Which factor most directly affects depth calculation from time-of-flight measurements in ultrasound?

• A. Attenuation
• B. Impedance mismatch
• C. Anisotropy of tissue
• D. Speed of sound in tissue

Answer: (D)

The speed of sound in tissue determines how far the pulse travels in a given time, which directly links time-of-flight to depth. In ultrasound, the instrument measures the round-trip time for a pulse to reach a boundary and come back. This time t relates to depth d by t = 2d / v, where v is the speed of sound along the path. Rearranging gives d = v × t / 2. So the depth you compute depends on knowing the speed of sound in the tissue along that path. If you assume a standard soft-tissue speed (about 1540 m/s) but the actual path contains faster or slower tissue, the calculated depth will be off.

Attenuation, impedance mismatch, and anisotropy affect signal quality, boundary visibility, or slight speed variations, but they do not directly set the depth from time-of-flight. The core factor for depth calculation is the speed of sound in tissue.