Aliasing occurs when the velocity exceeds what fraction of the PRF?

Aliasing in Doppler ultrasound occurs when the measured velocities exceed the Nyquist limit, which is defined as half of the pulse repetition frequency (PRF). The Nyquist limit is crucial because it represents the maximum velocity that can be accurately measured before aliasing occurs. If the velocity of blood flow exceeds this limit, the system can incorrectly display the flow direction and speed, resulting in an aliasing artifact.

When the velocity exceeds half of the PRF, the Doppler shift becomes ambiguous, causing the system to misinterpret the actual flow. Thus, the critical fraction of the PRF where aliasing begins to occur is indeed 1/2, accurately matching the half of the PRF that defines the Nyquist limit. Understanding this concept is vital for sonographers to ensure the accuracy of Doppler flow measurements and to troubleshoot potential issues during echocardiographic assessments.