Week 9 lecture 3 Doppler Artifacts Summary

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Doppler Artifacts

• Spectral Doppler Artifacts

  • Frequency aliasing

  • Range ambiguity (high PRF)

  • Spectral broadening

  • Spectral mirror artifact

• Colour Doppler Artifacts

  • Colour aliasing

  • Colour dropout

  • Colour bleed

  • Angle effects

  • Mirror image

Spectral Doppler Artifacts

1. Frequency Aliasing

• Spectral Doppler flow information appears above and below the baseline.

• Doppler shift frequencies are greater than 1/2 the PRF (Pulse Repetition Frequency).

• The Nyquist limit is exceeded, leading to aliasing.

• PRF is also referred to as the scale.

Correcting Spectral Aliasing

• Adjust the PRF/scale: Increase the velocity scale.

• Decrease the baseline: This is called baseline shifting.

• Increase the Doppler angle: But not beyond 60 degrees.

• Use a lower operating frequency: A lower operating frequency results in a lower Doppler shift.

  • Doppler Equation:

    

    • Where:

      • f_D is the Doppler shift frequency

      • v is the velocity of blood flow,

      • f_t is the transmitted frequency,

      • \Theta is the Doppler angle,

      • c is the speed of sound in the medium.

• Use continuous wave (CW) Doppler: Continuous wave Doppler does not have the same aliasing limitations as pulsed wave Doppler.

2. Range Ambiguity Artifact

• In PW Doppler, the ultrasound system sends another pulse before all echoes from the previous pulse are received.

  • This leads to the range ambiguity artifact.

3. Intrinsic Spectral Broadening

• The spectral window is filled in (vertical thickening).

• This is seen with turbulent flow conditions.

4. Spectral Mirror Artifact

• Flow is seen on opposite sides of the baseline.

• Often seen when flow is perpendicular to the beam.

Colour Doppler Artifacts

1. Colour Aliasing

• Colour Doppler has a reduced PRF compared to spectral Doppler, therefore aliasing is more commonly seen.

• Wrapping around of colour map.

• Opposite flow direction to Doppler shift.

2. Colour Dropout

• Colour is not displayed when it should be.

• Causes:

  1. Doppler shift signal is too weak.

  2. Colour Doppler settings are not optimized.

• Solutions:

  • Reduce Doppler angle.

  • Use Power Doppler.

• Adjust Colour Doppler settings.

  • Avoid having Wall Filters set to Max and Gains lowered!

3. Colour Bleed

• Colour pixels appear outside of the vessel wall.

• Most likely caused by tissue movement.

4. Colour Angle Effects

• Different hues and direction are displayed for blood flowing at the same velocity and direction.

• It's important to understand the different Doppler angles to interpret what is going on.

5. Mirror Image

• Duplication of a vessel or colour Doppler shift on the opposite side of a strong reflector.

• The mirror vessel will demonstrate colour flow.

• Decreasing colour gain may reduce or eliminate this artifact.