Doppler velocity measurements are most reliable when the Doppler angle is within which range?

• A. 0 to 60 degrees
• B. 60 to 90 degrees
• C. 90 to 120 degrees
• D. 120 to 180 degrees

Answer: (A)

Doppler velocity depends on the cosine of the angle between the ultrasound beam and the direction of flow. When the angle is small, cos θ is large (near 1), so velocity estimates are less sensitive to small errors in the angle. As the angle grows toward 90 degrees, cos θ shrinks toward zero, which amplifies any angle error and makes the velocity calculation highly unstable or even undefined (no Doppler shift at 90 degrees). Angles well past 90 degrees also reduce the measurable component of flow along the beam and are not reliable for velocity measurement. For these reasons, keeping the Doppler angle in a small range where cos θ remains large—typically 0 to 60 degrees—yields the most reliable velocity measurements.