This copy is for personal use only. To order printed copies, contact [email protected] 1888 Soft-Tissue Infections and Their Imaging Mimics: From Cellulitis to Necrotizing Fasciitis1 Mohammad Reza Hayeri, MD, MPH Pouya Ziai, MD Infection of the musculoskeletal system can be associated with high MULTISYSTEM/GENERAL Monda L. Shehata, MD mortality and morbidity if not promptly and accurately diagnosed. Oleg M. Teytelboym, MD These infections are generally diagnosed and managed clinically; Brady K. Huang, MD however, clinical and laboratory findings sometimes lack sensitiv- ity and specificity, and a definite diagnosis may not be possible. In Abbreviations: AZT = azidothymidine, HIV = uncertain situations, imaging is frequently performed to confirm human immunodeficiency virus, NF = necrotiz- the diagnosis, evaluate the extent of the disease, and aid in treat- ing fasciitis, STIR = short t inversion-recovery ment planning. In particular, cross-sectional imaging, including RadioGraphics 2016; 36:1888–1910 computed tomography and magnetic resonance imaging, provides Published online 10.1148/rg.2016160068 detailed anatomic information in the evaluation of soft tissues due Content Codes: to their inherent high spatial and contrast resolution. Imaging find- ings of soft-tissue infections can be nonspecific and can have dif- 1From the Department of Radiology, Univer- sity of California at San Diego Teleradiology ferent appearances depending on the depth and anatomic extent of and Education Center, 8899 University Center tissue involvement. Although many imaging features of infectious Ln, Suite 370, San Diego, CA 92103 (M.R.H., B.K.H.); and Department of Radiology, Mercy disease can overlap with noninfectious processes, imaging can help Fitzgerald Hospital, Darby, Pa (P.Z., M.L.S., establish the diagnosis when combined with the clinical history O.M.T.). Recipient of a Certificate of Merit and laboratory findings. Radiologists should be familiar with the award for an education exhibit at the 2015 RSNA Annual Meeting. Received March 22, spectrum of imaging findings of soft-tissue infections to better aid 2016; revision requested May 2 and received the referring physician in managing these patients. The aim of this June 7; accepted June 17. For this journal-based SA-CME activity, the authors, editor, and re- article is to review the spectrum of soft-tissue infections using a viewers have disclosed no relevant relationships. systematic anatomic compartment approach. We discuss the clinical Address correspondence to M.R.H., Depart- ment of Radiology, University Hospitals of Case features of soft-tissue infections, their imaging findings with empha- Medical Center, 11100 Euclid Ave, Cleveland, sis on cross-sectional imaging, their potential mimics, and clinical OH 44106 (e-mail: [email protected]). management. ©RSNA, 2016 ©RSNA, 2016 • radiographics.rsna.org SA-CME LEARNING OBJECTIVES After completing this journal-based SA-CME activity, participants will be able to: Introduction ■■Understand the spectrum of soft-tissue Soft-tissue infections are common in clinical practice, encompassing infections, organized by an anatomic ap- a wide spectrum of pathologic conditions that range from involve- proach. ment of the skin to the deep soft tissues and bone. Prompt diagnosis ■■Describe the imaging appearances is key in the management algorithm to avoid further progression of soft-tissue infections, particularly at cross-sectional imaging. and complications. Although soft-tissue infection is mainly a clinical ■■Discuss soft-tissue processes that mim- diagnosis, even an expert physical examination is not sufficient to ic infections and their imaging features. define the extent and nature of the infection. In such circumstances, See www.rsna.org/education/search/RG. imaging becomes pivotal in differentiating various patterns of soft- tissue infections, depicting the extent of the disease, and identifying collections potentially amenable to intervention (1). In this article, we systemically review the spectrum of musculoskeletal soft-tissue infections organized by anatomic compartments from superficial to deep, excluding the bones, while providing relevant case examples and differential diagnoses. RG • Volume 36 Number 6 Hayeri et al 1889 spatial and contrast resolution, providing exquisite TEACHING POINTS anatomic and pathophysiologic information about ■■ The microbiology of NF is either polymicrobial (type 1), with the extent and degree of involvement of both the a mixture of aerobic and anaerobic organisms, or monobacte- soft tissues and the underlying bone (4). Opti- rial (type 2), for which the main organisms are Gram-positive cocci. Type 1 infections are more common and usually in- mum protocols include T1-weighted images (for volve