Pseudotumors in Sports

Abstract

Musculoskeletal pseudotumors on imaging are not a rarity and may be increasingly detected due to an increased use of diagnostic imaging and expanding subspecialized clinical referral basis. A large variety of pseudotumors exist. Although there have been technical advances and there is a diversity of imaging modalities, the diagnosis of a pseudotumor remains challenging and its morphologic features may be difficult to distinguish from those of a malignant neoplastic lesion. It requires a systematic approach with awareness of the common types of pseudotumors, its imaging appearances, regional anatomy, injury mechanisms, and carefully obtained clinical history. This chapter illustrates the most important pseudotumors on imaging, their imaging characteristics on different imaging modalities, and possible clinical settings with keys to differential diagnosis. Emphasis is placed on MRI.

1 Introduction

Although there have been technical advances and there is a diversity of imaging modalities, the diagnosis of a pseudotumor remains challenging and requires a systematic approach. The combination of clinical history, physical examination, and imaging appearance, including anatomical location, is crucial (Perdikakis et al. 2017). In many instances it will be evidently clear that an apparent clinical mass is a direct consequence of a specific sports injury, such as thigh hematoma after impaction during a football game. Commonly an athlete is fully aware of the injury and its relationship to the newly developed abnormality. Many sports activities are associated with specific injuries and findings.

Some sports injuries may be minor and forgotten and consecutively may not be immediately perceived as being related to the current clinical problem (e.g., only one-quarter of children with a fat necrosis on MRI imaging recalled a history of trauma, Tsai et al. 1997). The minor repetitive trauma with no inciting event is one of the more challenging constellations in the diagnostic process. Also acute trauma associated with unusual radiological findings may confuse the diagnosis (e.g., complicated by secondary infection).

Contributory features include the presence of normal variants with possible overuse syndromes, specific body habitus, or patient’s history suggesting inappropriate training or equipment predisposing to overuse injuries. Some characteristic locations and patient age may be suggestive of the diagnosis, e.g., elastofibroma dorsi of the infra-scapular posterior thoracic wall.

Musculoskeletal pseudotumors on MRI are not a rarity and may be increasingly detected due to an increased availability of MRI and expanding subspecialized clinical referral basis. The growing sports activity especially in older children and adolescents predisposes to more injuries and overuse. A unique group in this context is female athletes, affected by the “female athlete triad” (osteoporosis, eating disorder, and amenorrhea).

A large variety of MRI pseudotumors exist. Some pseudotumors are clinically apparent as a mass or swelling. Some are only evident on imaging with morphologic features which may be mistaken for a neoplastic lesion. However pseudotumors are far more common than neoplasms. They can be classified either anatomically or by etiology. All anatomical structures may be involved.

2 General Imaging Principles

All modalities are involved in pseudotumor imaging. Often the radiologist is confronted with already performed imaging and may not be able to influence the initial choice of the imaging modality or protocol. Otherwise remaining with basic principles is prudent. Radiographs in two planes may show calcifications, zoning phenomena, fractures, presence of normal variants, or signs of overuse such as advanced degenerative joint disease not corresponding to patient’s age. Correlation with conventional radiographs is extremely important as many masses or focal abnormalities on MRI simulating neoplasms may have characteristic radiographic appearances and be easily diagnosed (Jelinek and Kransdorf 1995; Kransdorf and Murphey 1997). Examples include the peripheral calcific rim of myositis ossificans, retained radio-opaque foreign bodies, stress fractures, and hydroxyapatite deposition within tendons (see also Sects. 4.4, 5.1, 8, and 10).

Computed tomography can help with demonstrating the above characteristics; however analogue to the general musculoskeletal tumor diagnostic MRI is a mainstay with sports pseudotumors given its multiplanar capability, regional anatomic review, and a superior soft-tissue contrast resolution. Depending on the primary goal of the imaging the field of view may be adapted accordingly. If the main aim of the examination is to establish the presence of mass, visualization of the contralateral side may be helpful to detect the asymmetry, using a larger field of view. A smaller field of view with higher spatial resolution may be useful for detailed assessment of the lesion.

Due to technical advances high-field MRI scanners (1.5 T or higher) in connection with dedicated multichannel coils provide superior image quality. Analogue to the diagnosis of musculoskeletal tumor, there are standardized algorithms based on consensus guidelines of the European Society of the Musculoskeletal Radiology (Nöbauer-Huhmann 2019). Routine sequences in three planes are important, including T1-weighted and STIR sequences (short tau inversion recovery) in at least one plane.

Skin markers may also help in the detection of the lesion, as the absence of a true mass on imaging may be challenging.

Gradient echo sequences may be useful to enhance a “blooming” artifact associated with hemosiderin deposition, metallic foreign bodies, and gas. Though contrast administration may not necessarily change the diagnosis, it may increase the conspicuity of the findings (e.g., unique cases of muscle strains in professional athletes with negative findings on the T2-weighted imaging).

If a true mass is absent on MRI, the finding is associated with a large reactive soft-tissue component or an extensive hemorrhage and whether there has been a history of sports injury, trauma, or overuse with a possible pseudotumor should be included in the differential diagnosis.

The morphologic appearance on imaging (preferably both radiographs and MRI) in association with a detailed clinical history may help avoid an unnecessary biopsy with also sometimes inconclusive histopathological diagnosis (e.g., false-positive histopathological diagnosis of a soft-tissue osteosarcoma in case of a myositis ossificans, Ragunanthan and Sugavanam 2006). It is crucial to perform clinical and imaging follow-up examinations after suspecting a pseudotumor diagnosis, particularly with less experienced readers, in order to avoid a false-negative diagnosis regarding a neoplastic lesion.

As the pseudotumor diagnosis may be challenging, increased awareness of the common types of pseudotumors, its imaging appearances, regional anatomy, injury mechanisms, and carefully obtained clinical history (e.g., precise questionnaire) may significantly increase the specificity of the MRI diagnosis, which may often be a surprise to the referring clinician.

3 Keys to Differential Diagnosis and True Tumors

Typically true musculoskeletal tumors of either bone or soft-tissue origin present with an increasing mass effect and relatively minor pain compared to the size of the mass, unless there has been a history of bleeding, biopsy, or trauma. Neoplastic lesions tend to displace or invade the local anatomy. Pseudotumors may often only show a subtle distortion of the regional anatomy, so that the comparison with the contralateral side may be very helpful and sometimes essential in the diagnostic process.

Pseudotumors are commonly associated with a marked adjacent soft-tissue reaction, as classically seen with myositis ossificans (Walczak et al. 2015). Adjacent marked reactive soft-tissue changes or hemorrhage is unusual with a neoplastic lesion unless there has been a history of a direct trauma, bleeding, or biopsy.

