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Imaging of Musculoskeletal Soft Tissue Infection
Author’s Accepted Manuscript Imaging of Musculoskeletal Soft Tissue Infection Ching-Di Chang, Jim S. Wu
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To appear in: Seminars in Roentgenology Cite this article as: Ching-Di Chang and Jim S. Wu, Imaging of Musculoskeletal Soft Tissue Infection, Seminars in Roentgenology, http://dx.doi.org/10.1053/j.ro.2016.10.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Imaging of Musculoskeletal Soft Tissue Infection Ching-Di Chang, MD1 and Jim S. Wu, MD2
1. Department of Radiology Kaohsiung Chang Gung Memorial Hospital Chang Gung University College of Medicine 123 Ta-Pei Road, Niao-Sung District, Department of Radiology, Kaohsiung Chang Gung Memorial Hospital, Kaohsiung city, 833, Taiwan Email: [email protected] Phone Number: 886-7-7317123, ext. 2579 Fax Number: 886-7-7318762 2. Department of Radiology Beth Israel Deaconess Medical Center Harvard Medical School 330 Brookline Ave Boston, MA 02215 Email: [email protected] Phone: 617-667-1283 Fax: 617-667-8212 (Dr. Wu is the corresponding author) Disclosure statement The authors have nothing to disclose
Introduction Musculoskeletal soft tissue infections account for over 2 million emergency department visits in United States annually, mostly related to cellulitis or soft tissue abscess.1 Infection of musculoskeletal soft tissues can be classified by the anatomic structures they involve: (1) skin and subcutaneous tissues (cellulitis, abscess), (2) fascia (necrotizing fasciitis), (3) muscle (infectious myositis and pyomyositis), (4) bursae (infectious bursitis), (5) and tendons and tendon sheaths (suppurative tenosynovitis). It is important to describe the location and appearance of the infection as they have important treatment implications. Some conditions can be treated with antibiotics alone, whereas others require drainage or surgery. This characterization is best achieved with imaging.
Clinical features and diagnostic tests Predisposing factors for soft tissue infection include old age, immunosuppression, intravenous drug use, human immunodeficiency virus (HIV) infection, diabetes, peripheral vascular disease, alcoholism, malnutrition, and poor socioeconomic status.2-7 Infection can occur from hematogenous spread, but more commonly arises
via local spread from broken skin, such as a small cut, surgery, penetrating trauma, or diabetic ulcer.6,8 The gold standard for the diagnosis of infection is a positive culture from affected tissue. However, obtaining sufficient tissue for analysis can be a problem, especially if the site is anatomically difficult to access or if there are surface contaminants such as with skin ulcers. Clinical signs include fever, erythema, tenderness, swelling, and crepitus of the infected area. Blood tests can show leukocytosis and elevation of C-reactive protein, erythrocyte sedimentation rate, and procalcitonin.9
Imaging Modalities With soft tissue infections, management depends to a large extent on the location and extent of involvement, which can be best assessed on imaging. Radiographs should be the first line modality. They are relatively inexpensive, quick to perform, and can identify foreign bodies, osseous involvement, soft tissue swelling, and soft tissue gas as in necrotizing fasciitis (figure 1A). Ultrasound (US) can demonstrate fluid collections and is helpful in distinguishing between cystic and solid lesions. This can be especially important when searching for an abscess collection.10 US also can
be useful in the assessment of foreign bodies, which can be missed on CT (computed tomography) and radiographs if not radiopaque. Finally, US can provide real-time images and a means for performing fine needle aspiration, drainage, or biopsy.11,12 However, US is highly operator dependent, is limited for assessing deep complex lesions and osseous pathology, and may underestimate the extent of the lesion.13 CT is excellent for evaluating bony structures, but can also play an important role in the assessment of soft tissue infection. It is quick to perform and allows for multi-planar reformations, which can be particularly useful in the emergency department.14 CT is likely the best imaging modality to assess for soft tissue gas indicative of infection, such as in necrotizing fasciitis (figure 1). Both soft tissue and bone algorithms should be used, and the administration of intravenous contrast can help identify rim-enhancing abscess collections. Magnetic resonance imaging (MRI) is likely the best overall test to assess soft tissue infection. It has superb soft tissue contrast, can delineate the location and extent of disease involvement, and provides assessment of the adjacent bone marrow, which is especially important in cases with concurrent early osteomyelitis.14,15 If there is no contraindication, gadolinium-based contrast material should be given if there is a
suspicion for infection, to assess for an abscess, and to identify necrotic tissue for surgical debridement in necrotizing fasciitis. Disadvantages of MRI include its long imaging time, higher cost, and poor assessment for soft tissue gas and foreign bodies. Nuclear medicine imaging (bone and white blood cell scans) is less useful in soft tissue infection because of its low spatial resolution and long scan time. However, it can provide good overall assessment of the entire body and be useful as an alternative in patients with contraindication to MRI.6,16
