Phase cancellation: a cause of acoustical shadowing at the edges of curved surfaces in B-mode ultrasound images.

Acoustical shadowing occurring at the edges of curved objects is one of the most frequently observed artifacts in ultrasound imaging. This artifact has been generally ascribed to refraction and reflection effects at the boundary between the curved object and the surrounding tissues. However, the shadowing that would be produced by pure refraction and reflection may not correspond in all circumstances to what is most often seen clinically, i.e., a sharp, discrete shadow projecting down from the edge. We used a tissue-mimicking contrast detail phantom, speed of sound (SOS) 1477 m/s, containing cylindrically shaped wells to investigate the origin of these shadows. Using solutions of relatively high SOS (20% ethylene glycol), approximately equivalent SOS (distilled water), and low SOS (70% isopropyl alcohol), the phantom was scanned with the scanhead face oriented perpendicular to and parallel to the central axes of the cylinders. Shadowing could be produced in both cases when there was a SOS difference between the contents of the cylinders and the phantom. When scanning perpendicular to the cylinders, refraction and reflection effects could have contributed to any shadowing produced, but when the scan planes were oriented parallel to the central axes of the cylinders, neither refraction nor reflection could be occurring to a significant degree. The shadowing produced in these circumstances could be better explained by a phenomenon well known in transmission ultrasonography called phase cancellation. Phase cancellation would produce shadowing independent of scan plane orientation, and could contribute to the shadowing generated in clinical imaging.