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Factors Affecting Color Flow Images: Angle Dependency - 2

The following diagrams illustrate the changes in the color display as the flow angle becomes increasingly closer to parallel with the transmitted beam.

At position 1, where flow is perpendicular to the beam of sound, there are no detectable frequency shifts. Black indicates a flow velocity of zero. At position 2, where the angle of incidence between the beam of sound and the flow is slightly more parallel, there is detection of a small frequency shift. Dark blue indicates this low velocity flow. At position 3, where the angle of incidence is even more parallel, a lighter blue indicates a higher velocity.

As the angle between the ultrasound beam and the flow becomes almost parallel, a larger, more accurate frequency shift is calculated. If the actual frequency shift exceeds the Nyquist limit, the resulting velocity has color reversal. This has happened at position 4. Although neither the flow speed nor the direction changed, the angle of incidence altered, and therefore, the color velocity changed.

Click below to view a real-time image of aortic arch flow demonstrating how a change in flow direction also changes the color displayed on the image. module230_AOARCH~1.AVI