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Color flow Doppler is used in combination with B‐mode ultrasonography. It allows you to see blood flow within a vessel and helps to determine the direction of that flow. Doppler is best when the flow is parallel with the sound beam. Color signatures are usually set up so that flow toward the probe is red – remember by “you are getting warmer” – and flow away from the probe is blue – remember by “you are getting colder” or that “blue” and “away” have the same number of letters – although this can be set on most machines to user preference. Color flow Doppler has its limitations, with low velocities and is also affected by patient movement, a phenomenon called “jumping” (see Chapter 25).

To maximize accurate image acquisition and proper interpretation, the sonographer should be familiar with settings that affect color flow Doppler (Pozniak et al. 1992). Briefly, the Big 3 are gain, pulse repetition frequency, and wall filter setting, and their effects are as follows.

 Gain: the overall sensitivity to flow signals.

 Pulse repetition frequency (also known as PRF, scale, or velocity scale): user‐defined setting. Use a lower PRF with low velocities; use a higher PRF with higher velocities to correct aliasing. If too high a PRF is selected, low velocities may not be recorded, and a vessel may appear as if thrombosed (no flow); conversely, too low a PRF will lead to overlap of the signal on the image.

 Wall filter: removes unwanted low‐velocity Doppler signals; however, if set too high, it will remove important low‐velocity signals.

For more information, the reader is referred to more detailed texts (Nyland et al. 2002; Evans et al. 1989) and the machine's technical support team.

An alternate form of color flow Doppler, called power Doppler imaging (PDI), can be employed. Similar to color flow, this shows flow of fluid but at much lower velocities. The trade‐off is a lack of directionality. Blood flowing at 0.5 cm/sec away from the probe will have the same color signature as blood flowing at 0.5 cm/sec toward the probe. Consider using PDI for small vessels in POCUS studies such as the optic artery and vein

Pearl: Color signatures are usually set up so that flow toward the probe is red and flow away from the probe is blue – remember “red is getting warmer and toward you” and “blue is away from you and getting colder” or that “away” and “blue” have the same number of letters.

Pearl: Color flow Doppler is optimized by understanding its settings and their effect on the image: gain, pulse repetition frequency, and wall filter setting.