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Artifacts

Artifacts

Artifacts are images that cannot be correlated with true anatomical structures. In ultrasound, artifacts can produce structures that appear in the image but are not present anatomically. Structures which are present anatomically may be completely absent. Artifacts can cause structures that are present to be visualized in an incorrect location.

Basic Assumptions

There are several assumptions made by an ultrasound machine during imaging and the process of image generation.

1.      Sound travels in a straight line

2.      Reflections are produced by structures along the beams main axis

3.      Sound travels at exactly 1540 m/s

4.      Intensity of a reflection directly corresponds to a reflectors scattering strength

5.      The imaging plane is very thin. Sound beams travel directly to a reflector and back to source.

Commonly Encountered Artefacts

Artifacts occur because the basic assumptions of ultrasound imaging are violated. Some artefacts may hinder imaging, whilst others may lead to potential false diagnosis. In some organs such as the lung, artefacts play an important role in detecting pathology.

  1. Mirror Imaging Artifact
  2. Acoustic Shadowing Artifact.
  3. Posterior Acoustic Enhancement.
  4. Edge Shadowing Artifact.
  5. Reverberation Artifact.
  6. Comet Tail Artifact.
  7. Ring Down Artifact.
  8. Side Lobe Artifact.

Mirror Image Artifact

Fig 1.10. The liver is mirrorred in the lung above the diaphram, a strong reflector. Dwyer, Kristin & Rempell, Joshua. (2016). Young Woman with a Fever and Chest Pain. Western Journal of Emergency Medicine. Image available via license: Creative Commons Attribution 4.0 International

Edge Shadowing Artifact

Fig 1.13. Edge artifacts are caused by scattering and refraction of sound energy by a curved reflector. This leads to reduced intensity of the reflected ultrasound beam to the transducer, causing a shadow appearance.

Ring Down Artifact

Fig 1.16. Example of ring down artifact. These are B lines, seen in lungs when the alveoli are fluid filled. They are not the same as comet tails, although the 2 labels are wrongly used interchangeably at times.

The source of ring-down artifact is a small pocket of fluid trapped by surrounding air bubbles. Ultrasound waves hit a pocket of trapped fluid, the sound waves resonate within the fluid, and a continuous sound wave is transmitted back to the transducer. The resonant vibrations detected by the ultrasound transducer are displayed as bright vertical lines extending deep to the trapped fluid.

Acoustic Shadow

Fig 1.11. Ultrasound beam does not penetrate bone well as it is a strong reflector. As a result the lung beneath the ribs are not isonated and appear to be shadowed.

Reverberation Artifact

Fig 1.14. Reverberation artifacts are ghost images caused by reflection of ultrasound waves between 2 strong reflectors. This causes delayed return of the beam to the transducer leading the machine to interpret the presence of multiple ghost images at equidistant intervals below the true image.

Side Lobe Artifact

Fig 1.17. Image by Shigemura, available via license: Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International. Side and grating lobe artifacts result in the blurring of the edges of a displayed object (reduce lateral resolution); the assumption that the ultrasound waves are infinitely thin is violated.

Acoustic Enhancement

Fig. 1.12. Acoustic enhancement refers to increased echoes deep to structures that transmit sound exceptionally well. This is characteristic of fluid-filled structures such as cysts and the gallbladder.

Comet Tail Artifact

Fig 1.15. Comet tail artifacts are well described in lung ultrasound. They are not the same as B-Lines. Comet tail artifacts arise from the pleural surface, are typically less than 1cm in length and can be seen in all areas of the lung, best with a high frequency probe. They are caused by a reverberation mechanism.

Video: Artifacts

Video from: American Thoracic Society: Ultrasound Artifacts: Authors Sachita Shah, M.D. Amy Morris, M.D. Manjiri Dighe, M.D. Institution University of Washington