Thiết kế beamformer kỹ thuật số cho mảng transducer siêu âm tần cao FPGA

DESIGN OF DIGITAL BEAMFORMERS FOR HIGH FREQUENCY ULTRASOUND TRANSDUCER ARRAYS USING FIELD PROGRAMMABLE GATE ARRAY (FPGA) by Changhong Hu ___________________________________________________________ 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) August 2006 Copyright 2006 Changhong Hu UMI Number: 3238308 Copyright 2006 by Hu, Changhong All rights reserved. UMI Microform 3238308 Copyright 2007 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code.

ProQuest Information and Learning Company 300 North Zeeb Road P. Box 1346 Ann Arbor, MI 48106-1346 ii ACKNOWLEDGEMENTS This thesis is the result of several years of hard work whereby I have been accompanied and supported by many people. It is a pleasant aspect that I have now the opportunity to express my gratitude for all of them. I would fist like to thank my advisor, Professor K.

Kirk Shung who gave me the opportunity to work in the NIH Ultrasonic Transducer Resource Center at University of Southern California (USC). I am deeply grateful for his helping, stimulating and encouraging me in all the time of research. I wish to express my warm and sincere thanks for my thesis committee: K. Kirk Shung, Jonathan Cannata, Jesse Yen and E.

Kim for helping me on the preparation of this dissertation. I would also like to thank the faculty, staff and my colleagues, especially Peijie Cao, Qifa Zhou, Ruibin Liu, Kevin Snook, Xiaochen Xu, Jin Ho Chang, Jay Williams and Joe Han for their assistance with this work. I owe my loving thanks to my wife Xuemei Deng, my daughter Ziyan Hu. They give me many supports.

Without their encouragement and understanding it would have been impossible for me to finish this work. My special gratitude is due to my parents, my brother, my sisters and their families for their loving support. iii TABLE OF CONTENTS ACKNOWLEDGEMENTS ii LIST OF TABLES vi LIST OF FIGURES vii ABSTRACT xii Chapter 1: HIGH-FREQUENCY ULTRASONIC IMAGING AND HIGH-SPEED ELECTRONIS 1 1.2 HIGH FREQUENCY ULTRASOUND IMAGING: SINGLE ELEMENT 5 1.3 HIGH FREQUENCY ULTRASOUND IMAGING: ARRAYS 8 1.4 HIGH SPEED ELECTRONICS 14 1.5 THESIS OUTLINE 18 Chapter 2: BEAMFORMING PRINCIPLE 19 2.2 DIGITAL BEAMFORMING METHODS 23 2.1 DIRECT DELAY-SUM BEAMFORMING 23 2.3 BANDPASS TIME-DOMAIN BEAMFORMING 26 2.4 FRACTIONAL-DELAY INTERPOLATION BEAMFORMING 27 2.5 FREQUENCY DOMAIN BEAMFORMING 31 2.3 DELAY CALCULATION 31 Chapter 3: DIGITAL BEAMFORMING IMPLEMENTATION 34 3.1 FIELD PROGRAMMABLE GATE ARRAY (FPGA) SELECTION 34 3.2 FPGA STRUCTURE OVERVIEW 35 3.4 SIMULATION RESULTS 44 iv Chapter 4: DEVELOPMENT OF A HIGH FREQUENCY ULTRASOUND DIGITAL BEAMFORMER WITH ANUULAR ARRAY TRANSDUCERS 47 4.2 FABRICATION AND EVALUATION OF THE TRANSDUCER 49 4.1 ANALOG FRONT-END CIRCUIT AND TRANSMIT BEAMFORMER 57 4.2 DIGITAL BEAMFORMER CIRCUIT 59 4.5 DISCUSSION AND CONCLUSIONS 62 Chapter 5: DEVELOPMENT OF A HIGH FREQUENCY ULTRASOUND DIGITAL BEAMFORMER WITH LINEAR ARRAY TRANSDUCERS 67 5.2 LINEAR ARRAY FABRICATION AND DESIGN CONSIDERATIONS 71 5.1 ANALOG FRONT CIRCUIT AND TRANSMIT BEAMFORMER 84 5.2 DIGITAL BEAMFORMING CIRCUIT 89 5.3 GRAPHIC USER INTERFACE 93 5.5 DISCUSSION AND SUMMARY 94 Chapter 6: SUMMARY AND FUTURE WORK 100 6.2 FUTURE WORK 102 BIBLIOGRAPHY 112 APPENDICES 122 APPENDIX A 122 v APPENDIX B 126 APPENDIX C 130 vi LIST OF TABLES Table 5.1: Parameters of the Linear Array Used in this Study 72 Table 5.2: Comparison of the –6 dB lateral beam width and the depth of focus with variable active elements for linear array (30 MHz) between simulation and theoretical calculation 78 Table 5.3: Comparison of the –6 dB lateral beam width and the depth of focus with variable active elements for linear array (35MHz) between simulation and theoretical calculation 79 Table 6.1: Number of control signals require for current design 108 Table 6.2: Number of control signals require for new design 108 vii LIST OF FIGURES Figure. Geometry of an ultrasound transducer.

