What is the phenomenon of "aliasing" in Doppler ultrasound?

Aliasing in Doppler ultrasound occurs when the frequency shift exceeds the Nyquist limit, which is defined as half of the pulse repetition frequency (PRF). In simpler terms, when the velocity of the blood flow being measured is significantly high, the system may not be able to accurately interpret the frequency shift. This misinterpretation leads to an erroneous display of flow, where the signal appears to reverse direction or cycles back on itself, resulting in misleading readings.

The Nyquist limit serves as a threshold for the Doppler system. If the frequency shift due to the moving blood cells is greater than this limit, the system cannot process the signal correctly, which leads to the characteristic "wrap-around" effect known as aliasing. This situation necessitates careful adjustment of the ultrasound machine settings or the use of different imaging techniques to accurately assess high-velocity flow. Recognizing and addressing aliasing is essential in clinical practice to ensure accurate Doppler readings, especially in cases of high blood flow velocities, such as in situations with arterial stenosis or regurgitation.