Improving ultrasound images with elevational angular compounding based on acoustic refraction

Abstract

Ultrasound imaging is affected by coherent noise or speckle, which reduces contrast and overall image quality and degrades the diagnostic precision of the collected images. Elevational angular compounding (EAC) is an attractive means of addressing this limitation, since it reduces speckle noise while operating in real-time. However, current EAC implementations rely on mechanically rotating a one-dimensional (1D) transducer array or electronically beam steering of two-dimensional (2D) arrays to provide different elevational imaging angles, which increases the size and cost of the systems. Here we present a novel EAC implementation based on a 1D array, which does not necessitate mechanically rotating the transducer. The proposed refraction-based elevational angular compounding technique (REACT) instead utilizes a translating cylindrical acoustic lens that steers the ultrasound beam along the elevational direction. Applying REACT to investigate phantoms and excised tissue samples demonstrated superior suppression of ultrasound speckle noise compared to previous EAC methods, with up to a two-fold improvement in signal- and contrast-to-noise ratios. The effects of elevational angular width on speckle reduction was further investigated to determine the appropriate conditions for applying EAC. This study introduces acoustic refractive elements as potential low cost solutions to noise reduction, which could be integrated into current medical ultrasound devices.

Speckle reduction in ultrasound endoscopy using refraction based elevational angular compounding

“…To acquire the same number of images for compounding using different lenses, the translation step (δ) for each acoustic lens was selected by dividing the length of each lens ( l ) by the number of acquired images (N = 100; δ: 200 µm, 200 µm, 150 µm, 50 µm, 24 µm, for the 0°, 2.5°, 5°, 15°, and 30° acoustic lenses, respectively). The effective deflection angle was calculated using Snell’s law as c 1 sinØ 2 = c 2 sinØ 1 (where c 1 and c 2 are the longitudinal wave velocities, and Ø 1 and Ø 2 are incidence and exit angles in materials 1 and 2, respectively) 21 , 22 and confirmed experimentally in a similar manner to that reported in our previous study 18 . (“See supplementary Fig.…”

“…To minimize speckle artefacts, compounding methods are preferable to image processing techniques because they yield both high quality images and reveal fine structures obscured by speckle noise, which are otherwise irretrievable using the aforementioned techniques 15 , 16 . EAC is a preferred spatial compounding technique with both high despeckling efficiency and good temporal resolution, which makes it favorable for real time imaging 18 . However, tight anatomical constraints of hollow organs prevent implementation of EAC in EUS using 2D or tilting 1D transducer arrays.…”

“…In contrast, elevational angular compounding (EAC), which relies on capturing partially correlated images by steering the imaging plane with small angular steps in the elevational direction (perpendicular to the imaging plane) 16 , is ideal for radial EUS because its geometry is suitable for capturing sequential frames in a radial configuration. In addition, EAC allows imaging of the same region of interest in all sequential frames, therefore eliminating the need for spatial alignment, which is desirable for real time imaging 16 – 18 . However, no EAC implementations for EUS have yet been introduced, likely due to spatial and cost constraints.…”

“…Our group recently introduced a refraction-based elevational angular compounding technique (REACT), wherein a customized refractive element imparts a fixed linear array with elevational angular steering capabilities 18 . REACT demonstrated more efficient ultrasound despeckling compared to the previous EAC implementations, primarily because the fixed transducer array minimized motion artefacts.…”

“…REACT demonstrated more efficient ultrasound despeckling compared to the previous EAC implementations, primarily because the fixed transducer array minimized motion artefacts. However, the refractive element of the REACT prototype was designed for linear arrays, and was therefore not suitable for use in radial ultrasound endoscopy 18 .…”