Acute infectious cellulitis is an infection of the subcutaneous fat not extending beyond the superficial fascial planes (Fig. 21.5). It is usually associated with a hemolytic group A Streptococcus infection. Cellulitis will only on rare occasions present as a soft tissue mass [174]. An association with abscesses, ascending lymphangitis, regional lymphadenitis, osteomyelitis, and pyoarthrosis is possible [79]. Lymphedema may mimic infectious cellulitis; but the latter is more localized than lymphedema, which tends to affect the entire extremity.

CT shows diffuse infiltration of the subcutaneous fat and thickening of the skin. On MRI, cellulitis appears as an ill-defined area, hypointense on T1-weighted sequences and hyperintense on T2-weighted sequences [150]. Cellulitis may be diagnosed when T2-weighted images reveal subcutaneous thickening with fluid collections and when subcutaneous tissue, superficial fascia, or both show contrast enhancement [165]. The depth of soft tissue involvement of the infection can be best evaluated on T2-weighted images [150]. However, as the sensitivity of magnetic resonance imaging exceeds the specificity, the extent of the deep fascial involvement can be overestimated [165], leading to the wrong diagnosis of necrotizing fasciitis.

Necrotizing fasciitis is a rare soft tissue infection, involving deep fascial planes. It has a predilection for older patients, especially for those with malignancy, poor nutrition, and alcohol or drug abuse. It can also be found after trauma, or around foreign bodies in surgical wounds. However, it is important to remember that it can also appear in otherwise healthy subjects with no known risk factors. Early recognition is critical, since this entity is a surgical emergency. The clinical course can be fulminant and the mortality rate can be as high as 73 % [150]. The causative organisms are mostly group A hemolytic Streptococcus and Staphylococcus aureus, on occasion acting in synergy. Other both aerobic and anaerobic pathogens may also be involved [34].

Necrotizing fasciitis has similar signal behavior on MR as cellulitis, except for a deeper extension. A hyperintense signal on T2-weighted images in deep fasciae with fluid collections, thickening, and peripheral enhancement after intravenous contrast medium injection suggests necrotizing fasciitis [165]. However, this is not typical for necrotizing soft tissue infection, as other non-necrotizing conditions can have similar MR signal characteristics [112]. Therefore, clinical correlation is of utmost importance [34].

Lymphedema is classified as primary or secondary lymphedema. The primary form is more common in children and is associated with a variety of hereditary or genetic syndromes [194]. Secondary lymphedema has no age preference. Local causes include trauma, surgery, infection, chronic inflammation, and radiotherapy (Fig. 21.6). It can also be associated with systemic disease, with a more generalized edema. Secondary lymphedema is usually a clinical diagnosis.

MRI of chronic lymphedema reveals deformity of lymphatics at different tissue levels [107]. In the subcutaneous tissue, it shows a diffuse edema or a honeycomb pattern consistent with reticular lymphangiectasia and “lakes” with increased signal intensity on T2-weighted images [107].

(MR) lymphography and lymphoscintigraphy can give additional information on lymphatic morphology and function [123, 194].

A soft tissue abscess is a well-delineated fluid collection surrounded by a well-vascularized fibrous pseudocapsule. It can present as a soft tissue mass without a suggestive history or symptoms [85], and the radiologist must therefore always consider a possible infectious origin of a mass with undetermined characteristics. In one-third of cases, abscesses are multiple [85]. Associated inflammation is possible, distorting normal muscle anatomy and fascial planes [16]. Depending on the causal organism and degree of inflammation, the margins of an abscess can be well-defined or infiltrating.

Conventional radiography has little value, unless there is gas development within the abscess (Fig. 21.7). Ultrasound shows an elongated or lobulated fluid collection, which may show peripheral vascularity on color Doppler (Fig. 21.8). In case of intralesional gas, dirty shadowing may be present (Fig. 21.9). It is also useful in guiding an aspiration biopsy or percutaneous catheter drainage.

