can be housed in the
narrow casing. The connector traces are 15 µm wide, with the same pitch as the array (i.e.
37 µm). At the connector side, the traces spread out and widen at the same time, in order
to cope with the connector specifications.
A 25-µm-thick polyimide surface was used as the supporting layer of the flexible
circuit. Then it was sputtered with 50 nm of chrome and 250 nm of gold on top. The
pattern of the traces was then created using the photolithography technique. After
cleaning, another layer of 25-µm-thick polyimide was put on the surface of the circuit,
serving as a protective layer. Finally the circuit was cut out from the whole supporting
piece by laser (Laserod Inc., Torrance, CA).
2.3.2.3. Transducer Assembly
With the acoustic stack and the flexible circuit ready, the final steps would be

Figure 2-7 a) shows the pre-fabricated backed block made from Epotek 301, and b) shows the bonding
process of the acoustic to the backing block.
b) a)
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
34

Figure 2-8 a) and b) shows the finished prototype transducer accommodated in 3mm and 4mm needle
housing respectively, and c) shows the corresponding pulse-echo experiment result of a representative
element.
1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1
-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0.05
Time ( s)
Amplitude (V)
10 15 20 25 30
-40
-35
-30
-25
-20
-15
-10
-5
0
Frequency (MHz)
Amplitude (dB)
a)
c)
b)
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
35

assembling the components into the designed phased array. First the flexible circuit was
placed on the bottom surface of the composite (i.e. the surface with separated electrodes)
and carefully aligned with the elements. Then a small amount of Epotek 301 was applied
at the interface and a constant pressure was maintained across the whole assembly for a
good bonding. After that it was cured in a dry nitrogen environment for at least 3 days,
allowing a complete setting of the epoxy.
The assembly was then bonded to a pre-fabricated backing block made of Epotek
301 epoxy, as shown in Fig. 2-7a) and b). The backing block was made using the molding
method, and its diameter was slightly smaller than the inner lumen of the housing for the
ease of positioning the transducer. Again the acoustic stack (with the flexible circuit) was
bonded to the backing block with Epotek 301 as a very thin adhesive layer. After curing
in the dry nitrogen environment, the finished acoustic stack was encased in a needle
housing with an outer diameter of 3 mm. The housing was made of aluminum to provide
RF shielding. Finally, 25 µ m of Parylene C are coated on the surface of the transducer for
both acoustic matching and electrical insulation. After the prototype is finished, it was
poled in air under room temperature for 10 minutes using a DC field of around 3 kV/mm.
2.3.3. Performance Evaluation
The finished prototype is shown in Fig. 2-8 a) and b). The first batch of prototypes
was housed in 4 mm needle casing for a higher yield rate, as shown in Fig. 2-8 b). Efforts
has been made in order to reduce the transducer size to 3 mm, and the experimenting with
the flexible circuit bending angles and reducing the thickness of the housing wall.
Prototype with 3 mm diameter was successfully fabricated after several optimizations,
which is shown in Fig. 2-8 a). In this section, mainly the performance of the 3 mm
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
36

transducer array will be evaluated unless specified.
In order to demonstrate the feasibility of this transducer array to be used for
intervention guidance, the developed prototype was first characterized using the standard
pulse-echo experiment. Then phantom imaging experiments were carried out to quantify
the image resolution and to evaluate the performance of the prototype in imaging
biological tissue.
2.3.3.1. Transducer Characterization
Pulse-echo responses of the array elements were measured. Each element was
excited using a Panametrics 5900PR 200-MHz pulser/receiver (Panametrics Inc.,
Waltham, MA). The transmit energy and receive gain for the Panametrics 5900PR were
set at 1 µ J and 20 dB respectively. The transducer was immersed in a deionized water
bath, and the echo signal from a quartz plate was then received and recorded using an
analog-to-digital CompuScope hardware (CS122G1, Gage Applied Technologies Inc.,
Montreal, Quebec, Canada) set at 50 Ω coupling. The measured pulse-echo characteristic
of a representative element was shown in Fig. 2-8 c). The number of working elements is
47 out of the 48 total elements, with one dead element at the end (element #48). The
distribution of the center frequencies and bandwidths are shown in Fig. 2-9 a). It can be
seen that the performance of the elements are reasonably uniform across the entire
aperture. The average center frequency is around 21 MHz and the bandwidth is around
42%, which is satisfactory for an imaging transducer.
Other than the pulse-echo testing, the combined electrical and mechanical crosstalk
was measured between adjacent elements. For this measurement the phased array was
placed in a deionized water bath with no reflector. A function generator (AFG 3252,
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
37

Tektronix Inc., Beaverton, OR) set in sinusoid burst mode was used with amplitude of 5
Vpp. An element was excited at discrete frequencies from 15 MHz to 25 MHz, and the
peak applied voltage was recorded, as shown in Fig. 2-9 b). The maximum crosstalk was

Figure 2-9 a) shows the center frequencies and bandwidths of all the elements in the transducer array
(element #48 is not working), and b) shows the crosstalk measurements of the prototype transducer. The
maximum crosstalk value is -28 dB over the frequency range tested.
a)
b)
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
38

around -28 dB across the nearest element and -35 dB across the next-nearest element. The
average crosstalk values were -32 dB and -39 dB across the nearest and next-nearest
element, respectively.
2.3.3.2. Imaging Experiment
Although the performance of the prototype transducer from the pulse-echo testing
suggests that it might be suitable for the proposed application, the ultimate indication
would still be its imaging capability. Therefore imaging experiments were carried out for
further evaluation.
Signal Routing
A house-developed imaging system [74] was used as the back-end for the image
formation. The signal routing was achieving by an array connector board, and a cable
assembly. The array connector board consists of a ZF5S-50-01-T-WT connector and a
BTH-120-01-X-D-A connector (Samtec Inc., New Albany, IN) .The flexible circuit on

Figure 2-10 illustrates the signal routing scheme from the phased array transducer to the back-end imaging
system.
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
39

the transducer was directly inserted into the ZF5S-50-01-T-WT connector to make the
contact connection. Connection between the array connector board and the cable
assembly was achieved by the BTH connector pair, as illustrated in Fig. 2-10.

