If the round-trip time doubles, the reflector distance changes by what factor?

• A. The distance remains the same
• B. The distance increases by four
• C. The distance doubles
• D. The distance decreases by half

Answer: (C)

The key idea is that the round-trip time is directly proportional to how far the sound travels. In ultrasound, the pulse goes to a reflector and back, so the round-trip distance is 2 times the reflector distance, and the time taken is T = (2D) / c, where c is the speed of sound in the tissue.

If the round-trip time doubles, then (2D) must double while c stays the same, which means D must double as well. So the reflector distance increases by a factor of two.

The other possibilities don’t fit because they would require different changes in time: remaining the same would need the time unchanged, quadrupling the distance would need quadrupling the time, and halving the distance would require halving the time.