Wideband time reversal processing for ultrasound NDE imaging

Time-reversal technique based on the decomposition of the time-reversal operator (TR-DORT) using large arrays of transducers has been used in ultrasound imaging for many years. However, this technique forms an image based on the center frequency of emission and has limitations such as poor target resolution, sensitivity to noise, and the requirement of large number of array elements with dense spatial sampling. In order to overcome these limitations, we investigate wideband TR imaging techniques emerged in radar. Wideband imaging bears clear advantages over its narrowband counter, most notably, enhanced resolution and improved robustness with noise. In this paper, we employ two wideband imaging techniques in the context of nondestructive evaluation of materials, namely, wideband Multiple Signal Classification (WB-MUSIC) and the Time-Domain DORT (TD-DORT) algorithms. Both techniques utilize wideband transmitted ultrasound signals, and can be applied using monochromatic signals or time-pulse. The capability of these techniques in providing improved target localization and efficient sensor deployment in low signal-to-noise ratio environments is demonstrated.