Figure 2-11 a) shows the image of the wire phantom with a 30 dB dynamic range. b) and c) shows the
axial and lateral line spread functions at the focal point respectively.
a)
b) c)
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
40

Wire Phantom Imaging
The image resolution of the prototype was evaluated by imaging a wire phantom.
The wire phantom comprises four 20 µ m diameter tungsten wires (California Fine Wire
Co., Grover Beach, CA), and each wire was positioned at a different locations, in both the
axial and azimuth directions. With a center frequency of 20 MHz, the corresponding
wavelength is 75 µ m, thus the 20 µ m wires could be safely assumed as four point targets.
The wire phantom image obtained is shown in Fig. 2-11 a). A single transmit focus
at 6 mm was achieved by delay-and-sum beamforming. On reception, dynamic receive
beamforming was employed to form the image. One of the wires was positioned at the
focal point in order to obtaining the axial and azimuth line spread functions, as shown in
Fig. 2-11 b) and c). It can be seen that the four wires can be visualized clearly without
reasonable image quality. From the line spread functions, the -6 dB axial and azimuth
resolutions were around 80 µ m and 210 µ m respectively, which are typical values for a
20 MHz transducer array.
Ex Vivo Imaging
As the phased array is intended to use alongside with an intervention device such as
biopsy needle, we have obtained the image of a needle so as to evaluate the guiding
capability of the array. A stainless steel needle with an outer diameter of 3 mm was
inserted into a cow liver, mimicking the actual biopsy process.
The experimental setup and the obtained images were shown in Fig. 2-12. A
dynamic range of 45 dB was used for both images. The needle wall can be clearly
visualized from the image and the wall boundary was well-defined after insertion into the
cow liver. The image quality is also reasonable as seen from the fine speckle pattern.
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
41

Figure 2-12 a) shows the experimental setup of the ex vivo imaging experiment, and b) shows the image
of the same needle inserted in a cow liver.
a)
b)
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
42

From the image, it can be observed that the penetration depth is around 10 mm,
which might be sufficient in some cases - but the compromise between penetration and
resolution should be further investigated to find a better balance for this application.
2.4. Summary and Future Works
Summary
In this chapter, we have developed a miniature phased array transducer then can be
inserted into the body for interventional guidance. The fabricated 48-element 20 MHz
phased array has an outer diameter of 3mm and showed good pulse-echo characteristics
with an acceptable inter-element crosstalk. The ultimate goal of this array is to image
small blood vessels and/or pathological tissues inside the patient’s body, increasing the
reliability of the guiding process.
Imaging experiments has been carried out, and the capability of the phased array in
positioning the intervention device was demonstrated. The performance of the proposed
phased array transducer demonstrates the feasibility of such an approach for
interventional guidance.
Future Works
As discussed, the penetration depth of the phased array might be insufficient for
some applications. Therefore one of the future directions would be investigating the
performance of phased arrays with different center frequencies, for example, 15 MHz
array could be built and the performance between the prototypes could be compared and
evaluated.
Also, the size of the transducer array can be further reduced by considering the use
Chapter 2. Fabrication and Characterization of Miniature Phased Array for Intervention Guidance
43

of a non-circular housing. By investigating the possibility of accommodating the
transducer array into smaller housing with different geometry, such as rectangle, it could
become more favorable for intervention procedures.

Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
44

Chapter 3.
Design and Fabrication of a Ring-shaped Array for
Photoacoustic Tomography
3.1. Chapter Overview
In the previous chapter, the development of a miniature phased array for a new
intervention guidance approach was described, demonstrating how new transducer array
can help in opening up new areas in the medical field. In this chapter, another new
application using specialized transducer array is being investigated and presented.
Specifically, the goal of this project is the early detection of arthritis via the
combination of light and sound. This is based on the photoacoustic effect, which is the
basic principle for photoacoustic imaging. In this work, a ring-shaped transducer array
was developed for photoacoustic tomography. As a relatively new imaging modality,
photoacoustic imaging has a large potential in diagnostic medicine. The rest of this
chapter has been organized as follow:
 Section 3.2 reviews the principle of photoacoustic imaging and discusses its
potentials in the early detection of rheumatoid arthritis.
 Section 3.3 presents the design and development of the ring-shaped array.
 Section 3.4 evaluates the performance of the developed array.
 Section 3.5 summarizes the main features of the developed array and describes the
ongoing works.

Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
45

3.2. Background
Rheumatoid arthritis has been one of the most common and disabling type of
inflammatory polyarthritis [75, 76]. It is a chronic systemic disease, and its
manifestations are primarily in the joints, causing pain, swelling, stiffness, or even the
loss of joint function. It has been estimated that about 0.6 percent of the U.S. adult
population suffer from rheumatoid arthritis [77], which would be very likely to pose
adverse effect on their life quality in both short-term and long-term manner.
3.2.1. Overview of Rheumatoid Arthritis
Rheumatoid arthritis falls under the category of autoimmune disease, meaning that
the joint tissues are actually being attacked by the patients’ own immune system, causing
inflammation and damage. A simple illustration of rheumatoid arthritis is shown in Fig.
3-1. First, the synovium will be thickened and swollen, becoming filled with white blood
cells. The white blood cells will in turn attack the cartilage and bone tissue within the