On MR imaging, an abscess is hypointense to isointense relative to muscle tissue on T1-weighted images. On T2-weighted images, the central portion of the abscess is usually hyperintense, but the capsule may display an isointense or hypointense signal intensity relative to subcutaneous fat [85]. On T1-weighted images, the pseudocapsule can have a variable signal intensity compared to skeletal muscle. After intravenous contrast medium injection, a peripheral rim of enhancement is seen, corresponding to the inflammatory and cellular component of the abscess (Fig. 21.10). Enhancement after intravenous contrast medium injection can also be absent [85].

A variable degree of peripheral edema in muscle and subcutaneous tissue can be seen, displaying a hyperintense signal intensity on T2-weighted images. Inhomogeneity on T2-weighted sequences may be a consequence of intralesional gas bubbles and/or necrotic material [190].

The described signal characteristics can be different in an immunocompromised host [85]. 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 [9]. If the content is sufficiently viscous, it can even show mild increased signal intensity on T1-weighted images [21]. In the proximity of bone, a soft tissue abscess is often associated with osteomyelitis or a periosteal reaction (Fig. 21.11).

Pyomyositis, also called bacterial myositis, is a rare cause of single or multiple abscesses of skeletal muscle of unknown etiology. It was initially mainly found in tropical regions where lack of footwear, insect bites, and minor trauma, if untreated, may lead to pyomyositis. Diabetes, HIV infection, and malignancy can, as immune-compromising conditions, however, also predispose to pyomyositis [35, 90, 144, 147], as such contributing to an increased incidence of this disease in industrialized regions with a more temperate climate. It is considered one of the most common musculoskeletal complications of AIDS [144, 154]. In 70–90 % of cases, the infection is caused by Staphylococcus aureus [35, 90, 144, 147]. Other pathogens such as Streptococcus pyogenes, Mycobacterium tuberculosis, Mycobacterium avium-intracellulare, Nocardia asteroides, Cryptococcus neoformans, and Toxoplasma, Salmonella, and Microsporidia species have also been reported [195].

In general, normal skeletal muscle has a high intrinsic resistance to bacterial infection and abscess formation. Therefore, some authors suggest that underlying muscle damage may facilitate the onset of pyomyositis. This is supported by the presence of previous trauma to the affected muscles in 20–50 % of cases [35, 72].

The clinical course of disease can be divided into three stages [39]. The first stage or invasive phase occurs during the first 2 or 3 weeks after infection, with subacute presentation such as local swelling, mild pain, variable fever, and anorexia. Diagnosis in this stage is difficult due to multiple differential diagnoses. During the second or suppurative phase, definitive abscess and pus are clearly evident. High fever, chills, local tenderness, swelling, and myalgia are frequent findings. It should be noted that local erythema and regional adenitis are usually absent in pyomyositis [45]. Aspiration of the lesion at this time reveals pus. The third stage of the disease is so severe that even toxic shock syndrome has been reported [83].

However, symptoms may be absent when the lesion is deep-seated or due to a superimposed transient bacteremia [144].

Laboratory data are not specific for pyomyositis. Leukocytosis and an increase in the erythrocyte sedimentation rate are often seen, but may be absent in patients with neutropenia or end-stage acquired immunodeficiency syndrome. Eosinophilia is reported only in tropical cases and may be due to a parasitic cause of infection [44, 45]. Interestingly, creatine phosphokinase and aldolase levels remain normal. Blood cultures are positive in only 5–30 % of patients and in 1.8 % the outcome is fatal due to sepsis and shock [110, 195]. Culture of aspirated pus has been reported as negative in 15–30 % of cases [35, 44].

The muscles of the thigh and gluteus region are most often affected. Pyomyositis has also been described in the obturator, serratus anterior, deltoideus, triceps, biceps, iliopsoas, gastrocnemius, abdominal, and paraspinal muscles (Fig. 21.12) [35, 79, 90, 147]. In AIDS patients, pyomyositis may be multifocal 43 % of cases in the study of Fleckenstein et al. [58]. Multiplicity of lesions in AIDS patients is not specific for pyomyositis and may be found in other pathologic conditions such as polymyositis, Kaposi sarcoma, and lymphoma [58].

