The y-axis of an M-mode display represents which quantity, derived from the time-of-flight information?

• A. Time
• B. Reflector depth
• C. Amplitude
• D. Color

Answer: (B)

In M-mode imaging, the vertical axis reflects depth. This depth comes from the time it takes for the ultrasound pulse to travel to a reflector and back (time-of-flight). Because the speed of sound in tissue is treated as nearly constant, a longer round-trip time means the reflector is deeper, and depth is computed from that time-of-flight using depth ≈ (speed of sound × time-of-flight) / 2. The horizontal axis represents the passage of time, showing how the reflector’s position changes as the heart (or other structure) moves. Amplitude is shown by the brightness of the echo at a given depth and time, not by the axis, and color is not used in standard M-mode.