Figure 3-1 illustrates the basic concept of rheumatoid arthritis. In the normal joint (left hand side), the
joint capsule is lined with synovium. In the diseased joint (right hand side), the synovium becomes
inflamed and the joint is eroded eventually.
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
46

joint. As the disease progresses, the surrounding tissues will also be affected and
weakened, thus unstabilizing the joint itself.
Patients suffering from rheumatoid arthritis may be experiencing the following
symptoms [76]:
 Swollen joint – decrease in mobility
 Fatigue and a loss of energy
 Occasional fevers
 Pain and stiffness at the diseased region
As this disease is very often chronic in nature, it will affect the patient for a long period
of time. Some patients may even suffer from psychological or social issues other than
physical disabilities.
3.2.2. Diagnosis and Treatment
Diagnosis
The diagnosis of rheumatoid arthritis can be challenging. The first screening of
rheumatoid arthritis is mostly clinical, in which the clinicians would identity the
corresponding symptoms, such as bone erosion [78]. The patients would be asked to
carry out activities with the diseased joint and the performance would be observed and
evaluated.
If a possibility of rheumatoid arthritis is identified, a series of laboratory tests would
be useful in confirming the diagnosis. The general tests should include a complete blood
cell count with differential, rheumatoid factor, and erythrocyte sedimentation rate or
C-reactive protein. Specific tests such as testing for anticyclic citrullinated peptide
antibody (anti-CCP) [79, 80] have also been proven useful.
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
47

Medical imaging is another diagnostic tool commonly employed. The conventional
X-ray radiography has long been used in determining the degree of joint destruction [81,
82]. MRI has also been used sometimes, and it is able to provide a comprehensive
assessment of the pathology due to its excellent soft tissue contrast [83, 84]. Ultrasound is
a convenient imaging tool which is also used in detecting bone erosion [85] other than
radiography and MRI.
Treatment
Currently, there exists no standard protocol in treating rheumatoid arthritis and it is
very hard to completely cure the disease. Various approaches can be employed, and are
chosen by the clinicians based on each patient’s individual situation. For most patients,
medications would be subscribed, including anti-inflammatory drugs, pain relief, etc. The
medications are mostly for reducing the inflammation and preventing the worsening of
the pathology.
Nevertheless, for patients with severe joint damage, surgeries might be required to
restore the joint function. Examples include joint replacement, arthrodesis and tendon
reconstruction. Performing surgery might be able to improve the patient’s ability to
perform daily activities, while like many other surgeries, it carries a much higher risk
than taking medications.
3.2.3. Early Detection of Rheumatoid Arthritis: Benefits
As discussed, the treatment of rheumatoid arthritis might be difficult yet inefficient.
In addition, the structural damage to the cartilage and bone tissue is irreversible. However,
recent studies have shown that early intervention may be crucial in preventing the joint
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
48

damage [86-88]. This means that if the rheumatoid arthritis can be detected in early stage,
the treatment would be much more effective.
This in turn poses the need in the development of screening methods for early
detection of the pathology. The existing methods might not work well for this purpose
due to the following reasons. First, the symptoms differ for each patient, and therefore
only a few tests might be effective at this stage. Second, some symptoms only develop
over time, and might be overlooked before later stages. Finally, the test used for early
detection should be convenient and fast, serving as a simple screening test. Approaches
like the lab test would not serve well in this aspect.
In order to solve this problem, two goals can be identified. First, a common, if not
universal, feature that would occur since the early stage of the pathology should be
identified. Then a corresponding test for this feature should be developed. There are
various studies on this aspect, and different methods have been proposed, such as the
screening for some specific antibodies [89, 90] or by identifying soft tissue abnormalities
via MRI [91]. However, these methods might not serve well as the front line screening, as
the processing time is relatively long.
3.2.4. Proposed Solution: Photoacoustic Tomography
The proposed solution in this project is to identify the angiogenesis around the joint
using photoacoustic tomography (PAT). Studies have shown that angiogenesis and
increase vascularity are observed in the early stage of rheumatoid arthritis [92-95]. This
feature might be challenging to be pinpointed using common imaging practice, as the
vasculatures can be as fine as 10 µ m. PAT is a relatively new imaging modality, and is
potentially capable in visualizing the fine vasculatures [96, 97].
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
49

Photoacoustic Tomography
PAT is a hybrid imaging modality based on the photoacoustic effect, which refers to
the generation of sound wave through the absorption of light, or in general,
electromagnetic energy [98, 99]. Specifically, the absorption of photons by the targets, e.g.
biomolecules, will transiently heat up the molecules. Pressure waves are generated due to
the local thermal expansion. PAT works by detecting the light-induced acoustic wave and
form the corresponding images [96, 100]. Fig. 3-2 illustrates some imaging configuration
of PAT, including a linear-array setup in a) and a circular-array setup in b).
PAT can be considered as an ultrasound imaging modality with EM-enhanced

Figure 3-2 a) shows a linear-array PAT setup for noninvasive in vivo functional imaging of Methylene
Blue [101], and b) shows a circular-array PAT setup for recording the cerebral hemodynamic changes in a
rat [102].
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
50

contrast. The major difference between PAT and usual ultrasound imaging is the origin of
image contrast. The contrast in ultrasound imaging arises from the difference in acoustic
impedance within the region of interest, while for PAT, the contrast comes from the
difference in optical absorption, which will provide an entirely different information set.
Moreover, as the optical absorptions of most biomolecules are wavelength-dependent,
PAT can be highly selective by choosing the appropriate wavelength.
Detecting Angiogenesis with PAT
As discussed, detecting angiogenesis at the early stage of rheumatoid arthritis is
challenging due to the small vessel size. However, the presence of blood in the
vasculature opens up the possibility for detection using PAT. Blood comprises a variety of
chromophores, most importantly, hemoglobins. Hemoglobins exhibit a high optical
absorption in the visible to near-infrared spectral region, making it a good marker for PAT.
Besides, being closely related to ultrasound imaging, PAT also inherits the advantages of
being portable and having short imaging time. Therefore PAT shows a great potential as
an early screening test for rheumatoid arthritis.
3.2.5. Research Goal
Recent works have already demonstrated the feasibility of using PAT for detecting
angiogenesis [103-105]. Commercial transducers were employed in those studies to
capture the photoacoustic signal, and the whole data set was acquired by mechanically
translating the transducers. However, this will increase the total processing time and pose
addition complexity to the system. Therefore, in this project, the goal is to design and
fabricate a ring-shaped array used for PAT. Specifically, the transducer array should be:
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
51

