Tumor-like Soft Tissue Lesions

Necrotizing Fasciitis

Necrotizing fasciitis is a rare, rapidly evolving, and life-threatening soft tissue infection causing thrombosis of the blood vessels located in the fascia. However, it is often difficult to diagnose, especially in subacute forms. Thus, it is often underestimated or confused with cellulitis or abscess. Consequently, early diagnosis is missed or delayed in 85% to 100% of cases in large published series. Unlike cellulitis, it typically extends into deep fascial planes. Extension along the deep fascial planes is therefore an important sign of the presence of life-threatening necrotizing fasciitis. The causative organisms are mostly group A hemolytic streptococci and Staphylococcus aureus , on occasion acting in synergy. Other pathogens, both aerobic and anaerobic, may also be involved. The presence of gas within the necrotized fasciae is characteristic but may be absent. Known predisposing factors include older age, especially in combination with immunocompromising conditions, such as malignancy, poor nutrition, and alcohol or drug abuse. It can also be found after trauma or around foreign bodies in surgical wounds. However, it can also appear in otherwise healthy subjects with no known risk factors ( eFig. 96-1 ). The clinical course can be fulminant, making early recognition mandatory. Nevertheless, even with surgery, mortality rates of 20% to 40% have recently been reported. Therefore, imaging must never further delay surgical treatment once the diagnosis is established.

Ultrasonography is not recommended in adults, as the infiltration of the hypodermis may substantially degrade image quality. On MRI, this condition shows as a hyperintense signal intensity on T2-weighted images extending into the deep fasciae with fluid collections, with thickening of the deep fasciae due to fluid accumulation and reactive hyperemia. Signs supportive of necrotizing fasciitis include extensive involvement of the deep intermuscular fasciae (high sensitivity but low specificity), thickening to more than 3 mm, and partial or complete absence on postgadolinium images of signal enhancement of the thickened fasciae (fairly high sensitivity and specificity).

After intravenous contrast administration, peripheral enhancement is seen. However, this presentation can be seen in other non-necrotizing infectious conditions. When no deep fascial involvement is revealed, necrotizing fasciitis can be excluded.

Abscess

A soft tissue abscess is a well-delineated fluid collection surrounded by a well-vascularized fibrous pseudocapsule. Although there will often be a suggestive preceding event (e.g., puncture) or underlying illness, it can also occur without a suggestive history or symptoms. Abscesses can be multiple and can distort normal muscle anatomy and fascial planes because of their inflammatory nature. Their margins can be well defined or infiltrating, depending on the organism involved.

Conventional radiography has little or no value in the imaging work-up. Ultrasonography reveals an elongated or lobulated fluid collection and can assist in guiding a puncture and subsequent aspiration. Some authors have reported that ultrasonography is more sensitive than CT, with CT being nevertheless more specific for superficial soft tissue abscesses. On MRI, an abscess is hypointense to isointense compared with muscle tissue on T1-weighted images. On T2-weighted images, the central portion of the abscess is usually hyperintense, although the capsule may appear isointense or hypointense relative to subcutaneous fat. On T1-weighted images, the pseudocapsule can have a variable signal intensity compared with skeletal muscle. After intravenous contrast medium injection, a peripheral rim of enhancement is seen, corresponding to the inflammatory and cellular component of the abscess ( eFig. 96-2 ). Some recent reports suggest that diffusion-weighted images are highly sensitive for abscess and may add specificity to contrast-enhanced T1-weighed images of ring-enhancing lesions ( eFig. 96-3 ). The sensitivity and specificity of diffusion-weighted images for detecting soft tissue abscesses have been recently reported to be 92% and 80%, respectively. When abscess occurs near bone, an associated osteomyelitis or a periosteal reaction can be seen ( eFig. 96-4 ). Concomitant inflammatory edematous changes in the surrounding soft tissues (muscle, subcutaneous tissue) can sometimes be seen.

However, imaging characteristics may be different in an immunocompromised host. The peripheral edema usually seen on T2-weighted images is sometimes absent. Similarly, T1-weighted images will not always show the pseudocapsule. The infected fluid in the center of the abscess can have an inhomogeneous signal intensity. If the content is sufficiently viscous, it can even show mild increased signal intensity on T1-weighted images. Enhancement after intravenous contrast medium injection can also be absent.

Pyomyositis

Pyomyositis, also known as bacterial myositis, is a rare cause of single or multiple abscesses of skeletal muscle of unknown etiology. In general, normal skeletal muscle has a high intrinsic resistance to bacterial infection and abscess formation. Thus, some authors suggest that underlying muscle damage may facilitate the onset of pyomyositis.