Skin and subcutaneous tissue (cellulitis and abscess) Cellulitis refers to infection of the dermis and subcutaneous tissue17 that can cause pain, erythema, and tissue edema. It typically develops after disruption of the skin and invasion by microorganisms,18 especially Gram-positive cocci such as Staphylococcus aureusand-haemolytic streptococci.17,19 Cellulitis is generally a clinical diagnosis, and imaging is unnecessary in most cases.18 Indications for imaging are failure of the patient to improve on appropriate antibiotics, presence of complications (abscess formation or osteomyelitis), or suspicion of an alternative diagnosis. Radiographic signs of cellulitis are nonspecific,
with only soft tissue swelling and loss of fat planes typically seen. US can demonstrate a diffuse increase in echogenicity of involved skin and subcutaneous tissue. There can be a “cobblestone” appearance to the subcutaneous tissue, with anechoic strands passing between subcutaneous fat representing inflammatory exudate dissecting the tissue (figure 2A).20,21 On CT, cellulitis appears as skin thickening, increased stranding in the subcutaneous tissues, and superficial fascial thickening without involvement of the underlying deep structures.14 On MRI, cellulitis appears as skin thickening, ill-defined T2 hyperintense and T1 hypointense streaks in the subcutaneous tissue, due to inflammation and edema (figure 2B).22 With contrast administration, diffuse enhancement of the soft tissues typically occurs.23,24 Many of these imaging findings of cellulitis are nonspecific and can be seen with such non-infectious causes as trauma or systemic disorder ( heart failure, renal insufficiency, and liver disease). In these cases, it is important to scrutinize the relevant clinical history. Cellulitis can be complicated by an abscess, which is defined as a collection of purulent fluid in a confined tissue space.25 Doppler US can detect an abscess as an anechoic or hypoechoic fluid collection with increased periphery vascularity.21 An
abscess appears as a low-attenuation fluid collection with rim enhancement on CT, and as a lesion with central fluid-like signal intensity with a peripheral enhancing rim on MRI (figure 3).6 Abscesses, especially those >5cm, cannot be treated by antibiotics alone as the capsule will prevent the medication from reaching a sufficient drug concentration level to effectively treat the infection. Thus, surgical or percutaneous drainage of an abscess is needed.21,26
Fascia (necrotizing fasciitis) Necrotizing fasciitis (NF) is a rapidly progressing, deep infection that causes necrosis of subcutaneous tissue and fascia.14 Causes include post-surgical wound infection, trauma, cutaneous disease, and perirectal abscess.27 NF has a high mortality rate (about one-third) and is almost uniformly fatal without treatment.28 The process initially begins in the superficial fascial planes and progresses into deep fascial layers, causing necrosis by microvascular occlusion.29 Patients present with local pain, erythema, swelling, tenderness, and crepitus on palpation. NF is uncommon, though its incidence is rising due to more patients with such predisposing factors as diabetes, alcoholism, organ transplantation, immunodeficiency, and malignancy.14,30,31
Two-thirds of cases are caused by mixed aerobic-anaerobic flora, which are often accompanied by species of gas-forming anaerobic bacteria such as Enterobacteriaceae and Clostridium.31 Fournier`s gangrene is a specific form of NF that involves the perineum. NF is a clinical diagnosis, with imaging of value in early detection and to demonstrate extension of the infection. In patients with unstable vital signs, treatment should not be delayed for an imaging study. Plain radiographs are of limited value, but can demonstrate gas along fascial planes or subcutaneous tissues in infections caused by gas-forming organisms. CT is valuable for demonstrating soft tissue gas and is often easier and quicker to obtain than MRI.14 Typical CT features include inflammatory fat stranding, dermal thickening, crescentic fluid or gas along superficial or deep fascial planes, and soft tissue edema.14,29,32,33 Moreover, CT is the most sensitive modality for detecting abnormal gas in soft tissues (figure 4), the hallmark of NF14,29 but seen in only half the cases.32 The superior soft tissue contrast and excellent anatomic detail makes MRI an excellent imaging modality for the diagnosis of NF. MRI can show (a) deep fascial thickening (>3mm) and extensive deep multi-compartment involvement on
T2-weighted , fat-saturated images,; (b) hypointensity along the deep fascia representing gas accumulation, best seen on T2-weighted gradient echo sequences; and (c) focal or diffuse non-enhancing portions in the abnormal fascia (though gadolinium should be withheld in patients with renal failure).29,34,35 However, high signal intensity in deep fascial planes on T2-weighted images (representing fluid distributed along deep fascia) and variable enhancement patterns are not specific for NF,34,36-38 so it is crucial to correlate this appearance with other imaging findings as well as clinical symptoms and signs. Early diagnosis and extensive debridement is essential for improving prognosis and survival rate.27,28
Muscle (infectious myositis and pyomyositis) Infectious myositis refers to infection of skeletal muscle, which can be due to bacteria, viruses, fungi, or parasites. The term “pyomyositis” is often misused as an abscess within the muscle, but specifically refers to bacterial infection of skeletal muscle that frequently leads to abscess formation.6 The most common pathogen is Staphylococcus aureus (>75% cases).39 Causes include hematogenous spread from bacteremia or direct extension from skin injury.7,14,39 Infective myositis is most