Magnetic resonance imaging is the most precise technique to use in determining the exact location and defining the extent of the disease [51], and it plays a pivotal role in early diagnosis. On T1-weighed 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 [72, 110]. 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 [154]. Intravenous administration of contrast material can further discriminate between viable and necrotic muscle tissue, 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 extension of the perilesional inflammatory reaction and the possible association of cellulitis. However, previous surgery or local radiotherapy at the site of a sarcoma may be accompanied by locoregional stranding, which may cause difficulties in the differential diagnosis [72].

Scintigraphy is rarely used for detection [202], but may detect additional abscesses on a distance of the primary lesion [147].

Pyomyositis can be effectively treated by antibiotic therapy in the first stage of disease, but during the later stages, drainage should be initiated early and serve as the main component of therapy, either by percutaneous computed tomography (CT) or ultrasound-guided drainage or by open surgery [72].

Human hydatid disease is a parasitic infection due to the development in the organism of Echinococcus granulosus larvae, a parasitic tapeworm, with dogs being the definitive hosts, sheep the intermediate hosts, and humans the incidental intermediate hosts. Ingestion of contaminated water or food and contact with dogs are known causes of infection. The disease is still endemic in several regions and has the highest incidence in the Mediterranean Basin, Middle East, South America, New Zealand, Australia, Central Asia, and China [138]. According to various authors, the incidence of musculoskeletal echinococcosis including involvement of subcutaneous tissue is around 1–5.4 % among all the cases of hydatid disease [10]. Alveolar echinococcosis due to infection by Echinococcus multilocularis is more rare, but has a more invasive nature sometimes mimicking a malignant lesion. Muscular echinococcosis may be primary, but may also occur secondarily resulting from the spread cysts from other areas either spontaneously or after operations for cystic echinococcosis in other regions of the body [87, 88]. The growth of cysts within a muscle is difficult because of muscle contractility and the presence of lactic acid. The affinity for muscles of the neck, trunk, and limb roots could be explained by the volume of the muscle mass, the increased vascularity, and the decreased activity of these muscle groups [15].

Primary muscular cystic echinococcosis disease is rare, with only isolated cases being described in the literature [15, 201]. Only few cases of primary subcutaneous cystic echinococcosis of the inferior limbs have been reported. Cystic echinococcosis of superficial muscles presents as a painless soft tissue mass with fluctuating consistency and evolves slowly. This mass may be accompanied with neurovascular compression signs, inflammation in case of fissuring, infection of the cyst [69], or skin fistula. Deep muscular cystic echinococcosis especially of the psoas muscle which is relatively more frequent [158] may be discovered by chance. Hydatid serology is often negative [122]. It is particularly useful in postoperative monitoring when looking for a local or more remote recurrence [62].

Muscular cystic echinococcosis may have different appearances on imaging techniques.

MR has proven superior over ultrasound in detecting this multivesicular structure. More solid appearances are also possible, making it sometimes difficult to differentiate it with other soft tissue tumors [107]. Even in these cases, MR can often reveal the vesicular nature of the lesion, which if frequently still focally preserved.

Ultrasonography and magnetic resonance imaging are the imaging tools of choice to confirm the diagnosis before surgery [62]. The imaging characteristics of soft tissue involvement resemble those of hydatid cysts found in the liver. Ultrasonographic appearance of cystic echinococcosis may vary. The cyst wall usually manifests as an echogenic line. Simple cysts (type I) do not demonstrate internal structures. Detachment of the endocyst from the pericyst (type II) appears as well-defined fluid collection with a localized split in the wall and “floating membranes” inside the cavity [17]. Multivesicular cysts (type III) manifest as well-defined fluid collections in a honeycomb pattern with multiple septa representing the wall of the daughter cysts (Fig. 21.13). Multivesicular or inhomogeneous muscle hydatids may have a nonspecific appearance that may simulate hematoma, abscess, or necrotic soft tissue tumor [62, 68]. Calcified cysts may be seen. Computed tomography is useful in doubtful cases, in deep muscular localizations, and in appraising the exact localization of the cyst and its relationship with the neural and vascular elements as well as the state of the adjacent bone. MRI imaging characteristics may differ depending on the life cycle stage of the parasite [162]. Typically, the lesion consists of a mother cyst, containing multiple daughter vesicles. On T1-weighted images these daughter vesicles 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 [15]. A rim of low [15] signal intensity on T2-weighted images surrounds the lesion. This rim is composed of three layers: an endocyst, ectocyst, and adventitia. The adventitia develops as a reaction following compression and inflammation of surrounding tissue. It is well vascularized, enhancing after intravenous contrast injection [111, 120] (Figs. 21.14 and 21.15). Edema or acute inflammation caused by compression in soft tissue adjacent to the cyst is uncommon but may be seen.

