Which statement best describes how M-mode samples data in the body?

• A. It analyzes color flow velocity
• B. It samples along a single penetration and records depth changes over time
• C. It measures temperature variations
• D. It samples multiple depths to form a 3D image

Answer: (B)

M-mode ultrasound captures motion along a single line by sampling echoes along one penetration and recording how structures along that line move in depth over time. The display shows depth on one axis and time on the other, producing a time–depth trace that reveals rapid motion, such as heart wall movement and valve opening/closing, with very high temporal resolution. This is why the statement is best: it describes sampling along one line and tracking depth changes as time passes. It isn’t about color flow velocity (that’s Doppler), it doesn’t measure temperature variations, and it doesn’t create a 3D image from multiple depths (that would involve 3D imaging techniques).