Width of Ultrasound Beam

The Doppler spectrum just described assumes that the entire vessel is insonated when sampling. However, the size of the Doppler beam is actually quite narrow and only part of the vessel is sampled. If the beam passes through the center of the vessel only flow from the anterior and posterior aspects of the vessel will be sampled. The low velocity flow from the lateral walls will not be sampled. Therefore, the Doppler spectrum does not truly represent all of the flow velocities within the vessel.

The sample volume size is adjustable. The size of the sample volume used will affect the amount of spectral broadening in the signal. If the sample volume is large enough to encompass the entire vessel the resulting signal will have a large range of frequencies. This is because slower flow near the walls as well as faster flow at the center of the vessel are included. If the sample volume is small only flow specific to that region is detected. If the sample volume is small and placed closer to the vessel wall only slower velocities would be detected. Fastest flow is detected when the sample volume is placed at the center of the vessel.