Inflammatory myopathies include focal myositis, nodular myositis, proliferative myositis, and diabetic muscle infarction. Clinically, inflammatory myopathies often present as a diffuse swelling of the thigh or calf, with or without tenderness. Only on rare occasions, they present as a solitary soft tissue mass. These different entities are only distinguishable by their histologic appearances [85], often requiring a biopsy for correct diagnosis.

On MR imaging studies, myopathies are characterized by non-focal hypointense areas on T1-weighted images and hyperintense signal on T2-weighted images [86]. These diffuse signal changes are even better seen when a T2-weighted fat suppression sequence is used [74]. Differential diagnosis includes infectious myositis, trauma, muscular denervation, muscular dystrophy (such as Duchenne’s [106] or Becker’s muscular dystrophy), rhabdomyolysis, polymyositis, dermatomyositis [74], and soft tissue malignancy. In most of these cases, clinical and laboratory tests will permit to make the correct diagnosis.

Focal Myositis. Focal myositis is a relative rare usually self-limiting soft tissue pseudotumor. It is usually found in the lower extremities, 50 % of the cases being located in the thigh and 25 % in the lower leg. Other more rare locations include the neck, tongue, perioral region, forearm, hand, abdomen, eyelids, and paraspinal muscles [94]. There is no sex or age predilection [94].

Typically, focal myositis presents as a local intramuscular soft tissue swelling, which can rapidly grow in a few weeks (Fig. 24.16). In more than 50 % of the cases, pain is the main symptom. Usually the process is limited to one muscle, but involvement of multiple muscles has been reported. One-third of the patients with focal myositis evolve to polymyositis or a polymyositis-like syndrome, suggesting that focal myositis is a localized form of polymyositis [14].

On MRI, focal myositis is of increased signal intensity on T2-weighted images, in one or more muscle groups. In contradistinction to most soft tissue sarcomas, the internal muscle bundle is spared (Fig. 21.16) [184].

Diabetic Muscle Infarction. 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 a well-controlled patient without known diabetic complications [89]. Although the pathogenesis is still to be completely clarified, the most likely hypothesis is that the muscle infarction is secondary to vascular disease such as arteriosclerosis and diabetic microangiopathy [178], while some authors suggest an alteration in the coagulation-fibrinolysis system [140].

Diabetic muscle infarction typically presents as a sudden onset of severe pain in the thigh (especially in the quadriceps muscle) or calf, with diffuse enlargement of the involved muscle or muscle groups. After subsequent partial resolution, a painful palpable mass can be found in up to one-third of the cases [178]. Bilateral involvement has been described, with a reported frequency varying from 8 % to more than one-third of the cases [11, 89, 172, 178].

Clinically, it is frequently misdiagnosed as an abscess, neoplasm, or myositis, often requiring a biopsy for further evaluation [33, 47, 89]. Commonly there is elevated erythrocyte sedimentation rate, but no leucocytosis. This can be helpful in the differentiation from pyomyositis [89].

The diagnosis may be suspected on ultrasound if a well-marginated, hypoechoic intramuscular lesion with intralesional linear echogenic structures is present. There is typically absence of intralesional motion or swirling of fluid when pressure is applied with the transducer. This differentiates diabetic infarction from an abscess or a necrotic mass [47, 105].

