||Development of a Multifunctional Pulse Generator for High Frequency Ultrasound Imaging System
||Department of BioMedical Engineering
high frequency ultrasound imaging system
multifunctional pulse generator
arbitrary waveform generator
This paper presents a development of a multifunctional pulse generator for high frequency ultrasound imaging system, which includes short pulse signal and coded pulse signal. Currently, the short pulse is commonly used in high frequency ultrasound imaging system since it has wide bandwidth with high resolution. However, the energy would severely attenuate while applying ultrasound tissue imaging. The attenuation will lead to the decrease of penetration depth and signal-to-noise ratio. Hence, to improve the effects of attenuation, bipolar pulse is developed. However, the decrease of bandwidth could deteriorate the system resolution. In recent years, chirp coded pulse is used as the excitation pulse for high frequency ultrasound imaging system to improve the attenuation of high frequency. By elongating signal length, chirp pulse increases mean power to improve the penetration depth and SNR. However, it would reduce bandwidth and resolution while elongating signal length. So, chirp pulse must work with pulse compression in back-end signal processing for increasing the penetration depth, SNR and resolution. This multifunctional pulse generator system is designed based on FPGA and high speed DAC which facilitates compact and cost-effective implementation. The reconfigurable structure is competent for various applications; moreover, it opens a platform for future research and development. When evaluating imaging system, waveform analysis and frequency domain analysis are presented; the system and the instruments for commercial use are compared. The efficiency of the system compares favorably with the instruments. Furthermore, the results show that the coded pulse signal can acquire 21dB increase of SNR and 400% increase of penetration depth in contrast with unipolar pulse. Finally, with the help of compression filter, chirp pulse can efficiently enhance the penetration depth with a minimal sacrifice of axial resolution.
CHAPTER 1 INTRODUCTION 1
1.1 Motivation and Objective 1
1.2 Literatures Review 3
1.2.1 Bipolar Excitation 3
1.2.2 Chirp Excitation 4
1.2.3 Multifunctional Pulse Generator 6
1.3 High Frequency Ultrasound Imaging System 8
CHAPTER 2 THEORETICAL BASIS 9
2.1 The Basic Principles of Ultrasound 9
2.2 Transducer 11
2.2.1 Axial Resolution 12
2.2.2 Lateral Resolution 13
2.3 Excitation Pulse of Ultrasound Imaging. 14
2.3.1 Unipolar Excitation 14
2.3.2 Bipolar Excitation 15
2.3.3 Chirp Excitation 16
2.4 Pulse Compression 18
CHAPTER 3 SYSTEM DESCRIPTION 19
3.1 High Frequency Ultrasound Imaging System Structure 19
3.2 Pulse Generator System 22
3.2.1 FPGA Control Core 22
3.2.2 High Speed DAC 24
3.3 Signal Processing System 26
3.3.1 Matched Filter 26
CHAPTER 4 RESULTS AND DISCUSSION 28
4.1 Pulse Analysis 28
4.1.1 Frequency Domain Analysis 28
4.1.2 Waveform Analysis 32
4.2 Signal-to-Noise Ratio Analysis 34
4.2.1 The SNR of Echo Signal 34
4.3 Penetration Depth 37
4.3.1 Tissue-Mimicking Phantom 37
4.4 Wire Phantom Images 40
CHAPTER 5 CONCLUSION 42
CHAPTER 6 REFERENCES 43
Table 2.1 Acoustic Properties of Biological Tissues and Relevant Materials 9
Table 2.2 Average values of ultrasound parameters of bio-medical plastics 10
Table 4.1 The SNR comparison of different pulses 36
Table 4.2 The proportion of the tissue-mimicking phantom’s components 37
Table 4.3 The penetration depth comparison of different pulses 39
