What are some common artifacts seen in musculoskeletal Ultrasound?
It is important to be aware of common artifacts that are often seen with musculoskeletal US, not only to avoid misinterpreting a finding, but also because artifacts may provide valuable diagnostic clues about underlying pathology. Posterior acoustic shadowing is present when the ultrasound beam is refracted, reflected, or absorbed, resulting in an anechoic region deep to a structure or interface.
Several entities that may result in posterior acoustic shadowing include gas, bone, calcification, and even some foreign bodies. Posterior acoustic shadowing behind bone and calcifications is often referred to as “clean” shadowing and appears uniformly anechoic due to the majority of the ultrasound beam being absorbed. Posterior acoustic shadowing caused by gas is often referred to as “dirty” shadowing and results in multiple low-level echoes due to the degree of reflection at gas/tissue interfaces.
When comparing solid and fluid-containing structures, the ultrasound beam is gradually attenuated to a greater extent by a solid structure than by one containing fluid. Therefore, the strength of the ultrasound beam is greater after passing through fluid and results in stronger reflections which appear brighter than the reflections from the same interface located deep to solid tissues. This artifact is known as increased through transmission or posterior acoustic enhancement, and it appears on the image as a bright band extending deep to an object. This artifact may be useful in distinguishing between cystic and solid masses, but it is important to understand that some solid masses (i.e., nerve sheath tumors) may attenuate sound less than the adjacent soft tissues, resulting in increased through transmission of a solid mass.
Reverberation is another commonly observed artifact in musculoskeletal US. Reverberation occurs when the ultrasound beam reflects off of a smooth and flat interface (i.e., bone, metal implant, etc.) and back to the transducer multiple times, resulting in a series of linear echoes extending deep to the smooth, flat interface. If the reflected echoes appear continuous deep to the flat interface, the term ring down is often used. This artifact makes US an important modality to evaluate soft tissues/structures adjacent to surgical hardware because ring down artifact occurs deep to the hardware and will not obscure superficial regions of interest.