Ultrasound Imaging Using Multilayer Synthetic Aperture Focusing

Synthetic aperture focusing techniques (SAFT) have already been studied within several fields of non-destructive testing, but so far only with a single, relatively homogenous medium. In this article, we modify the original time-domain SAFT in order to image the interior of a multilayer structure. Standard focusing techniques assume that the wave velocity is constant within the volume to be imaged. The concept of the root-mean-square (RMS) velocity is used here to modify the classical delay-and-sum algorithm to handle layers with differing wave velocities. A scheme for iteratively using this to estimate the interfaces between the layers is also presented. The proposed method is demonstrated on ultrasonic B-scans of two test objects immersed in water, and it is shown that the increased lateral resolution and signal-to-noise ratio of standard SAFT is extended to the multilayer case. The increased resolution also makes it possible to accurately estimate the interfaces between consecutive layers, as long as the preceding interfaces are relatively smooth.