MR images display enlargement of the involved muscles, with uniform increased signal intensity on T2-weighted and inversion recovery images demonstrating the edematous and inflammatory changes [11, 33, 86, 89]. T1-weighted images show normal or decreased signal intensity in the involved muscles, the swelling being sometimes less appreciated on this sequence [89].

Perifascial and subcutaneous edema are best evaluated on inversion recovery and fat-suppressed T2-weighted images [84]. Additionally, MRI can detect subclinical muscle infarction months before the onset of clinical symptoms [89].

Atypical presentations have been reported as a high signal of the affected muscle on T1-weighted images, presumably reflecting intramuscular hemorrhage [169].

Bursae are spaces near joints containing small amounts of fluid, reducing friction between different structures. The clinical most important are the trochanteric, subdeltoideal, ischiogluteal, pes anserina, iliopsoas, retrocalcaneal, and olecranon bursae, because they are the most commonly affected ones.

The amount of fluid may increase due to inflammation following overuse, direct trauma (rheumatoid), arthritis, or infection, resulting in a soft tissue mass. Imaging of bursitis is further discussed in Chap. 20.

The term Morton’s neuroma is a misnomer as it does not represent a true neuroma but rather perineural fibrosis and nerve degeneration, most likely due to repetitive compression and irritation of the interdigital nerve. Therefore, the term Morton’s fibroma is preferred [81].

Morton’s fibromas are most commonly found in the second and third intermetatarsal spaces and less frequently in the first and fourth. More than one intermetatarsal space may be affected as well. Morton’s fibroma is most commonly diagnosed in middle age, with a female predominance, and is believed to be related to the use of high-heeled shoes with increased weight bearing on the forefoot [196]. Although Morton’s fibroma is a common cause of intermetatarsalgia, in many cases it is not associated with clinical symptoms. Lesions with a transverse diameter smaller than 5 mm are often asymptomatic, as studies on healthy volunteers have shown, with a prevalence of small (<5 mm) Morton’s fibromas of approximately 30 % in asymptomatic individuals [19]. Lesions with a transverse diameter of 5 mm or more are most likely symptomatic.

Ultrasound examination is reliable and the most cost-effective imaging technique in detecting Morton’s fibroma [24]. A well-defined hypoechoic mass in the plantar soft tissues is seen at the level of the metatarsal heads (Fig. 22.23a). Dynamic ultrasound examination using Mulder’s test can increase conspicuity and diagnostic confidence. For this test, the patient’s forefoot is held in the sonographer’s non-imaging hand while performing lateral compression of the metatarsals and applying the transducer to the plantar aspect of the intermetatarsal regions. When this maneuver is performed on patients with a Morton’s fibroma, the mass is compressed between the metatarsal heads eliciting characteristic pain before it is plantarily displaced, often coinciding with a palpable click (Mulder’s sign) [9]. Elastography may be useful to detect smaller and less-defined lesions [133]. Although ultrasound has been proposed as a useful technique for guidance to inject corticosteroid, local anesthetic, or alcohol [9, 197], according to other authors, this technique seems not to improve the efficacy and safety of therapeutic injections if the clinician is well trained and is familiar with forefoot anatomy [109, 143].

Although in experienced hands, clinical examination and ultrasound may suffice for the diagnosis of Morton’s fibroma [24], MR imaging has been proven to be highly sensitive and specific in diagnosing Morton’s fibroma and may be used for difficult cases, differential diagnosis, and precise preoperative assessment [42, 203, 205]. On MRI, Morton’s fibroma appears typically as a tear-shaped, spindle-shaped, or dumbbell-shaped lesion in the region of the neurovascular bundle on the plantar side of the deep intermetatarsal ligament (Fig. 21.23). Morton’s fibromas display typical signal intensities on MR imaging sequences: isointense to muscle on T1-WI and hypointense relative to fat tissue on T2-WI [205]. There is no typical enhancement pattern, varying from low to moderate to marked enhancement. In the appropriate clinical setting, administration of gadolinium contrast medium is not required for a reliable diagnosis of a Morton’s fibroma. MR plays an important role in excluding other pathologies that can mimic Morton’s fibroma such as intermetatarsal or subcapitometatarsal bursitis (which occurs dorsal to the transverse metatarsal ligament), osteonecrosis of the metatarsal heads, metatarsophalangeal joint synovitis or dislocation, tendon sheath ganglion, pigmented villonodular synovitis, stress fractures, and other disorders associated with metatarsalgia [204, 205].

