||Effects of Operational Mode and Attenuation on the Characterization of Biological Tissues with Ultrasonic Backscattering Signals and Statistical Analysis
||Institute of Medical Informatics
Nakagami statistical distribution
ultrasonic backscattering signal
probability distribution function
使用3.5MHz、7.5MHz和10MHz聚焦式換能器量測離體的豬肝組織及使用30MHz和50MHz聚焦式換能器量測活體大鼠的肝組織來進行實驗。豬肝組織分為健康豬肝及病變豬肝兩組，每組的數量為10，而十週大的Sprague Dawley (SD)大鼠肝臟組織數量為5。各個頻率的超音波換能器分別以1、3、5與10個週期的弦波訊號激發，弦波訊號由重複的脈衝串組成。衰減效應是利用在換能器及組織間擺放不同厚度的衰減假體作量測，而衰減假體的衰減係數為1.62 dB/mm．MHz。使用Nakagami統計模型包含Nakagami統計參數 (Nakagami-m)來描述及評估超音波逆散射包封訊號的機率密度函數。最後，使用組織學分析中的H&E染色，不僅檢測肝臟病理狀態，更可判別在解析體中散射子的分布情形。
The Nakagami statistical distribution of ultrasonic backscattering signals has shown it is capable of characterizing the variations of density and arrangement of scatterers in biological tissues. This analysis of ultrasound signals has also found with less dependency on the effect of attenuation and the estimated statistical parameter can be affected by the board-band attenuation in the tissues and distortion of acquired echo signals. Accordingly, as the employed ultrasound frequency and pulse duration, namely operational mode, were increased, several additional factors could come into play and further affect the precise estimation of the statistical parameters. To further investigate the addressed issues, experiments were arranged and performed using 3.5, 7.5, and 10MHz focused transducer in vitro porcine liver tissues and using 30, 50MHz focused transducer in vivo rat liver tissues. The porcine liver tissues were obtained with either healthy (N=10) or pathological fibrosis (N=10) were obtained from a local slaughter house, and rat liver tissues from 10-week-old male Sprague Dawley (SD) rat liver tissues (N=5). Various excitation cycles, including 1, 3, 5, and 10, of tone bursts at pulse repetition frequency corresponding to ultrasound frequencies of 3.5, 7.5, 10, 30 and 50 MHz were adjusted for driving the transducers. Various thickness of attenuated phantoms with the attenuation coefficient of 1.62 dB/mm．MHz were placed on the surface of tissue to be measured. The Nakagami statistical model, which includes the shape parameter (Nakagami-m), was implemented to assess variations of the probability density function (PDF) estimated from the acquired ultrasonic backscattering signals. Eventually, the histological analysis, using the H&E staining, not only detect the pathological fibrosis liver status but also measure the number of scatterers in the resolution cell, it’s corresponds to a volume of ultrasound signals within transducer pulse length and the lateral profile beamwidths. Results of in vitro porcine liver tissues indicated that the attenuation could significantly vary the shape of PDF of backscattered envelopes. Results of in vivo rat liver tissues indicated that the Nakagami-m increased with the increasing ultrasound frequencies and bandwidth, and that those associated PDFs were nearly pre-Rayleigh distributed. Results of histological analysis are shown the stronger scatterers distributed in the tissues, the characterization of tissues both describe to pre-Rayleigh distribution. All the results consistently demonstrated that the use of 3 cycles tone bursts may achieve the most appropriate performance to accommodate a trade-off between attenuation effect and image resolution. Current study also verified that the operational modes of incident ultrasound need to be properly assured before that the statistical model may be further applied to parametric imaging and clinical applications.
Table of Contents VIII
List of Tables XI
List of Figures XII
Chapter 1: Introduction 1
1.1 Ultrasound 1
1.2 Quantitative Ultrasound Parameters 3
1.3 Ultrasonic Tissue Characterization 4
1.4 Research Objectives and Specific Aims 7
Chapter 2: Theoretical Background 9
2.1 Fundamentals of Ultrasound Wave Propagation 9
2.1.1 Acoustic Wave Equation 9
2.1.2 Reflection and Refraction 14
2.1.3 Attenuation and Absorption 16
2.1.4 Ultrasonic Scattering 17
2.2 Ultrasonic Transducers 21
2.2.1 Sound Field 24
2.2.2 Axial and Lateral Resolution 25
2.3 Statistical Models for Ultrasonic Backscattered Signals 27
2.4 Structure of Biological Liver Tissue 35
Chapter 3: Materials and Methods 36
3.1 Experiments on phantoms 36
3.2 Experiments on Animals 38
3.2.1 In Vitro Porcine Liver 38
3.2.2 In Vivo Sprague Dawley Rat 38
3.3 Experimental Arrangement 41
3.4 Measuring Sound Velocity and Attenuation of Material 63
3.5 Histological Analysis 70
Chapter 4:Results 77
4.1 Distributions of the Ultrasonic Backscattered Envelopes 77
4.1.1 In Vitro Porcine Liver Tissue 77
4.1.2 In Vivo Rat Liver Tissue 91
4.2 The Statistical Model of Probability Density Distribution 93
4.2.1 In Vitro Porcine Liver Tissue 93
4.2.2 In Vivo Rat Liver Tissue 104
4.3 Histological Sections 109
4.3.1 In Vitro Porcine Liver Tissue 109
4.3.2 In Vivo Rat Liver Tissue 111
Chapter 5:Discussion 112
5.1 Nakagami Statistical Analysis of Operational Mode and Attenuation in vitro Porcine Liver Tissues 112
5.2 Nakagami Statistical Analysis of Operational Mode and Attenuation in vivo Rat Liver Tissues 119
5.3 Comparison the Nakagami Statistical Analysis of Operational Mode and Attenuation in Phantom, in vitro Porcine Liver Tissues, and in vivo Rat Liver Tissues 120
5.4 Histological Analysis 122
5.4.1 Common Faults of Histological Analysis 122
5.4.2 Verification of Number of Scatterers by Histological Analysis 123
Chapter 6: Conclusions and Future Works 125
6.1 Conclusions 125
6.2 Future Works 127
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