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Doppler Cosine Angle Insonation — FRCA Primary MCQ

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HardPhysics & Clinical MeasurementDoppler Cosine Angle InsonationFRCA Primary

Blood flows at 80 cm s−1, but a Doppler system calculates velocity as though the insonation angle were 0° when the actual angle is 60°. What velocity will it display approximately?

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Correct answer: B — 40 cm s−1

The best answer is “40 cm s−1”. The detected Doppler shift is proportional to velocity multiplied by cos theta. At 60 degrees, cos theta is 0.5, so interpreting the shift as if theta were zero displays about 80×0.5=40 cm s−1. Angle error becomes increasingly important as the beam approaches perpendicular to flow.

Reference: Magee, Essential notes on the physics of Doppler ultrasound: https://pmc.ncbi.nlm.nih.gov/articles/PMC7807840/