frequent in the young population,39 and the most commonly affected muscles are the quadriceps (>25%), iliopsoas, and gluteus.39,40 Pyomyositis has three stages – (1) diffuse muscle infection (phlegmon), (2) abscess formation, and (3) sepsis.39,41 Uncomplicated pyomyositis can be treated with antibiotics, while an abscess may require drainage. In the early stage of pyomyositis (phlegmon), there can be localized muscle edema that appears hypoechoic on US. There can be muscular enlargement with low attenuation and effacement of intermuscular fat planes on CT. On MRI, there is intermediate T1 signal and high intensity on fluid-sensitive sequences with enhancement following contrast medium administration (figure 5).6,14,41,42 Diffusion weighted MR imaging (DWI) can be helpful in equivocal cases by showing restricted diffusion in the abscess.43
Bursa (infectious bursitis) A bursa is a fluid-filled sac that is lined by synovial cells, which is often found around but outside of the joint and can become inflamed or infected. When inflamed, such as in rheumatoid arthritis or gout, it is called bursitis; when infected, the term
“infectious bursitis” should be used. Infectious bursitis is most often caused by Staphylococcus aureus44,45, with steroid and alcohol use the most frequent predisposing factors.45 The prepatellar and olecranon bursae are most commonly affected because of their superficial location and propensity to trauma resulting in skin breakage.45,46 No specific imaging finding can differentiate septic bursitis from inflammatory bursitis, though the presence of gas within the bursa suggests an underlying infectious process.6,47 If there is concern for infectious bursitis, aspiration and culture of bursal fluid is required. Antibiotic therapy is often sufficient to treat infectious bursitis, but percutaneous drainage or surgical management is required if the condition does not resolve with antibiotics alone.45 On plain radiographs, there may be a soft tissue lump in the expected location of the bursa, but deeper bursae are more difficult to visualize. US shows heterogeneous echogenicity within the distended bursa with peribursal edema and hyperemic changes in the bursal wall.6,41 On CT, the distended bursa can contain low-attenuation material, and there can be increased peripheral stranding and an enhancing wall (figure 6). On MRI, fluid within the bursa has variable signal intensity on fluid-sensitive sequences and low signal intensity on T1-weighted images.47 Lack of
bursal and peripheral soft tissue enhancement can help to exclude septic bursitis.47 Knowing the common location of bursae can help to differentiate bursitis from an abscess. In the absence of a reliable imaging modality to differentiate infectious bursitis from reactive inflammatory change, prompt aspiration should be performed if the precise diagnosis is unclear.
Tendon and tendon sheath (suppurative tenosynovitis) Suppurative tenosynovitis is an infection involving the closed synovial sheath of tendons.46 Predominantly caused by Staphylococcus aureus or Streptococcus pyogenes after penetrating trauma,46,48 it most commonly affects the hand and wrist, particularly the flexor tendons.46,48,49 Early diagnosis and aggressive surgical treatment is crucial to prevent tendon necrosis and extracompartmental spread of infection, especially in view of the high risk of morbidity with flexor tendon involvement.46 Chronic infection caused by atypical mycobacterium can have devastating results and even lead to amputation.49 Patients complain of pain, tenderness, and swelling along the distribution of the affected tendon sheath. Diagnosis of acute infectious tenosynovitis is made by culture of synovial fluid
obtained from either aspiration or surgery. Radiographs in acute infectious tenosynovitis show nonspecific soft tissue swelling. On US, there can be hyperemic changes in the tendon sheath with accumulation of fluid containing echogenic debris.41,46 MRI is the preferred imaging modality in diagnosing tenosynovitis because of its high sensitivity and ability to detect extension of disease. On MRI, fluid within the tendon sheath has high T2 and low-to-intermediate T1 signal intensity, with contrast enhancement of the synovial sheath (figure 7). There also may be thickening of the tendon itself and loss of its normal low signal.46 Chronic tenosynovitis, which can be caused by fungal or mycobacterial infection, manifests as scarce fluid within the tendon sheath; thickening of the synovial membrane; thickening, necrosis, or disruption of the tendon; and stenosis and hyperemia of the tendon sheath.6,41,46
Mimickers of soft tissue infection Muscle edema Muscle edema appears as increased signal on fluid-sensitive MRI sequences that reflects infectious myositis or early pyomyositis. However, this is a nonspecific
finding that also can be seen in acute dermatomyositis, polymyositis, trauma, radiation treatment, denervation, and compartment syndrome.50 In dermatomyositis and polymyositis, muscle edema typically appears as symmetric involvement of bilateral proximal muscles (figure 8), especially the vastus lateralis and vastus intermedius in the thigh.51 Trauma-related muscle edema is localized to the injured muscle area, while radiation-induced vasculitis and tissue injury results in muscle edema and subcutaneous injury with a sharp margin limited to the radiation field.52 Edema also can develop in acute and subacute denervation within 24-48 hours53 and may be associated with corresponding neuropathy. In patients with compartment syndrome, muscle edema is located within the affected compartment.