In most cases, therapy is conservative mainly comprising of footwear modifications, radiofrequency ablation, physical therapy, and local (corticosteroid and anesthetic) injections into the affected webspace. Surgical excision is only performed when necessary [9].

Traumatic neuroma is a nonneoplastic reactive hyperplasia of nerve tissue and usually occurs at the proximal end of a nerve trunk that has been severed, partially transected, or injured as a result of trauma. Traumatic neuromas are classified into neuroma-in-continuity (NIC) after partial nerve transection or end-bulb neuromas (EBN) after complete disruption. EBNs lack distal continuity with the parent nerve, while NICs are contiguous both proximally and distally [2].

The most common location for traumatic neuromas is the lower extremity after amputation, followed by the head and neck, where they have been reported to occur after the extraction of teeth.

After limb amputation, neuromas may be asymptomatic when not compressed, but can cause unexplained pain and discomfort particularly when a prosthesis is worn. Physical examination may reveal a painful nodule at the site of the transected nerve, local tenderness over the injured nerve with distally radiating tingling (Tinel sign), denervation atrophy of the muscles, and sensory or trophic changes [38].

On imaging, traumatic neuroma may mimic a peripheral nerve sheath tumor (PNST). The history of a previous trauma is the key finding to the correct diagnosis.

Ultrasound reveals a hypoechoic mass at the distal end of site of the amputated nerve in case of an EBN [76]. In a transected nerve, ultrasound may reveal a focal gap due to nerve discontinuity and scar tissue at the level of transection and EBN formation at the proximal and distal stump margin [206]. An NIC is seen as a spindle-shaped mass on the course of the involved nerve and may mimic a PNST. Ultrasound-guided steroid or alcohol injection in painful stump neuroma has been reported as a successful method for pain relief [37, 104].

On MRI (Fig. 21.24), an EBN is seen as a T2 hyperintense nerve terminating in a baseball-shaped mass resembling a balloon on a string or a green onion appearance [2, 38]. To differentiate an NIC from a PNST, administration of gadolinium contrast is not useful as both lesions may demonstrate contrast enhancement. The lack of a target sign on T2-WI in a traumatic neuroma in conjunction with the clinical history of a previous trauma is the most useful clue to the correct diagnosis of a traumatic neuroma [2].

Distal denervation muscle atrophy serves as a useful secondary sign of nerve injury on MR imaging [38].

A contusion is caused by a capillary rupture that provokes bleeding between the tissue muscle fibers, resulting in edema and inflammatory reaction. Contusions are not always painful, may present as a mass, and thereby cause clinical confusion. A soft tissue contusion appears on MR images as a diffuse interstitial infiltration due to edema. It is hyperintense on T2-weighted images but without architectural muscle distortion. However, the muscle may increase in volume, owing to inflammation. The history of a previous trauma is required to differentiate muscle contusion from focal myositis, which may have a similar imaging appearance.

The ultrasound appearance of hematomas is variable in time. Acute hematomas are hyperechoic and they become more hypoechoic with aging. They may have well-defined or irregular margins (Fig. 21.25a). Dynamic evaluation with muscle contraction is valuable for assessing disruption of muscle architecture. On CT, acute hematoma appears as a hyperdense area; MR imaging has replaced CT in the imaging of hematomas.

The MR imaging appearance of muscular hematomas (Figs. 21.25 and 21.26) reflects the pathophysiology of forming hemoglobin breakdown products, which are the main constituents of a hemorrhagic collection. Further discussion of the pathophysiology of hemoglobin degradation is beyond the scope of this chapter.

