A bending of the sound beam that results in lateral misregistration of structures posterior to the refracted beam.

• A. Refraction
• B. Diffraction
• C. Reflection
• D. Attenuation

Answer: (A)

Refraction is the bending of the ultrasound beam as it passes from one tissue into another with a different propagation speed. When the beam meets the boundary at an oblique angle, its path changes according to Snell’s law, so the transmitted beam travels in a new direction rather than continuing straight. This change in direction means echoes from deeper structures follow a different route, causing those structures to appear shifted sideways relative to their true position—posterior to the refracted beam you get lateral misregistration. The amount and direction of bending depend on the incidence angle and the speeds of sound in the two media: if the second medium is slower, the beam bends toward the normal; if faster, it bends away from the normal. This effect is distinct from diffraction, reflection, and attenuation.