Morel Lavallée lesion A Morel Lavallée lesion is a hemolymphatic mass located superficial to the deep fascia after a degloving injury, which causes skin and subcutaneous tissue to separate from the underlying deep fascia.54,55 This typically occurs over the greater trochanteric region of the femur and proximal thigh.54,55 MRI demonstrates variable signal characteristics based on the stage of the hematoma and the contents of the
lesion. High T1 signal represents acute or subacute hematoma.55 Water signal intensity may reflect seroma, while T2 hyperintensity with patchy internal enhancement is consistent with chronic organizing hematoma.55 Large collections can become superinfected or lead to tissue necrosis if not evacuated.
Diabetic myonecrosis Myonecrosis is an uncommon complication of longstanding, poorly controlled type 1 and type 2 diabetes, which usually affects lower limb muscles and is due to muscle infarction.56,57 This myonecrosis is believed to be caused by microangiopathy leading to muscle inflammation, ischemia, and hemorrhagic infarction.57,58 Patients typically experience the sudden onset of severe pain, with 40% having bilaterally involvement.59 On MRI, diabetic muscular ischemia appears as a hyperintense area on fluid-sensitive sequences and demonstrates homogeneous enhancement.59 Central non-enhancing foci surrounded by an enhancing rim on T1-weighted, fat-saturated images represents infarction or necrosis (figure 9), while focal T1 high signal within the non-enhancing region reflects hemorrhage.59 Although a typical clinical presentation is the key to diagnosis, aspiration or biopsy can be performed when an
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Figures
Figure 1. Necrotizing fasciitis. A 52-year-old man with pain and swelling in the elbow region. (A) Lateral radiograph shows abnormal gas (arrows) within soft tissue. (B) Corresponding axial CT image demonstrates abnormal gas (arrows) distributed along intermuscular septa, edematous change of muscle (white asterisks), and diffuse subcutaneous infiltration (arrowheads).
Figure 2. Cellulitis. (A) US and (B) corresponding coronal T1-weighted MR image in a 52-year-old man show skin thickening (arrows), septations, and edema of subcutaneous tissue (arrowheads). There is no involvement of deep structures. Note the “cobblestone” appearance in the subcutaneous soft issues due to perifascial fluid interposed between the echogenic subcutaneous fat.
Figure 3. Pyomyositis with abscess formation. (A) Coronal T2-weighted, fat-saturated image and (B) sagittal T1-weighted, post-contrast, fat saturated MR images in a 54-year-old woman demonstrate an abscess (arrowhead) with central region that is T2 hyperintense in the left adductor longus muscle. Note the rim enhancement (arrow) after contrast administration.
Figure 4. Necrotizing fasciitis. 22-year-old man who stepped on a fish hook. Axial CT image of the thighs demonstrates extensive gas throughout the anterior compartments of both thighs and the right gluteus maximus muscle (arrowheads).
Figure 5. Iliopsoas abscess. 57-year-old man with heavy alcohol consumption. Coronal contrast CT image shows multiloculated rim-enhancing fluid collection (white asterisk), with gas (arrowheads) in the left iliopsoas muscle.
Figure 6. Infectious trochanteric bursitis. Axial contrast CT image in a 72-year-old man shows a distended right trochanteric bursa with internal gas (arrowhead), enhancing wall (arrows), and prominent peribursal soft tissue edema. Aspiration yielded Staphylococcus aureus.
Figure 7. Suppurative tenosynovitis. (A) Axial T2 weighted, fat-saturated MR image and (B) axial T1 weighted, post-contrast, fat-saturated image in a 44-year-old female demonstrate fluid (black asterisk) distending the flexor tendon sheath of the wrist, with enhancement of the tendon sheath wall (arrows). The hypointense tendons remain intact (arrowheads).
Figure 8. Dermatomyositis. (A) Axial T2-weighted, fat saturated MR image in a 58-year-old male shows extensive muscle edema (white asterisks) in the anterior and medial compartments of both thighs. This nonspecific appearance can be seen with several muscle disorders. The patient had typical skin changes and symmetric proximal muscle weakness, and dermatomyositis was confirmed on surgical muscle biopsy.
Figure 9. Diabetic myonecrosis. 62-year-old woman with long-standing poorly controlled diabetes. T1-weighted, post-contrast MR image demonstrates rim-enhancing foci (arrowheads) in the muscles of the posterior thigh.
