Which ultrasound modality uses the range equation to determine the depth of origin of reflected echoes?

• A. Pulsed wave ultrasound
• B. Continuous wave ultrasound
• C. Doppler ultrasound
• D. A-mode ultrasound

Answer: (A)

The key idea is that depth information comes from time-of-flight: how long it takes a transmitted pulse to travel to a structure and back as an echo. Using the speed of sound in tissue, the depth is calculated from that round-trip time with the range equation (depth ≈ speed × time / 2). To get this timing, you need a pulsed source that emits a short pulse and then listens for echoes. That capability is intrinsic to pulsed wave ultrasound, which records echoes after each pulse and can map their depths to form the image.

Continuous wave ultrasound keeps transmitting continuously, so there isn’t a discrete emission and corresponding start time to anchor echoes in time, making depth localization unfixed. Doppler focuses on frequency shifts to measure motion or flow rather than depth. A-mode does provide depth information along a single line, but the modality most commonly associated with acquiring depth through the time-of-flight for imaging is pulsed wave ultrasound.