Dropout color artifact occurs at what Doppler angle?

• A. 0 degrees
• B. 90 degrees
• C. 180 degrees
• D. 45 degrees

Answer: (B)

Dropout color artifact happens when there is no Doppler shift detected because the flow is perpendicular to the ultrasound beam. Doppler shift depends on the component of velocity along the beam, described by Doppler shift ∝ cos theta. At a 90-degree angle, cos 90° = 0, so the shift is zero even if there is flow, and the color signal drops out. In practice, aligning the beam with the flow (0° or 180°) yields the strongest Doppler signal, while oblique angles like 45° produce reduced shift but not a true dropout. This is why a 90-degree angle is the scenario that leads to color dropout.