Lipomas are well-capsulated masses composed of mature adipose cells.

Although lipomas are most frequently superficially located, they may also be located intramuscularly.

Radiographs are usually normal, but a low to intermediate density may be seen in larger lesions.

On ultrasound, intramuscular lesions are often more difficult to distinguish from the surrounding muscle fibers. The reflectivity is highly variable ranging from hypo- to hyperreflective compared to subcutaneous fat (Fig. 3a).

CT shows a homogeneous mass with low density (−60 to −130 HU). Intramuscular lesions may contain thin fibromuscular septa. There is no significant contrast uptake.

On MRI, the lesion has similar signal characteristics as subcutaneous fat on all pulse sequences. Fat-suppression techniques show homogeneous suppression. Minor (less than 2 mm) internal septa and a low SI capsule may be seen. After administration of intravenous gadolinium contrast, there is no contrast enhancement, except for the peripheral fibrous capsule or the subtle internal septa (Vanhoenacker et al. 2006) (Figs. 3 and 4).

Table 2 summarizes the most useful criteria to differentiate between a usual lipoma and a liposarcoma.

Myxoma is histologically characterized by the presence of abundant, avascular, and myxoid stroma in which a small number of cells are embedded. Most myxomas are intramuscularly located (82 %). It is a tumor of adults. Areas most frequently involved are the large muscles of the thigh (Fig. 5), shoulder, buttocks, and upper arm. There is an association with polyostotic fibrous dysplasia (bones involved are usually in the vicinity of the myxoma) in Mazabraud’s syndrome (De Schepper and Bloem 2007) (Fig. 6).

Ultrasound shows a well-delineated solid or mixed hypoechoic lesion with fluid-filled clefts or internal cystic foci reflecting myxoid stroma. Usually, there is retroacoustic sound enhancement. A small amount of hyperechoic fat or edema may surround the upper and lower poles of the lesion (Zamorani and Valle 2007). On MRI, myxomas present as very low signal intensity lesions on T1-weighted images (lower than signal intensity of normal muscle) and as high signal intensity lesions on T2-WI (higher than signal intensity of fat). A small rim of fat corresponding to focal atrophy of surrounding muscle may be seen on T1-WI (Luna et al. 2005). On T2-WI, perilesional strands of high signal may be seen due to leakage of myxomatous tissue in surrounding muscle (Nishimoto et al. 2004). There may be moderate, central enhancement after administration of intravenous gadolinium contrast, having a “smoke”-like appearance (Fig. 5) (Peterson et al. 1991).

Extra-abdominal desmoids are rare benign soft tissue tumors arising from connective tissue of muscle, overlying fascia, or aponeurosis. They have been designated previously as desmoid tumor, aggressive fibromatosis, or musculoaponeurotic fibromatosis. The term “desmoid” means band-like or tendon-like lesions (De Schepper and Vandevenne 2006). Currently, the term desmoid type fibromatosis is used (Fletcher et al. 2013). The lesions can be regarded as deeply seated fibromatosis in contradistinction to palmar fibromatosis which is superficially located. Desmoids are infiltrative tumors, known for their frequent recurrences. Complete surgical excision is often difficult.

The most commonly encountered locations are upper arm (28 %), chest wall and paraspinal area (17 %), thigh (12 %), neck (8 %), knee (7 %), pelvis or buttock (6 %), and lower leg (5 %). Desmoids occur most frequently in patients in the second to fourth decades of life, with a peak incidence between the ages of 25–35 years (Murphey et al. 2009). The lesion is typically deeply seated and poorly circumscribed. Slow insidious growth is the rule and most lesions are painless (Murphey et al. 2009).

Plain radiographs are often normal. A nonspecific soft-tissue mass may be apparent in patients with larger lesions. Calcification is uncommon. Underlying pressure erosion or cortical scalloping may be seen.

Ultrasound may demonstrate a nonspecific poorly defined, hypoechoic soft-tissue mass. Large lesions may demonstrate posterior acoustic shadowing. Increased vascularity is seen on Doppler-ultrasound (Murphey et al. 2009; Chew and Vanhoenacker et al. in press) (Fig. 7a).

On CT, the lesion has variable attenuation and enhancement (up to 110 HU). The attenuation is usually similar to that of skeletal muscle, but lesions with more prominent collagen content may reveal mildly higher attenuation. Low attenuation is rare and is associated with myxoid components. The margins are often indistinct (Murphey et al. 2009).

On MRI, the lesion may be well demarcated or ill defined. The signal intensity of desmoid type fibromatosis is variable, and depends on the extent of collagen and degree of cellularity of the lesion. Low signal intensity areas on T2-WI correspond histologically to hypocellular and collagen-rich portions of the tumor, whereas more cellular areas with a large extracellular space are of high signal on T2-WI. In addition, high T2 signal may be seen in myxoid portions.

Postcontrast MR images typically reveal heterogeneous and often moderate to marked enhancement of these lesions. Hypocellular, collagenized bands do not enhance and therefore are often accentuated at postcontrast MR imaging (Murphey et al. 2009) (Fig. 7).