Vet Detective · Ultrasound physics teaching guide
Doppler Aliasing
How pulsed-wave sampling can make Doppler shifts wrap across a colour scale or spectral baseline.
Animation paused.
EXAMPLES:
Colour Doppler · oblique vessel
Example map: red toward the transducer, blue away. Colour maps can be inverted.
Wagon-wheel analogy
Sampling analogy only; not a Doppler frequency scale.
Illustrated velocity along vessel
60 cm/s
model input
cos θ
0.87
θ = 30°
Uncorrected beam-axis component
52 cm/s
V × cos θ
Displayed value in example scale
52 cm/s
after idealised wrap
What the model shows: Drag either slider. The illustrated velocity is projected onto the beam axis as V × cos θ, then mapped through a fixed ±100 cm/s example scale. Real pulsed-wave systems alias Doppler shift when it exceeds half the pulse repetition frequency. Converting that shift to a velocity scale also depends on transmit frequency and the angle setting.
Clinical takeaway: Aliasing is a sampling and display phenomenon, not a diagnosis. It indicates that sampled Doppler shift has exceeded the current Nyquist limit. In colour Doppler, abrupt colour reversal or mixed colour can arise from aliasing together with spatial variation in the beam-axis component, complex flow, variance display or noise.
A colour mosaic does not establish turbulence. High-speed laminar flow can alias, while slower disturbed flow may not. Optimise the colour settings and use spectral Doppler when velocity information is required.
A colour mosaic does not establish turbulence. High-speed laminar flow can alias, while slower disturbed flow may not. Optimise the colour settings and use spectral Doppler when velocity information is required.
How to reduce aliasing: Raise the velocity scale or PRF, recognising that this may reduce slow-flow sensitivity. Shifting the baseline redistributes the displayed range between directions but does not increase the Nyquist limit. Reduce sample depth when depth is limiting the achievable PRF, consider a lower Doppler transmit frequency, or use continuous-wave Doppler for high velocities. Continuous-wave Doppler avoids aliasing but does not provide range specificity.
Angle and absent colour: Optimise beam alignment; do not change the angle merely to hide aliasing. At an ideal 90° angle, the modelled Doppler shift is zero. In practice, absent colour may also reflect low flow, scale or PRF, gain, wall filter, depth, sampling, motion or noise, or occlusion. It cannot determine vessel patency by itself. Align the Doppler cursor with flow and use angle correction only when appropriate for the measurement.
Angle and absent colour: Optimise beam alignment; do not change the angle merely to hide aliasing. At an ideal 90° angle, the modelled Doppler shift is zero. In practice, absent colour may also reflect low flow, scale or PRF, gain, wall filter, depth, sampling, motion or noise, or occlusion. It cannot determine vessel patency by itself. Align the Doppler cursor with flow and use angle correction only when appropriate for the measurement.