Infographics / Ultrasound Physics

Aliasing and the Nyquist limit in pulsed-wave Doppler

Two panels show the same Doppler signal sampled at the pulse repetition frequency. On the left the sampling rate is at least twice the maximum Doppler shift, so the samples capture every cycle and the true velocity is displayed. On the right too few samples per cycle are taken, so the reconstructed trace is a false slow wave that wraps across the baseline. This is aliasing, which occurs when the Doppler shift exceeds the Nyquist limit, equal to half the pulse repetition frequency. Below, the fixes are listed: raise the pulse repetition frequency or use high pulse repetition frequency mode, lower the transmit frequency, or increase the insonation angle.

Nyquist limit and aliasing in pulsed Doppler ultrasound Two panels show the same Doppler signal sampled at the pulse repetition frequency. On the left the sampling rate is at least twice the maximum Doppler shift, so the samples capture every cycle and the true velocity is displayed. On the right too few samples per cycle are taken, so the reconstructed trace is a false slow wave that wraps across the baseline. This is aliasing, which occurs when the Doppler shift exceeds the Nyquist limit, equal to half the pulse repetition frequency. Below, the fixes are listed: raise the pulse repetition frequency or use high pulse repetition frequency mode, lower the transmit frequency, or increase the insonation angle. Pulsed Doppler samples at the PRF; Nyquist limit = PRF / 2 a Doppler shift above PRF / 2 cannot be sampled correctly and aliases Adequate: PRF ≥ 2 × fₕ 0 enough samples per cycle → true velocity read Under-sampled: PRF < 2 × fₕ 0 wraps to opposite side too few samples → false slow trace (aliasing) To reduce aliasing, raise the Nyquist limit or shrink the Doppler shift: increase PRF, or use high-PRF mode — but higher PRF lowers Rₖₖₓ → range ambiguity lower the transmit frequency f₀ — a smaller Doppler shift fₕ for the same velocity increase the insonation angle toward 90° — smaller cosθ (at the cost of accuracy)
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