the trunk and perineum, whereas type 2 infections are anatomic details) and short t inversion-recovery more frequently found in the extremities. (STIR) sequences or T2-weighted images that are ■■ The characteristics of involvement of the deep fasciae signifi- fat suppressed (for detection of soft-tissue edema). cantly associated with necrotizing infectious fasciitis include Intravenous contrast medium can be used to (a) extensive involvement of the deep intermuscular fascia, define extent of disease, outline abscess collections not just the area contiguous to the deep peripheral fascia, (b) thickening of fascia measuring 3 mm or more at STIR or or sinus tracts, and detect nonenhancing devital- fat-suppressed T2-weighted imaging, (c) involvement of three ized soft tissue. Contrast medium administration or more compartments, and (d) low signal intensity with fat- should be avoided if the renal function is moder- suppressed T2-weighted imaging in the deep fascia with cor- ately (ie, glomerular filtration rate [GFR] of 30–44 responding nonenhancement on postcontrast images. mL/min) to severely (GFR <30 mL/min) impaired ■■ Healthy muscles are generally resistant to infection, but if they to avoid risk of developing nephrogenic systemic are affected, a predisposing factor such as diabetes, drug use, fibrosis (5). Complementary sequences such as malnutrition, human immunodeficiency virus (HIV) infection, immunodeficiency, malignancy, or trauma usually coexists. diffusion-weighted imaging with apparent diffu- ■■ An intramuscular abscess is the hallmark of pyomyositis. In sion coefficient maps may be helpful by identifying contrast, viral myositis does not progress to abscess formation. abscesses (6). ■■ MR imaging findings in early subacute muscle denervation include diffuse T2 hyperintense signal and normal T1 intensity Anatomic signal of the muscles and absence of subcutaneous edema. Considerations and Terminology Anatomically, musculoskeletal infections can be classified into superficial and deep infections. Superficial infections are considered to include Imaging Techniques infections involving the skin and hypodermis, Radiography generally constitutes the initial whereas deep infections involve the soft tissues examination for patients with soft-tissue infec- at and below the level of the fascia (Fig 1). This tions. Findings pointing to possible inflammatory classification is oversimplified and arbitrary, as changes on radiographs include soft-tissue swell- infection spreads not only in horizontal planes ing, effacement of fat planes, and skin discon- but also vertically, invading the boundaries tinuity in the setting of deep ulcers. However, between the superficial and deep layers. This is these findings are nonspecific and can be seen in important in larger areas of the body, such as the other settings, such as trauma, systemic causes of trunk and extremities, where the natural lack of subcutaneous edema, venous insufficiency, and fibrous boundaries between subcutaneous tissues deep venous thrombosis. The presence of gas in and fascia enables widespread infection (7). the soft tissues or the identification of foreign The skin is the most superficial layer of the bodies generally raises concern for underlying soft tissues and consists of the epidermis and infectious processes. Moreover, radiography can dermis. These two layers of skin cannot be help exclude other causes of soft-tissue swelling, resolved with imaging. Therefore, diagnosis of such as underlying fractures (2). Ultrasonography epidermal infections (such as impetigo or furun- (US) can aid physicians in excluding noninflam- culosis) and dermal infections (such as erysipelas matory causes of soft-tissue swelling, such as deep or folliculitis) is almost always clinical, with a venous thrombosis and soft-tissue tumors. It also limited role for imaging in the diagnosis and provides localization of superficial nonradiopaque treatment of skin infections. Below the dermis is foreign bodies and real-time guidance for percu- the subcutaneous tissue or hypodermis, which taneous interventions and tissue sampling (3). is a fatty layer of variable thickness. The fascia is Computed tomography (CT) plays an important the next anatomic layer, with the most confus- role in the assessment of soft-tissue infection in ing and ambiguous terminology in the literature the emergency department due to its wide avail- and between surgeons and anatomists. Anato- ability, scanning speed, high spatial resolution, and mists classify the fascia into superficial and deep multiplanar reformatting capabilities (1). Overall, layers. Some designate the whole hypodermis as CT provides higher sensitivity for detection of superficial fascia, whereas others consider su- soft-tissue gas and