Resolution Enhancement in Ultrasonic TOFD Imaging by Combining Sparse Deconvolution and Synthetic Aperture Focusing Technique(Sparse-SAFT)

The shallow subsurface defects are difficult to be identified and quantified by ultrasonic time-of-flight diffraction(TOFD)due to the low resolution induced by pulse width and beam spreading.In this paper,Sparse-SAFT is proposed to improve the time resolution and lateral resolution in TOFD imaging by combining sparse deconvolution and synthetic aperture focusing technique(SAFT).The mathematical model in the frequency domain is established based on the l1 and l2 norm constraints,and the optimization problem is solved for enhancing time resolution.On this basis,SAFT is employed to improve lateral resolution by delay-and-sum beamforming.The simulated and experimental results indicate that the lateral wave and tip-diffracted waves can be decoupled with Sparse-SAFT.The shallow subsurface defects with a height of 3.0 mm at the depth of 3.0 mm were detected quantitatively,and the relative measurement errors of flaw heights and depths were no more than 10.3%.Compared to conventional SAFT,the time resolution and lateral resolution are enhanced by 72.5 and 56%with Sparse-SAFT,respectively.Finally,the proposed method is also suitable for improving resolution to detect the defects beyond dead zone.

Improvement in resolution of fiber-laser photoacoustic tomography based on a virtual-point concept

In this study,a virtual-point concept was introduced into fiber-laser photoacoustic tomography to improve the elevational image resolution.The flexible fiber laser was bent into an arc shape to conform to the ultrasound wavefront,which formed an ultrasound focus at the center of the arc.The synthetic aperture focusing technique was utilized to reconstruct the images;as a result,the elevational resolution particularly within the out-of-focus region was considerably improved compared to the resolution of an image retrieved by multiplexing the PA time-resolved signals with sound velocity.The all-optical fiber-laser photoacoustic tomography system with a high spatial resolution has potential for various applications,including biomedical research and preclinical/clinical diagnosis.