 Having a diameter large enough to accommodate a human finger for imaging the
inflamed joint; and
 Having sufficient sensitivity and bandwidth for yielding PAT image with
satisfactory image quality; and
 Capable in performing real-time ultrasonography for additional information in
both positioning the array and as an anatomical reference.
3.3. Ring-shaped Transducer Array for PAT
As discussed, the developed ring-shaped array should be able to accommodate a
human finger, as shown in Fig. 3-3. The joint will be illuminated by bundles of optical

Figure 3-3 illustrates the imaging setup with the ring-shaped transducer array.
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
52

fibers aligned on the array to produce the photoacoustic signal. As the geometry of the
array is not very common, we first would have to decide on the transducer parameters
using simulations.
3.3.1. Transducer Design
First we would have to decide on the center frequency of the transducer. As the
received signal is also acoustic wave, the image resolution of PAT also improves with
increasing center frequency. Compromising between the resolution and penetration depth,
also with the fabrication challenges, a center frequency of 10 MHz is decided.
For the diameter of the inside imaging cavity, around 50 mm would be appropriate
in order to accommodate some inflamed joints which might be swollen severely in some
cases. As the element count of the array would be 256 for the ease of fabrication, a
diameter of 48.89 mm was chosen so that the pitch would be an integer multiple of the
wavelength. The complete specifications of the array are given in Table 3-1.
Table 3-1. The specifications of the ring-shaped array
Center frequency 10 MHz
Element no. 256
Array diameter 48.89 mm
Element pitch 600 μm
Element kerf 60 μm
Elevation focus 18 mm
Simulation Studies: Acoustic Beam Profile
Due to the unusual geometry of the array transducer, a simulation study was
conducted to investigate the feasibility of the array to be used for imaging purpose. A
MATLAB-based (The Math Works Inc., Natick, MA) software Field II [106, 107] was
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
53

used to simulated the acoustic behavior of the ring-shaped array. Half of the elements, i.e.
128 consecutive elements, are excited and dynamic focusing was employed upon
reception. The corresponding two-way acoustic beam profile was obtained, as shown in
Fig. 3-4. Due to the nature of Field II, some computational errors exist near the surface of
the transducer, resulting in the singularities.
From the simulated beam profile, it can be seen that the beam was highly focused at
the center of the curvature. The -6 dB beamwidths are 153 µ m and 230 µ m along the X
and Z direction respectively, which is reasonable at the center frequency of 10 MHz.
a) b)

c) d)

Figure 3-4 a) and b) show the beam profile of the ring-shaped array, while c) and d) show the pressure
intensities at the focal point along the X-direction and Z-direction respectively.
z [mm]
x [mm]
Intensity profile in XZ plane
-20 -15 -10 -5 0 5 10 15 20
0
5
10
15
20
25
30
35
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
54

Simulation Studies: Phantom Imaging
As the pressure intensity outside the focal point is relatively low, the image
resolution is expected to be decaying rapidly when moving away from the focus.
Therefore the B-mode images acquired from a wire phantom and a cyst phantom are
simulated respectively to study this effect.
The wire phantom consists of 9 point scatterers around the ring center, with around
1.4 mm spacing in between adjacent points. The cyst phantom is a 15 mm × 15 mm
square phantom (seen on the X-Z plane) with two cysts, and the diameters of which are 2
mm and 4 mm respectively. The wire phantom aims in evaluating the image resolution at
different spatial location, while the cyst phantom is for assessing the imaging capability
of the array in general.
Again, for each transmission event, a group of consecutive 128 elements (e.g.
element no. 1 to element no. 128) was excited and the echo signals were received by the
same group of elements with dynamic focusing to obtain a single scanline. Then the next
scanline was obtained by repeating the same process with the next group of elements
a) b)

Figure 3-5 a) shows the wire phantom images obtained by using the 256-element array and b) shows the cyst
phantom image.
x(mm)
z(mm)
-10 -5 0 5 10
15
20
25
30
35
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
55

(element no. 2 to element no. 129), like imaging with a linear-switched array. A total of
256 scanlines were obtained to form a complete image.
A hamming apodization window is applied to both the transmit and receive aperture
in an attempt to increase the image quality. Although theoretically the image quality can
be improved significantly by using advanced beamforming techniques like plane wave or
synthetic aperture imaging, the implementation would not be trivial due to the high
computation load.
A scan conversion is applied to visualize a 24 mm × 24 mm field of view centered at
the focal point. The image is displayed after the standard log compression with a dynamic
range of 40 dB, as shown in Fig. 3-5. We can see that the point targets in the wire
phantom can be visualized with a good resolution, which remains reasonable even when
moving far away from the center. For the cyst phantom, the boundaries of the cyst are
quite well-defined, and the speckle pattern suggests that the image resolution is
reasonable within the imaging region.
Transducer Materials
From the simulation results, the imaging capability of the ring-shaped array with the
designed specifications was demonstrated. Then the materials used in fabricating the
array transducer should be determined, in order to achieve both satisfactory sensitivity
and bandwidth.
Piezocomposite (1-3 or 2-2) materials are good candidates for making this array
transducer as it has the following advantages: (1) high sensitivity, (2) high bandwidth,
and (3) high flexibility to be conformed into the desired curvature. We have compared
different piezo-materials, i.e. PMN-PT single crystals and PZT-5H for the fabrication of
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
56

piezocomposites. From Table 3-2, it can be seen that by using PMN-PT, we can
potentially achieve a high sensitivity for the transducer. However, we have decided to
fabricate the array transducer using conventional PZT-5H (C82-H, Fuji Ceramics Crop.,
Japan) due to several reasons.

