Spurious spectral broadening (ultrasound)
Pulse-wave curves from a healthy CCA obtained to illustrate artifactual causes of spectral broadening.
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All images show the common carotid artery in a longitudinal orientation with the Doppler beam steer angled against the direction of flow to ensure appropriate Doppler-angle despite the perpendicular course of the vessel.
- On the first image the sampling gate is small, and positioned in the middle, thus the pulse wave curve is thin, as only the Doppler-shift of the red blood cells in the middle of the vessel (which have the highest velocity) contribute to it. Note that the curve is not "pencil thin" as even under normal circumstances the velocity of the RBCs is somewhat varying, particularly during the slower, diastolic phase.
- On the second image the sampling gate is inappropriately broad, almost as wide as the diameter of the vessel. This results in inclusion of signal generated by slower moving RBCs along the vessel wall in the pulse wave curve. The consequence is a characteristic "fill-in" of the area under the curve which is commonly termed spectral broadening.
- The third image shows another possible problem, an off-center position of the otherwise correct sampling gate, also resulting in the inclusion of slow-flow signal, and causing broadening of the spectrum.
Inappropriately broad or/and off-center position of the sampling gate are the most common causes of spurious spectral broadening in Doppler-ultrasound.