Vary the sample volume size. Changing the sample volume size changes the axial length of the sample volume. A large sample volume size can be used to perform a sweep of a vessel to detect very low flow or a high velocity jet. Once the flow has been detected the sample volume size can be decreased to remove unwanted noise or spectral broadening from the signal. If a higher Doppler frequency is used a narrower beam is produced creating a smaller sample volume size. For the most accurate peak velocity spectral display keep the sample volume size as small as possible and place it within the stream of fastest flow.
Check the location of the transmit focus. Flow detection is maximized at the focal zone.
Decrease the wall filter. Wall filters are used to eliminate the lowest Doppler shifts from the display. Their function is to eliminate clutter from the signal that is caused by moving solid structures. However in doing so low velocity blood flow is also eliminated. Interpretation of the Doppler spectrum takes into account the waveform shape and the presence or absence of diastolic flow in the signal. Loss of the diastolic waveform indicates increased peripheral resistance. To ensure this portion of the waveform is not lost or undetected use the lowest Doppler wall filter setting.
Accurate angle correction. Doppler systems can calculate a velocity measurement from a Doppler shift frequency. In order to do this the sonographer must input angle into the Doppler system. Assuming that flow is parallel to the vessel wall this angle is estimated by using the ultrasound image as a guide. The angle is lined up with the axis of the vessel. Velocity should not be estimated if this angle is above 60 degrees. Color Doppler is useful in identifying the true axis of flow when an off axis jet is present with a stenosis.
