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Doppler angle error — FRCA Primary MCQ

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HardPhysics and Clinical MeasurementDoppler angle errorFRCA Primary

During Doppler assessment of blood flow, the measured velocity is much lower when the beam is nearly perpendicular to the vessel. What is the most likely explanation?

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Correct answer: C — Doppler shift depends on the cosine of the insonation angle

The Doppler shift is proportional to the cosine of the angle between ultrasound beam and blood flow; at 90 degrees the cosine approaches zero. The speed of sound in blood does not explain this directional error. Red cells continue to scatter ultrasound, but the measured frequency shift depends on motion along the beam. Aligning the beam more parallel to flow improves velocity estimation.

Reference: Primary FRCA physics texts; Dorsch and Dorsch Understanding Anesthesia Equipment