【作者】 杨春；

【导师】 秦开怀；

【作者基本信息】 清华大学 ， 计算机科学与技术， 2014， 博士

【摘要】 合成孔径聚焦技术(SAFT)在工业超声无损检测领域得到了初步应用，但仅局限于单一介质的超声成像处理，而不能对分层物体实现快速准确的图像重构。本文深入研究了当前分层物体的SAFT超声成像技术中亟待解决的多种重要问题，主要研究工作包括：1.从计算机图形学视角分析了时域SAFT技术的原理，以正向映射的观点将原DAS和全范围动态聚焦过程重新解释为在图像上画圆弧操作。将光栅图形学中圆的扫描转换方法引入SAFT技术中，并扩展为圆弧扫描转换算法ArcDrawing，从而提出了时域SAFT正向映射实现算法AD-SAFT。实验表明，AD-SAFT算法的成像速度比原时域SAFT实现方法快两倍。2.基于正向映射的观点，将原时域SAFT结合射线跟踪技术(SAFT+RayTracing)重新解释为在图像上画折射线移动轨迹曲线的操作过程，提出了射线移动轨迹追踪技术。该技术无需求折射点，避免了原射线跟踪技术所需的迭代计算。同时，也不再需要求解合成孔径有效长度，避免了该参数在分层物体的重构成像中不能准确计算而导致的成像误差。为了快速计算折射线，计算机图形学中基于几何的折射公式和直线扫描转换算法被运用到该技术中，从而提出了射线移动轨迹追踪算法SAFT-Trajectory。实验结果显示，该算法的成像效果优于原SAFT+RayTracing方法，其计算速度比后者提升了两个数量级。3.针对原相移迁移(PSM)技术只允许被测对象在深度方向上存在异种介质而在水平方向上必须为同种介质的受限情况，在相移因子的计算中考虑了非规则分层物体在水平和深度两个方向上均存在的声波速度变化现象，提出了变波速相移迁移技术(Generalized PSM, GPSM)。为了避免冗余计算，进一步提出了支持任意非0值输入的SRFFT输入截断算法(SRFFT-IP)和支持任意单输出的SRFFT单输出截断算法(SRFFT-SOP)。GPSM技术能对非规则分层物体实现准确成像，其计算时间低于SAFT+Ray Tracing方法，但略长于原PSM技术、远长于SAFT-Trajectory算法。4.分别提出了AD-SAFT和SAFT-Trajectory算法的GPU并行加速方法，基于CUDA并行计算平台，实现了单一介质和分层物体的实时成像。更多还原

【Abstract】 Synthetic aperture focusing technique (SAFT) has widely been used in the field ofindustrial ultrasonic nondestructive testing. But its applications are limited to imageobjects with a certain single medium, and cannot achieve the purpose of fast andaccurately reconstructing the images of layered objects with multiple media. Thelayered objects are the objects of several layers with anisotropic and inhomogeneousmedia in depth, lateral, or both directions. This dissertation studies several importantissues existing in SAFT for ultrasonic imaging of multi-layered objects. The mainresearch work and contributions of this dissertation includes:(1) The principle of the time-domain SAFT is re-interpreted from the perspectiveof computer graphics and the process of DAS and all-range dynamic focusing can behandled by drawing circular arcs in the image, which is a forward mapping of theultrasonic testing procedure while the original SAFT is in an inverse pattern.Furthermore, the traditional scan conversion approach of circles developed in rastergraphics is introduced to SAFT and generalized to the scan conversion algorithm ofcircular arcs, called ArcDrawing. Based on ArcDrawing, this dissertation proposes aforward mapping algorithm, AD-SAFT, for time-domain SAFT. The experiments showthat the reconstruction speed of AD-SAFT is two times faster than that of the traditionalimplementation of time-domain SAFT.(2) Base on the forward mapping theory, this dissertation adds the sound waverefraction effects into AD-SAFT and re-interprets the principle of the traditional methodthat combines the time-domain SAFT with ray tracing technologies as drawingtrajectory curves of focusing points in the image. The trajectory drawing procedure is ina forward pattern while the traditional method is inverse. On the basis of there-interpretation, a ray motion trajectory tracking approach is proposed. Because theunknown conditions in the traditional inverse method are changed to be knownconditions in the forward pattern, this new approach does not need to calculate theeffective length of the synthetic aperture but the traditional method does, so it avoids thedefects of inaccurate images that brought out by the inaccurately calculated value of theeffective length for layered targets. The new approach also avoids the iterative computation required by ray tracing as the refraction points are known conditions andno longer need to be computed. In order to fast compute the ray lines, the geometricalrefraction formula and scan conversion algorithm of lines in computer graphics areadopted in the new approach, based on which, the ray motion trajectory trackingalgorithm, SAFT-Trajectory, is presented in this dissertation. The experimental resultsindicate that the new approach speeds up the ultrasonic imaging procedure ofmulti-layered objects by two orders of magnitude with better effects than the traditionalmethod.(3) The phase shift migration (PSM) technique, as a frequency-domainimplementation of SAFT, can be adopted for imaging of regularly layered objects thatare inhomogeneous only in depth but isotropic and homogeneous in the lateral direction.In order to deal with irregularly layered objects that are anisotropic and inhomogeneousin both the depth and lateral directions, the phase shift factor with a constant soundvelocity in PSM is extended to a generalized phase shift factor by considering thevariation of the sound velocity in both the depth and lateral directions. With the newphase shift factor, this dissertation proposes a generalized technique, called GPSM,which breaks through the limitation of PSM that the sound velocity must be constant inthe lateral direction. In addition, SRFFT (split-radix fast Fourier transform) input/outputpruning algorithms are developed and employed in GPSM to speed up the imagereconstructions. The experiments show that GPSM is capable of reconstructing accurateshapes and interfaces of irregularly layered objects. The computing time of GPSM ismuch less than the time-domain SAFT combined with the ray-tracing technique, but alittle bit longer than PSM while much longer than SAFT-Trajectory.(4) Noting that SAFT-Trajectory is the most efficient approach among the existingreconstruction techniques related to SAFT for ultrasonic imaging of layered objects, andthat AD-SAFT and SAFT-Trajectory algorithms are very suitable for parallelimplementation, this dissertation proposes a GPU-accelerated AD-SAFT and aGPU-accelerated SAFT-Trajectory algorithm, respectively, and implements them withCUDA to achieve real-time ultrasonic imaging for both the single medium objects andthe multi-layered objects.更多还原