摘要
医学超声工程发展体现两个方面,一是应用领域的扩展,如定量超声技术测量骨密度来诊断骨质疏松。二是传统技术的改进,如超声成像技术的进步带来的成像质量的提高。定量超声技术则利用超声透射法来测量骨密度,反映骨骼的微观结构。超声成像是医学超声诊断技术最重要的技术,它是对人体软组织进行成像协助医生对组织病症进行诊断。
本文在超声测量骨密度和超声成像两个方向进行研究,研究方法采用理论分析,仿真说明和实验验证三者结合的方式。先对传统方法存在的问题进行分析,针对这些问题提出改进的测量方法和成像算法,并进行仿真和实验,结果表明这些方法可以提高测量精度和成像质量。具体研究成果如下：
1.研究设计基于任意波发射的超声骨密度测量系统,并利用该系统做了相关的测量实验。
2.提出了一种新的用于骨密度测量的编码方案,它用Chirp信号调制Golay互补码,产生Golay_Chirp信号,该信号具备了Golay和Chirp共同的优点。仿真和实验表明该编码信号可以进一步的增加超声透射深度和抗干扰能力。
3.将通信中的M序列引入到BUA测量中来,相对其他编码信号,M序列产生容易,发射简单,且码长较长,因此更加适合于骨密度测量。实验表明该编码相对传统的脉冲波,其测量精度得到有效的改善。
4.将AR模型引入BUA的测量,并提出了针对BUA测量AR模型定阶方法,仿真和实验表明采用高阶AR模型可以平滑衰减曲线,提高测量精度。实验时采用宽带超声传感器测量BUA,发现BUA在0.4-1.8MHz区域内与频率仍具有较好的线性,从而扩展了测量带宽。
5.在SOS测量方面,采用线性拟合法去除系统的固有延时,有效的提高SOS测量精度。
6.对超声成像的原理进行分析,并对编码发射技术与自适应波束形成技术进行深入研究,提出一种将两种方法相互结合的超声成像方案,并采用FieldⅡ进行仿真,结果表明该方法可以提高图像的对比度和分辨率。
超声测量骨密度和超声成像作为医学超声的诊断技术,一直是相关领域的研究热点。本文研究可以提高测量精度,改进成像质量,从而降低误诊率。这些研究不仅具有一定的理论创新,而且更具有较高的实用价值。
这些工作得到两个国家自然基金项目(60471057和60871087)及2008年中国科学技术大学研究生创新基金的支持。
The development in medical ultrasonic engineering can be presented in the following two aspects:1. Application extension, such as quantitative ultrasound in the assessment of bone mineral density and osteoporosis; 2. Improvement in traditional techniques, such as enhancement of imaging quality owing to improved ultrasound imaging techniques. Quantitative ultrasound takes advantage of through-transmission techniques to assess bone density as well as the micro-structure of bones. Ultrasound imaging serves as the most fundamental in the field of medical ultrasound, and images of human's soft tissue can provide further assistance to doctors for making clinical diagnosis.
This paper explores researches in quantitative ultrasound assessment of bone mineral density and ultrasound imaging in a combined way, from theoretical analysis, simulations to experimental validation. Firstly, we introduce limitation existing in current studies and propose some new measurement techniques and imaging algorithms for further improvement. Moreover, corresponding simulations and experiments are conducted and the results show that precision and high quality are improved to a great extent, which can indicate the availability of those methods. Detailed research findings are as follow:
1. Designing the first nation-wide random-wave-emitting-based ultrasound bone mineral density measurement system, which is used to carry out experiments.
2. Proposing a new coding scheme for measuring bone mineral density, which uses Chirp signal to modulate Golay signal,so that Golay_Chirp signal is generated. This resulted signal is equipped with advantages of both Golay and Chirp. Simulations and experiments show that this coding scheme can further strengthen the anti-noise ability and transmission depth of ultrasound signals.
3. Introducing the principle of M series in communication to the measurement of broadband ultrasonic attenuation. Compared to other coding signals, M series can be easily generated, simply emitted and with longer code length, therefore it is more suitable in bone mineral density measurement, also improvement in precision can be displayed from experiments.
4. Incorporating AR model in broadband ultrasonic attenuation measurement to overcome the problem of noise influence, acute ups-and-downs in the attenuation curve. Proposing the method for determining order in the application of AR model. Simulations and experiments show that. adopting high order in AR model can smooth . attenuation curve and improve measuring precision. Applying broad-band ultrasound traducer in measuring broadband ultrasonic attenuation and a linear relationship of broadband ultrasonic attenuation and frequency is found in the frequency range of 0.4-1.8MHZ. This result is instructive in its ability of further widening band width.
5. In the speed-of-sound measurement, the inherent delay of the system is removed by linear fitting method, which improves the speed-of-sound measurement accuracy effectively.
6.Analyzing the principles of ultrasound imaging and making in-depth exploration in the code-emitting and adaptive beamforming technique, novel imaging method with a combination of the above two is proposed and Field II is used to make simulations. An obvious improvement in contrast and resolution is observed in the results.
As major techniques of medical ultrasound, quantitative ultrasonic assessment of BMD and ultrasonic imaging have always been the research focus in related fields. This paper explored the improvement in measuring precision and imaging quality, which can lower the rate of erroneous diagnosis. The research results introduced here is not only unique in theory innovation but also of high practical value.
The author acknowledges the support of the National Natural Science Foundation of China (Grant Nos.60471057,60871087) and 2008 Innovation Foundation for Graduates, USTC.
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