In ultrasound imaging, which parameter primarily determines depth calibration and time-of-flight calculations in soft tissue?

• A. Acoustic impedance.
• B. Speed of sound in tissue (c).
• C. Attenuation coefficient.
• D. Reflection coefficient.

Answer: (B)

Depth calibration and time-of-flight calculations hinge on how fast sound travels through tissue. The echo’s round-trip time t relates to depth by the equation depth = (c × t) / 2, since the pulse must go to the reflector and back. Therefore, the speed of sound in soft tissue, c (about 1540 m/s in typical models), directly sets how time measurements convert to physical depth. If c is assumed incorrectly, the depth mapping will be distorted.

Other factors influence signal strength and boundary behavior but don’t set the time-to-depth relationship. Acoustic impedance affects how much energy reflects at interfaces, the attenuation coefficient describes how rapidly signal amplitude decays with distance, and the reflection coefficient depends on impedance mismatch. These determine echo amplitudes and boundary reflections, not the fundamental conversion from travel time to depth.