Which ultrasound mode maps Doppler frequency shifts as colors?

Color Doppler is the ultrasound mode that maps Doppler frequency shifts as colors.

In medical ultrasonography, moving blood cells alter the frequency of returning sound waves. The system analyzes that shift and assigns colors to represent flow direction and relative velocity, commonly displaying flow toward and away from the probe in contrasting hues. Color Doppler is usually overlaid on a grayscale anatomical image.

B-mode, or brightness mode, creates the familiar two-dimensional structural ultrasound image. M-mode records motion along a single scan line, making it useful for timing moving cardiac structures. A-mode displays echo amplitude against depth and is much less common in routine diagnostic imaging. None of those modes, by themselves, is the color-coded flow display described in the question.

Color Doppler is not a photograph of blood and its colors are not inherently “red artery” and “blue vein.” The palette indicates motion relative to the transducer, and the displayed velocity can be affected by the insonation angle. Spectral Doppler is a neighboring technique: instead of painting a region with colors, it plots measured velocities against time, often allowing more precise velocity estimates.