If you make a mistake in the measurement of the Doppler angle, the error in velocity estimation will be greatest at which Doppler angle?

If you make a mistake in the measurement of the Doppler angle, the error in velocity estimation will be greatest at which Doppler angle?

• A. 0 degrees
• B. 30 degrees
• C. 45 degrees
• D. 70 degrees

Answer: (D)

The key idea is that Doppler velocity calculation relies on the angle between the blood flow and the ultrasound beam. The frequency shift is proportional to v cos θ, so solving for velocity gives a value that depends on 1/cos θ. When the angle is off by a small amount, the resulting velocity error grows with tan θ. In other words, dv ≈ v tan θ × (angle error). So the larger the Doppler angle, the more sensitive the velocity estimate is to angle misestimation. Among the options, the largest angle is 70 degrees, which means the velocity calculation is most affected by any mistake in that angle. At nearer-perpendicular angles, even a small error can drastically change the velocity value, whereas at smaller angles the impact is smaller.

The key idea is that Doppler velocity calculation relies on the angle between the blood flow and the ultrasound beam. The frequency shift is proportional to v cos θ, so solving for velocity gives a value that depends on 1/cos θ. When the angle is off by a small amount, the resulting velocity error grows with tan θ. In other words, dv ≈ v tan θ × (angle error). So the larger the Doppler angle, the more sensitive the velocity estimate is to angle misestimation. Among the options, the largest angle is 70 degrees, which means the velocity calculation is most affected by any mistake in that angle. At nearer-perpendicular angles, even a small error can drastically change the velocity value, whereas at smaller angles the impact is smaller.