Ultrasonic Synthetic Aperture Focusing Technique With Finite Source Element for Focused Transducers

A spherically focused transducer (SFT) can concentrate the ultrasounds to a focal zone that yields a better lateral resolution at a certain axial range. Unfortunately, this result is always accompanied by a loss of resolution outside the focal zone. The synthetic aperture focusing technique (SAFT) is a type of digital signal processing technique that is commonly based on the point-source wavefront backpropagation (WFBP) theory to improve the axial resolution of the image. However, the ultrasound is refracted when it propagates through the couplant–specimen interface to detect the flaws in specimens; thus, the capability of a virtual point-source SAFT in refocusing ultrasonic energy is deteriorated caused by the phase aberration by an SFT. Therefore, in this study, the finite source element (FSE) refraction corrected WFBP SAFT imaging method will be introduced to reduce the phase aberration effect when refocusing a defocused ultrasonic image. Study results show that the axial resolution of the image is largely improved by this technique, but improvement is not yet attained in the lateral resolution. In addition, the signal-to-noise ratio of the image is enhanced. Therefore, for cases with a strong phase aberration, the proposed method is recommended for obtaining better image resolution.