Which angles require no angle correction in Doppler?

• A. 0 and 180 degrees
• B. 90 degrees
• C. 45 degrees
• D. 135 degrees

Answer: (A)

In Doppler velocity calculations, the measured shift is tied to the cosine of the angle between the ultrasound beam and the direction of blood flow. If the beam is exactly along the flow (angle is 0 degrees) or directly opposite the flow (angle is 180 degrees), cos theta equals ±1, so the velocity factor is at its maximum and no additional angle correction is needed to determine speed—the Doppler shift directly corresponds to velocity (with direction indicated by the sign). At 90 degrees, cos theta is zero, giving no Doppler signal; at intermediate angles like 45 or 135 degrees, you must apply the cosine correction to recover true velocity. Therefore, the angles that require no angle correction are 0 and 180 degrees.