The Doppler angle of incidence is the angle between the Doppler beam and what?

• A. The vessel wall
• B. The direction of blood flow
• C. The transducer surface
• D. The color map

Answer: (B)

The Doppler angle of incidence is defined as the angle between the Doppler beam and the direction of blood flow. This matters because the Doppler frequency shift depends on the cosine of that angle; the closer the beam is to flowing along the blood direction, the larger the measurable shift and the more accurate the velocity estimate. If the beam is perpendicular to flow, the cosine term becomes zero and no Doppler shift is detected, making velocity measurement impossible. The angle is not measured relative to the vessel wall, the transducer surface, or the color map—it's specifically between the beam path and the direction the blood is moving.