4 Soft Tissue-Related Pseudotumors

4.1 Muscle Tear, Morel-Lavallée Syndrome, and Rhabdomyolysis

Muscle strains and tears, soft-tissue hematomas, but also less common injuries like Morel-Lavallée syndrome or calcific myonecrosis may mimic true soft-tissue tumors on imaging.

Muscle tears often occur in professional athletes. The correct diagnosis and grading of the muscle injury may be relevant especially in elite athletes (Thierfelder et al. 2019). Most muscle injuries (around 96%) occur due to an indirect mechanism of accident and result in muscle tear (Mueller-Wohlfahrt et al. 2013). Some muscle injuries occur more often in particular sporting activities (Fig. 1). For instance muscle injuries of hip and thigh (hamstrings, adductors, and quadriceps strains) are frequent among soccer players, and rotator cuff tears occur frequently in tennis and baseball players (Ekstrand et al. 2013). The correct interpretation of the radiological findings including the type and grading of the injury may be relevant for the therapy options and prognosis. Apart from the clinical grading systems, which consider bulge size and loss of strength, one most commonly used MRI-based grading system distinguishes four different muscle injury grades (Mueller-Wohlfahrt et al. 2013). Grade 0 stands for normal muscle, grade 1 stands for intramuscular edema with no fiber discontinuity, grade 2 stands for partial muscle tear, and grade 3 stands for complete muscle tear (Ekstrand et al. 2013). In case of a grade 3 injury, which may frequently require a surgical therapy, the exact extension of the tear should be reported. Another predictive factor in muscle tears is the longitudinal extension of the injury, which is considered to have the highest predictive value as it correlates with the amount of the muscle units separated from the aponeurosis (Connell et al. 2004). Morel-Lavallée syndrome, muscle hernia, and calcific myonecrosis are possible complications of a muscle injury, which are treated in a separate paragraph.

Fig. 1

(a, b) Muscle injury and rhabdomyolysis. 23-Year-old male Swiss wakeboarder with unilateral lower back soft-tissue swelling with side asymmetry. (a) STIR parasagittal image shows extensive muscle signal change and partial tear. (b) Sagittal T1-weighted image (WI) shows absence of fracture. Hematological results showed extensively increased muscle enzymes. After further review a clinical history of acute trauma was obtained. Surgical fasciotomy was performed. Follow-up MRI demonstrated marked fatty replacement of unilateral paravertebral posterior muscles consistent with posttraumatic compartment syndrome

4.1.1 Hematoma

Superficial soft-tissue hematomas may be associated with minimal and often forgotten trauma. Deep-muscle hematomas are more frequently associated with more obvious trauma, coagulation disorders (e.g., hemophilia), or derailed anticoagulation therapy. Often the MRI appearances of hematomas are complex due to the different stages of blood content as there may be a repeated hemorrhage within movement. The MRI appearance of hematoma depends on the state of the hemoglobin molecule, whether it is intra- or extracellular in nature (Kransdorf and Murphey 1997; Dooms et al. 1985; Gomori et al. 1985; Rubin et al. 1987). In the hyperacute phase blood is isointense on T1 weighting and decreased on T2 weighting, reflecting the earliest phase of oxygenated hemoglobin to deoxyhemoglobin. Due to cell lysis in the subacute phase (1 week to 3 months old) hemoglobin molecules are released into the extracellular space and become methemoglobin, which is characteristically increased in signal intensity on T1 and T2 weighting. Weeks to months later the methemoglobin breaks down into hemosiderin (Fig. 8) with decreased signal intensity on T1 and T2 weighting. Breakdown of blood products in a hematoma is not a uniform process. The accelerated peripheral breakdown will show a low-signal rim whereas the central region may show higher, inhomogeneous signal intensity. It may sometimes be difficult to distinguish a simple hematoma from a large hemorrhagic neoplastic lesion (Jelinek and Kransdorf 1995). A solid, enhancing nodularity or rim may be helpful in distinguishing between these lesions (Jelinek and Kransdorf 1995). However an organizing hematoma may show some internal enhancement due to enhancing fibrovascular tissue and should be followed up to resolution, especially in an atraumatic clinical setting. Secondarily infected hematomas may present a more complex MRI appearance with peripheral and surrounding soft-tissue reaction.

4.1.2 Morel-Lavallée Syndrome

Soft-tissue injuries like the muscle strains and tears stated above may present months or even years after a trauma, which makes the diagnosis more difficult. A posttraumatic cystic lesion, known as Morel Lavallée syndrome, occurs after a severe trauma to pelvis or thigh in an abrupt shearing force mechanism (De Coninck et al. 2017). It is a degloving injury where soft tissue is sheared from the underlying fascia which is accompanied by a repeated hemorrhage. Morel-Lavallée lesion presents as a subcutaneous cystic mass surrounded by a postinflammatory, fibrous capsule situated in the peritrochanteric region in between the iliotibial band and fascia lata (McLean and Popovic 2017). The contents of the lesion may accumulate slowly over a longer period of time, due to the anatomical predisposition of this region. The subcutaneous localization of the bone, the increased mobility of the soft tissues, as well as the rich vascular supply of the dermis on this anatomical site may result in continuous drainage of blood into the lesion. The lesion presents with fluid-equivalent signal, but it may also be homogeneously hyperintense on T1- and T2-weighted images with no signal dropout after fat saturation due to chronic hemorrhagic contents. After gadolinium administration there is no or minimal peripheral enhancement (due to granulation tissue), seen on subtracted images due to hyperintense appearance on native T1-weighted images. The peritrochanteric region is the most frequent site; however Morel-Lavallée lesions have also been reported in the suprapatellar region, abdominal wall, buttocks, and lower lumbar spine (M. erector spinae). The nonspecific signal intensity may be challenging in the diagnostic process; however the typical localization abutting a fascial plane in the peritrochanteric region should be suggestive. A differential diagnosis of a fat necrosis (typically less homogeneous signal intensity), abscess (typically locules of air and strong rim enhancement), and a soft-tissue hematoma (different localizations) should be considered.

4.1.3 Myocele/Hernia

Muscle hypertrophy, which is commonly found in sporty individuals, may lead to a myocele, which is a muscle protrusion through an acquired or congenital fascial or myofascial defect, mimicking a mass lesion. It is a rare and painful lesion, which may occur in athletes due to exertional fatigue and muscle hypertrophy (Sanders et al. 2011). It occurs most frequently in lower extremities and may be undetectable during rest periods which requires dynamic visualization during muscle contraction (Thierfelder et al. 2019). Less frequent localization is the lateral abdominal wall, which may be acutely or chronically injured in a variety of athletic endeavors and may result in a lumbar hernia formation (Stensby and Baker 2016). Regarding dynamic examination ultrasound may be here advantageous (Karantanas 2019).