D is the focal 3 distance, A is the diameter, and DOF is the depth of focus Figure. The general diagram of UBM system developed (Chen 2000). Diagram of the linear array (Top) and phased array (Bottom) 10 Figure. Electronic scanning with a linear array.

The subgroup is first selected, fired to form one line in an image, and the same 12 pattern is then repeated for the set of subgroup along the array, in a sequential and repetitive way Figure. If the elements are triggered imultaneously (Top), the formed beam patter is wider that 13 that formed by triggering outer element first and center element late (Bottom) Figure. The diagram of the PCB trace impedance calculation. W is the 18 trace width, t is trace thickness, and h is the layer thickness Figure.

The beamforming process 20 Figure. Diagram of the zero-padding beamforming 25 Figure. Simulation using the zero-padding algorithm. (A) An echo sampled at 1 GHz.

(B) The same echo sampled at 100 MHz. (C) 25 Zero-padding the echo in B by a factor of 10. (D) Low-pass filtered echo from C Figure. Illustration of the band-pass procedure 27 Figure.

Comparison of the ideal waveforms and the outputs from FD 30 filters using Lagrange filter Figure. Diagram of the beamformer in frequency domain 32 Figure. Phase delay before and after the beamforming 33 viii Figure. Delay calculation diagram 33 Figure.

The input/output block (IOBs) of Virtex-E FPGAs 37 Figure. Detailed structure of CLBs of Virtex-E slice 38 Figure. Delay implementation for one channel, D is the course delay 39 coefficient and d is the fine delay coefficient Figure. Coarse delay structure, cock delay is decided by the 40 sampling rate Figure.

Fine delay structure 41 Figure. The diagram of the finite element machine used in dynamic focusing procedure, each circle presents the state when the time 43 coefficient is popped out Figure. The logic structure of the implemented beamformer, it 44 includes dynamic receiving focus and sum logic Figure. Simulation result of two-channel with dynamic focusing 45 Figure.

First zoom part of Figure 3. Second zoom part of Figure 3. The simulation beam pattern (8X2 mm) of focused annular array transducer in lateral-depth (XZ) plane. The 6 element 53 annular array has 3 mm diameter with 30 µm kerfs size Figure.

The measured beam pattern (8X2 mm) of focused annular array transducer (3 mm diameter, 30 µm kerfs size) in lateral- 54 depth (XZ) plane Figure. The measured cross-section view (2X2 mm) of the beam at focal point, where x, y are the two lateral directions 55 ix Figure. Block diagram of the imaging system. The imaging system consists of a transducer, a six-channel transceiver, an FPGA 57 based beamformer, a DSP microprocessor-based scan converter and a PC computer for displaying the image Figure.

The photograph of the assembled transceiver and beamformer 60 board Figure.6 Wire phantom (20 µm Tungsten) images. The ex vivo corneal images of an excised rabbit eye using the 64 annular array Figure.8 The comparison of lateral intensity profile of measurement (solid line) and simulation (dot line) along the focal point (6. The 66 –6dB width is 150 µm, 200 µm for simulation and measurement, respectively Figure. The –6 dB contour of simulation beam pattern (Top) and lateral intensity profile at focal point (Bottom) of linear array transducer 76 (30 MHz, 100 µm pitch size) in lateral-depth (XZ) plane with different active elements Figure.

