光声成像中超声信号采集及预处理技术研究

【作者】 李伟；

【导师】 张建秋；

【作者基本信息】 复旦大学 ， 电路与系统， 2012， 硕士

【摘要】 光声成像是近年来发展起来的一种无损医学成像方法,它结合了纯光学成像和纯超声成像的优点,可以提供高分辨率和高对比度的组织成像。光声成像中两项关键技术是信号采集和图像重建。本文在信号采集及预处理方面展开了如下内容的工作和研究：本文首先设计并实现了一套基于“单元旋转扫描”架构的高准确度光声成像系统,主要构建了由可编程控制的数字示波器、可调谐激光器和数控转台等组成的硬件平台,并开发了配套的控制软件。本文解决了超声传感器的电缆线屏蔽、各仪器间响应延迟等实际应用中的问题,一方面节省了硬件开支,另一方面也使得系统能更协调地工作。本文的构建方法可望发展为一种无损伤的生物组织结构与功能成像系统。在上述平台进行光声成像的过程中,我们发现当水面高度较小时的重建结果会产生伪影,这种现象在水深的时候是不存在的。本文从超声波在水中的传播特性出发,分析了光声成像技术中图像伪影的产生机理,分析表明：伪影的产生是由超声信号在水面的反射延迟造成的。进而本文提出了通过延迟相减来控制消除反射伪影的方法,并给出了在实际应用中如何控制水面高度的依据。通过构建的光声成像平台进行实验验证,结果表明：提出的方法能有效地消除重建图像中的伪影,提高了成像图像的质量。此方法对光声成像的实际应用有一定的指导意义。最后,针对光声成像数据采集系统中获得的超声数据存在杂波的问题,本文提出了提高光声成像图像质量的Renyi熵方法,即从超声信号的时频分布得到的Renyi熵出发,计算Renyi熵直方图,从而确定超声信号分割的阈值,达到把杂波部分分离的目的。实验结果表明该方法能有效地分割出超声采集数据中的杂波从而将其滤除,有效提高重建图像的质量。更多还原

【Abstract】 Photoacoustic tomography (PAT) has become a novel noninvasive medical imaging technology. It combines the advantages of pure optic imaging and pure ultrasound imaging and can provide tissue image of high resolution and high contrast tissue. The key technologies in photoacoustic tomography are signal acquisition and image reconstruction. Based on the aspects of signal acquisition and data preprocess, this research contents in this thesis are as follows:At first, a photoacoustic tomography system of high accuracy based on the architecture of "Unit Rotation Scanning" has been designed and implemented in this thesis. The system is a platform composed of digital oscilloscope, tunable laser and rotary table which are programmable. And the controlling software has also been developed. The problems of ultrasonic sensor’s cable shielding and response delay between devices have been solved. The advantages are saving the cost of hardware and making the system coordination run better. This method of constructing the photoacoustic tomography system is expected to be a noninvasive biological tissue and functional imaging system.In the process of photoacoustic tomography on the above platform, we found that the reconstructed image had artifacts when the water was shallow. The mechanism of artifacts in the photoacoustic tomography is analyzed based on the propagation characteristics of ultrasonic waves in the water. The analysis shows that artifacts are generated from the delay caused by the reflection of the ultrasonic signal upon the water surface. Then a method through the delay subtraction is proposed to control these artifacts. The reference is also provided on how to control the water height in the practical application. The method is validated with our experimental platform. From the experiment, it is shown that the proposed algorithm can eliminate artifacts effectively and the quality of the reconstructed image is better.At last, to overcome the problem of the noise wave in the ultrasound data collected from the photoacoustic tomography system, a new method is proposed to firstly separate ultrasound signals and noise wave based on Renyi entropy and then to reconstruct the image using the separated signals. The experiment on the photoacoustic tomography platform shows that the noisy part in the ultrasound data acquisition can be segmented out through the threshold obtained by the histogram of Renyi entropy. The method is validated through image quality assessment. It is proved that the proposed algorithm can segment ultrasound signals effectively and enhance the quality of the reconstructed image.更多还原