4.1.4 Rhabdomyolysis

Rhabdomyolysis may occur due to endurance exercise and weight lifting in athletes or sudden increase in physical activity in less trained individuals (Furman 2015). Its clinical presentation may be very subtle in early phase, which makes the diagnosis challenging. Muscular pain or swelling, limited range of motion, and excretion of dark urine in young males with a history of extensive physical activity should raise concern for this condition. Undiagnosed and untreated rhabdomyolysis may lead to severe complications such as renal failure, compartment syndrome, and dysrhythmias (Furman 2015). On MRI imaging there is a diffuse T2 hyperintensity of the affected muscle group due to edema, sometimes occurring bilaterally. Together with the laboratory testing results (increased creatinine kinase level) it may be consistent with rhabdomyolysis. MRI imaging may be helpful to detect the exact distribution and extent of the affected muscle group.

4.1.5 Calcific Myonecrosis

Calcific myonecrosis is a rare entity, which occurs as a late complication of a lower limb trauma, often as a late sequel to compartment syndrome and injury of the common peroneal nerve (Fig. 2) . It is a fusiform mass with peripheral calcification and central liquefaction or cyst formation. Since there may be a long delay (up to few decades) between the injury and the presentation with a soft-tissue mass, the association with a traumatic event initially may not be considered in the diagnostic process, which may be challenging to differentiate from a neoplastic lesion. The common site of the calcific myonecrosis is the calf. On radiographs calcific myonecrosis presents as a well-defined lesion with distinct, plaque-like, peripheral calcification replacing the leg musculature. Due to the very slow development process the lesion may erode the outer cortex of the adjacent bone, sometimes leading to a complete destruction of a diaphyseal segment (in particular in thin tubular bones like the fibula). On MRI the lesion presents on both T1- and T2-weighted sequences as a heterogeneous mass formation with some T2 hyperintense components due to the cystic changes. The chronic masses may increase in size over a period of months, several years, or even many decades due to the repeated intralesional hemorrhage. The lesion may show fluid-fluid levels, possibly due to precipitation of calcium salts and layering out of the old hemorrhage. The differential diagnosis of this rare condition includes malignant soft-tissue tumors with calcifications, e.g., synovial sarcoma and soft-tissue osteosarcoma. In contrary to the calcific myonecrosis the mineralization in synovial sarcomas commonly shows diffuse distribution throughout the lesion. Synovial sarcomas are frequently localized in a periarticular way within 5 cm of the joint. While synovial sarcomas may show some contrast enhancement, calcific myonecrosis typically shows no enhancement after gadolinium administration. Other conditions like dermatomyositis, polymyositis, and diabetic myonecrosis may also demonstrate calcifications; however they commonly show systemic manifestations with no history of a trauma.

Fig. 2

(a–d) Calcific myonecrosis. 45-Year-old male, Swiss yachtsman, who continues with cross-training which includes 100 km bike riding, presents with a fluctuating lower limb mass. History of a prior trauma 30 years ago at a Sydney Harbour New Year’s Eve party was obtained, with lower limb boat crush injury and compartment syndrome, which at the time had resulted in near amputation. (a) Sagittal T1-WI, anterior compartment muscle altered signal with peripheral decreased signal, mass-like effect, and posterior compartment extensive muscle fatty replacement. (b) Sagittal STIR sequence, marked peripheral decreased signal with central variable increased fluid signal. (c) Axial T1-WI, mass-like expansion of two anterior compartment muscles and absence of normal muscle structure compared to normal muscle adjacent to proximal fibula and extensive fatty atrophy of the posterior compartment muscles. Extensive fasciotomy scar is not demonstrated here. (d) Axial STIR sequence shows the equivalent of anterior muscle replacement by peripheral calcified margin, central fluid collection, and dense calcification of the adjacent muscle

4.2 Overuse and Tear of Ligaments and Tendons

Tears and overuse of ligaments, tendons, and muscles, both acute and chronic, may mimic neoplastic lesions on imaging. The exact localization of the tear often depends on patient’s age. The typical site in young athletes is the non-fused apophysis (Fig. 3). In older patients the tear is commonly localized in the tendon, due to degenerative changes as a predisposing factor. In adult athletic patients the tears occur most commonly in the myotendinous junction, which is the weakest anatomic site (Thierfelder et al. 2019).

Fig. 3

Avulsion anterior superior iliac crest and tensor fascia lata tear. 16-Year-old female gymnast who presents with bilateral flank pain and possible right-sided mass for review. Coronal STIR image demonstrates iliac crest avulsion fracture fragment on the right with focal tear of tensor fascia lata. Less marked left-sided focal tear is also noted (arrows)

Acute and chronic avulsion injuries which commonly affect immature bone structure may be mistaken for a soft tissue or bone tumor due to reactive bone overgrowth and fragmentation, associated tendinopathy, and peritendinitis (Perdikakis et al. 2017). In acute phase, when the MRI appearance may be suggestive for an aggressive lesion, plain radiographs may be helpful to detect the avulsed bone fragment and thus to narrow the differential diagnosis. Partial avulsion of the adductor muscles from the femoral diaphysis in children due to sports injuries or overuse may simulate a malignant sarcoma on MRI (Anderson et al. 2001). There is typical widespread intramedullary edema but absence of stress fracture lines as well as bone or soft-tissue mass. This condition has been studied using bone scans in adults during army training and was called “adductor insertion avulsion syndrome” or “thigh splints.” The cause is thought to be due to excessive adductor contraction with stripping of the femoral periosteum anteromedially. Knowledge, particularly of the MRI findings with an appropriate clinical setting, can help physicians to make the correct diagnosis and avoid unnecessary biopsy. Proximal adductor avulsions near the symphysis pubis (Schneider et al. 1976) and intramuscular strains (Yoshioka et al. 1994) have been described in adults. Chronic muscle avulsion injuries in a variety of lower limb sites in children have been described (Anderson et al. 2004a, b). Overuse with excessive prolonged carrying of large babies may rarely lead to masses of the wrist, found to be due to de Quervain’s tenosynovitis.

Posttraumatic tendon tears of certain muscles may result in compensatory hypertrophy of other muscle groups due to changed weight load of the affected region (Sutter et al. 2013). The hypertrophic region may mimic a mass lesion. An example of this phenomenon is the hypertrophy of the ipsilateral tensor fasciae latae muscle due to tears of the gluteus medius or minimus tendon.