The –6 dB contour of simulation beam pattern (Top) and lateral intensity profile at focal point (Bottom) of linear array transducer 77 (35 MHz, 50 µm pitch size) in lateral-depth (XZ) plane with different active elements Figure. The comparison of the –6 dB contour of simulation beam pattern 80 using apodization functions Figure. The comparison of lateral intensity profile using apodization 81 functions at focal point Figure. The comparison of –6 dB width using different apodization 81 functions x Figure.

Block diagram of the overall imaging system 83 Figure. Excitation pulse and its spectrum 85 Figure. Schematic of the 4th Butterworth filter 86 Figure. Spectrum of the filter (solid line) and the spectrum of the signal 87 (dot line).

Block diagram of the crosspoint switch 89 Figure. The flow chart of the firmwork developed for FX2 96 Figure. The flow chart of the data acquisition software 97 Figure. Excised rabbit eyeball image from 30 MHz linear array with the pitch size of 100 µm (A) and the 35 MHz, 64 element 99 linear array with the pitch size of 50 µm (B) Figure.

Spectrums of the negative spike and bipolar pulser 103 Figure. Snapshot of the bipolar pulser 104 Figure. The peak-to-peak value of echo from quartz target using one element from the 35 MHz, 64 elements linear array transducer 106 with variable peak-to-peak value of the monocycle Figure. Schematic of proposed 64-channel digital beamformer 110 Figure.

Assembly high-speed connector and cable form Samtec 111 Figure. Layout of digital control board. Layout of analog front board. Layout of digital beamformer board.

Layout of the amplification and cross-switch board. Layout of digital contrl board. Layout of digital beamformer board. 129 xii ABSTRACT Ultrasound imaging is a well-established and widely used clinical technique that shows the cross-sectional image of human tissues.

Conventional ultrasound systems are targeted at imaging the heart or abdominal organs and the spatial resolution at this frequency range is on the order of a few millimeter. An ultrasound system will be capable of providing a better spatial resolution if higher frequencies are used. High frequency ultrasonic imaging (> 20 MHz) using single element transducers has been shown to be clinically useful in ophthalmology, dermatology, small animal and intravascular imaging. In recent years, more studies have been carried out on the development of high frequency array transducers.

Due to the fact that beamforming electronics is not yet commercially available for the high frequency arrays, prototypical digital beamformers for the annular and linear arrays were specifically developed for the purpose of testing the annular array and linear array developed at the NIH Transducer Resource Center at University of Southern California (USC). Field Programmable Gate Arrays (FPGAs) have been demonstrated to be an ideal platform for beamformer development. They provide the processing speed necessary for real-time beamformer with high frequency array transducers. Two digital beamformers were developed for the 8- element annular array and 64-element linear arrays.

xiii An imaging system composed of an annular array transducer, an eight- channel analog front-end, a field programmable gate array (FPGA) based beamformer, and a DSP microprocessor based scan converter was deveolped. A PC computer is used as the interface for image display. The beamformer that applies delays to the echoes for each channel is implemented with the strategy of combining the coarse and fine delays. The coarse delays that are integer multiples of the clock periods are achieved by using a First-In-First-Out (FIFO) structure and the fine delays are obtained with a Fractional Delay (FD) filter.

Using this principle, dynamic receive focusing is achieved. The image from a wire phantom obtained with the imaging system was compared to that from a single element transducer using one channel of the system. The improved lateral resolution and depth of field from the wire phantom image were observed. Images from an excised rabbit eye sample were also obtained, and fine anatomical structures were discerned.

A real-time digital beamformer for high frequency (>20 MHz) linear ultrasonic arrays was designed and developed. The system can handle up to 64- element linear array transducers and excite 16 channels and receive simultaneously at 100 MHz sampling frequency with 8-bit precision. Using fractional delay filters, fine delays as small as 1ns can be implemented. A frame rate of 30 frames per second was achieved.

Wire phantom (20 µm tungsten) images were xiv obtained and –6 dB axial and lateral widths were measured.