In Doppler ultrasound, what does aliasing refer to?

Aliasing in Doppler ultrasound refers to the phenomenon where higher velocities exceed the Nyquist limit, which results in a misrepresentation of the flow direction and speed. This occurs when the frequency shift of the returning echoes from moving blood cells is greater than half the sampling frequency. When aliasing happens, the spectrum appears to "wrap around," causing the high velocities to be misinterpreted as lower velocities in the opposite direction on the display.

This wrapping effect can lead to confusion in the assessment of blood flow, as the actual velocity may be significantly higher than what is displayed. Understanding aliasing is crucial for sonographers, as it informs them to adjust the Doppler settings, such as increasing the scale or adjusting the sample volume, to accurately measure and interpret blood flow velocities.