4.3 Nodular Fasciitis (Pseudosarcomatous Fasciitis)

Though quite rare nodular fasciitis is the most common pseudosarcomatous lesion, first described by Konwaler in 1955, most often occurring in young, athletic patients (20- to 35-year-olds) (Rani and Gupta 2019). It is a benign, mass-forming, proliferative fibrous lesion considered to be reactive after trauma or infection. However the exact etiology is unknown. The lesions may grow rapidly and show spontaneous regression. The most common clinical presentation is a painful subcutaneous nodule. There is a predilection for the upper limb (especially forearms), followed by trunk and abdominal wall; however it may occur anywhere in the body (Rani and Gupta 2019). There are three forms of nodular fasciitis based on its localization: subcutaneous, fascial, and intramuscular. The most common is the subcutaneous form, presenting as named above subcutaneous nodules. The intramuscular form commonly appears as a focal round or ovoid mass attached to the fascia with sometimes infiltrative borders, which may be challenging to distinguish from a sarcoma. The fascial form usually spreads along superficial fascial planes with no circumscribed margins and may sometimes show stellate growth pattern. Early lesions typically have a high T2 signal intensity reflecting the myxoid histology (Jelinek and Kransdorf 1995; Kransdorf and Murphey 1997). Older lesions may have decreased signal intensity on T2 weighting reflecting the predominantly fibrous histology (Jelinek and Kransdorf 1995). On T1-weighted images nodular fasciitis shows a homogeneous low signal intensity with slightly inhomogeneous enhancement after contrast media administration. There may also be some subtle soft-tissue edema and the lesion may show some aggressive features like transcompartmental growth, osseous, or articular involvement which make the diagnosis even more challenging (Coyle et al. 2013). The lesions are mostly smaller than 4 cm; 72% of them are smaller than 2 cm. Overall the MRI appearance is nonspecific.

Not only the radiological diagnosis may be challenging but also the pathological diagnosis based on the cytology (fine-needle biopsy) may be demanding due to the hypercellular and polymorphic cell population. The awareness of the possible cytological misdiagnosis (false positive for sarcoma) is an important component of the diagnostic process and may require a tight collaboration between the radiologist and pathologist (Rani and Gupta 2019). A suspected nodular fasciitis diagnosis should be followed up for the expected spontaneous regression.

4.4 Myositis Ossificans and Nora Lesion (BPOP)

4.4.1 Myositis Ossificans

Myositis ossificans is a common pseudotumor on MRI mostly related to a skeletal muscle with the highest prevalence for the flexor muscles of the arm and extensor muscles of the thigh (Boutin et al. 2002; Walczak et al. 2015). It may occur in association with other structures such as tendons and fascia.

Myositis ossificans is a solitary, self-limiting, abnormal ossifying soft-tissue mass mostly associated with trauma, typically occurring in young, active males (Walczak et al. 2015; Wang et al. 2018). The pathophysiology of MO is not completely understood. It is assumed to be caused by an inappropriate differentiation of fibroblasts into osteogenic cells induced by a skeletal muscle injury (Walczak et al. 2015; Wang et al. 2018). The history of trauma is often not initially apparent, which can make the diagnosis difficult (Kwee and Kwee 2019). Thus carefully obtained detailed clinical history may play an important role in the diagnostic process.

The variability in appearance depending on the stage of the disease may also be challenging in the diagnostic process. Three stages are described in the evolution of myositis ossificans: early stage (<4 weeks), intermediate stage (4–8 weeks), and mature stage (>8 weeks) (Walczak et al. 2015). The awareness of the possible clinical presentation and the radiological appearance during all stages of the disease may be the key to diagnosis. The morphologic features of the early stage of myositis ossificans may often be mistaken for a soft-tissue sarcoma due to the lack of typical ossification pattern. This may lead to an unnecessary biopsy which may predispose to even more extensive ossification and a worse outcome (Kransdorf and Murphey 1997).

In the initial phase of myositis ossificans (<4 weeks) the calcifications are frequently inapparent as the lesion includes mainly fibroblasts and myoblasts with only a small amount of osteoid formation (Wang et al. 2018). The progression of the calcification process may only partially be defined by the stage of the disease, as the maturation process of the bones strongly depends on the age of the patient, similar to the callus formation (Kwee and Kwee 2019). During the initial, active phase the mass-like region may be clinically warm, painful, and woody in consistency. Biopsy during the early phase of development may give a false osteosarcoma diagnosis, as there is a florid osteoblastic activity. Serial radiographs are useful to demonstrate the initial development of amorphous bone, which may mimic the osteoid matrix on imaging and may be difficult to differentiate from an extraskeletal osteosarcoma.

During the intermediate phase (4–8 weeks) the amorphous bone progresses to a peripheral rim of calcification with a lucent center. In the mature phase (>8 weeks) the dense calcification is separate from bone, frequently running parallel to the long axis of the bone in a zonal pattern. The parosteal localization with distribution along the cortex may mimic melorheostosis. Characteristic sclerotome distribution in melorheostosis and lack of zonal ossification pattern should be helpful in the differentiation between these (Greenspan and Azouz 1999).

The presence of a cleft between the mass and the adjacent bone in myositis ossificans may be helpful in the differentiation from parosteal osteosarcoma. Also the localization of the lesion may be helpful in the diagnosis. Lesions located in MO-specific anatomical regions like the anterior femoral cortex together with other characteristics are suggestive of myositis ossificans (Walczak et al. 2015). Lesions in atypical localizations such as hands, feet, ribs, head, and neck should raise concern for malignancy (Walczak et al. 2015). Recurrent giant-cell tumor of bone in the soft tissue is a rare differential diagnosis of myositis ossificans with almost identical appearance on imaging (eggshell peripheral calcifications) (Walczak et al. 2015). One rare variant of myositis ossificans occurring in the fingers and less commonly in the toes is a fibro-osseous pseudotumor of the digits. Florid reactive periostitis and Nora lesion (Fig. 4) (bizarre parosteal osteochondromatous proliferation/BPOP) are considered to be a part of the same spectrum of lesions and may occur as a continuum (Dorfman and Czerniak 1998) (see separate paragraph).

Fig. 4

(a, b) Nora lesion. 50-Year-old female after sports injury. (a) Ultrasound shows hyperechogenic rimmed lesion with posterior shadowing, at the site of previous trauma, with a history of decreasing mass size and soft-tissue swelling. (b) Lateral thumb radiograph confirms calcification (image courtesy of Dr. G. Stuckmann, Kantonsspital Baden, Switzerland)

CT is the best modality for detecting the typical zonal calcification pattern of myositis ossificans even before it becomes apparent radiographically. Mature calcification pattern is typical for MO; however on radiographs alone it may not be diagnostic (differential diagnosis parosteal sarcoma), which makes the intermodal approach crucial.

