What is the ultrasound findings of musculoskeletal infection?
Infection of the superficial soft tissues, or cellulitis, has a variety of sonographic appearances depending on the time course of imaging. In the acute setting, cellulitis appears as swollen, hyperechoic subcutaneous tissue with hyperemia on color Doppler imaging. As time goes on, cellulitis may appear hypoechoic with the development of anechoic branching channels and distortion of soft tissue. In the setting of cellulitis, the observation of perifascial fluid and hyperechoic foci of gas with associated dirty shadowing and/or comet tail artifact may represent necrotizing fasciitis, a surgical emergency.
An abscess may have a variety of appearances on US but typically presents as a fairly well-defined hypoechoic, heterogeneous fluid collection with increased posterior acoustic enhancement and peripheral hyperemia on Doppler imaging. It is not uncommon to observe central foci/pockets of gas and a thickened, echogenic hyperemic abscess wall. The use of transducer pressure to induce the swirling of internal contents is helpful to differentiate an abscess from a seroma. Additionally, changing scanning parameters such as increasing the field of view and depth may make posterior acoustic enhancement through a suspected abscess appear more conspicuous compared to adjacent background structures.
Septic bursitis refers to the involvement of a bursa by infection. On US, septic bursitis may appear as synovitis and complex fluid within a bursa. Rarely, foci of gas with associated comet tail artifact may be identified. When attempting to differentiate septic bursitis from an abscess, the most important thing to consider is location. Septic bursitis will occur in the location of a known bursa and often appear more well defined than an abscess. Infectious myositis may appear as an enlarged hypoechoic muscle with associated hyperemia. It is important to look for intramuscular fluid collections to exclude pyomyositis.
If the soft tissue infection is located adjacent to bone, osteomyelitis may be present. If US demonstrates cortical irregularity or erosions, it is important to consider osteomyelitis, which may be more fully characterized with MRI. Septic arthritis may appear identical to inflammatory arthritis upon US examination. If infection cannot be definitively excluded, direct sampling of joint fluid is indicated. In the setting of septic arthritis, fluid is identified distending a joint recess. Unfortunately, the appearance of the fluid (i.e., hypoechoic versus hyperechoic) and associated hyperemia is not useful in discerning between infection and inflammation.
Comet tail artifact is another form of reverberation which appears as a short trail of posterior bright echoes that narrow as they extend from the source of the artifact. This is often seen with soft tissue gas and allows for ease of identification.
Refraction is often seen as shadowing at the edge of a curved surface such as those typically present in cystic structures. Ultrasound waves oriented at a tangential angle to a cyst wall or curved surface are scattered and refracted, resulting in the appearance of a linear shadow due to a lack of echoes returning to the transducer from the lateral cyst wall and anything in a direct path posterior to it. Occasionally, this artifact may confuse the unwary into assuming a cystic structure has a defect within it. This spurious assumption is easily corrected by changing the angle of the ultrasound beam.