The x-axis in B-mode ultrasound represents which quantity?

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

Answer: (C)

In B-mode imaging, echoes from each depth along the beam are mapped along the horizontal axis by converting the round-trip travel time into distance from the transducer. Faster echoes from shallower depths plot closer to the transducer, while deeper echoes plot farther along the axis. The brightness at each point represents the strength of the echo from that location. So, the quantity represented on the x-axis is the depth of reflectors. Time is used to determine that depth, but the axis itself reflects distance, not time. Velocity and amplitude are not the axis here—the brightness encodes amplitude.