Aliasing and range ambiguity
| View | Pulsed-wave and colour Doppler; CW and HPRF as alternatives. |
|---|---|
| Timing | Peaks are cut off and wrap to the opposite side of the baseline when velocity exceeds the Nyquist limit. |
| Caliper / trace | Shift the baseline, lower the transducer frequency, reduce sample depth (raising PRF), or switch to CW/HPRF accepting range ambiguity. |
Correct example
An LVOT PW signal that wraps in diastole because of aortic regurgitation is re-recorded with CW, which displays the full AR velocity above the baseline; the LVOT systolic velocity itself is kept on PW with baseline shift.
Common error
Measuring the peak of a wrapped signal, mis-reading its direction, or assuming an HPRF or CW velocity came from the sample-volume depth.
Effect on interpretation
Aliasing precludes accurate peak velocity and confuses direction; CW and HPRF remove aliasing but cannot locate the depth of the peak, so the origin of a high velocity must be inferred from imaging and colour.
Why
The Nyquist limit is half the PRF, and PRF falls with sample depth; a non-coaxial angle separately underestimates velocity without any visible sign, which is why stenotic jets are interrogated from multiple windows.