Underestimating the Doppler angle of incidence will result in which error?

• A. The frequency shift will be underestimated
• B. No Doppler frequency shift will be detected
• C. Mirror imaging of the Doppler spectrum will occur
• D. The velocity estimation will be inaccurate

Answer: (D)

The key idea is that the Doppler shift is tied to the angle between the flow and the ultrasound beam through a cosine factor, and velocity is calculated from that shift using the angle. The measured Doppler frequency shift depends on velocity and cos(theta). When you estimate the angle as smaller than it actually is, you effectively use a larger cos(theta) in the velocity calculation. Since velocity is inversely proportional to cos(theta) in the standard equation, the result is a velocity value that is too small. So the error you see is in the velocity estimate becoming inaccurate (usually underestimated). The Doppler shift you measure itself isn’t erased by this angle misestimation, and the other artifacts aren’t caused simply by underestimating the angle.