Which proximal renal artery angle is correct if the artery runs parallel to the vessel and is associated with a lower peak systolic velocity reading?

• A. 60 degrees
• B. 22 degrees
• C. 45 degrees
• D. 0 degrees

Answer: (C)

Doppler velocity readings depend on the angle between the ultrasound beam and the direction of blood flow. The measured velocity is the true velocity multiplied by cos(theta). So as the insonation angle increases from 0 toward 90 degrees, the reported peak systolic velocity decreases.

If the proximal renal artery runs parallel to the vessel, achieving a very small insonation angle would tend to push the reading higher (cosines near 1). To obtain a lower, yet still reliable, PSV without venturing to the limits of accuracy, an angle around 45 degrees is used. Cosine of 45 degrees is about 0.707, which provides a meaningful reduction in the measured velocity while staying within a common, dependable range for Doppler measurements. Angles like 22 degrees reduce the effect only slightly, and 0 degrees would maximize the reading; 60 degrees can further reduce the velocity but increases angle-correction error and can compromise accuracy. Therefore, 45 degrees is the best balance in this scenario.