Spectrum-Domain Plane Wave Imaging: A Novel Approach to Studying Multilayered Medium

Multilayered composite media are widely used in various industries, and the presence of small defects like voids or pores could lead to reduced mechanical properties. Ultrasound imaging with full-matrix capture (FMC) is a well-established modality to detect the small defect. However, the sequential emission of probe element combined with full-matrix reception results in heavy computational complexity and low frame rates, limiting real-time implementation. Furthermore, conventional FMC methods are only suitable for single-layer media and will be inaccurate for multilayered structures. To overcome these limitations, an efficient approach called spectrum-domain plane wave imaging (SD-PWI) was proposed to imaging multilayered media. By modifying the exploding reflector model to be applicable to PWI in multilayered imaging scenarios, the received wavefield was accurately extrapolated to the top of the objective layer, and the entire layer of interest was successfully reconstructed, employing fast Fourier transform based beamforming. Experimental findings demonstrated the effectiveness of SD-PWI. Compared with two classical FMC approaches, such as ray-tracing synthetic aperture and extended phase shift migration, multiangle compounded SD-PWI achieved improved image quality and higher efficiency. The side-drilled holes with diameters of 1–2.5 mm can be effectively detected, showcasing its ability to diagnose minor defects. Moreover, SD-PWI achieved a frame rate of 15 Hz for 3-layer medium imaging using a 192-element phased array. It is demonstrated that the proposed SD-PWI method is an accurate and efficient modality to studying multilayered media in industrial applications.