Unit 21: Acoustic Artifacts in Ultrasound

Definition of Artifacts

• Artifacts: Elements within ultrasound images that do not represent true anatomy.
  • Can cause false anatomy to appear or real anatomy to disappear.
  • Can alter the shape, size, location, or brightness of real anatomy.

Causes of Artifacts

• Machine assumptions leading to invalid results:
  • Sound travels at 1540 m/s (typical for soft tissue).
  • Sound travels directly to a reflector and back.
  • Reflections only come from the anatomy and the main beam path.
  • Sound does not change direction.
  • Beam is narrow in all dimensions.
• Mechanical errors (e.g., broken crystal) and operator errors (e.g., excessive gain reduction).
• External energy sources can cause distortion.

Categories of Artifacts

• Resolution Artifacts
• Position Artifacts
• Attenuation Artifacts

Resolution Artifacts

• Occur when image detail does not represent true anatomy.
• Types:
  • Axial Resolution: Poor resolution when objects are parallel to the beam and close together.
  • Lateral Resolution: Poor resolution when objects are perpendicular to the beam and close together.
  • Elevational Resolution: Related to beam thickness; known as slice thickness artifact.
• Correction: Use high-frequency transducers, position focus correctly, consider 1.5 D array transducers or harmonics.

Position Artifacts

• Anatomy appears in the wrong location.
• Types:
  • Refraction Artifact: Exact replica placed laterally.
  • Mirror Artifact: Exact replica placed deep to true anatomy due to interaction with strong reflectors (e.g., diaphragm).
  • Multi-path Artifact: Similar anatomy displayed deeper due to sound bouncing off multiple reflectors.
  • Reverberation Artifact: Equally spaced reflectors due to bouncing sound.
  • Comet Tail (Ring Down) Artifact: Dense line of echoes due to close structures.
  • Lobe Artifact: Lateral echoes displayed due to side lobes.
  • Speed Error Artifact: Anatomy appears broken due to varying sound speeds.
  • Range Ambiguity: Echoes from previous pulse appear in current image.
• Correction: Adjust transducer angle, gain, or depth.

Attenuation Artifacts

• Occur when sound interacts with a reflector affecting attenuation.
• Types:
  • Shadowing: Dark areas due to high attenuation (e.g., gallstones).
  • Edge Shadow: Dark lines due to refraction and divergence at curves.
  • Enhancement: Bright areas due to low attenuation.
  • Focal Enhancement: Horizontal bright band due to increased intensity at focus.
• Correction: Adjust angle, increase depth, or use TGCs.

Other Artifacts

• Speckle: Grainy appearance due to interference; use higher frequency transducers.
• External Interference: Noise from electronic equipment or biological activity.
• Techniques to reduce artifacts:
  • Spatial Compounding: Using images from different angles.
  • Frequency Compounding: Combining images of different frequencies.
  • Coded Excitation: Uses complex pulses for reduction.

Conclusion

• Recognize artifact types by appearance (e.g., reverberation, enhancement, shadowing).
• Understand underlying physics (e.g., sound speed, beam travel).
• Know corrective techniques.