Table 3-2. Properties of the transducer materials
Materials c [m/s] Z [MRayl] k
t
k
33
d
33
[pC/N] Κ
ε
PZT-5H (C82-H) 3900 34 0.51 0.73 600

1500
PMN-PT 3700 29 0.58 0.94 2000 900
Epo-Tek 301 2650 3.0 N/A N/A N/A N/A
Silver-loaded epoxy 1900 7.3 N/A N/A N/A N/A

The first reason is the difficulty in fabrication and the instability of the
PMN-PT/epoxy composite. Preliminary studies have been conducted, and we have
experienced with the single crystal PMN-PT from two different manufacturers: 1) IBule
Photonics Co., Ltd., Korea, and 2) CTG Advanced Materials (formerly known as HC
Materials), IL, USA. The 2-2 composite was fabricated using the mechanical dicing
approach by IBule Photonics Co., Ltd.. Although 1-3 composite would potentially give a
better acoustic performance over 2-2 composite, it was much harder to fabricate for
single crystal PMN-PT due to its fragile nature. However, it was hard to maintain strong
and intact junctions between the single crystal and epoxy along the azimuth dimension,
which is over 40 mm. Therefore most of the composite pieces failed upon handling and
the yield rate was too low for practical usage.
The second reason was the fact that we could fabricate 1-3 composite using PZT-5H
but not with single crystal PMN-PT. The fabricated composites were stable and the 1-3
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
57

composite configurations compensated for the less ideal acoustic properties. When
conforming the composite into the quarter-ring configuration, the physical stability poses
an advantage and can greatly improve the yield rate.
Finite Element Analysis
A finite element analysis study was conducted to investigate the expected
performance of the array transducers built with PZT-5H. The commercial virtual
prototyping tool PZFlex (Weidlinger Associates Inc., Los Altos, CA) was used to build
the transducer model. Besides the performance prediction, piezocomposites with
difference volume fractions were built for insights on designing the piezocomposites.

The simulation results are shown in Table 3-3 and Fig. 3-6. As expected, the
sensitivity of the transducer increases with a higher volume fraction. However the
sensitivity increase from 60% to 70% is not prominent, probably due to the compensation
from the change in effective coupling coefficient. On the other hand, the bandwidth of the
transducer is the best at around 50% - 60% volume fraction, due to the better matching in
the acoustic transmission path. Therefore a volume fraction of around 60% should yield
promising results for this project.

Table 3-3. Results of the finite element modeling
V olume fraction Center frequency [MHz] Max. amplitude Bandwidth Impedance [Ω]
50% 9.55 0.032 69% 147
60% 9.62 0.035 71% 112
70% 9.66 0.036 61% 92
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
58

a)

b)

Figure 3-6 Pulse-echo simulation results of PZT-5H/Epo-Tek 301 piezocomposites for different
volume fractions (50%, 60% and 70%): (a) time-domain echo and b) frequency-domain spectrum.
3.3.2. Fabrication of the Ring-shaped Array
The ring array transducer is fabricated in a modular approach, specifically, four
individual quarter-ring array transducers, which can form a complete ring. Each
quarter-ring array together with the corresponding connections to the back-end is
completely separated from others for easier maintenance and troubleshooting. Each
quarter-ring array transducer comprises two major components: 1) the acoustic stack, and
2) the underlying electronic connection board. A Verasonics V1 imaging system
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
59

(Verasonics Inc., Kirkland, WA) will be employed and the back-end system for data
processing and image display.
3.3.2.1. Fabrication of the Acoustic Stack
Preparation of individual layers
Unlike the acoustic stack fabrication process described in Chapter 2 for the phased
array, the piezoelectric element and the matching layers are built separately this time. For
the piezocomposite, the dice-and-fill method was again employed as shown in section
2.3.2.1, while this time a 1-3 composite was prepared instead of a 2-2 composite. Both
surfaces of the composite were then cleaned and sputtered with a 500/1000 Å Cr/Au as a
conduction layer. One side of the composite was scratch-diced, i.e. diced only 5–10 µm
deep into the material, to separate the electrodes on individual elements.
The matching layers and lens were prepared separately. The lens material was
chosen to be polymethylpentene (TPX) (RT13, Mitsui Chemicals, Tokyo, Japan). It was
first lapped to the desired thickness, and the 2
nd
matching layer (Epo-Tek 301) was casted
on top of the TPX and lapped down. On top of that, the 1
st
matching layer (2-3 µ m silver
a) b)

Figure 3-7 a) and b) shows the composited bonded to the matching layers with electrodes aligned.
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
60

particles loaded epoxy) was again casted and lapped down. Cr/Au was also sputtered on
top of the 1st matching layer. Finally the surface of the 1
st
matching layer was
scratch-diced like the piezocomposite. The finished piezocomposite and the matching
layers are shown in Fig. 3-7 a). The bonding process, as shown in Fig. 3-7 b), will be
described later. One point to note is that the matching layer is 8 mm long in the elevation
direction, while the piezocomposite is only 4 mm long – this is to accommodate for the
electrical connection scheme, the details of which will be described in the next section.
The backing block was made from Epo-Tek 301 by molding procedures. The
Epo-Tek 301 was casted into a premade RTV mold followed by overnight curing in a dry
nitrogen environment. The backing block was removed for the mold and it was already in
the quarter-ring configuration. As Epo-Tek 301 is not conductive, Cr/Au was sputtered on
its surface as the electrode.
Elevation focusing
As mentioned previously, TPX lens was employed in order to achieve the elevation
a) b)

Figure 3-8 a) shows the milling process of the TPX lens. The ball-end mill was used to mill the lens surface
in a back-and-forth manner, with a small increment in the milling depth for each pass. The finished lens
surface is shown in b).
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
61

a)

b)

