Which artifact is caused by slower speed of sound in tissue, causing reflectors to appear deeper than they are?

• A. Propagation Speed Error
• B. Ring Down
• C. Comet Tail
• D. Side Lobes

Answer: (A)

When tissue slows down the speed of sound, the time it takes for an ultrasound pulse to travel to a reflector and back increases. The machine translates that round-trip time into depth using an assumed propagation speed (often around 1540 m/s for soft tissue). If the actual speed is slower than the assumed speed, the same time corresponds to a greater distance, so reflectors are displayed deeper than they truly are. This mismatched speed leading to depth distortion is called propagation speed error.

The other artifacts have different causes: ring down arises from resonances of gas bubbles causing multiple short echoes; comet tail results from reverberations near a strong reflector; side lobes are secondary beams off to the side that generate additional echoes.