Research On The Synthetic Aperture Focusing Technique For Ultrasonic Imaging Of Layered Objects

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.