Whereas pyomyositis was initially encountered in tropical regions (e.g., after minor trauma or insect bite), in recent years, there has been an increased incidence of this disease in industrialized regions with a more temperate climate, especially in the presence of predisposing immune-compromising conditions, including diabetes, human immunodeficiency virus infection, and malignancy. In this regard, pyomyositis is considered one of the most common musculoskeletal complications of acquired immunodeficiency syndrome (AIDS). The causative organism is mostly S. aureus , although a number of other pathogens such as Streptococcus pyogenes and Mycobacterium tuberculosis have also been reported. Typically, pyomyositis arises from hematogenous spread of these organisms, usually with abscess formation. Blood cultures are positive in only 5% of cases, however, with a fatal outcome in 1.8% due to sepsis and shock. The muscles of the thigh and gluteus region are most often affected ( eFig. 96-5 ), although the infection can appear in many other locations.

On T1-weighted images, the abscess collection has a low signal intensity compared with surrounding muscle tissue. On occasion, a high-intensity peripheral rim is noted, probably representing blood breakdown products or granulation tissue. This has been described as very specific for infection.

Pus in the abscess can have an intermediate to high signal on T1-weighed images depending on the protein content. T2-weighed images reveal a hyperintense collection in the affected muscle, with increased signal in the surrounding muscle tissue representing edema, organized phlegmonous collections, or hyperemia. Intravenous administration of contrast material can further discriminate between viable and necrotic muscle tissue, with the latter lacking enhancement.

On occasion, the imaging presentation of pyomyositis can be confused with a sarcomatous lesion, especially when further clinical and biochemical information is inconclusive. Key elements in the differential diagnosis favoring an infectious origin are the extent of the perilesional inflammatory reaction and the possible association of cellulitis (in the absence of previous surgery or local radiotherapy).

Hydatid Cystic Disease

Hydatid cystic disease is a parasitic disease usually caused by the tapeworm parasite Echinococcus granulosus . Infection by E. multilocularis is more rare but has a more invasive nature, sometimes mimicking a malignant lesion. Hydatid cystic disease is a rare finding in Western countries. It is more common in parts of South America, the Middle East, Africa, Australia, and Mediterranean areas with sheep rearing, where the parasite is endemic. Although it may affect any organ, soft tissue involvement is unusual (1.75% to 2.42%) because intramuscular growth of a cyst is countered by muscle contractility and lactic acid. Soft tissue hydatid cysts are nevertheless usually intramuscular and most frequently found in the head, neck, trunk, and the root of the extremities. A subcutaneous localization is also possible.

The imaging characteristics of soft tissue involvement resemble those of hydatid cysts found in the liver, showing a multiseptate or multicystic mass surrounded by a rim. Typically, the lesion consists of a mother cyst, containing multiple daughter cysts. On T1-weighted images, these daughter cysts are seen as hypointense cysts within the intermediate signal of the mother cyst. The signal intensity of the daughter cysts on T2-weighted images can be high or low, with some authors suggesting a relation with the presence and absence, respectively, of viable scolices. Still, the value of MRI in determining the viability of the cysts remains controversial.

A rim of low and/or high signal intensity on T2-weighted images surrounds the lesion. This rim is composed of three layers: an endocyst, ectocyst, and pericyst. The pericyst develops as a reaction after compression and inflammation of surrounding tissue. It is well vascularized, enhancing after intravenous contrast injection, and thus resembles a pseudocapsule of sarcoma.

More solid appearances are also possible, making it sometimes difficult to differentiate it from other soft tissue tumors. Even in these cases, MRI can often reveal the vesicular nature of the lesion.

Focal Myositis

Focal myositis is a relatively rare, usually self-limiting soft tissue pseudotumor. Although it can occur in many locations, it is usually found in the lower extremities, with 50% of cases located in the thigh and 25% in the lower leg. There is no sex or age preference. In approximately one third of patients, focal myositis evolves to polymyositis or a polymyositis-like syndrome, suggesting that focal myositis may be a localized form of polymyositis.

Focal myositis usually presents as a sometimes painful local intramuscular soft tissue mass, which can grow rapidly in a few weeks ( eFig. 96-6 ). Although the process is normally limited to a single muscle, involvement of multiple muscles is possible. MRI reveals a heterogeneous signal pattern, with increased signal intensity on T2-weighted images, in one or more affected muscle groups. MRI can also clearly depict the extensive surrounding edema. A focal mass, when visualized, may enhance less than the surrounding edema.

Diabetic Muscle Infarction

Diabetic myopathy encompasses a spectrum of diseases, including muscle inflammation, ischemia, hemorrhage, infarction, necrosis, fibrosis, and fatty atrophy. Diabetic muscle infarction is a rare complication of diabetes mellitus. Patients with poorly controlled type 1 insulin-dependent diabetes mellitus and severe end-organ damage are most frequently affected, although it may occur in type 2 diabetic patients or even in a well-controlled individual without known diabetic complications. Although the pathogenesis is still to be completely clarified, diabetic myopathy is believed (like other long-term complications of diabetes) to be secondary to a microvascular pathologic process inducing inflammation, ischemia, and infarction of the affected muscles. The typical clinical presentation is an abrupt onset of pain and swelling of the affected muscles. However, MRI can detect subclinical muscle infarction months before the onset of clinical symptoms. In about 80% of cases, the large muscles of the thigh are most frequently affected, with an involvement of calf muscles in 20%. Whether MRI in conjunction with additional clinical and biochemical information can reliably establish the diagnosis remains nevertheless unclear. As a consequence, the place of biopsy in the diagnosis remains controversial.

