In M-mode, the x-axis represents what?

• A. Time
• B. Depth
• C. Amplitude
• D. Nothing

Answer: (A)

In M-mode, the display is a time-motion trace along a single ultrasound line. The horizontal axis represents time, letting you see how structures along the beam move as the heart goes through its cycle. Depth is shown on the vertical axis, indicating how far away each reflecting interface lies from the transducer at each moment. The brightness along the trace represents echo strength, not a separate axis, so amplitude is conveyed by brightness rather than position on the axis. This setup makes it easy to track movement and timing of cardiac structures over time, such as valve motion or wall movement, which is why time on the horizontal axis is the correct interpretation. Depth belongs on the vertical axis, and amplitude is shown by brightness.