Aliasing occurs in Doppler ultrasound when the pulse repetition frequency (PRF) is less than twice the Doppler shift frequency (Nyquist limit). When this condition is met, the Doppler signals are not sampled frequently enough to accurately measure the frequency shifts, resulting in the misrepresentation of the flow velocities. This causes the aliasing artifact, where high-velocity flows are displayed incorrectly as wrapping around the baseline, leading to potential diagnostic errors.
Reference: ARDMS Sonography Principles and Instrumentation (SPI) Review, Doppler Artifacts section.