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Probe to use when evaluating for skin & bone: High resolution (7.5 MHz & above) linear transducer Cellulitis: (image 1) In cellulitis, anechoic (black) strands randomly traverse the subcutaneous tissues giving rise to a "cobblestone mucosa" appearance. These anechoic strands are felt to be subcutaneous edema or inflammatory exudate which forms between interlobular septa, separating fat lobules. (1-3) It should be noted that the appearance of cobblestone mucosa may occur in some cases of peripheral edema. Abscess: (image 2) The typical appearance of an abscess on ultrasound is that of an anechoic (black) mass with an echogenic (bright) wall. An abscess may contain internal echogenic material consistent with debris or the comet-tail artifact seen with gas 9discussed below). Occasionally, an abscess does not present itself in the typical fashion. It may be hyperechoic or isoechoic to its surrounding tissues, making diagnosis much more difficult. You can always tap over the area in question with the transducer and look for moving contents to confirm liquifaction. Subcutaneous Air: (image 3) When air is surrounded by tissue, it appears echogenic (bright white). The air also creates an echogenic tail distally (i.e. the comet-tail artifact, see arrow in image 3). As air accumulates, the tail portion of this artifact may look more grey than white & is sometimes described as a "dirty shadow". This is a very important finding clinically in an abscess, suggesting a gas producing anaerobic infection (i.e. gangrene, necrotizing fasciitis, etc). Note that the comet-tail artifact can also be prouced by metal (i.e. foreign bodies). Hematoma: (image 4) The appearance of hematomas on ultrasound are typically a hypoechoic mass with homogenous internal echoes. In other words, hematomas are often not as dark as free fluid. Additionally, as the blood cooagulates, it becomes more isoechoic with surrounding tissues. Keep in mind, that it is not always easy to differentiate an abscess from hematoma when looking at an anechoic or hypoechoic mass. Effusion: (image 5) Simple effusions tend to be anechoic without any internal signals. In addition, they tend to be very thin walled. If the effusion is from an inflammatory condition such as gout, there maybe small crystal fragments visible within the effusion. (1) Loyer,Dubrow,David, et al. Imaging of superficial soft-tissue infections:sonographic findings in cases of cellulitis or abscess. AJR 1996; 166: 149-52. (2) Craig. Infection:ultrasound-guided procedures. Rad Clinics Nor Amer 1999;37(4): 669-678 (3) Bureau,Chemm,Cardinal. Musculoskeletal infections: US manifestations. Radiographics 1999;19:1585-92. |