Ultrasound artifacts: understanding and interpreting them (reverberation, comet tail, shadow cones, etc.)

Clinical ultrasound is a diagnostic tool that is increasingly used in various medical specialties to enrich the physical examination. However, the quality and accuracy of the diagnosis largely depends on the correct understanding and interpretation of the ultrasound images. 

An artifact is a non-real, even parasitic effect that can sometimes complicate the diagnostic process. To do this, it is imperative to know how to distinguish between echoes that do not correspond to an anatomical structure and those that represent real anatomical structures. It is also important to realize that, against all odds, artifacts can be very useful in interpreting ultrasound images. 

Artifacts can have various origins: they can be linked to the physical phenomena of ultrasound, to the type of ultrasound scanner you're using, to a possible malfunction of your device or to the environment. 

This article aims to provide a basic understanding of some common types of artifact in ultrasound imaging. Through a non-exhaustive list, it explores their definition and examines their impact on diagnostic imaging related to the physical phenomenon of ultrasound.

The posterior shadow cone, pure or impure

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The posterior shadow cone occurs when ultrasound encounters a structure that blocks or absorbs its propagation, creating a shadow area behind that structure. The posterior shadow cone can be classified into two categories: pure and impure.

A pure (homogeneous) posterior shadow cone occurs when the ultrasound is completely blocked or absorbed by the structure, resulting in a complete absence of echo in the area behind. This artifact is often seen behind calcified structures, such as gallstones, kidneys, or bladders, and can help confirm their presence.

On the other hand, an impure (heterogeneous)posterior shadow cone occurs when the ultrasound is partially blocked or absorbed by the structure, resulting in a partial reduction of echoes in the area behind. This artifact can be seen behind structures such as fibrous tissue or air interfaces such as the lung or intestinal gases.

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Posterior reinforcement

Posterior reinforcement occurs when ultrasound passes through a structure that conducts sound well, such as a fluid structure or cyst, and continues to propagate in a straight line, resulting in an increase in the amplitude of posterior echoes to the structure. This artifact is also known as acoustic enhancement or transmission augmentation.

Posterior enhancement is often seen behind fluid-filled structures and can help confirm their presence. However, it can also be seen behind solid structures, such as benign tumors or lymph nodes, which can complicate diagnostic imaging.

It's important to note that posterior enhancement can also be observed in the presence of other artifacts, such as reverberations or spurious reflections.

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Stray reflections 

Stray reflections occur when ultrasound is reflected from a structure that is not in the imaging plane, creating ghosting images that can be mistaken for real structures. This artifact is also known as phantom echoes or multiple echoes.

Stray reflections can be caused by highly reflective interfaces, such as stones or air interfaces, or by moving structures, such as blood flow. These multiple reflections can create phantom images that appear at different depths from the actual structure, which can complicate diagnostic imaging.

1. The reverberation artifact 

The reverberation artifact occurs when ultrasound is reflected multiple times between two strongly reflective interfaces, creating a series of equidistant echoes behind the reflective structure. 

The A-lines seen in lung imaging, where ultrasound is reflected by the pleural line, are a case in point.

2. The comet-tail artefact 

The comet-tail artifact is a specific form of reverberation artifacts. These artifacts manifest as small, downward-facing, triangular deviations, often associated with structures such as the pleural line, metal objects such as guidewires, and valve replacements. Unlike artefactial echoes of resonance, or "ring down" (e.g., pulmonary B-lines), which persist with depth, these reverberation artifacts fade. The B lines are indicative of pulmonary edema, inflammation, or fibrosis, highlighting their clinical importance in the evaluation of pulmonary conditions.

To reduce spurious reflections, it's important to optimize imaging parameters such as depth, gain and frequency.

3. Mirror images

Mirroring occurs when ultrasound is reflected from a highly reflective interface, creating a duplicated image on the other side of the interface. This artifact may look like an actual structure, but is actually a replica of the structure on the other side of the reflective interface.

4. Edge shadow cone

Edge shadow cone occurs when ultrasound bypasses a structure, creating a shadow area behind the structure. This artifact is often seen when imaging convex structures, such as ovarian follicles or thyroid cysts. The edge shadow cone can help identify the shape and contours of a structure, but can also complicate diagnostic imaging by masking the structures behind the convex structure. 

Edge shadowing can be reduced by adjusting the ultrasonic angle of incidence.

5. Anisotropy

Anisotropy occurs when ultrasound passes through a structure that has different acoustic properties in different directions. This can cause the echo intensity to vary depending on the angle of incidence of the ultrasound, creating an inaccurate ultrasound image of the structure.

Anisotropy is often observed in the imaging of tendons and muscles, where fibers have a preferential orientation. When ultrasound passes through these fibers at an oblique angle, it can lead to a reduction in echo intensity, giving the impression that the structure is thinner or more irregular than it actually is. 

To reduce anisotropy, it's important to optimize the angle of incidence of ultrasound.

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Conclusion

Understanding and correctly interpreting artifacts in ultrasound is crucial for an accurate diagnosis. While these artifacts can sometimes complicate diagnostic imaging, they can also provide useful information about tissue properties and pathological conditions. By being aware of these artifacts and their impact on ultrasound images, you can significantly improve your diagnostic accuracy and refer your patients more effectively.

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