Which ultrasound display shows amplitude as a function of depth in one dimension (often used in ophthalmology)?

• A. Time vs depth
• B. Lateral position vs amplitude
• C. Amplitude vs depth
• D. Depth vs amplitude

Answer: (C)

This is an A-mode display. In A-mode you plot the amplitude of each echo as a function of depth along a single line, so you get a one-dimensional depth profile with spikes whose heights represent the echo strength. The depth axis comes from the round-trip travel time of the pulse, using the known speed of sound in tissue. This simple amplitude-versus-depth plot is exactly what ophthalmology uses to measure distances inside the eye, since only depth information along one axis is needed. The other modes produce two-dimensional images (B-mode) or track motion over time (M-mode), which is why they don’t fit the description as well.