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S0037-198X(16)30053-0 http://dx.doi.org/10.1053/j.ro.2016.10.001 YSROE50579
To appear in: Seminars in Roentgenology Cite this article as: Ching-Di Chang and Jim S. Wu, Imaging of Musculoskeletal Soft Tissue Infection, Seminars in Roentgenology, http://dx.doi.org/10.1053/j.ro.2016.10.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Imaging of Musculoskeletal Soft Tissue Infection Ching-Di Chang, MD1 and Jim S. Wu, MD2
1. Department of Radiology Kaohsiung Chang Gung Memorial Hospital Chang Gung University College of Medicine 123 Ta-Pei Road, Niao-Sung District, Department of Radiology, Kaohsiung Chang Gung Memorial Hospital, Kaohsiung city, 833, Taiwan Email: [email protected] Phone Number: 886-7-7317123, ext. 2579 Fax Number: 886-7-7318762 2. Department of Radiology Beth Israel Deaconess Medical Center Harvard Medical School 330 Brookline Ave Boston, MA 02215 Email: [email protected] Phone: 617-667-1283 Fax: 617-667-8212 (Dr. Wu is the corresponding author) Disclosure statement The authors have nothing to disclose
Introduction Musculoskeletal soft tissue infections account for over 2 million emergency department visits in United States annually, mostly related to cellulitis or soft tissue abscess.1 Infection of musculoskeletal soft tissues can be classified by the anatomic structures they involve: (1) skin and subcutaneous tissues (cellulitis, abscess), (2) fascia (necrotizing fasciitis), (3) muscle (infectious myositis and pyomyositis), (4) bursae (infectious bursitis), (5) and tendons and tendon sheaths (suppurative tenosynovitis). It is important to describe the location and appearance of the infection as they have important treatment implications. Some conditions can be treated with antibiotics alone, whereas others require drainage or surgery. This characterization is best achieved with imaging.
Clinical features and diagnostic tests Predisposing factors for soft tissue infection include old age, immunosuppression, intravenous drug use, human immunodeficiency virus (HIV) infection, diabetes, peripheral vascular disease, alcoholism, malnutrition, and poor socioeconomic status.2-7 Infection can occur from hematogenous spread, but more commonly arises
via local spread from broken skin, such as a small cut, surgery, penetrating trauma, or diabetic ulcer.6,8 The gold standard for the diagnosis of infection is a positive culture from affected tissue. However, obtaining sufficient tissue for analysis can be a problem, especially if the site is anatomically difficult to access or if there are surface contaminants such as with skin ulcers. Clinical signs include fever, erythema, tenderness, swelling, and crepitus of the infected area. Blood tests can show leukocytosis and elevation of C-reactive protein, erythrocyte sedimentation rate, and procalcitonin.9
Imaging Modalities With soft tissue infections, management depends to a large extent on the location and extent of involvement, which can be best assessed on imaging. Radiographs should be the first line modality. They are relatively inexpensive, quick to perform, and can identify foreign bodies, osseous involvement, soft tissue swelling, and soft tissue gas as in necrotizing fasciitis (figure 1A). Ultrasound (US) can demonstrate fluid collections and is helpful in distinguishing between cystic and solid lesions. This can be especially important when searching for an abscess collection.10 US also can
be useful in the assessment of foreign bodies, which can be missed on CT (computed tomography) and radiographs if not radiopaque. Finally, US can provide real-time images and a means for performing fine needle aspiration, drainage, or biopsy.11,12 However, US is highly operator dependent, is limited for assessing deep complex lesions and osseous pathology, and may underestimate the extent of the lesion.13 CT is excellent for evaluating bony structures, but can also play an important role in the assessment of soft tissue infection. It is quick to perform and allows for multi-planar reformations, which can be particularly useful in the emergency department.14 CT is likely the best imaging modality to assess for soft tissue gas indicative of infection, such as in necrotizing fasciitis (figure 1). Both soft tissue and bone algorithms should be used, and the administration of intravenous contrast can help identify rim-enhancing abscess collections. Magnetic resonance imaging (MRI) is likely the best overall test to assess soft tissue infection. It has superb soft tissue contrast, can delineate the location and extent of disease involvement, and provides assessment of the adjacent bone marrow, which is especially important in cases with concurrent early osteomyelitis.14,15 If there is no contraindication, gadolinium-based contrast material should be given if there is a
suspicion for infection, to assess for an abscess, and to identify necrotic tissue for surgical debridement in necrotizing fasciitis. Disadvantages of MRI include its long imaging time, higher cost, and poor assessment for soft tissue gas and foreign bodies. Nuclear medicine imaging (bone and white blood cell scans) is less useful in soft tissue infection because of its low spatial resolution and long scan time. However, it can provide good overall assessment of the entire body and be useful as an alternative in patients with contraindication to MRI.6,16