MR images display enlargement of the involved muscles, with uniform increased signal intensity on T2-weighted and short tau inversion recovery (STIR) images, demonstrating the edematous and inflammatory changes ( Fig. 96-4 ). T1-weighted images show normal or decreased signal intensity in the involved muscles, with the swelling being sometimes less appreciated on this sequence. The role of intravenous contrast medium injection in the differentiation with other entities such as pyomyositis, muscle abscess, or a necrotic tumor is not yet clearly established.

Bursitis

Bursae are defined as spaces near joints containing small amounts of fluid, thereby reducing friction between different structures. Whereas more than 140 different bursae have been described, the most frequently affected ones are the trochanteric, subdeltoideal, ischiogluteal, pes anserina, iliopsoas, and retrocalcaneal and olecranon bursae.

The amount of fluid may increase as a result of inflammation, which can have an infectious or noninfectious origin (overuse, direct trauma). In noninfectious bursitis, repeated movements cause microtrauma in the tendon, tendon sheaths, and/or bursae. Bursitis may also be a first presentation of rheumatoid arthritis.

Infectious bursitis is a more rare pathologic process, usually associated with Staphylococcus aureus infection or, on rare occasions, with β-hemolytic streptococci. It frequently affects the olecranon and prepatellar and infrapatellar bursa, probably because of their superficial location, which makes them susceptible to trauma and subsequent infection ( Fig. 96-5A ).

When superficially located, bursae can be easily detected with ultrasound, on occasion demonstrating increased vascular signal on power Doppler imaging corresponding to infection or inflammation (see Fig. 96-5B ). On MRI, the increased fluid is hypointense on T1-weighted images and hyperintense on T2-weighted images. After intravenous administration of contrast medium, enhancement of hypertrophied synovium and surrounding soft tissue edema can be seen in both infectious and noninfectious bursitis ( Fig. 96-6 ).

Although no single imaging feature is able to reliably distinguish infectious from noninfectious bursitis, the combination of bone erosions with marrow edema is more suggestive for septic bursitis. Other features favoring an infectious origin are marked synovial thickening, synovial edema, soft tissue edema, and a complex appearance of the lesion. The anatomic location of bursitis is often characteristic (e.g., iliopsoas bursitis) ( Fig. 96-7 ), further adding to the correct diagnosis.

Sarcoidosis

Sarcoidosis is a multisystem disorder of unknown etiology that involves multiple organs. Muscle involvement in sarcoidosis is rare, reported in only 1.4% to 6% of patients with known sarcoidosis. Three main clinical presentations of muscular sarcoidosis can be distinguished: an acute myositic form, a diffuse atrophic form, and a nodular form. The acute myositis type occurs exclusively in the early stage of sarcoidosis, presenting as myalgia secondary to inflammation. MRI is usually negative, presumably because of the sparse distribution and small size of epithelioid cell granulomas.

In the diffuse atrophic myopathic form, patients can present with myalgia, muscle weakness, and atrophy. The muscles of the proximal portions of the extremities are frequently involved. MRI findings are nonspecific, revealing proximal muscle atrophy with fatty replacement.

The least common form is the nodular presentation, presenting as single or multiple, often bilateral, sarcoid nodules ( eFig. 96-7 ). These nodules appear elongated and extend along muscle fibers. On ultrasound examination, sarcoid nodules present as a hyperechoic center and a hypoechoic peripheral zone. They may also present with well-defined borders and an overall hypoechogenic aspect.

On MRI, the nodules may have a star-shaped hypointense center on all axial pulse sequences (“dark star” sign) that is believed to correspond with fibrous tissue, which does not enhance after intravenous administration of a contrast agent. However, this central structure is not present in the acute stage of the disease. It can also be absent in small nodules (<10 mm), presumably because of the short time of granulomatous inflammation in these small structures.

The peripheral area of the nodules is slightly hyperintense compared with muscle on T1-weighted images and has homogeneous high signal intensity on T2-weighted images. There is homogeneous enhancement after intravenous administration of a contrast medium, secondary to the high cellularity of granulomas and edema.

Coronal and sagittal images may show the “three stripes” sign, consisting of a hypointense inner stripe and hyperintense outer stripes. After corticosteroid therapy, the sarcoid nodules may disappear, or only the inner stripe may be visualized.