Reflectors appear in the correct number but at an incorrect depth; slower speeds place them too deep, displacing structures axially. Which artifact is this?

• A. Propagation Speed Error
• B. Ring Down
• C. Side Lobes
• D. Focal Banding

Answer: (A)

Depth accuracy in ultrasound comes from converting the round-trip travel time into distance using the assumed speed of sound. If the tissue’s actual speed is slower than what the system assumes, the echoes take longer to return, but the machine still uses the faster assumed speed to translate that time into depth. The result is that reflectors appear deeper than they really are, while the number of reflectors remains the same because the echoes occur at the correct times—only their axial positions are shifted. This mismatch is known as propagation speed error.

Other artifacts have different causes: ring down stems from rapid, successive reflections within a small highly reflective object like a gas bubble; side lobes produce off-axis echoes that can appear in unintended locations; focal banding relates to beam focusing and attenuation effects near the focal zone.