Davies Publishing SPI Practice Test 2026 – Your All-in-One Guide to Exam Success!

Aliasing happens when the Doppler frequency shifts produced by blood flow aren’t sampled fast enough to be represented accurately. In Doppler ultrasound, the signal is gathered at a pulse repetition frequency (PRF). If the true frequency shift from motion exceeds half of that PRF (the Nyquist limit), the sampled data cannot distinguish it properly, causing those higher frequencies to fold back into the lower part of the spectrum. The result is that high velocities can appear falsely low or even seem to be flowing in the opposite direction.

This is different from reverberation, which comes from echoes bouncing between surfaces and creates ghost signals; or spectral broadening, which broadens the range of velocities due to a mix of flow speeds or limitations in resolution; or noise, which is random background signal. Aliasing is specifically tied to the sampling rate and the Nyquist limit, leading to misrepresented velocity readings in the Doppler spectrum.