In Doppler ultrasound, the Doppler frequency shift equation is: Δf = (2 × f0 × v × cosθ) / c. To maximize the frequency shift and thereby improve sensitivity for low-velocity flow detection, the angle θ between the ultrasound beam and the vessel should be:

• A 0 degrees (parallel to flow) ✓
• B 90 degrees (perpendicular to flow)
• C 45–60 degrees (oblique to flow)
• D Greater than 70 degrees to avoid aliasing

Explanation

The Doppler shift is maximal when cosθ = 1 (i.e., θ = 0°, beam parallel to flow), yielding the maximum frequency shift Δf = 2f0v/c. At 90°, cosθ = 0 and no Doppler shift is detected (the beam is perpendicular to flow). Practically, true 0° alignment is rarely achievable, so Doppler angles of 30–60° are used clinically with angle correction. At angles >70°, small angular errors cause large velocity measurement errors. In colour Doppler, absence of signal in a vessel should prompt repositioning to achieve a lower insonation angle rather than assuming absent flow.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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