The x-axis of an A-mode display represents _______ which is derived from the _______ of the sound pulse.

• A. Echo depth; Pulse duration
• B. Reflector depth; Time-of-flight information
• C. Amplitude; Echo duration
• D. Depth of reflectors; Speed of sound

Answer: (B)

In A-mode ultrasound, the horizontal axis represents how deep the reflecting interfaces are. That depth is calculated from the time it takes for the sound pulse to travel to a reflector and back—the time-of-flight. Since the pulse must cover twice the distance to reach and return from the reflector, depth is determined by the round-trip time using the known speed of sound in tissue (depth ≈ speed × time / 2). So the x-axis shows reflector depth, derived from the time-of-flight information of the sound pulse. The other ideas aren’t the primary way the axis is defined: amplitude shows on the vertical axis, and pulse duration or echo duration aren’t used to place features along the x-axis.