Skin and subcutaneous tissue (cellulitis and abscess) Cellulitis refers to infection of the dermis and subcutaneous tissue17 that can cause pain, erythema, and tissue edema. It typically develops after disruption of the skin and invasion by microorganisms,18 especially Gram-positive cocci such as Staphylococcus aureusand-haemolytic streptococci.17,19 Cellulitis is generally a clinical diagnosis, and imaging is unnecessary in most cases.18 Indications for imaging are failure of the patient to improve on appropriate antibiotics, presence of complications (abscess formation or osteomyelitis), or suspicion of an alternative diagnosis. Radiographic signs of cellulitis are nonspecific,
with only soft tissue swelling and loss of fat planes typically seen. US can demonstrate a diffuse increase in echogenicity of involved skin and subcutaneous tissue. There can be a “cobblestone” appearance to the subcutaneous tissue, with anechoic strands passing between subcutaneous fat representing inflammatory exudate dissecting the tissue (figure 2A).20,21 On CT, cellulitis appears as skin thickening, increased stranding in the subcutaneous tissues, and superficial fascial thickening without involvement of the underlying deep structures.14 On MRI, cellulitis appears as skin thickening, ill-defined T2 hyperintense and T1 hypointense streaks in the subcutaneous tissue, due to inflammation and edema (figure 2B).22 With contrast administration, diffuse enhancement of the soft tissues typically occurs.23,24 Many of these imaging findings of cellulitis are nonspecific and can be seen with such non-infectious causes as trauma or systemic disorder ( heart failure, renal insufficiency, and liver disease). In these cases, it is important to scrutinize the relevant clinical history. Cellulitis can be complicated by an abscess, which is defined as a collection of purulent fluid in a confined tissue space.25 Doppler US can detect an abscess as an anechoic or hypoechoic fluid collection with increased periphery vascularity.21 An
abscess appears as a low-attenuation fluid collection with rim enhancement on CT, and as a lesion with central fluid-like signal intensity with a peripheral enhancing rim on MRI (figure 3).6 Abscesses, especially those >5cm, cannot be treated by antibiotics alone as the capsule will prevent the medication from reaching a sufficient drug concentration level to effectively treat the infection. Thus, surgical or percutaneous drainage of an abscess is needed.21,26
Fascia (necrotizing fasciitis) Necrotizing fasciitis (NF) is a rapidly progressing, deep infection that causes necrosis of subcutaneous tissue and fascia.14 Causes include post-surgical wound infection, trauma, cutaneous disease, and perirectal abscess.27 NF has a high mortality rate (about one-third) and is almost uniformly fatal without treatment.28 The process initially begins in the superficial fascial planes and progresses into deep fascial layers, causing necrosis by microvascular occlusion.29 Patients present with local pain, erythema, swelling, tenderness, and crepitus on palpation. NF is uncommon, though its incidence is rising due to more patients with such predisposing factors as diabetes, alcoholism, organ transplantation, immunodeficiency, and malignancy.14,30,31
Two-thirds of cases are caused by mixed aerobic-anaerobic flora, which are often accompanied by species of gas-forming anaerobic bacteria such as Enterobacteriaceae and Clostridium.31 Fournier`s gangrene is a specific form of NF that involves the perineum. NF is a clinical diagnosis, with imaging of value in early detection and to demonstrate extension of the infection. In patients with unstable vital signs, treatment should not be delayed for an imaging study. Plain radiographs are of limited value, but can demonstrate gas along fascial planes or subcutaneous tissues in infections caused by gas-forming organisms. CT is valuable for demonstrating soft tissue gas and is often easier and quicker to obtain than MRI.14 Typical CT features include inflammatory fat stranding, dermal thickening, crescentic fluid or gas along superficial or deep fascial planes, and soft tissue edema.14,29,32,33 Moreover, CT is the most sensitive modality for detecting abnormal gas in soft tissues (figure 4), the hallmark of NF14,29 but seen in only half the cases.32 The superior soft tissue contrast and excellent anatomic detail makes MRI an excellent imaging modality for the diagnosis of NF. MRI can show (a) deep fascial thickening (>3mm) and extensive deep multi-compartment involvement on
T2-weighted , fat-saturated images,; (b) hypointensity along the deep fascia representing gas accumulation, best seen on T2-weighted gradient echo sequences; and (c) focal or diffuse non-enhancing portions in the abnormal fascia (though gadolinium should be withheld in patients with renal failure).29,34,35 However, high signal intensity in deep fascial planes on T2-weighted images (representing fluid distributed along deep fascia) and variable enhancement patterns are not specific for NF,34,36-38 so it is crucial to correlate this appearance with other imaging findings as well as clinical symptoms and signs. Early diagnosis and extensive debridement is essential for improving prognosis and survival rate.27,28
Muscle (infectious myositis and pyomyositis) Infectious myositis refers to infection of skeletal muscle, which can be due to bacteria, viruses, fungi, or parasites. The term “pyomyositis” is often misused as an abscess within the muscle, but specifically refers to bacterial infection of skeletal muscle that frequently leads to abscess formation.6 The most common pathogen is Staphylococcus aureus (>75% cases).39 Causes include hematogenous spread from bacteremia or direct extension from skin injury.7,14,39 Infective myositis is most
