Artifacts Chapter 21

Study Notes on Ultrasound Artifacts

Introduction to Artifacts in Imaging

Artifacts in medical imaging, particularly with ultrasound technology, are defined as errors that manifest in images due to various factors. They are typically categorized based on their characteristics and underlying causes. Artifacts can be described as non-real images not seen in the actual scan, characterized by incorrect shapes, sizes, positions, or brightness levels.

Types of Artifacts

Artifacts can generally be classified into several categories based on their effects or how they appear on ultrasound images:

• Reverberation: Appears as multiple evenly spaced echoes on display, caused by the bouncing of sound waves between two strong reflectors that are parallel to the ultrasound beam.

  • Characteristics:
  • Multiple echoes appear equally spaced
  • Artifacts are parallel to the sound beam's main axis
  • Located at increasingly deeper levels
  • The appearance resembles Venetian blinds.

• Comet Tail: A type of reverberation that appears as a single long hyperechoic line directed downward, often associated with very high propagation speeds, such as those near mechanical heart valves.

• Shadowing: This artifact appears as hypoechoic (darker) or anechoic (completely dark) regions on the image, resulting from high attenuation. It can obscure visualization of underlying tissues but may provide valuable diagnostic information regarding tissue characterization, unrelated to sound speed in the medium.

  • Characteristics:
  • Typically results from an object that has greater attenuation than the surrounding tissue
  • Prevents visualization of the anatomy directly beneath it.

• Edge Shadowing: A specific type of shadow that forms a hypoechoic region extending downward from the edge of a curved reflector, caused by the refraction of sound waves at the edges.

  • Characteristics:
  • The sound beam diverges and decreases in intensity after interacting with a curved reflector.

• Enhancement Artifact: This artifact creates a hyperechoic region beneath tissues with abnormally low attenuation, opposite to shadowing, providing diagnostic value.

  • Characteristics:
  • Appears as increased echogenicity beneath low attenuation structures
  • Represents changes in tissue characterization.

• Focal Enhancement: Involves a side-to-side brightening effect at a focal point on the image, characterized by an increased intensity perceived in specific areas compared to adjacent depths.

• Mirror Image: An artifact caused when sound reflects off a strong reflector and is redirected toward another structure, typically resulting in a second echo that appears deeper than the actual structure.

  • Characteristics:
  • Exists on a straight line between the artifact and the true reflector.

• Crosstalk: A mirror image artifact that shows up in spectral Doppler displays; often mistaken for genuine reflections.

• Speed Error: Arises when sound travels through a medium at speeds different from that of soft tissue, leading to overstated or understated reflector depths.

  • Characteristics:
  • If the medium's speed is slower, distances to reflectors will be overstimulated, making them appear deeper than they truly are.
  • If the medium's speed is faster, it will underestimate the distances.

• Lobes: Occurs when sound energy transmits in directions other than that of the main beam axis, degrading the quality of lateral resolution. Both side lobes, created by single crystal transducers, and grating lobes, from array transducers, can lead to artifact issues.

• Refraction: Occurs when a sound pulse changes direction as it travels across a boundary at an angle, contributing to lateral resolution degradation.

• Slice Thickness (Section Thickness) Artifact: Related to beam dimension and impacts the ability to visualize hollow structures such as cysts.

• Temporal Resolution Artifacts: Best reduced with high frame rates to capture moving structures accurately. Spatial resolution relates to the image detail, determined by line density; low line density corresponds to less detail.

Additional Artifacts and Errors

• Multipath Artifact: Arises when sound pulses glance off a second structure before forming a primary reflector, causing subtle nonspecific changes that cannot be identified accurately on the image.
• Range Ambiguity Artifact: Due to reflecting structures being deeper than the imaging depth; mitigated by increasing the pulse repetition period for deeper imaging to avoid confusion in reflector depth indications.
• Noise: Refers to small amplitude echoes resulting from electrical interference or signal processing errors, significantly affecting low-level hypoechoic regions, resulting in speckled images formed from constructive and destructive interference of small sound wavelets.

Summary Table: Artifacts Appearance and Causes