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Common Artifacts

Artifacts are any perceived distortion, error, or specious addition to the image created during ultrasound signal processing. Failing to recognize an artifact exists may make you believe you see an image on the display when it actually does not exist. Here we will review the most common artifacts you will encounter in clinical practice. The way to troubleshoot these typically involve changing the angle of your probe and interrogating the area of interest on multiple views.

Reverberation Artifact

Reverberations are formed when a sound wave bounces between 2 strong specular reflectors (the reflectors are at 90 degrees to the ultrasound beam). These strong reflectors cause the ultrasound beam to bounce back and forth between them. Once this reverberated ultrasound beam comes back to the machine, it interprets an image as if it is from deeper structures since it took longer to get back to the transducer. The echoes that appear at increasing depths do not correspond to any anatomic structure. Think of the ultrasound beam as being partially trapped in between them and only returning at a later time. The result is of equally spaced images beyond the actual structure of interest. 

To eliminate this effect attempt tilting the transducer so that the reflective surface is no longer at 90 degrees to the ultrasound beam by altering the angle of incidence between the beam and the structure.

 

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Pleural line

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A-line

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Examples of reverberation artifacts seen while examining the lung. 1, the display shows multiple, equally spaced echoes creating equally spaced hyperechoic A-lines representing reverberations of the pleural line. Blue arrows denote the direction of the ultrasound beam. Notice that in i the beam reflects back but is trapped by the pleural line in ii, causing a delay of the return of signal to the probe; 2, Comet-tail artifacts forming B lines that originate from the pleural line and appear hyperechoic that extend indefinitely erasing A-lines and moving in concert with lung sliding. 

Acousic Shadowing

When an a structure reflects off most of the ultrasound beam that structure is typically bone, calcium or metal. This casts a shadow on the structures that lie deeper. Shadows appear hypoechoic or anechoic and you will not be able to see any structure deeper than the barrier that reflected the ultrasound beam. This artifact is common when interrogating in between rib spaces.

Air has a greater attenuation coefficient than that of bone, and creates dropout shadows on the structures that lie beyond. This is the reason it may be challenging to evaluate the lung in patients who have subcutaneous emphysema.

Below you we see air in the stomach on a patient and thus causes an acoustic shadow on deeper structures. Also rib shadow on the right upper quadrant creating hypoechoic regions and dropout of the liver. 

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Acoustic shadowing. In 1, we appreciate a full stomach and the air within that content creates an acoustic shadow obscuring deeper structures. The arrows on the clip mark the limit of the shadowing artifact. In clip 2 we can also see ribs (arrow) creating this effect and obscuring the liver and the kidney.

Acoustic Enhancement

Acoustic enhancement occurs when ultrasound energy passes relatively unattenuated through a medium with a low attenuation coefficient such as fluid. Beyond this fluid collection the structures appear hyperechoic. As the ultrasound beam encounters an area beyond a low-attenuated medium (urinary bladder), there is augmentation of the returning signal that  appears brighter. The time gain compensation can help adjust the gain on the deeper structures so that they appear darker.  In the images below we see enhancement caused by a pleural effusion and the internal jugular vein. 
 

Acoustic enhancement noted by the arrows in each of the clips.

Refraction artifacts

Refraction artifacts occur when the ultrasound beam is bent from its original direction as it passes through a boundary between tissues having different sound speeds.

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Refraction artifacts examples. In clip 1,  a needle acts as a strong reflector that bends the ultrasound beam. In clip 2 we can appreciate side lobe artifact (blue arrow). This is due ultrasound side lobes reflecting on a strong reflector outside of the central beam. These side lobe beams are secondary beams as crystals also expand and contract radially. Side lobe beams are low intensity beams that surround a central beam.  

References:

1. Antonakakis, John G. MD*; Sites, Brian MD† The 5 Most Common Ultrasound Artifacts Encountered During Ultrasound-guided Regional Anesthesia, International Anesthesiology Clinics: Fall 2011 - Volume 49 - Issue 4 - p 52-66 doi: 10.1097/AIA.0b013e318219b67a