frequent in the young population,39 and the most commonly affected muscles are the quadriceps (>25%), iliopsoas, and gluteus.39,40 Pyomyositis has three stages – (1) diffuse muscle infection (phlegmon), (2) abscess formation, and (3) sepsis.39,41 Uncomplicated pyomyositis can be treated with antibiotics, while an abscess may require drainage. In the early stage of pyomyositis (phlegmon), there can be localized muscle edema that appears hypoechoic on US. There can be muscular enlargement with low attenuation and effacement of intermuscular fat planes on CT. On MRI, there is intermediate T1 signal and high intensity on fluid-sensitive sequences with enhancement following contrast medium administration (figure 5).6,14,41,42 Diffusion weighted MR imaging (DWI) can be helpful in equivocal cases by showing restricted diffusion in the abscess.43
Bursa (infectious bursitis) A bursa is a fluid-filled sac that is lined by synovial cells, which is often found around but outside of the joint and can become inflamed or infected. When inflamed, such as in rheumatoid arthritis or gout, it is called bursitis; when infected, the term
“infectious bursitis” should be used. Infectious bursitis is most often caused by Staphylococcus aureus44,45, with steroid and alcohol use the most frequent predisposing factors.45 The prepatellar and olecranon bursae are most commonly affected because of their superficial location and propensity to trauma resulting in skin breakage.45,46 No specific imaging finding can differentiate septic bursitis from inflammatory bursitis, though the presence of gas within the bursa suggests an underlying infectious process.6,47 If there is concern for infectious bursitis, aspiration and culture of bursal fluid is required. Antibiotic therapy is often sufficient to treat infectious bursitis, but percutaneous drainage or surgical management is required if the condition does not resolve with antibiotics alone.45 On plain radiographs, there may be a soft tissue lump in the expected location of the bursa, but deeper bursae are more difficult to visualize. US shows heterogeneous echogenicity within the distended bursa with peribursal edema and hyperemic changes in the bursal wall.6,41 On CT, the distended bursa can contain low-attenuation material, and there can be increased peripheral stranding and an enhancing wall (figure 6). On MRI, fluid within the bursa has variable signal intensity on fluid-sensitive sequences and low signal intensity on T1-weighted images.47 Lack of
bursal and peripheral soft tissue enhancement can help to exclude septic bursitis.47 Knowing the common location of bursae can help to differentiate bursitis from an abscess. In the absence of a reliable imaging modality to differentiate infectious bursitis from reactive inflammatory change, prompt aspiration should be performed if the precise diagnosis is unclear.
Tendon and tendon sheath (suppurative tenosynovitis) Suppurative tenosynovitis is an infection involving the closed synovial sheath of tendons.46 Predominantly caused by Staphylococcus aureus or Streptococcus pyogenes after penetrating trauma,46,48 it most commonly affects the hand and wrist, particularly the flexor tendons.46,48,49 Early diagnosis and aggressive surgical treatment is crucial to prevent tendon necrosis and extracompartmental spread of infection, especially in view of the high risk of morbidity with flexor tendon involvement.46 Chronic infection caused by atypical mycobacterium can have devastating results and even lead to amputation.49 Patients complain of pain, tenderness, and swelling along the distribution of the affected tendon sheath. Diagnosis of acute infectious tenosynovitis is made by culture of synovial fluid
obtained from either aspiration or surgery. Radiographs in acute infectious tenosynovitis show nonspecific soft tissue swelling. On US, there can be hyperemic changes in the tendon sheath with accumulation of fluid containing echogenic debris.41,46 MRI is the preferred imaging modality in diagnosing tenosynovitis because of its high sensitivity and ability to detect extension of disease. On MRI, fluid within the tendon sheath has high T2 and low-to-intermediate T1 signal intensity, with contrast enhancement of the synovial sheath (figure 7). There also may be thickening of the tendon itself and loss of its normal low signal.46 Chronic tenosynovitis, which can be caused by fungal or mycobacterial infection, manifests as scarce fluid within the tendon sheath; thickening of the synovial membrane; thickening, necrosis, or disruption of the tendon; and stenosis and hyperemia of the tendon sheath.6,41,46
Mimickers of soft tissue infection Muscle edema Muscle edema appears as increased signal on fluid-sensitive MRI sequences that reflects infectious myositis or early pyomyositis. However, this is a nonspecific
finding that also can be seen in acute dermatomyositis, polymyositis, trauma, radiation treatment, denervation, and compartment syndrome.50 In dermatomyositis and polymyositis, muscle edema typically appears as symmetric involvement of bilateral proximal muscles (figure 8), especially the vastus lateralis and vastus intermedius in the thigh.51 Trauma-related muscle edema is localized to the injured muscle area, while radiation-induced vasculitis and tissue injury results in muscle edema and subcutaneous injury with a sharp margin limited to the radiation field.52 Edema also can develop in acute and subacute denervation within 24-48 hours53 and may be associated with corresponding neuropathy. In patients with compartment syndrome, muscle edema is located within the affected compartment.
