Doppler frequency shifts come from moving red blood cells and give us the spectral display that we see in Continuous Wave and Pulse Wave Doppler. The velocities obtained from these frequency shifts can also be displayed in Color Doppler, superimposed on top of the 2D ultrasound image. This is a method of displaying the mean or average velocity, while the spectral Doppler display can quantify specific velocities, like the peak systolic and end diastolic velocities, at a point in time.
Because the Doppler frequency shift is relative to the position of the observer, this is important when placing the Doppler sample and angle of insonation. The Doppler sample volume is the "observer" and the frequency shift created by the moving red blood cell is what we are evaluating on the spectral display. But what happens if the observer changes position? The velocity observed will be different. This is extremely important for Doppler Angle. The Society for Vascular Ultrasound recommends that scanning angle be maintained between 45-60 degrees. While we know that the closer to 0 degrees, the more accurate the velocity result because of the math and the cosine of the angle, try getting that on a your average carotid Doppler. It is often not attainable and so for reporting and to maintain consistency in the lab, it is best to stay within the range of what can be easily obtained on most exams.
Lara Miller, RDMS, RVT, RDCS, RCCS
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