MRI is the best modality for imaging of soft-tissue masses; however due to limitations in the detection of calcifications it should be interpreted together with recent radiographs.

On ultrasound myositis ossificans may appear as an infiltrative muscle lesion with peripheral hyperechogenic calcifications and no detectable blood flow on Doppler study (Fonseca et al. 2019).

If the trauma involves deep-muscle injury in some cases there may be an associated periosteal reaction of the adjacent bone, referred to as periostitis ossificans. The appearances of myositis ossificans on MRI reflect the phase of development and the zonal phenomenon of the histology. In the early phase before ossification the lesion is usually isointense to muscle on T1-weighted images with no distinct borders. It is centrally hyperintense on T2-weighted images with marked adjacent soft-tissue edema (De Smet et al. 1992). The marked perilesional muscle edema (more than the double size of the central lesion) is specific but not pathognomonic for myositis ossificans in early/intermediate stage and may help to differentiate from malignant soft-tissue lesions (Zubler et al. 2020).

Since hematomas often occur in the initial phase, MO may show a heterogeneous signal intensity with high-signal areas on T1-weighted images due to the blood products. T2 hypointensity may represent hemosiderin deposition or calcifications (Walczak et al. 2015). At the initial stage there may be a subtle peripheral rim. In subacute lesions with early peripheral calcification there is a low-signal-intensity rim, like a tidemark on the beach when the wave is residing, reflecting this calcification. The center of the lesion may be isointense to muscle or slightly increased in signal intensity on T1-weighted images.

The T2 hyperintensity of the central portion may delineate the decreased signal of the peripheral rim more clearly (Jelinek and Kransdorf 1995; De Smet et al. 1992). At the acute and subacute stages the adjacent soft-tissue edema may be very prominent, which is unusual in malignant tumors, unless they have undergone a biopsy or an intralesional hemorrhage (Jelinek and Kransdorf 1995). In the chronic phase, the central portion of the lesion is slowly ossifying, so that the signal intensity on T1- and T2-weighted images is analogous to yellow bone marrow (De Smet et al. 1992). Marked intralesional enhancement should raise concern for sarcoma.

4.4.2 Nora Lesion (Bizarre Parosteal Osteochondromatous Proliferation/BPOP)

BPOP is an uncommon mineralized lesion of mesenchymal origin, usually affecting the proximal and middle phalanx of hand and feet, metacarpals, and metatarsals with commonly parosteal localization adjacent to bone surfaces. Hands are four times more often affected than feet (Torreggiani et al. 2001). Less common documented localizations are long bones, skull, and maxilla. The lesion presents commonly as a rounded calcified mass attached to the adjacent bone with no alteration of the underlying cortex. It consists of bone, cartilage, and fibrous tissue (Gruber et al. 2008). BPOP affects most commonly patients in the third and fourth decades; there is no sex predilection. The exact etiology of the lesion is unclear. It may occur within a reparative process due to trauma (Mahajan et al. 2012). However in many affected patients no trauma has been recorded in the clinical history. According to its remarkably high recurrence rate (29–55%) and aggressive appearance it may be difficult to distinguish from a malignant tumor (Gruber et al. 2008; Dhondt et al. 2006).

4.5 Inflammatory Myopathies

As a heterogeneous group of muscle diseases, inflammatory myopathies may mimic other pathologies (Maurer and Walker 2015). Early diagnosis may be important for the outcome due to the amenability of the immunosuppressive therapy (Maurer and Walker 2015). Inflammatory myopathies are most usually diffuse in nature, commonly bilateral in the thighs or calves, associated with weakness or tiredness and elevated blood enzyme levels (Fujimoto et al. 2002). Muscle edema, fatty replacement, and muscle atrophy may be visualized on MRI imaging (STIR and T1-weighted images). A rare entity of pseudosarcomatous myofibroblastic proliferations (proliferative myositis, proliferative fasciitis, nodular fasciitis) may mimic soft-tissue sarcomas both clinically and on imaging (Jarraya et al. 2014). Proliferative myositis may present as a rapidly growing nodule, localized around the neck, head, or upper extremities. Due to the rapid growth the definite diagnosis mostly requires a biopsy. The MRI appearance may include some interspersed muscle fascicles within the mass, which may suggest the correct diagnosis and help to distinguish from nodular fasciitis (Jarraya et al. 2014). Inflammatory myopathies may occur in sporting individuals.

4.6 Muscle Denervation

Muscle denervation may mimic a tumor due to edema and swelling on MRI imaging in the acute phase. There are predisposing sporting activities leading to specific neural compression syndromes. Not uncommonly, labral tear associated with suprascapular notch ganglion may be associated with acute to chronic supraspinatus and infraspinatus muscle denervation. Quadrilateral space syndrome is a relatively rare condition with symptoms caused by a mechanical compression (e.g., labral tear with paralabral cyst) of the axillary nerve inferoposterior to the glenohumeral joint in the quadrilateral space leading to the atrophy of teres minor muscle and less commonly deltoid muscle (Robinson et al. 2000). It is associated with overhead sports like baseball, volleyball, swimming, yoga, or triathlon (Brown et al. 2015). Repeated arm abduction and external rotation may also lead to compression of the posterior circumflex humeral artery (PCHA), resulting in a vascular quadrilateral space syndrome (vQSS) due to thrombosis, aneurysm formation, and distal emboli (Brown et al. 2015). Quadrilateral space syndrome may present with neurogenic symptoms (weakness, paresthesia, pain). It is commonly underdiagnosed, which may lead to a permanent disability of overhead athletes and threaten their career (Brown et al. 2015). Repetitive traction and microtrauma within overhead motion in athletes may lead to the injury of the suprascapular nerve in the suprascapular notch with consecutive denervation of both supraspinatus and infraspinatus muscle. Inflammatory, infectious, or idiopathic conditions are the cause of suprascapular neuropathy in Parsonage-Turner syndrome (Ahlawat et al. 2015).

Iatrogenic nerve injuries, e.g., common peroneal nerve injury within knee surgery after minor sports-related trauma, may also lead to muscle denervation mimicking a tumor in the acute phase with muscle edema.

4.7 Stress Reactions in Soft Tissue

Superficial fat impingement, which mimics a mass lesion, is a relatively rare condition due to repetitive pressure with abnormal positioning or equipment over a longer period of time. Fortunately, this is infrequent; however, occasionally obvious clinical histories may have been forgotten.

