A 2-D Circular Array Transducer for Endoscopic Ultrasound Imaging of Tube

The concept of endoscopic ultrasound (EUS) has been introduced to nondestructive testing (NDT) for the inspection of tubular structure. However, the fixed normal focusing beam of the present EUS transducers obstructs imaging flexibility and limits the robustness of the inspection, particularly for planar reflectors. To address this challenge, a 2-D circular array (2-D CA) designed for 3-D focusing is developed in this article. A prototype 2-D CA transducer is fabricated and validated, which is demonstrated as a central frequency of 10.10 MHz and an average pulse duration of no more than 344 ns. An independent-dual-focusing (IDF) beamforming scheme is further proposed, providing delay laws in two orthogonal directions. The acoustic field simulation results confirm that the 2-D CA is capable of achieving focusing in any direction, thereby enhancing the flexibility of EUS imaging. The imaging performance of the 2-D CA is evaluated through the immersion EUS inspection of a stainless-steel tube specimen. All the quasiplanar reflectors, including ring grooves with narrow-width, small-diameter flat bottom holes (FBHs), and longitudinal grooves, are successfully detected in the 2-D CA imaging results. These reflectors could hardly be recognized by the conventional CA with a fixed-normal beam, affirming the superior detection robustness of the 2-D CA. The detection signal-to-noise ratio (SNR) and error of quantitative characterization of the 2-D CA are 26.12 dB higher and 40.37% lower than those of conventional CA, respectively. The proposed 2-D CA enables novel and advanced EUS imaging modalities, which have potential applications in both medical imaging and NDT domains.