In an acute hematoma, the signal characteristics are dominated by intracellular deoxyhemoglobin. The lesion is iso- or slightly hypointense on T1-weighted images compared with muscle. Susceptibility effects also lead to low signal on T2-weighted images [28].

A hematoma in the early subacute stage is characterized by the presence of intracellular methemoglobin. This produces a high signal intensity on T1-weighted images, often visualized as a high-intensity peripheral rim. This high-intensity rim is a useful sign, as it may be the only clue that the mass is a hematoma. Susceptibility effects persist on T2-weighted images.

When loss of cell compartmentalization occurs in late subacute hematomas, extracellular methemoglobin results in Tl shortening. On T2-weighted images, the hematoma may be outlined by an area of high signal intensity (Fig. 21.25b-c). Diffuse edema is also present within the muscle in acute and subacute hematomas.

Finally, hemosiderin in a chronic hematoma also produces susceptibility effects on T2-weighted images. This results in low signal intensity on T1-weighted images and particularly on T2-weighted images. Blooming artifact is seen when using gradient echo imaging. Furthermore, this phenomenon is accelerated at the periphery of the collection, resulting in a peripheral hypointense rim, whereas the central portion of the hematoma may remain hyperintense [28].

It is important to differentiate hematoma from hemorrhagic tumor, because hemorrhage may obscure tumor tissue. T1-weighted images with fat suppression can aid in the differentiation, further discrimination methemoglobin from fatty tissue [39]. The best features suggesting hematoma are the progressive decrease in size of the lesion, the presence of fluid-fluid levels, and the time-dependent signal intensity changes. Conversely, the presence of enhancing nodules after contrast medium administration may suggest the presence of tumor [85]. Nevertheless, organized hematomas can show some enhancement.

When in doubt a biopsy should be performed to establish a confident diagnosis, especially if there is an increase in size of the hemorrhagic mass.

A chronic expanding hematoma is an entity characterized by its persistence and increasing size for more than 1 month after the initial hemorrhage [108]. MR reveals a heterogeneous signal intensity on both T1- and T2-weighted images, with a peripheral rim of low signal intensity [6]. Although the absence or presence of contrast enhancement is often used to distinguish a hematoma or a hemorrhagic neoplasm, intralesional nodular enhancement pattern may be seen in chronic expanding hematoma [108].

Foreign bodies such as plastic, wood, glass, and silica may penetrate the soft tissues and produce an inflammatory reaction. Clinically, a foreign body reaction first appears as a painful soft tissue swelling, and after a quiescent period of weeks or months, the symptoms may reappear [175]. If not removed immediately, a foreign body can become encapsulated with fibrous tissue and form a granuloma. The presence of histiocytes and giant cells with a surrounding inflammatory reaction is useful in establishing the diagnosis.

Different imaging methods have each their advantages and limitations in the evaluation of foreign bodies. Radio-opaque bodies can be demonstrated on conventional radiographs. When the lesion is in or near the bone, radiographs can show osteolytic and/or osteoblastic bone lesions [99, 159]. Since a positive history of penetrating trauma is not always obvious, and some lesions can be radiolucent, a foreign body reaction can be mistaken for a neoplasm [99, 159].

Ultrasound is a primary imaging tool when conventional radiographs fail in detecting the foreign object. Its use in the detection and guided retrieval of a suspected foreign body has been well established [29]. Ultrasound shows large differences in acoustic impedance between hyperechoic foreign bodies and hypoechoic surrounding inflammatory tissue (Fig. 21.27).

On MR imaging studies, the foreign body itself is usually low, due to the presence of few mobile protons in the commonly found foreign materials (glass, wood, metal, etc.) [85, 99, 124, 182]. However, dispersed oil droplets can induce a granulomatous reaction and present as subcutaneous or intramuscular soft tissue masses with high signal on T1-weighted images [101]. An intralesional fat-fluid level may be seen at the interface of fatty and necrotic components in case of injection granulomas in bodybuilders. The combination of the clinical history and the typical location at usual locations for injection (such as the shoulder and gluteus muscles) are the clues to the correct diagnosis of these pseudotumoral soft tissue masses [8, 185].