ARTIFACTS IN MSK ULTRASOUND

Artifacts in most imaging techniques (Plain film, CT scans, & MRI) effect image quality and reduce resolution. MSK ultrasound can also be affected leading to improper diagnosis. Certain MSK ultrasound artifacts are sometimes .beneficial in assisting with the diagnosis.

ANISOTROPY (Anisotropic Reflectors) Tendons are highly anisotropic and to a much lesser degree are Muscles, Ligaments, and Nerves. Anisotropy occurs when the transducer probe’s Angle of Incidence is less than (90 degrees) or perpendicular to the underlying anatomical structure. This results in fewer returning echoes to the probe resulting in a hypoechoic (dark) image. This image artifact commonly occurs at tendon and fascia attachment to bone (Achilles & Plantar Fascia). This phenomenon can lead to misinterpretation and can be overcome by simple movement of the probe side to side in the transverse (axial) image and heel to toe in the longitudinal (sagital) image. If the hypoechoic area of the anatomical structure disappears with this technique when the probe becomes perpendicular to the structure, you can interpret this as an artifact. This movement of the transducer probe can be beneficial when a tendon is surrounded by fat that has the same echogenicity as the tendon, and will appear hypoechoic to the fat.

SHADOWING – This artifact occurs at a highly reflective interface (Bone, Air, & Calcifications) in which most of the sound wave energy is reflected and only a very minimal amount of energy passes through resulting in a signal void. The hypoechoic image deep to this interface will cast a shadow due to the differences in acoustic impedance.

DIRTY SHADOWING – This artifact is a characteristic exhibited when gas is present within the soft tissues. False echos will occur deep to the highly reflective soft tissue/gas interface.

REFRACTILE SHADOWING or CRITICAL ANGLE SHADOWING – This artifact is seen at curved surfaces ( Tendons, Diaphysis Region of Bones, Large Vessels) along the lateral margins of the object, and at the retracted torn ends of tendons and muscle. This shadowing occurs from a combination of refraction and reflection. Clinically this is helpful when imaging torn tendons as this is indicative of a full thickness tear. The degree of tendon separation can be seen as the distance between the acoustic shadowing deep to the torn tendon ends.

ENHANCED THROUGH-TRANSMISSION – This artifact is seen when the sound wave passes through a fluid filled structure (Cyst, Bursa) resulting in a false impression of increased echogenicity of the deeper structure. This occurs due to signal not being attenuated within the fluid and time gain compensation.

COMET TAIL ARTIFACT – Metal and glass will cause the sound wave to reflect between the anterior and posterior surfaces (Reverberation) of the object imaged, resulting in characteristic bands with increased echogenicity inferior to the object. The signal intensity of the bands will decrease with depth and give the appearance of a comet tail.

REVERBERATION – This artifact can cause phantom structures or mirror images when imaging near the tibia and calf region. The sound wave is reflected back and forth resulting in a mirror image on the underside of the tibial cortex.

REFRACTION – This artifact occurs below the interface of two dissimilar structures with different acoustic impedances and is also dependant upon the angle of incidence. The sound wave is “bent” below the interface resulting in the deeper structures being in a false location. This artifact can be minimized by placing the transducer probe perpendicular (90 degrees) to the object being imaged.