Multiple virtual source aperture imaging for non-destructive testing

In recent years, the field of non-destructive testing has seen an expansion in the use of synthetic aperture imaging techniques, from the conventional synthetic aperture focusing technique (SAFT) and the total focusing method (TFM) through to the virtual source aperture (VSA) technique. Each technique shares a common imaging principle, which allows for improved lateral resolution and signal-to-noise ratio (SNR) by synthesising a larger aperture and the averaging effect. However, the computational burden of the advanced imaging algorithm limits the speed at which such systems can operate, which is further compounded by the acquisition of a larger number of raw amplitude data. Previous work has sought to limit transmission cycles in an effort to achieve a real-time data acquisition and imaging system, but at the detriment of image quality. This paper introduces several improvements to the VSA technique that allow for real-time imaging at lateral resolution and SNR levels comparable to and exceeding those of TFM, which is often referred to as the ‘gold standard’ in image quality. For this, three improvements are presented: firstly, by applying an apodisation function on transmit it is shown that improvements in side-lobe suppression can be achieved while maintaining a wide main lobe; secondly, two VSA scanning strategies are introduced, allowing for a greater averaging effect due to the additional virtual sources; and finally, a self-adaptive weighting function is applied to each pixel in the B-scan image based on the principles of incoherent and coherent summation.