Artifacts

An artifact is any phenomenon that affects the acquisition or interpretation of an ultrasound image.  Artifacts can occur because of properties within the tissue itself, or be created by the anesthetist.  The most commonly seen artifacts are air artifact, shadow artifact, reverberation, and acoustic enhancement.

air artifact labeled IIAir artifact occurs when the transducer does not fully contact the skin.  Because of the large disparity in acoustic impedance between air and tissue, sound cannot penetrate the skin and is reflected off.  This phenomenon is common when imaging smaller anatomical surfaces such as the wrist or ankle.  In this image, air artifact is indicated by the yellow arrows.  To prevent or correct this, apply sufficient gel to the transducer, and apply even pressure.

Shadow artifact results from attenuation that occurs when an ultrasound wave comes in contact with tissues that have a high attenuation coefficient.  The major concern is the provider’s inability to identify structures in the shadow.  In the supraclavicular image to the left, shadowing (red arrows) occurs beneath the first rib, and the pleura is no longer visible.  Increasing the gain or adjusting the approach may reduce the effects of shadow artifact.

Reverberation occurs when ultrasound contacts a strong specular reflector such as this block needle.  The sound is reflected creating the initial image.  The remaining wave passes through the shaft until it contacts the back surface, where part of it is reflected back to the transducer.  This continues over and over, with sound wave “bouncing” in the shaft of the needle before returning to the transducer.  The strength of the echo is continually decreasing, and the time it takes to return is prolonged, resulting in multiple needles of decreasing intensity appearing.

Acoustic enhancement occurs when sound passes through an area of very low acoustic impedance into tissue whose impedance is much higher.  In the image of the femoral artery below, acoustic enhancement (yellow arrows) is seen at the bottom of the artery because sound traveling through the artery meets little impedance from the blood creating a higher than normal mismatch when it comes in contact with the intima on the inferior surface of the artery.  This causes the tissue to appear more echogenic than any other part of the artery, and can be misinterpreted as plaque formation within the vessel.  Viewing the vessel in a longitudinal view will confirm that this is indeed enhancement and not plaque.


References

Gray AT. Atlas of ultrasound-guided regional anesthesia. Philadelphia, PA. Saunders, Elsevier; 2010:8-9.

Sites B D, Brull R, Chan V W, et al. Artifacts and pitfall errors associated with ultrasound-guided regional anesthesia. part II: a pictoral approach to understanding and avoidance. Reg Anesth Pain Med. 2007; 32(5):419-433.

Pollard BA, Chan VW. An introductory curriculum for ultrasound-guided regional anesthesia: a learner’s guide. Toronto. University of Toronto Press Inc.; 2009:8-11.

 

 

 

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