The brightness of the shade of gray that color Doppler is allowed to write over is adjusted by which control?

• A. Priority control
• B. Gain
• C. PRF
• D. Threshold

Answer: (D)

Color Doppler overlays color onto a grayscale image using a velocity gate. The threshold is the control that sets this gate: it defines the minimum Doppler velocity that will be colored. Signals below the threshold stay gray, while signals above it are colored, effectively changing how much of the grayscale image is overwritten by color. By adjusting the threshold, you control how much of the gray background remains visible versus how much of it gets replaced by color. A higher threshold restricts coloring to faster flows, leaving more gray, while a lower threshold allows more slow flow to be colored. Gain changes overall color intensity but does not determine which gray areas get overwritten, PRF sets the velocity range and aliasing characteristics, and priority affects overlay order rather than grayscale brightness.