Auto-focused virtual source imaging with arbitrarily shaped interfaces

This work presents a new method, named auto-focused virtual source imaging (AVSI), for synthetic aperture focusing through arbitrarily shaped interfaces with arrays. First, the shape of the component surface is obtained by time-of-flight (TOF) measurements. Then, a set of virtual source/receivers is created by focusing several array subapertures at the interface normal incidence points. Finally, the synthetic aperture focusing technique (SAFT) is applied to the received signals to generate a high-resolution image. The AVSI method provides several advantages for ultrasonic imaging in a two-media scenario. First, knowledge of the probe-part geometry is not required, because all information needed for image formation is obtained from a set of ultrasonic measurements. Second, refraction complications in TOF calculations are avoided, because foci at the interface can be considered as virtual source/ receivers, and SAFT can be performed in the second medium only. Third, the signal-to-noise ratio is higher than with synthetic aperture techniques that use a single element as emitter, and fourth, resolution is higher than that obtained by phased-array imaging with the same number of active elements, which reduces hardware complexity. The theoretical bases of the method are given, and its performance is evaluated by simulation. Finally, experimental results showing good agreement with theory are presented.