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The first image and shows the CCA scanned with an inappropriately low PRF (1.4 kHz) for the high velocity arterial flow. As a result flow above the upper threshold of is displayed with blue hues, during the systolic peak.
The second image shows the same vessel after the PRF has been increased (5.15 kHz).
The cine loop shows how aliasing gradually disappears as the PRF is adjusted (Note: the frame rate has been reduced to increase visual clarity).
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The first image shows that low PRF results in a "wraparound" of the spectral curve, with the peak displayed below the baseline.
With a slight increase of PRF and adjustment of the baseline the spectral curve is displayed normally.
Case Discussion
Demonstration of color and spectral Doppler aliasing.