During Doppler acquisition the angle of blood flow may not be directly toward or away from the transducer. To ensure accurate display of blood flow velocity, the angle between the ultrasound beam and blood flow is included in the velocity calculation.
Accuracy of the mean velocity calculation depends on an average of all velocities within the spectrum. If a small sample volume size is used for this type of measurement, only flow from the center of the vessel will be included. The slower flowing blood near the vessel wall will not be part of the spectral information. This will mean an overestimation of the true mean velocity. The lower blood velocities are not included in the average measurements and the values will be falsely elevated.
The ultrasound images below show measurements obtained with two different sample volume sizes and the resulting mean velocity measurements. Click on an image for an enlarged view.
