What is refraction in ultrasound and how can it create artifacts?

• A. Change in propagation direction at a boundary due to speed mismatch; can cause misplacement of reflectors.
• B. Shadowing due to attenuation.
• C. Speckle due to interference.
• D. Mirror image artifact.

Answer: (A)

Refraction in ultrasound is the bending of the sound wave as it crosses an interface where the speed of sound is different, and the incidence is not perpendicular. The changing speeds on either side of the boundary cause the transmitted ray to change direction according to Snell’s law. Because the imaging system assumes the wave travels in straight lines and uses travel time to place reflectors, this bent path makes a reflector appear in a different place than it truly is, often shifting it laterally or distorting its position.

This is why refraction creates artifacts: the display maps time into position along a straight-line path, while the actual beam traveled along a curved path. The effect grows with larger speed differences, oblique incidence, and particular boundary geometry. Other listed phenomena—shadowing from attenuation, speckle from interference, and mirror-image artifacts from strong reflectors—are caused by different processes and not by refraction.