Chronic compression of the metatarsal region may lead to formation of a subcutaneous synovial lined intermetatarsal bursa. This is a painful fluid collection in between the metatarsal heads, deep to the transverse metatarsal ligament (Van Hul et al. 2011) with fluid-equivalent signal intensity and possible subtle enhancement in the periphery after gadolinium administration. If the fluid collection measures more than 3 mm in transverse diameter it is perceived as a cutoff suggesting intermetatarsal bursitis. The diagnosis should be correlated with the clinical symptoms.

Another similar entity in the metatarsal region developed due to chronic stress and friction is an adventitious bursa. In contrary to the native named above, permanent bursa, this fluid collection is not lined with mesothelium, but with synovium-like columnar cells, being gradually formed within the process of chronic friction in the subcutaneous soft tissues. Due to the fluid content the lesion also shows low signal intensity in T1-weighted sequences and high signal intensity on T2-weighted sequences, often with some intralesional band-like low-intensity structures (Van Hul et al. 2011).

Fortunately, rarely chronic fat impingement may occur due to shoe equipment wear and repetitive fat impingement (Fig. 5).

Fig. 5

Fat impingement. 45-Year-old starting back to weekend sports activities. Gluteal fat impingement due to hard wear. Vasculitis and systemic disorders were excluded. Fat apron resection was performed and tumor excluded

Fig. 6

(a–c) Occult posttraumatic fracture. A 38-year-old male with clinical concern for mass lesion, impingement, or labral tear. Additional history by radiology team elicited heavy sports tackle injury. (a) Coronal STIR of the right hip shows bone marrow edema extending from femoral head to intertrochanteric region. (b) Coronal T1-WI of the pelvis shows decreased signal right proximal femur, contralateral left intact. (c) Coronal T1-WI with fat suppression after gadolinium contrast administration of the right hip outlines femoral head fracture line (arrow). Small ganglion is noted as well. There was no osteopenia or osteoporosis on bone density review. Orthopedic follow-up confirmed normal return to function

5 Bone-Related Pseudotumors

5.1 Fractures, Occult Fractures, and Stress Reactions

Posttraumatic bony abnormalities such as occult fracture, stress reactions, and stress fractures may mimic bone tumors. Rarely high-end sports individuals may present after forgotten sports trauma, and imaging appearances may initially be alarming, but with careful review for fracture lines and specific clinical questioning on trauma the diagnosis can be made (Figs. 6 and 7). Stress reactions and fractures may be mistaken for sarcomas among other trauma-related lesions on imaging. Due to increased athletic activity and advances in imaging technology stress fractures may be a common finding in athletes and military recruits (Breer et al. 2012; Behrens et al. 2013). Stress fractures may be subdivided due to their etiology into fatigue and insufficiency fractures (Breer et al. 2012). Fatigue fractures occur due to abnormal or repetitive stress on a normal bone structure and are typical for athletes (Fig. 5). Insufficiency fractures mostly affect weakened bone structure (e.g., due to osteoporosis, vitamin D insufficiency, steroid use, anorexia, bisphosphonate therapy) and are often seen in postmenopausal women (Fig. 8). Female athletes with eating disorder and lower bone density are often affected by stress fractures (e.g., female athlete triad) (Behrens et al. 2013).

Fig. 7

(a–c) Tibial stress fracture. 57-Year-old-patient with knee pain and swelling. Additional clinical history revealed short-term period of heavy weight lifting. (a) Coronal T1-WI reveals medial tibial stress fracture. (b) Sagittal STIR shows reactive marrow changes, posterior medial tibial stress fracture line, and prominent soft-tissue reaction. (c) Coronal STIR stress fracture at the same level as in (a)

Fig. 8

(a, b) Insufficiency fracture of calcaneus. 50-Year-old female presents with soft-tissue swelling of ankle and Achilles tendon dysfunction, with concern for mass, after increase in walking activity. (a) Sagittal T1-WI reveals a fracture line at the posterosuperior calcaneus and small Achilles tendon enthesophyte. (b) Coronal STIR shows the fracture line, marrow change, and minor imbibition of pre-Achilles Kager’s fat pad

The most common sites of stress fractures in athletes are metatarsals, tibia, fibula, and tarsal navicular (Behrens et al. 2013). Dancers and runners are commonly affected athletes (Fig. 9).

Fig. 9

(a, b) Bilateral tibial stress fractures. 18-Year-old male marathon runner, including mountain running. Presents after 14-km mountain run with concern for tumor and osteoid osteoma. Coronal STIR bilateral tibia demonstrates right-sided incomplete stress fracture at the proximal to mid-third junction with adjacent soft-tissue reaction and left proximal tibial stress reaction. (b) Coronal T1-WI with right diaphyseal cortical periosteal and intramedullary stress reaction and more diffuse region of stress reaction at the left proximal tibia

Many sporting activities are associated with some specific sites and injury patterns (Pavlov 1995) such as junction of pelvis and ischium in long-distance runners, and hook of hamate in golf and tennis players. Some localizations of stress fractures are practically pathognomonic for specific athletic activities such as spinal pars defects of the thoracic and lower lumbar spine in cricketers and unilateral defects often involving the side opposite to the bowling arm in fast-spin bowlers (Hardcastle et al. 1992). Some sites of stress fractures may predispose to delayed union, e.g., anterior tibial diaphysis, lateral femoral neck, patella, medial malleolus, navicular, fifth metatarsal bone, and talus (Behrens et al. 2013). Fracture sites with lower risk for delayed union are posteromedial tibia, metatarsals, calcaneus, cuboid, cuneiform, fibula, medial femoral neck, and femoral shaft (Behrens et al. 2013).

The patient’s age predisposes to specific fracture types. Both acute and chronic but also acute-on-chronic avulsion injuries in the immature skeleton may simulate bone-forming surface tumors or intraosseous sarcomas. Stress fractures in the immature skeleton are frequently mistaken for sarcomas on imaging (particularly localized in the proximal tibia and distal femur) (Shimal et al. 2010).

MRI is the imaging modality with the highest sensitivity and specificity for stress fractures (Behrens et al. 2013). Any linear low signal intensity on T1-weighed sequence associated with increased signal intensity of T2-weighted sequence which becomes more obvious after contrast administration and with fat suppression should prompt a diagnosis of a stress fracture.

Radiographs should always be performed as it may provide the correct diagnosis in many cases (e.g., stress fracture of the metatarsals). Along with attentive review of the clinical history (e.g., altered or excessive training, changed equipment) the awareness of the typical sites may suggest the correct diagnosis.

