pt 3 Spectral Doppler Artifacts and Corrections

Spectral Doppler Artifacts

• Doppler ultrasound can encounter artifacts that lead to inaccurate flow information representation.

• Common artifacts include:

  • Aliasing: The most prevalent artifact in Doppler ultrasound.

  • Origin of the term: Comes from Middle English "elles", Latin "alius", and Greek "allos" meaning "other" or "otherwise".

  • Definition: Indicates improper representation due to insufficient sampling.

  • Optical temporal aliasing: For example, a wagon wheel appears to rotate differently when filmed at varying frame rates.

Nyquist Limit

• The Nyquist limit refers to the maximum detectable Doppler shift frequency by pulsed wave Doppler instruments.

• The limit is defined as half the pulse repetition frequency (PRF).

  • Typical PRF range for Doppler is 5 to 30 kHz.

• If Doppler shift exceeds the Nyquist limit, aliasing occurs.

• Solutions to aliasing include:

  • Increase PRF

  • Change Doppler angle: Increases the angle and decreases the Doppler shift for a given flow.

  • Baseline shift: Moves the aliased peaks to the correct location without legitimate Doppler shift interference.

• Continuous wave Doppler does not experience aliasing but lacks depth localization.

Correction Methods for Aliasing

• Methods to address aliasing:

  • Shift the baseline

  • Increase pulse repetition frequency (PRF)

  • Increase Doppler angle

  • Use lower-frequency transducer

  • Switch to continuous wave operation

• Note: Baseline shifting is effective if desired information is not obscured.

Range Ambiguity

• Increasing PRF can lead to range-ambiguity:

  • Occurs when a new pulse is sent before all echoes from the previous pulse are received.

  • The system cannot differentiate between echoes from the last and previous pulses leading to potential misinterpretation of flow.

Mirror Image Artifact

• A spectral mirror image appears on the opposite side of the baseline for unidirectional flow.

• Caused by:

  • High Doppler gain setting leading to overloading of the amplifier, resulting in leakage (crosstalk) of signals.

Doppler Noise and Color Doppler Artifacts

• Noise: Electronic noise appears if Doppler gains are set too high, leading to speckled appearance in Doppler spectra.

• Color Doppler Artifacts:

  • Similar artifacts as those observed in gray-scale sonography including:

  • Aliasing: Incorrect flow direction when Doppler shift exceeds Nyquist limit.

  • Solutions: Adjust flow speed range (PRF increase), baseline shifting to reduce aliasing effects.

  • Mirror Image Artifact: Similar to spectral displays, can duplicate images across strong reflectors.

  • Shadowing: Loss of Doppler-shift information beyond an object creating a shadow effect.

  • Clutter: Results from motion of tissues, heart walls, or valves that obscures true flow representation.

  • Noise effects: Mimics flow in regions with low echoes, leading to misinterpretation.

  • Twinkling artifact: Observed at highly reflective surfaces, possibly due to phasing issues in Doppler detection.