If the go-return time is 39 µs, the reflector depth is approximately ______.

If the go-return time is 39 µs, the reflector depth is approximately ______.

• A. 1 cm
• B. 4 cm
• C. 3 cm
• D. 2 cm

Answer: (C)

Go-return time is the round-trip time for a sound pulse to travel to a reflector and back. The depth is found from depth = (speed of sound in tissue × go-return time) / 2. Using a typical tissue speed of about 1540 m/s and t = 39 µs (39 × 10^-6 s), depth ≈ (1540 × 39e-6) / 2 ≈ 0.030 m, which is about 3 cm. So the reflector is approximately 3 cm deep. For reference, a shallower reflector would produce a shorter go-return time (about 12–13 µs for 1–1.3 cm) and a deeper one would take around 52 µs for 4 cm, placing 39 µs squarely near 3 cm.

Go-return time is the round-trip time for a sound pulse to travel to a reflector and back. The depth is found from depth = (speed of sound in tissue × go-return time) / 2. Using a typical tissue speed of about 1540 m/s and t = 39 µs (39 × 10^-6 s), depth ≈ (1540 × 39e-6) / 2 ≈ 0.030 m, which is about 3 cm. So the reflector is approximately 3 cm deep.

For reference, a shallower reflector would produce a shorter go-return time (about 12–13 µs for 1–1.3 cm) and a deeper one would take around 52 µs for 4 cm, placing 39 µs squarely near 3 cm.