Module 5 Artefacts

Module Five: Artefacts in Ultrasound Imaging

Introduction to Artefacts

  • Definition of Artefact: Any appearance on the ultrasound image that does not correlate with the actual anatomy being scanned.

  • Importance of Understanding Artefacts: Artefacts can lead to misdiagnosis in imaging.

  • Common Causes of Artefacts:

    • Assumptions Made: Misunderstandings of the imaging principles.

    • Machine Fault: Equipment issues that introduce errors.

    • Operator Error: User errors in handling imaging controls.

Assumptions in Sonographic Imaging

  • Pulse Echo Principle: Several key assumptions that must be true to ensure accurate imaging:

    1. Sound Travels in Straight Lines: Refractive changes can occur with trajectory changes through tissues of varying acoustic velocities, causing multipath artefacts.

    2. Ultrasound Beam Thickness: While the beam is assumed to be infinitely thin, it has a finite width, which can lead to beam width artefacts.

    3. Speed of Sound Consistency: Assumed constant speed of sound (1540 m/s), yet actual speed varies across tissues (e.g., fat, muscle).

    4. Constant Rate of Attenuation: Assumption that attenuation remains constant, but varying attenuation rates exist across different tissues.

  • Consequence of Violated Assumptions: Misrepresentation occurs on images if any assumptions are broken.

Categories of Common Artefacts

  • Acoustic Shadowing

    • Definition: A region of no echoes resulting from sudden severe attenuation of the ultrasound beam beneath reflective structures like stones or bones.

    • Examples:

    • Kidney Stones: Create shadows due to strong reflection preventing echoes from returning.

    • Gallbladder Calculi: Also create distinct acoustic shadows, indicating their presence.

    • Image Demonstrations:

    • Kidney images showing stones with corresponding shadows.

    • Difference between clean shadows (calculi) and noisy shadows (gas) due to resonance phenomena in gas bubbles.

  • Acoustic Enhancement

    • Definition: Enhancement seen behind low attenuating structures, indicated by increased echogenicity.

    • Examples:

    • Renal Cyst: Bright echoes return from behind a cyst due to less attenuation.

    • Bile in Gallbladder: Shows increased echogenicity due to low attenuation.

  • Reverberation Artifacts

    • Definition: Occur due to multiple reflections between high mismatch interfaces, causing erroneous depiction at greater depths.

    • California Classically Causes: Occurs in subcutaneous layers from structures such as skin and fascia.

  • Partial Volume Artifacts

    • Two Types:

    1. Beam Width Artifacts: Causative interfaces are sensed as arising from the beam’s center.

    2. Slice Thickness Artifacts: Causative interfaces are not seen directly due to out-of-plane scanning.

  • Velocity Artifacts

    • Sound Speed Variation: Sound travels differently through various tissues, leading to apparent bending of images (e.g., needle appearing bent in fatty tissue vs. breast parenchyma).

  • Side Lobe Artifacts

    • Side Lobe Energies: Strong reflectors can cast signals from off-axis, causing artifacts, particularly evident in areas surrounding anechoic structures like the bladder.

  • Mirroring Artifacts

    • Definition: Caused by reflection off a large specular interface, producing duplicated images of structures (e.g., liver duplication above diaphragm).

  • Ghost Artifacts

    • Definition: Artifacts specific to abdominal midline structures, resulting from refraction through rectus abdominis muscles, causing doubling of structures (e.g., aorta appearing twice).

  • Pseudofractured Kidney Artifact

    • Region Specific: Seen on the left kidney when imaged through the spleen, creating an appearance of fracture due to combined edge and velocity effects.

  • Comet Tail and Ring Down Artifacts

    • Comet Tail: Results from reverberations occurring with small high mismatch interfaces (e.g., gallbladder wall).

    • Ring Down: Caused by resonance phenomena in gas pockets, often indicating abscesses due to gas formations.

  • Anisotropy

    • Definition: This artifact appears in musculoskeletal imaging when tendons are scanned at different angles than 90 degrees, mimicking tears due to poor echo return away from the ideal approach.

  • Equipment and Operator Error Artifacts

    • Broken Transducer Elements: Can cause patchy, non-uniform imaging.

    • Poor Setup Settings: Improper TGC (Time Gain Compensation) and gain controls can lead to significant artefacts, affecting image quality.

Conclusion

  • Understanding these artifacts is crucial for accurate imaging and diagnosis in sonography. Each type of artifact has implications on the interpretation of ultrasound images, necessitating careful scanning technique to minimize their impact. Best of luck in your studies!