How do impedance and speed of sound in tissue influence time-of-flight calculations used for depth measurements in ultrasound?

• A. Time-of-flight is independent of tissue speed; use 1540 m/s as fixed; no calibration.
• B. Time-of-flight uses a standard speed of sound around 1540 m/s; variations alter calculated depths; calibration used.
• C. Time-of-flight assumes speed of sound varies per pixel.
• D. Time-of-flight uses temperature only.

Answer: (B)

Understanding how depth is calculated in ultrasound relies on how fast sound travels through tissue. Time-of-flight measures the round-trip time for a pulse to reach a boundary and return, and depth is inferred from how long that travel took using the relationship depth ≈ (speed of sound × time) / 2. In soft tissue, a practical standard speed around 1540 m/s is used because tissue speeds vary and the system needs a consistent reference. If the actual speed in the path differs from this assumed value, the depth derived from the same travel time shifts: faster actual speeds make the calculated depth shallower, slower speeds make it deeper. To keep measurements accurate, calibration with known-depth phantoms or patient-specific adjustments help align the assumed speed with reality.

Impedance, which describes how strongly a tissue interface reflects ultrasound, mainly affects echo brightness and boundary visibility rather than the travel time itself. It determines how strong the echo is at a boundary, while the depth calculation depends on speed of sound and the measured time. So you use the standard speed, apply time-of-flight, and rely on calibration to account for tissue speed variations; impedance shapes image contrast but not the basic time-to-depth conversion.