Pulsed-Wave Spectral Doppler:
©Walter Rasmussen, R.D.C.S.
A sample volume is set within the 2-D image.
When pulsed wave spectral Doppler is activated, a small box, a pair of bars or a small dot on the cursor line indicates where the sample volume is placed. The transducer emits a pulse of ultrasound specifically for the sample volume position and then waits for the pulse to return. The closer the sample volume is placed to the transducer, the higher the scale available because pulse transit time is reduced. When Pulsed-Wave Doppler is active, only fairly low velocities of between 1-2 meters/sec can be recorded. When sampling a velocity that exceeds the scale, the spectral Doppler tracing will be too tall to fit within the graph and this is called aliasing or wrap-around because the peak of the profile will be seen cut-off at the top and placed emerging from the bottom of the graph. Moving the baseline helps counteract the effect of aliasing. The next step is to increase the scale which increases pulse repetition frequency (PRF).
Above: Aliasing with wrap-around on PW Doppler. Peak velocity appears at opposite side of graph.
When the PW Doppler scale is at its maximum, high pulse repetition frequency is automatically activated. When using High PRF, the machine will indicate that multiple sample volumes are incorporated into the spectral Doppler recording by showing more than one sample volume on the cursor and a notation such as, “HPRF” near the strip chart.
High PRF Enabled, showing multiple gates where blood flow is sampled.
Using high PRF Doppler should be avoided because of the uncertainty introduced with mixing multiple sampling sites. While exploring areas where the exact location of the sampled blood flow is of critical importance, high PRF should not be used and instead, CW Doppler may be more useful as long as the area of interest is typically the highest velocity.
The most common areas where the use of high PRF should be avoided those with low velocity such as the pulmonic valve, pulmonary veins, the left ventricular outflow tract and atrial shunts.