In Doppler imaging, the angle of the beam to the vessel is approximately:

• A. 0 degrees
• B. 60 degrees
• C. 90 degrees
• D. 45 degrees

Answer: (C)

Doppler shifts depend on the angle between the ultrasound beam and the direction of blood flow. The observed Doppler frequency shift is proportional to the cosine of that angle. When the beam is aligned with the flow (a small angle), cosine is close to 1, giving the largest, most reliable shift and the best velocity information. As the angle increases, the cosine decreases, so the Doppler shift gets smaller and velocity estimates become less accurate. At a right angle to the flow (90 degrees), the cosine is zero, so there is effectively no Doppler shift and flow cannot be measured. That’s why, in Doppler imaging, we aim to keep the beam as parallel to the vessel as possible. In practice, clinicians work with angles roughly within 0 to 60 degrees, with 30–45 degrees often being a convenient compromise between signal strength and ease of alignment.