Doppler angle of incidence is defined as the angle between which two things?

• A. The Doppler beam and the vessel wall
• B. The Doppler beam and the blood flow
• C. The transducer and the patient’s skin
• D. The color map and velocity scale

Answer: (B)

In Doppler ultrasound, the angle of incidence is the angle between the ultrasound Doppler beam and the direction of blood flow. This angle matters because the Doppler shift used to calculate velocity depends on the cosine of that angle; aligning the beam with the flow minimizes measurement error and yields the most accurate speed estimate. So the rate at which the frequency of the returned signal changes is tied to how closely the beam follows the actual flow direction. The angle between the beam and the vessel wall isn’t what sets the velocity measurement, and the angle between the transducer and the patient’s skin is about probe placement rather than flow direction. The color map and velocity scale are display parameters, not geometric factors in the Doppler calculation.