Figure 3-9 a) shows the assembly process of the acoustic stack, and b) shows the actual conformation
of the acoustic stack using the fixture.
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
62

focusing. The lens would either be concave or convex in shape, depending on the
acoustic impedance of the material. The focal length z
f
of a lens is given by:
𝑧 𝑓 =
𝑅 𝑐 1 −
1
𝑛
(3-1)
where R
c
is the radius of curvature and n

is the ratio between the sound velocities in the
lens and the loading medium. Given a required focal length of 18 mm, the lens was then
machined using a 6.3 mm radius ball-end-mill to give a concave configuration. Finally,
the lens surface was polished to provide a smooth surface finish. The milling process and
the finished lens are shown in Fig. 3-8.
Assembling the acoustic stack
The assembly process is illustrated in Fig. 3-9 a). First the piezocomposite was
bonded to the matching layers with Epo-Tek 301. The scratch-diced electrodes on both
wafers were aligned carefully to avoid short connection between elements. The
conformation is done under elevated temperature, i.e. 80
o
C at which the stack will
become softer. The pre-fabricated backing block was then bonded using Epo-Tek 301 to
the conformed stack as a support upon cooling, as shown in Fig. 3-9 b). The acoustic
stack was ready after curing in a dry nitrogen environment overnight.
3.3.2.2. Electrical Connections
Signal routing from the acoustic stack
Each quarter-ring array transducer will be connected to a hard prototype circuit
board (PCB), which in terms routes the signal to the back-end Verasonics system via a
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
63

cable assembly. Each PCB contains of three layers. Specifically, the outer two layers are
ground plates, and the signal traces are sandwiched in between the outer layers.
The purpose of this design is to reduce the electrical crosstalk and the ground plates also
serves as a RF shield. The signal traces run from the acoustic stack to a Samtec QSE
connector (Samtec Inc., New Albany, IN), as shown in Fig. 3-10.
a)

b)

c)

Figure 3-10 a) and b) shows the front and back side of the PCB. Noted that the signal traces are not
visible as they are layered between the ground plates. c) shows the pattern of the signal traces running
from the array connector pads to the Samtec QSE connector.
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
64

a)

b)

Figure 3-11 a) shows a finished quarter-ring array connected to the PCB and b) shows the cable
assembly used to connect the array to the imaging system.
The acoustic stack will sit on the front side of the PCB, with the conductive backing
block contacting the ground pad, as illustrated in Fig. 3-8 a). The matching layer, and
hence the front side of each element will be connected to the respective signal pad on the
back side of the PCB using conductive epoxy. The finished quarter-ring array is shown in
Fig. 3-11.
Connections to the imaging system
As discussed, the array was connected to the Verasonics system via two cable
assemblies. Each cable assembly (shown in Fig. 3-11) has a Cannon DL5-260PW-6A ZIF
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
65

connector (ITT Cannon, Shakopee, MN, USA) on one end, and two custom-made mating
boards on the other end, each of which would be connected to an individual quarter-ring
array. The Verasonics system can accommodate two Cannon 260 ZIF connectors, each
routing 128 transducer elements. Hence two cable assemblies were required to route
complete ring array.

a)

b)

Figure 3-12 a) shows the completed ring array transducer comprising four individual quarter-ring
array transducer, and b) shows the housing of the transducer, with the bowl-like configuration at the top
of the array for optical fibers placement.

Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
66

3.3.2.3. Transducer Housing
The complete array was cased in a specially designed housing for this application, as
shown in Fig 3-12. The housing was made with aluminum for RF shielding. Each quarter
array will be positioned with the help of three positional poles aligned with the holes at
the corners of the PCBs.
The patients can place their fingers inside the cavity in the center of the housing for
the imaging experiment. On the top cover of the housing there is a bowl-like structure.
This allows the placement of the optical fibers as close to the transducer array, with is
around 5 mm away from the center of the transducer array. A water cup can be attached to
the bottom of the housing, so that the inner cavity can be filled with water for imaging
without the need to submerge the housing into a water-bath.
3.3.3. Evaluation of the Ring-shaped Array
3.3.3.1. Overall Acoustic Performance
The overall acoustic performance of the fabricated transducer array including the
center frequency, bandwidth, number of working elements, etc., were first evaluated by a
series of experiments.
The resulted pulse-echo response of a representative element was shown in Fig. 3-13
a). The measured center frequency is around 10.29 MHz, with a bandwidth of 70.0%.
These values agree with the predicted values from simulation. The measured center
frequencies of all the elements are also shown in Fig. 3-13 b). It can be seen that the
frequencies of all the elements lie within the range of 9-11 MHz, which can be
considered as consistent. A total of 246 elements are working out of the 256 elements,
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
67

with most of the dead elements located towards the end of each quarter ring array – this
might be due to some broken connections during the assembling process, as the four
quarter rings were closely packed.
A two-way insertion loss measurement was also carried out to quantify the
sensitivity of the array elements. The array was place in a water bath, and a quartz target
a)

b)

Figure 3-13 a) shows the pulse-echo response of a representative element of the array transducer, and
b) shows the frequency distribution of all the array elements.
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
68

was place at around 18 mm from the transducer surface. The measurement was done over
the frequency range of 5 MHz to 15 MHz, with a frequency increment of 0.5 MHz. At
each frequency, a 30-cycle sine wave burst with 5 V peak-to-peak voltage was sent to the
array element, and the amplitudes of the returned echoes were logged.
After compensating for attenuation of propagating in water, reflective loss of the
quartz target and a diffraction loss [108], the maximum insertion loss is found to be -22.6
dB, which is comparable to the pulished values.
3.3.3.2. One-way Pulse Response
As the developed array would be used for photoacoustic imaging, the one-way pulse
response of the array transducer was first evaluated for some insights of its performance,
assuming a two-way symmetry. A needle hydrophone HPM04/1 (Precision Acoustics,

Figure 3-14 shows the experiment setup for the one-way pulse measurements using the hydrophone.
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
69

Dorchester, UK) was placed 18 mm away from the transducer surface. The experiment
setup was shown in Fig 3-14. One of the elements of the array prototype was excited with
a function generator with signal amplification. A 30 mV peak-to-peak (before 40 dB
amplification) single cycle sine wave was employed with a pulse repetition frequency of
200 Hz. The measured voltage on the hydrophone was digitized and displayed on the
a)

b)

c)

Figure 3-15 a) and b) shows the pressure profile in the azimuth and the elevation direction respectively.
The symmetric shape of the profiles suggests that the hydrophone is aligned with the array prototype. c)
shows the pressure time response at the focus, with a center frequency and bandwidth of 9.35 MHz and
74.6% respectively.
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
70

control workstation, and the position of the needle hydrophone was adjusted by the
motorized stage to get the maximized signal.
The measurement results are given in Fig. 3-15. The hydrophone was scanned in the
azimuth and elevation direction respectively to find the maximal pressure point, and the
scan results were shown in a) and b). Upon locating the focus, the time response of the
array element was recorded and the corresponding frequency spectrum was computed.
The measured center frequency of the element is 9.35 MHz, with a bandwidth of 74.6%,
which is reasonable for imaging purpose. The down-shifting of the center frequency
might be caused by several reasons, including 1) lowered effective acoustic velocity from
dicing ceramic into sub-elements; 2) frequency-dependent attenuation of the lens; 3)
electrical impedance mismatch with the pulser circuit, etc [109].