Furthermore some subtle findings like a specific pattern of the accompanying soft-tissue edema may also suggest the diagnosis (e.g., soft-tissue edema localized next to the MCL in a subchondral insufficiency fracture of the knee) (Wilmot et al. 2016). Any periosteal reaction associated with muscle hypertrophy in an athlete should prompt a review of the type of training and accurate pain characteristics, as the MRI findings may be due to overuse.

5.2 Avascular Necrosis

Avascular necrosis may occasionally mimic a cartilage lesion on radiographs, but the presence of the classical double-geographic line sign should make this diagnosis obvious on MRI. Young children practicing overhead sports like baseball may present with humeral capitellum avascular necrosis (Panner’s disease) (Sakata et al. 2015).

Long-term use of steroids may be a predisposing factor in the development of avascular necrosis (Min et al. 2008).

5.3 Rheumatological or Degenerative Joint Disease

Young athletes may also first present with a monoarticular joint disease which should be differentiated from overuse within advanced degenerative joint disease. Laboratory screens are helpful here.

6 Normal Variants

Normal variants and their overuse may present as MRI pseudotumors. Normal bone marrow may occasionally cause concern for tumors; however use of T1/T2-weighted and STIR sequences usually excludes real pathology. The bone marrow composition may be assessed using chemical shift encoding-based water-fat MRI (Baum et al. 2015).

Supernumerary bones (styloid bone, accessory ossification centers for scaphoid tuberosity) (Capelastegui et al. 1999), tarsal coalition, and accessory muscles (e.g., soleus muscle and extensor digitorum brevis manus muscle) (Capelastegui et al. 1999) are some targets. Normal variants like accessory muscles are usually asymptomatic. However, sometimes they may cause symptoms due to the mass effect of the supernumerary muscle on adjacent structures like nerves, vessels, or tendons, especially in trained athletes with hypertrophic muscles (Perdikakis et al. 2017). Visible swelling may mimic a soft-tissue tumor (Fig. 10) (Desimpel et al. 2017). The awareness of the regional anatomy and normal variants together with MRI imaging may help in differentiation between both conditions.

Fig. 10

(a, b) Bony anatomic variant (prominent tuberculum peroneum) associated with tendon partial tear and inflammatory reaction of soft tissues. 50-Year-old weekend sportsman with clinical concern for overuse or tumor mass. (a) Axial fat-suppressed T1-WI after administration of gadolinium contrast shows lateral calcaneal tubercle prominence associated with peroneal tendinosis, adjacent tenosynovitis, and skin contour deformity. (b) Sagittal STIR, peroneal tendon partial longitudinal split tear, and soft-tissue reactive change

Occasionally tarsal coalition and calcaneal spurs associated with peroneal tendon inflammation and partial tear may be associated with a marked synovial reaction and tenosynovitis, mimicking a pseudotumor.

Awareness of these syndromes and review of the radiographs usually allow correct diagnosis. A frequent finding among the normal variants is a periosteal desmoid presenting with an avulsive cortical irregularity of the distal femur often misinterpreted as a suspicious finding by less experienced readers.

7 Infection

Pyomyositis may occur in healthy sporty individuals; however the clinical presentation prior to MRI strongly suggests infection with reddened skin, pain, and often systemic features such as fever. Patients with immunosuppression may present with less signs of inflammation, which may be confusing both clinically and radiologically. MRI features (Soler et al. 1996, 2000; Ogilvie et al. 2001; Abdelwahab et al. 2003) suggesting an abscess include a thickened peripheral rim on T1 and T2 weighting, which may have increased signal intensity on T1 weighting and demonstrate marked contrast enhancement. Immunosuppressed patients commonly have an absence of marked adjacent soft-tissue and subcutaneous edema. Fortunately, atypical organisms such as cysticercosis, Echinococcus granulosus in hydatid disease, and Coccidioides immitis in coccidioidomycosis may rarely cause abscess formations with a mass-like appearance on MRI (Jelinek and Kransdorf 1995). HIV-positive patients may rarely present with a highly vascularized, angiomatosis-focal infection in soft tissues, which may erode and involve adjacent bone, mimicking a sarcoma, due to infection with Bartonella henselae, a gram-negative bacillus (Biviji et al. 2002). This organism may also be involved in “cat scratch disease” (Dong et al. 1995), as the carrier is usually a cat and is usually associated with single lymph node enlargement.

8 Calcium Deposition Disorders

Calcium deposition disorders—hydroxyapatite, gout, and calcium pyrophosphate dihydrate crystal deposition disorders with calcium deposition in tendons, ligaments, and bursae—may create MRI pseudotumors. Patients with a provisional clinical diagnosis of tumor associated with ligament calcification may be referred for MRI (Anderson et al. 2003a, b). The MRI appearance may include a focal thickening of the lateral collateral ligament associated with adjacent marked soft-tissue reaction and intravenous gadolinium contrast enhancement, which may correlate on radiographs with soft-tissue calcifications. In the resorptive phase the calcifications may migrate inside the tendon, which may cause severe pain due to the inflammatory process (Bianchi and Becciolini 2019). Extratendinous and intramuscular migration of hydroxyapatite crystals has also been documented, which due to muscle edema and inflammation may be misdiagnosed as neoplasm on MRI (Pereira et al. 2016) (Figs. 11 and 12). With its acute clinical onset and dissipation on follow-up radiographs, this entity is presumed to occur due to hydroxyapatite deposition.

Fig. 11

Calcific tendinosis. 40-Year-old female, weekend tennis player. Axial STIR sequence shows calcification of subscapularis tendon, extratendinous subdeltoid extension, and reactive muscle changes. Additional history was obtained by the radiologist for sudden marked pain increase. Radiograph confirmed calcific tendinosis

Fig. 12

(a, b) Calcific tendinosis. 35-Year-old female, weekend sports, presents with a pseudopalsy of upper limb and concern for tumor. (a) Axial STIR sequence shows calcific tendinosis, calcification rupture, and adjacent reactive signal change in the teres minor muscle. (b) Coronal STIR shows soft-tissue reactive changes adjacent to the focal calcification

Awareness of this entity is critical for a correct MRI diagnosis of a pseudotumor.

Review of radiographs at the time of MRI may be helpful in detecting calcifications. This condition is more characteristic within the rotator cuff tendons of the shoulder.

