The x-axis of an M-mode display represents which quantity?

• A. Depth
• B. Time
• C. Velocity
• D. Amplitude

Answer: (B)

In M-mode imaging, you’re watching motion along a single line over time, so the display is built to track how structures move as the heart beats. The depth from the transducer forms the vertical axis, while time progresses along the horizontal axis. This layout lets you see how far a structure moves and when—for example, how a ventricular wall shifts during the cardiac cycle or when a valve opens and closes—by watching the trace unfold from left to right as time advances. Velocity is not shown as an axis; it’s inferred from how fast the structures move across time, and amplitude is represented by the brightness of the echoes along the depth line, not on the axis itself.