3.3.3.3. Integration with Imaging System
One of the final goals of this project would be performing real-time B-mode
imaging and with the array transducer. Therefore after evaluating the properties of the
array prototype, the final step would be integrating it with the back-end imaging system.
As discussed previously, the array transducer was connected to the Verasonics
system via two custom-made cable assemblies. The signal flow of the system is
illustrated in Fig. 3-16, and Fig. 3-17 shows the prototype array connected to the
Verasonics V1 system.

Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
71

Figure 3-16 a) illustrates the signal flow of the imaging system (with a Verasonics V1 back-end) and b)
shows the actually connection of the prototype to the system.
a)
b)
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
72

System Verification
Pulse-echo tests were carried out so as to test the functionality of the system and the
cable interconnection. In this test, only one element was excited each time and the
corresponding echo received was recorded and evaluated.
A bipolar pulse with center frequency of 9 MHz (the maximum frequency that was
supported by the V1 system) with a peak-to-peak voltage of 3.2 V was used as the
excitation signal. Coupled using water bath, a glass reflector was placed at the elevation
focus, i.e. 18 mm, and the received echo at a representative channel was shown in Fig.
3-17. The center frequency measured was 8.8 MHz, with a bandwidth of around 60%.
The downshifting of the frequency was again due to the possible reasons discussed in
section 3.3.3.1, and also because of the fact that the center frequency excitation pulse was
9 MHz instead of 10 MHz.

Figure 3-17 The echo data received using the Verasonics system.
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
73

3.4. Summary and Future Works
Summary
A ring-shaped transducer array for photoacoustic tomography was developed in this
project. The array will be integrated with a Verasonics imaging system for data
acquisition and image processing. The ultimate goal of this project is to obtain
photoacoustic images of the finger joints, hence enabling the early detection of
rheumatoid arthritis by identifying the angiogenesis around the pathological tissues.
The one-way pulse response of the fabricated prototype has been evaluated, and the
frequency and bandwidth matched the designed values. The functionality of the prototype
together with the imaging system was also evaluated by carrying out the pulse-echo
experiment using the Verasonics V1 system.
Future Works
One future research directions would be trying to improve the performance of the
transducer array. As observed in the developed prototype, there exist some non-functional
elements towards the end of each quarter-ring array. Speculation would be the some
broken connections caused during the assembly process. Therefore by fabricating the
transducer array as a whole or in half-ring configuration might help in reducing the
number of dead elements and improving the overall uniformity.
Another future work would be performing the imaging experiments with the array
and the Verasonics system. In collaboration with the University of Michigan, we would
setup the photoacoustic imaging setup with their laser system. After characterizing the
imaging capability of the system, in vivo experiments involving patient scans would be
Chapter 3. Design and Fabrication of a Ring-shaped Array for Photoacoustic Tomography
74

performed and the true capability of this system in the early detection of rheumatoid
arthritis will be evaluated.

Chapter 4. Proposal Summary and Future Directions
75

Chapter 4.
Proposal Summary and Future Directions
4.1. Chapter Overview
In the previous two chapters, the development of two different ultrasonic transducer
arrays for specific applications has been reported. The purpose of this concluding chapter
would be to summarize the key features of the developed transducer arrays. In addition,
the ongoing and future works of each project would be discussed.
The rest of this chapter is organized as follow:
 Section 4.2 reviews the significance of this research work on fostering the
development of ultrasound imaging;
 Section 4.3 discusses the ongoing and future works of the research projects.
4.2. Summary of Thesis Study
4.2.1. Overall Scope
This dissertation study has been dedicated to the development of specialized
ultrasonic transducer arrays. Despite the availability of a variety of commercial ultrasonic
transducer arrays, ultrasound imaging nowadays has been emerging into new areas in
which some specific requirements might not be catered by using the existing commercial
arrays. Therefore in order to facilitate the development of new applications for medical
ultrasound imaging, it would be important to design and develop ultrasonic transducers
that can fully meet the specifications for the projects.
Chapter 4. Proposal Summary and Future Directions
76

Two different projects were investigated in this dissertation study. The first one was
the development of a miniature phased transducer array used for guidance of
interventional procedures, and the second project aimed at the development of a
ring-shaped transducer array for the early detection of rheumatoid arthritis using
photoacoustic tomography. Prototype arrays have been fabricated for both projects and
some preliminary testing results demonstrated the feasibility of these approaches for the
corresponding applications.
4.2.2. Specific Developments
Miniature Phased Array
A miniature phased transducer array has been developed for interventional guidance.
The fabricated 48-element 20 MHz phased array was housed in a 3-mm-diameter needle.
To the best of our knowledge, this is one of the smallest high frequency forward-looking
phased array.
The prototype showed good pulse-echo characteristics with an acceptable
inter-element crosstalk, and the imaging capability was evaluated by imaging a stainless
still needle inserted in a bovine liver. The fine image resolution would allow the phased
array to image small blood vessels and/or pathological tissues inside the patient’s body,
increasing the reliability of the guiding process.
Ring-shaped Array
A prototype ring-shaped transducer array has been fabricated. The 10 MHz array has
a diameter of 48.89 mm, which is large enough to accommodate a human finger, even
with swollen tissue. Together with a laser system, it is expected to give the PAT image of
Chapter 4. Proposal Summary and Future Directions
77

