Which expression correctly defines the depth in the ultrasound range equation?

Which expression correctly defines the depth in the ultrasound range equation?

• A. Depth = speed of sound in tissue x round trip travel time
• B. Depth = 1/2 x speed of sound in tissue x round trip travel time
• C. Depth = 1/2 x speed of sound in tissue x one-way travel time
• D. Depth = speed of sound in tissue x round trip travel time x 2

Answer: (B)

The depth is determined by how far the sound travels in tissue, but the time you measure is for a round trip—there and back. So you multiply the speed of sound in tissue by the round-trip travel time to get the total distance the pulse would cover in that time, and then you divide by two to get the depth to the reflector (one-way distance). That gives depth = (speed of sound in tissue × round-trip travel time) / 2, which is the same as 1/2 × speed × round-trip time. This accounts for the round-trip nature of the measurement, turning the total travel time into the single-way depth. Using the one-way travel time would be inconsistent with how the instrument records time (it records round-trip time), and multiplying by 2 would overestimate depth.

The depth is determined by how far the sound travels in tissue, but the time you measure is for a round trip—there and back. So you multiply the speed of sound in tissue by the round-trip travel time to get the total distance the pulse would cover in that time, and then you divide by two to get the depth to the reflector (one-way distance). That gives depth = (speed of sound in tissue × round-trip travel time) / 2, which is the same as 1/2 × speed × round-trip time.

This accounts for the round-trip nature of the measurement, turning the total travel time into the single-way depth. Using the one-way travel time would be inconsistent with how the instrument records time (it records round-trip time), and multiplying by 2 would overestimate depth.