Module 6.0

Course Overview

• Course Code: PHYS 254

• Title: DMS Physics 2

• Institution: Southern Alberta Institute of Technology

Artifacts

• Definition: An artifact is anything that does not accurately display the structures or functions that are imaged.
    - Examples include:
      - Simple artifacts: Echoes present in a cystic structure that shouldn’t exist.
      - Complex artifacts: Duplication of a structure where only one truly exists.

Types of Artifacts

• Helpful Artifacts: Some can assist the Sonographer in diagnosing or delineating structures.
    - Examples: Enhancement, shadowing.

• Confusing Artifacts: Others may lead to confusion.
    - Examples: Beam width artifact, refractive duplication.

Causes of Artifacts

Artifacts can result from various factors, including:

• Malfunctioning equipment.

• Defective recording devices.

• Improper operation of equipment.

• Acoustic properties of tissues and sound wave propagation.

Propagation Artifacts

Overview

Artifacts can occur due to misconceptions about how sound propagates in tissue, such as:

• All tissues are assumed to have the same acoustic velocity ($1540 ext{ m/s}$).

• The sound beam travels straight.

• Echo strength represents organ echogenicity solely.

• The distance to each reflector equals the round trip time.

Categories of Artifacts

• Not Real: Do not represent actual interfaces.

• Missing: Reflectors that are absent.

• Improperly Located: Echoes in the wrong position.

• Improper Brightness, Shape, or Size: Echoes that are misrepresented in visual characteristics.

Groups of Artifacts

1. Propagation Group

2. Attenuation Group

Detailed Propagation Artifacts

1. Axial Resolution

• Description: Objects less than half the spatial pulse length (SPL) apart in the axis of the beam appear as a single echo.

• Source: Own work.

2. Lateral Resolution

• Description: This artifact, also known as point spreading, occurs when reflectors are smeared across the screen in areas where the beam is wide.

• Solution: Reduce overall gain or time gain compensation (TGC) and adjust transducer focus properly (near zone).

3. Slice Thickness

• Description: Known as volume averaging, this causes anechoic structures to appear filled with false debris.

• Cause: Assumes returning echoes originate from the beam's center, with echoes from surrounding areas compressed into 2D.

• Solution: Position the probe to align the narrowest portion of the Z-axis with the desired structure.

4. Acoustic Speckle

• Cause: Result of multiple off-axis reflections creating heterogeneous brightness through constructive and destructive interference from scattered reflections in parenchyma.

• Control: Persistence averages frames to normalize tissue brightness.

5. Reverberation

• Description: Occurs from sound reflecting between the transducer and a strong interface.
    - Appearance: Multiple equally spaced linear lines, resembling rungs of a ladder which weaken with depth.

• Solution: Change scanning window or utilize harmonics to minimize this artifact.
  

Types of Reverberation Artifacts

1. Comet Tail: Short-range reverberation between closely spaced reflectors. Common causes include metal clips or calcium deposits.

2. Ring Down: Continuous stream of long echoes seen due to bouncing sound from gas bubbles, helpful for diagnoses.

3. Mirror Image: Results from a duplication of an object due to angled strong reflectors; e.g., diaphragm reflects sound towards an object.

4. Multipath: Occurs when sound takes an off-axis path before returning to the probe, leading to misallocated echoes.

5. Refraction: Happens when the sound beam is redirected after striking an interface at an angle between media with different velocities.
      - Examples: Edge shadowing, refractive duplication, refractive malposition, refractive enhancement.

Edge Shadow

• Description: Shadows occur at edges of strong curved reflectors due to non-perpendicular incidence.

• Solution: Change the scanning angle to mitigate the shadow.

Refractive Duplication

• Description: A confusing artifact resulting in lateral duplication of echo interfaces due to beam refraction by a strong interface, e.g., rectus abdominus acting as an acoustic lens.

• Correction: Align perpendicular to the structure and adjust scanning angle.

Side Lobes

• Description: Linear echogenic bands present due to returning weak side beams.

• Correction: Insulator rings and TGC manipulation to mitigate side lobe effects.

Grating Lobes

• Description: Caused by array element length and width vibrations, leads to confusing images.

• Correction: Apodization and sub-dicing help reduce these artifacts.

Propagation Speed Error

• Issue: Misallocation of reflectors based on incorrect assumptions about sound speed, prevalent in deeper fat tissue.

• Awareness: No correction is possible; understanding the error is important for diagnosis.

Range Ambiguity

• Description: Leads to echoes appearing closer to the transducer than they are; occurs more frequently in Doppler imaging due to excessively high pulse repetition frequencies (PRFs).

• Correction: Decrease frame rate or PRF to mitigate.

Attenuation Artifacts

Overview

1. Shadowing - Lack of echoes beneath highly attenuating structures due to absorption or reflection of sound waves.
      - Solutions: Change window angles, increase frequency, adjust focus, reduce overall gain.

2. Enhancement - Brightness seen behind structures attenuating less than surrounding structures, aiding in diagnosis.

3. Focal Enhancement - Illusion of brightening in specific areas within the focal zone caused by beam intensity differences.
      - Correction: TGC is used to accommodate for this artifact.

Doppler Artifacts

Overview

• Artifacts in Doppler scanning include:
    - Aliasing
    - Slice Thickness
    - Reverberation (including twinkle artifact)
    - Mirror Imaging
    - Flash or Clutter
    - Noise
    - Refraction

Specific Doppler Artifacts

1. Aliasing - Can help identify high-velocity flows or turbulence for pathologies.

2. Doppler Slice Thickness - Multiple flow velocities within one flow envelope due to beam width effect.

3. Doppler Noise - Results from high gain settings causing color bleed into non-flowing structures. Can mislead diagnosis by artificially elevating peak velocities.

Additional Equipment-Related Artifacts

• Electronic Interference: Regular or irregular echo patterns due to radio frequency signals from other devices interfering with ultrasound data.

• Faulty Equipment - Causes:
    - Malfunctioning crystal elements
    - Cracked damping materials
    - Defective recording devices
    - Improper transducer shielding

Conclusion

• Understanding and recognizing these artifacts is crucial for accurate imaging and diagnosing within sonography practices. Awareness of equipment functions and limitations is key to reducing the impact of these artifacts in clinical settings.