the joint.
A one-way pulse response measurement was carried out and the center frequency
and bandwidth of the prototype were reasonably closed to the simulation results. A simple
pulse-echo experiment using the Verasonics system also suggests that the connection
scheme was working as designed.
4.3. Future Research Directions
In the previous chapters, some of the future research directions for each project have
been mentioned. In this section, a more detailed discussion would be given.
4.3.1. Miniature Phased Array
Transducer Size
In the current design of the transducer array, we managed to reduce the outer
diameter of the housing to 3 mm, which is considered to be acceptable for interventional
procedures. However as the process is invasive, it is always preferable to have the from
factor of the transducer array be as small as possible. Therefore efforts can be made to
further reduce the size of the transducer array by investigating the possibility of
accommodating the transducer array into smaller housing with different geometry, such
as rectangle, it could become more favorable for intervention procedures.
Flexible Circuit and Cable Connection
The other possible research direction is to replace the current flexible circuit design
which wrap around both ends of the array, instead of just one end. This is currently not
possible as the resultant circuit would be double in length, which is not practical with the
Chapter 4. Proposal Summary and Future Directions
78

current fabrication process. However this can be achieved, it can be expected that the
transducer yield rate will be increased – as now the tension can be balanced out from both
sides and the disbanding issue should be less severe. Also, the electrical crosstalk level
should be further reduced.
To overcome the technical difficulties, one way would be developing a more
compact flexible circuit that can be accommodate completely in the needle housing, and
with coaxial cables soldered directly onto the flexible circuit. In this case the length of the
flexible circuit would not be a limitation, and also be able to provide a more robust
connection.
4.3.2. Ring-shaped Array
Performance Improvement
One future research direction would be trying to improve the performance of the
transducer array. As discussed in the previous chapter, some of the dead elements in the
prototypes might be due to the broken connections caused during the assembly process.
This issue might be addressed by investigating the possibly of fabricating the ring
transducer array as a whole or in half-ring configuration, as the number of elements at the
‘end’ is effectively reduced. This would also help in the element alignment – as now it is
inevitable to have some misalignments between the quarter-ring arrays, either due to the
imperfection in positioning the transducers on the PCBs, or in positioning the device in
the housing.
However this might lead to a lower yield rate as handling the long thin strip of
acoustic stack would be more challenging. Also the replacement cost would be higher
than the quarter-ring approach. The compromise between the yield and performance
Chapter 4. Proposal Summary and Future Directions
79

should be further investigated.
System Integration
The integration of the prototype array with the back-end Verasonics system is
currently ongoing. The integration includes the setup for both conventional B-mode
imaging and photoacoustic tomography. The B-mode images would be used to position
the finger due to its relatively high frame rate, and it could also provide additional
anatomical information.
After B-mode imaging has been implemented, the system will be used for
photoacoustic tomography. A ring of optical fiber bundles will be placed on top of the
transducer array (as illustrated in Fig. 3-3) to illuminate the tissues with laser. The laser
will be set to operate in synchronization with the Verasonics back-end, so that the
photoacoustic signals can be acquired accordingly. Then the data will be processed [102,
104] offline for the photoacoustic images.
Imaging Experiment
In vivo imaging experiments will be carried out when the system integration is
completed. Patients with rheumatoid arthritis will be invited to take part in this project in
a volunteering manner and photoacoustic tomography images of their diseased joints
would be acquired. Based on the acquired data, the feasibility of this approach in early
detection of rheumatoid arthritis will be evaluated.

80

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Abstract (if available)

Abstract Background: For more than half century, ultrasound imaging has been widely employed as a valuable diagnostic tool. Although it has often been viewed as a mature imaging modality, its emergence into new areas has been continuing. Active researches are going on and new applications using ultrasound imaging are being explored. However, despite the availability of a variety of commercial ultrasonic transducer arrays, researchers often would come across certain specific requirements that cannot be accommodated by the existing transducers. Although some may try to use the standard transducer arrays, the resulting images might not be ideal for research purposes. In this dissertation study, the goal is to design and develop ultrasonic transducers that can fully meet the specifications for some specific projects. It is expected that by employing the newly-developed transducer array, the research projects can be advanced and the possibilities of using ultrasound in various aspects can be further explored. ❧ Interventional Procedure Guidance: The first project focused on the use of ultrasound imaging in guiding interventional procedures such as biopsy. Currently the ultrasound probes are often placed at the body surface of the patients, leading to several drawbacks including the limitation of penetration and image quality. A miniature phased array transducer that can be placed adjacent to the intervention device has been developed. The transducer comprised 48 elements housed in a 3-mm-diameter needle. The center frequency and the bandwidth were around 21 MHz and 42% respectively. The imaging capability of the transducer was evaluated by acquiring the B-mode images of a needle in a cow liver. The performance of the proposed phased array transducer demonstrates the feasibility of such an approach for interventional guidance. ❧ Early Detection of Rheumatoid Arthritis: The second project aimed at the early detection of rheumatoid arthritis using photoacoustic tomography (PAT). As it is known that angiogenesis correlates with the early stage of rheumatoid arthritis, using PAT for screening purpose seems promising due to its high sensitivity for blood. A ring-shaped transducer array would be fabricated to accommodate a human finger so as to acquire the PAT image at the finger joint. The one-way pulse measurement of the prototype showed that the array has a center frequency and bandwidth of 9.35 MHz and 74.6% respectively, suggesting the array has promising acoustic performance for this application.

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Asset Metadata

Creator Chiu, Chi Tat (author)

Core Title Design and development of ultrasonic array transducers for specialized applications

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Biomedical Engineering

Publication Date 09/26/2016

Defense Date 09/01/2016

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag OAI-PMH Harvest,ultrasonic imaging,ultrasonic transducer array development

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Shung, K. Kirk (committee chair), Chen, Mike (committee member), Zhou, Qifa (committee member)

Creator Email chitatch@usc.edu,chiuharry@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-306786

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Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

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