A Circular Total Focusing Method with Eccentricity Correction and Intensity Compensation for Endoscopic Ultrasound Imaging of Dual-layered Media.

Abstract

Present endoscopic ultrasound (EUS) imaging methods for circular array suffer from the non-uniform spatial resolution in the imaging of a dual-layered media, such as the tubes' immersion EUS inspection. The problem is mainly attributed to the restricted focus and beam de-focusing at the interface. In this paper, a circular total focusing method (CTFM) is proposed, which leverages the concept of the conventional total focusing method and makes three vital improvements to overcome the challenges. First, to obtain the accurate time-of-flight (TOF) in the dual-layered media, a fourth-order equation of Snell's law is built and solved in polar coordinate system. Second, a fast geometric approximation method is derived to correct the TOF distortion caused by the transducer's eccentricity. Third, the intensity compensation is applied to flatten the imaging intensity at different positions by considering the directivity of element, transmission at interface, and divergence in media. The CTFM is validated on a tube's immersion EUS using a 10 MHz circular array with 128 elements. Experiment results demonstrate that the proposed CTFM outperforms existing imaging methods. The lateral and axial resolution are 0.71 mm and 0.30 mm, which are 27.5% and 33.3% higher than those of the classic delay-and-sum method. The CTFM image shows high and uniform SNR which is 33.6% higher than that of delay-and-sum images. The CTFM provides a novel EUS imaging modality which can be applied in both medical and non-destructive testing domains.