9 Metabolic Disorders

Metabolic disorders may be associated with MRI pseudotumors such as amyloid deposition and hyperparathyroidism, though they are most unusual in sporting individuals. Secondary amyloidosis is the most common of this unusual group, often seen in association with chronic renal failure. This condition occurs due to the deposition of beta-2 microglobulins within joint capsules and tendon sheaths. It may rarely be associated with pseudotumors near joints. Characteristically there is a decreased signal intensity on T2 weighting, which appears to reflect the collagen-like nature of the amyloid (Kransdorf and Murphey 1997). The hypointense appearance on T2-weighted imaging may already narrow the differential diagnosis as it is less common. Besides amyloidosis, hypointense signal may occur more typically with hyperacute hemorrhage or chronic hemosiderin deposition, cartilaginous loose bodies, tenosynovial giant-cell tumor, rheumatoid arthritis, hemochromatosis, gout, calcium pyrophosphate arthropathy (CPPD), hydroxyapatite deposition disease (HADD), tumoral calcinosis (Steinbach et al. 1995), arthrofibrosis, and iatrogenic lesions with fibrosis (Wadhwa et al. 2016).

Unusual in athletes, though mentioned for completeness sake, secondary hyperparathyroidism may be associated with brown tumors of bone which are nonspecific on MRI. The most common clinical setting is chronic renal failure with high serum calcium, low serum phosphorus, and high parathormone level. Idiopathic tumoral calcinosis may present as an MRI pseudotumor with large septated regions of variable signal intensity in periarticular regions on both T1 and T2 weighting; however review of the radiographs allows for this diagnosis.

10 Foreign-Body Reactions

Foreign-body reactions due to trauma or as a consequence of trauma such as retained surgical swabs (“cottonballoma”) (Kalbermatten et al. 2001) fortunately are less common, though they should be considered in the appropriate clinical setting. Close inspection for traces of intramedullary screws, subtle metallic artifacts, or direct evidence of retained foreign bodies, such as glass, should be sought for.

Surfers may be rarely exposed to foreign-body reactions, as described in a case report presenting a “Surfinoma” (Squire et al. 2010). A fragment of a fiberglass surfboard, located in M. latissimus dorsi, has been left after a surgical closing procedure, forming a sinus to the axillary region. A year after the injury the patient presented with a mass in the left axillary region, redness of skin, and a small fluid collection consistent with an abscess formation inferior to the foreign bodies (Squire et al. 2010). This rare case may help to raise awareness for any foreign-body reaction and its possible appearance in athletes.

11 Vascular Pseudoaneurysms

Less common MRI pseudotumors include pseudoaneurysms of the hand arteries, which may present as soft-tissue masses without further symptoms (Anderson et al. 2003a, b). Due to their superficial location and proximity to the bones, the arteries of the hand are frequently exposed to minor or repetitive trauma, which may lead to a pseudoaneurysm formation (Gardiner Jr. and Tan 2017; Conn Jr. et al. 1970).

The most common location of the arterial injury due to blunt trauma is the hypothenar region, where the superficial branch of the ulnar artery passes directly over the hook of hamate within the Guyon’s canal (Gardiner and Tan 2017). This condition has been described as the hypothenar hammer syndrome and is associated with ischemia of the hand and fingers. The damage of the endothelial cells results in platelet aggregation and thrombus formation with a further cascade of a traumatic insult of the vessel wall, which may eventually lead to an aneurysm formation (Gardiner and Tan 2017; Glichenstein et al. 1988). The compression of the sensory branch of the ulnar nerve may also occur (Glichenstein et al. 1988). The hypothenar hammer syndrome has been usually described in adult men with industrial occupations involving repetitive blunt trauma to the hands (Newmeyer 1993; Little and Ferguson 1972). It has also been described in sports-related injuries in handball players and baseball catchers (Glichenstein et al. 1988; Newmeyer 1993; Little and Ferguson 1972).

The thenar hammer syndrome (Janevski 1979) involves an acute or chronic compression of the radial artery between the first and second metacarpal, due to its superficial location being covered only by the muscle of flexor pollicis brevis and subcutaneous fat.

The commonest pseudoaneurysms involve the popliteal artery, which in up to 75% of cases may occur bilaterally if due to arteriosclerosis (Kransdorf and Murphey 1997). Additional MR angiography sequence may give the diagnosis here.

12 Posttreatment/Intervention (Steroid-Induced Osteoporosis, Insufficiency Fracture)

Though rare in sporting individuals, post-therapeutic appearance on MRI, including postradiation changes, may be associated with an MRI pseudotumor. Due to increasing popularity of physical activity among the elderly population with possible oncologic background, these conditions may gain in significance. Postradiation findings on MRI with signal alterations in soft tissue and bone marrow may initially raise concern for a true neoplasm; however on close review of the imaging characteristics, there is usually no real mass. The attentive review of the patient’s history (e.g., radiotherapy) usually allows a correct diagnosis (Richardson et al. 1996).

Things to Remember

• 1. Check for the history of trauma—think myositis ossificans.

  Review the history for specific sport and repetitive activity—think overuse/tears of ligaments, muscles, and stress reactions or fractures.

• 2. Ascertain whether there is true focal mass present. Consider larger FOV to look for asymmetries and compare to the contralateral side.

• 3. Always correlate MRI with radiographs and/or CT.

• 4. 4. In case of superficial lesion—think hematoma/fat necrosis; in case of intramuscular lesion—think hematoma/infection/myositis ossificans (tidemark sign).

• 5. Remember age-specific lesions:

  • Adolescents: Stress reactions and chronic avulsion fractures may mimic neoplastic lesion.

  • Aging population with more sporting activity.

  • Aging population—more osteoporosis and insufficiency fractures.

  • Check risk factors for AVN.

• 6. Joint lesions: Remember that systemic disorders may first present in one joint.

• 7. Marked irregular synovitis—think of rheumatologic disorder or infection.

  Abscess wall sign—increased signal on T1 weighting within thickened wall.

• 8. Ensure T1 weighting to review for normal marrow signal.

• 9. Exclude normal variants with overuse—check radiographs.

Imaging Boxes

Plain Radiography/CT

• Radiopaque or radiolucent foreign bodies.

• Evaluation of acute or chronic avulsion fractures.

• (Mature) myositis ossificans.

• Calcifications.

Ultrasound

• Foreign bodies (wood, glass, …).

• Muscle/tendon trauma and Morel-Lavallée lesions.

• Early detection and characterization of myositis ossificans.

• Myocele/muscle hernia: muscle contraction aids in confident diagnosis.

MR Imaging

• Imaging in three planes required.

• Imaging protocol should include at least T1-WI and STIR or fat-suppressed T2-WI.

• Gradient echo imaging: useful to detect hemosiderin, metallic foreign bodies, and gas.

• Should always be correlated with clinical history and plain films/CT.