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Factors Affecting Color Flow Images: Summary

Angle dependency affects the accuracy of the color velocity information displayed. As blood flow becomes parallel to the sound beam, the ultrasound system calculates more accurate velocity estimates. As blood flow becomes more perpendicular to the sound beam, less accurate, lower-than-actual velocity estimates are calculated. Larger packet sizes offer greater velocity accuracy, but produce lower frame rates. Smaller packet sizes produce higher frame rates, but velocity information is less accurate. Decreasing the color sector width increases the frame rate without affecting the packet size or color accuracy. CFI line density increases result in improved resolution and a slower frame rate. A decrease in CFI line density decreases color resolution, but increases color frame rate. Use an enhanced velocity map to detect mean velocity differenced between adjacent sample sites. To see the degree of turbulence within a site, as well as its mean velocity, use a variance map. Yellow in an enhanced velocity map indicates high velocity blood flow toward the transducer. Yellow in a variance map indicates a high degree of turbulence moving toward the transducer. Setting the color gain too low will cause you to underestimate the size of a blood flow jet. Clutter filters help eliminate low velocity wall motion, also eliminating the concordant blood flow signal. Increasing the color flow imaging PRF increases the maximum measurable mean velocity, but decreases the system’s sensitivity to low velocity blood flow. Decreasing the color flow imaging PRF decreases the maximum measurable mean velocity, but increases the system’s sensitivity to low velocity blood flow. Using a lower transducer frequency increases the velocity measurement capability at any depth. Blood Flow Patterns in the Heart Once you understand the basic concepts of color flow imaging, you can begin to recognize and evaluate both normal and abnormal blood flow patterns from various imaging windows. This section presents examples of normal blood flow, and some frequently seen cardiac abnormalities. Whether your evaluation is normal or abnormal blood flow, remember the following points: The color bar indicates both the toward/away color orientation and the color map used. It is normal to see a small amount of localized turbulence adjacent to rapidly moving values. Thoroughly interrogate each blood flow area in all three dimensions Be aware that CFI lets you visualize blood flow from imaging windows not typically useful in PW Doppler. Unlike PW Doppler, which only provided information from the sample volume, CFI offers spatial information about an area of blood flow. In any blood flow jet, even one with a main vector nearly perpendicular to the transducer, there are usually more parallel components that CFI can detect. For example, in the parasternal long-axis view, where normal blood flow is almost perpendicular to the transducer, PW Doppler produces a very poor quality spectral display. However, CFI produces a good estimate of the spatial distribution of the blood flow, the relative blood flow velocities, and whether the blood flow is laminar or turbulent.