Morel Lavallée lesion A Morel Lavallée lesion is a hemolymphatic mass located superficial to the deep fascia after a degloving injury, which causes skin and subcutaneous tissue to separate from the underlying deep fascia.54,55 This typically occurs over the greater trochanteric region of the femur and proximal thigh.54,55 MRI demonstrates variable signal characteristics based on the stage of the hematoma and the contents of the
lesion. High T1 signal represents acute or subacute hematoma.55 Water signal intensity may reflect seroma, while T2 hyperintensity with patchy internal enhancement is consistent with chronic organizing hematoma.55 Large collections can become superinfected or lead to tissue necrosis if not evacuated.
Diabetic myonecrosis Myonecrosis is an uncommon complication of longstanding, poorly controlled type 1 and type 2 diabetes, which usually affects lower limb muscles and is due to muscle infarction.56,57 This myonecrosis is believed to be caused by microangiopathy leading to muscle inflammation, ischemia, and hemorrhagic infarction.57,58 Patients typically experience the sudden onset of severe pain, with 40% having bilaterally involvement.59 On MRI, diabetic muscular ischemia appears as a hyperintense area on fluid-sensitive sequences and demonstrates homogeneous enhancement.59 Central non-enhancing foci surrounded by an enhancing rim on T1-weighted, fat-saturated images represents infarction or necrosis (figure 9), while focal T1 high signal within the non-enhancing region reflects hemorrhage.59 Although a typical clinical presentation is the key to diagnosis, aspiration or biopsy can be performed when an
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Figures
Figure 1. Necrotizing fasciitis. A 52-year-old man with pain and swelling in the elbow region. (A) Lateral radiograph shows abnormal gas (arrows) within soft tissue. (B) Corresponding axial CT image demonstrates abnormal gas (arrows) distributed along intermuscular septa, edematous change of muscle (white asterisks), and diffuse subcutaneous infiltration (arrowheads).
Figure 2. Cellulitis. (A) US and (B) corresponding coronal T1-weighted MR image in a 52-year-old man show skin thickening (arrows), septations, and edema of subcutaneous tissue (arrowheads). There is no involvement of deep structures. Note the “cobblestone” appearance in the subcutaneous soft issues due to perifascial fluid interposed between the echogenic subcutaneous fat.
Figure 3. Pyomyositis with abscess formation. (A) Coronal T2-weighted, fat-saturated image and (B) sagittal T1-weighted, post-contrast, fat saturated MR images in a 54-year-old woman demonstrate an abscess (arrowhead) with central region that is T2 hyperintense in the left adductor longus muscle. Note the rim enhancement (arrow) after contrast administration.
Figure 4. Necrotizing fasciitis. 22-year-old man who stepped on a fish hook. Axial CT image of the thighs demonstrates extensive gas throughout the anterior compartments of both thighs and the right gluteus maximus muscle (arrowheads).
Figure 5. Iliopsoas abscess. 57-year-old man with heavy alcohol consumption. Coronal contrast CT image shows multiloculated rim-enhancing fluid collection (white asterisk), with gas (arrowheads) in the left iliopsoas muscle.
Figure 6. Infectious trochanteric bursitis. Axial contrast CT image in a 72-year-old man shows a distended right trochanteric bursa with internal gas (arrowhead), enhancing wall (arrows), and prominent peribursal soft tissue edema. Aspiration yielded Staphylococcus aureus.
Figure 7. Suppurative tenosynovitis. (A) Axial T2 weighted, fat-saturated MR image and (B) axial T1 weighted, post-contrast, fat-saturated image in a 44-year-old female demonstrate fluid (black asterisk) distending the flexor tendon sheath of the wrist, with enhancement of the tendon sheath wall (arrows). The hypointense tendons remain intact (arrowheads).
Figure 8. Dermatomyositis. (A) Axial T2-weighted, fat saturated MR image in a 58-year-old male shows extensive muscle edema (white asterisks) in the anterior and medial compartments of both thighs. This nonspecific appearance can be seen with several muscle disorders. The patient had typical skin changes and symmetric proximal muscle weakness, and dermatomyositis was confirmed on surgical muscle biopsy.
Figure 9. Diabetic myonecrosis. 62-year-old woman with long-standing poorly controlled diabetes. T1-weighted, post-contrast MR image demonstrates rim-enhancing foci (arrowheads) in the muscles of the posterior thigh.