Figure 1.1 Schematics of the bipolar pulse generator 4
Figure 1.2 The ideal timing signals generated in the timing control circuit and the output of the bipolar pulse generator 4
Figure 1.3 The pulse echoes excited by bipolar and unipolar pulse 4
Figure 1.4 Schematic of the programmable modulated excitation imaging system 6
Figure 1.5 Block diagram of a multifunctional pulse generator 7
Figure 1.6 Unipolar pulse and their spectrum 7
Figure 1.7 35MHz bipolar pulse 7
Figure 1.8 60Vpp, 25 to 75MHz Chirp pulse 7
Figure 2.1 Direct piezoelectric effects 11
Figure 2.2 Reverse piezoelectric effects 12
Figure 2.3 The echo and imaging of high frequency ultrasound 12
Figure 2.4 The echo and image of low frequency ultrasound 12
Figure 2.5 The relationship between acoustic beam and lateral resolution 13
Figure 2.6 Unipolar pulse and its frequency domain analysis from MODEL 5900PR 15
Figure 2.7 Bipolar pulse and the frequency domain of bipolar pulse 16
from Tektronix AFG3252 16
Figure 2.8 Chirp pulse and the frequency domain of chirp pulse from Tektronix AFG3252 17
Figure 2.9 The compression echo signal by compression filter 18
Figure 3.1 System block diagram 19
Figure 3.2 The frequency response of pulse echo and the acoustic beam pattern 20
Figure 3.3 Altera DE2-70 development and education board 22
Figure 3.4 Block diagram of an arbitrary waveform pulse generator 23
Figure 3.5 The Altera DE2-70 development and education board 24
Figure 3.6 THDB-ADA, High-Speed A/D and D/A Development Kit 25
Figure 3.7 The chirp waveform from MATLAB and the schematic of data rate of DAC 25
Figure 3.8 The unipolar, bipolar and chirp waveform from FPGA 25
Figure 3.9 The original chirp and the reverse chirp pulse model 26
Figure 4.1(a) Model 5900PR (b) Tektronix AFG 3252 28
(c) The implemented multifunctional pulse generator 28
Figure 4.2(a) Unipolar pulse and frequency domain analysis from FPGA 29
Figure 4.2(b) Unipolar pulse and frequency domain analysis from Model 5900PR 29
Figure 4.2(c) Bipolar pulse and frequency domain analysis from FPGA 30
Figure 4.2(d) Bipolar pulse and frequency domain analysis from Tektronix AFG3252 30
Figure 4.2(e) Chirp pulse and frequency domain analysis from FPGA 31
Figure 4.2(f) Chirp pulse and frequency domain analysis from Tektronix AFG3252 31
Figure 4.3 The rising time (from extreme to zero voltage) and the convergence time 32
(the ringing of the pulse) of the unipolar pulse from FPGA and Model 5900PR 32
Figure 4.4 The symmetry and the convergence time (the ringing of the pulse) 33
of the bipolar pulse from FPGA and Tektronix AFG 3252 33
Figure 4.5 The unipolar echo signal from Model 5900PR 34
Figure 4.6 The unipolar echo signal from FPGA 34
Figure 4.7 The bipolar echo signal from Tektronix AFG3252 35
Figure 4.8 The bipolar echo signal from FPGA 35
Figure 4.9 The chirp echo signal from Tektronix AFG3252 35
Figure 4.10 The chirp echo signal from FPGA 36
Figure 4.11 The gelatin powder and the graphite powder 37
Figure 4.12 The tissue-mimicking phantom image of unipolar pulse 38
from Model 5900PR and FPGA 38
Figure 4.13 The tissue-mimicking phantom image of bipolar pulse 38
from Tektronix AFG3252 and FPGA 38
Figure 4.14 The tissue-mimicking phantom image of chirp pulse 38
from Tektronix AFG3252 and FPGA 38
Figure 4.15 The wire phantom and the detailed schematic diagram 40
Figure 4.16 The wire phantom image of unipolar pulse from Model 5900PR and FPGA 40
Figure 4.17 The wire phantom image of bipolar pulse from Tektronix AFG3252 and FPGA 41
Figure 4.18 The wire phantom images of chirp pulse from Tektronix AFG3252 and FPGA 41
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