When a reflector is 3 cm deep, a pulses time-of-flight is _____.

When a reflector is 3 cm deep, a pulses time-of-flight is _____.

• A. 52 µs
• B. 26 µs
• C. 78 µs
• D. 39 µs

Answer: (D)

Time-of-flight is the round-trip travel time of the pulse—from the transducer to the reflector and back. So it uses twice the depth divided by the speed of sound in tissue. Depth is 3 cm, which is 0.03 m. Round-trip distance is 0.06 m. Using the typical speed of sound in soft tissue, about 1540 m/s, the time is 0.06 / 1540 ≈ 3.9 × 10^-5 seconds, which is 39 microseconds. So the pulse takes roughly 39 µs to travel to the reflector and return. (One-way time would be about 19.5 µs, but time-of-flight is the round-trip.)

Time-of-flight is the round-trip travel time of the pulse—from the transducer to the reflector and back. So it uses twice the depth divided by the speed of sound in tissue.

Depth is 3 cm, which is 0.03 m. Round-trip distance is 0.06 m. Using the typical speed of sound in soft tissue, about 1540 m/s, the time is 0.06 / 1540 ≈ 3.9 × 10^-5 seconds, which is 39 microseconds.

So the pulse takes roughly 39 µs to travel to the reflector and return. (One-way time would be about 19.5 µs, but time-of-flight is the round-trip.)