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Appendix A: Flow Velocity Accuracy
Figure A-6. Horizontal and Diagonal Vessel Relative Lengths and Parabolic Flow Development
Beam Width Effects
When TAMEAN is measured on an ultrasound system, typically the Doppler range gate is set to
include the entire diameter of the vessel. The intensity-weighted average velocity representing the
mean Doppler shift is affected by attenuation of the beam when traveling through tissue (phantom
material), and loss of low velocity readings due to wall filter settings used in the presets and/or in
settings chosen by the sonographer.
It is important to understand that TAMEAN is the average velocity in the sample volume, established
both by the Doppler range gate and by the ultrasound beam width. Because the sample volume may
cover the tubing partially, or even asymmetrically, TAMEAN might not yield accurate estimates of
the mean velocity for the vessel. As illustrated in Figure A-7, if the vessel diameter exceeds the
scanner’s beam width, some of the lower velocities near the vessel wall may not be sampled.
The sample volume is 3-dimensional, established by:
•
Beam width in the image plane
•
Beam width perpendicular to the image plane (slice thickness)
•
Axial region defined by the range gate position and length
Figure A-7. Beam Width And Vessel Diameter Sampling Issues
Beam width
Beam width
