11 Soft Tissue Disease
Magnetic resonance imaging (MRI) is far superior to computed tomography (CT) for the visualization of soft tissue pathology because of greater soft tissue contrast and an overall improved tissue characterization based on signal behavior on different pulse sequences and relaxation parameters. Compared with MRI, CT is more sensitive for the diagnosis of both tiny soft tissue calcifications and air collections and facilitates differentiation between the two.
For CT, the contrast characteristics of soft tissue disease depend on the relative proportions of fat, water, and mineral. Normal muscles are of soft tissue density and are separated from each other by fatty septa. In many muscle diseases, the muscle fibers become necrotic and degenerate or are replaced by fat and connective tissue. Fatty replacement of muscle may be complete and homogeneous or incomplete and inhomogeneous, but it is not characteristic for a specific disease. It is observed with muscular dystrophies, neuropathies, ischemias, and metabolic and systemic myopathies, as well as idiopathically (Fig. 11.1). CT is of little use in the differentiation of these conditions, but it may play an important role in the localization, distribution, and assessment of the extent of muscular involvement.
CT is useful in the evaluation of soft tissue masses. It allows definition of the exact dimensions of a lesion and its relationship to nearby neurovascular structures and bone. Certain limitations of CT in the evaluation of soft tissue masses, however, must be recognized. With the exception of a few lesions, such as lipomas (Fig. 11.2) and cysts (Fig. 11.3), CT rarely allows a specific histologic diagnosis based on attenuation values and appearance. Besides lipomas and cysts, other lesions may contain adipose tissue or fluid, respectively. The density of fat ranges from − 50 to − 100 HU. A fluid-containing lesion appears hypodense to muscle and has a density similar to water or slightly above it (range 0–20 HU). Fat-containing lesions are summarized in Table 11.1 and cystic lesions in Table 11.2.
Lipoma (subcutaneous, intermuscular, intramuscular, and synovial)
Lipoma arborescens (diffuse synovial lipoma in the knee)
Neural fibrolipoma (fibrolipomatous hamartoma usually of the median nerve in the wrist)
Macrodystrophia lipomatosa (neural fibrolipoma with macrodactyly)
Lipoblastoma (lipoma in infancy and early childhood)
Hibernoma (brown fat tumor, typically involving the shoulder region, chest wall, or thigh)
Elastofibroma (between the inferior margin of the scapula and chest wall)
Overestimation of a soft tissue mass with CT is possible because of adjacent soft tissue edema. Differentiation of actual invasion of neighboring structures such as the neurovascular bundle and bone from simple distortion and pressure defects by the adjacent mass is not always feasible.
Soft tissue calcifications can easily be appreciated by CT. They can be classified as metabolic (metastatic), dystrophic, or idiopathic.
Metabolic (metastatic) calcifications are associated with a disturbance in calcium/phosphorous metabolism, resulting in the deposition of calcium in normal tissues (Fig. 11.4). Such conditions include renal osteodystrophy (less commonly, primary hyperparathyroidism), hypoparathyroidism, hypervitaminosis D, milk–alkali syndrome (prolonged excessive intake of milk and alkali for heartburn in peptic ulcer disease, often associated with renal insufficiency), sarcoidosis, and processes associated with massive bone destruction (e.g., metastases, multiple myeloma, and leukemia).
Dystrophic calcifications represent calcium deposits in damaged tissue without metabolic derangement. They are associated with traumatic, ischemic, neuropathic, infectious, and neoplastic conditions. Besides calcification of a hematoma, other traumatic causes include sequelae of previous surgery, irradiation, and thermal injuries. Foreign body and injection granulomas also frequently calcify. Subcutaneous fat necrosis resulting in calcification of the subcutaneous adipose tissue is found with pancreatic disorders, Weber–Christian disease (nonsuppurative nodular panniculitis with subsequent necrosis and fibrosis in the subcutaneous fat and all visceral adipose tissues), and vascular insufficiency, in which calcified varicose veins or arteriosclerotic arteries are frequently also present. In the infectious group, calcified abscesses and granulomas are encountered. Soft tissue calcifications are also found in a variety of parasitic infestations, such as cysticercosis and echinococcosis. Tumor calcifications occur in both benign and malignant neoplasms. In benign tumors, the calcifications may be central (e.g., in lipomas [Fig. 11.5] and hemangiomas) or peripheral (e.g., in myxomas and xanthomas). In malignant neoplasms, such as synovial sarcoma, and less commonly in other soft tissue sarcomas, such as malignant fibrous histiocytoma, leiomyosarcoma, and rhabdomyosarcoma, both necrosis and hemorrhage may lead to secondary calcifications. Extraskeletal chondrosarcomas and osteosarcomas may demonstrate irregular, poorly marginated calcific deposits, whereas calcifications in their benign counterparts (chondromas and osteomas) tend to be well defined.
Phleboliths are dystrophic calcifications in organizing thrombi. They present as circular or elliptical calcifications with radiolucent centers measuring < 1 cm in their longest diameter. They are commonly found in hemangiomas and varicosities. They are quite characteristic but may occasionally be simulated by extra-articular (tenosynovial) chondromatosis, cysticercosis, and the calcified fatty deposits in Ehlers–Danlos syndrome (connective tissue disease with joint hyperextensibility and multiple musculoskeletal and other anomalies).
Idiopathic soft tissue calcifications are limited to the con nective tissue disorders (e.g., scleroderma [Fig. 11.6 ], dermatomyositis, and occasionally systemic lupus erythematosus) and idiopathic calcinosis (Fig. 11.7). In the latter conditions, the calcifications may be widespread and arranged in longitudinal bands (calcinosis universalis) or in multiple, well-demarcated masses of calcium often about articulations (tumoral calcinosis), or they may be localized (circumscript calcinosis, or calcinosis circumscripta). In tumoral and circumscript calcinosis, calcium–fluid levels are sometimes observed. Soft tissue calcifications are summarized in Table 11.3; Table 11.4 lists soft tissue calcifications commonly found in para-articular distribution.
Soft tissue ossification is diagnosed when cancellous bone is surrounded by cortical bone. Because only a limited number of diseases presenting initially with soft tissue calcification may eventually progress to soft tissue ossification, the differential diagnosis of the latter is accordingly smaller. A common cause of soft tissue ossification is traumatic myositis ossificans. It is characterized by a peripheral ring of ossification (“eggshell” calcification) surrounding a more lucent center (Fig. 11.8). Furthermore, the lesion is typically separated in its entire length from the adjacent cortex when located near a bone. In the absence of a history of trauma, the same lesion is often referred to as a pseudomalignant osseous tumor of soft tissue. A parosteal osteosarcoma presents as a radiodense lesion attached in a sessile fashion to the external cortex (Fig. 11.9a). In contrast to myositis ossificans, ossification of the tumor proceeds from the base of the lesion to its periphery. An osteochondroma is composed of a medullary cavity that is contiguous with the bone from which it arose and is surrounded by sharply defined cortical bone and a thin cartilaginous cap with varying degrees of calcifications (Fig. 11.9b). Soft tissue ossifications with a predilection for the para-articular regions are found in melorheostosis, a condition in which the bony alterations are diagnostic. Heterotopic bone formation occurs commonly after surgery, especially after insertion of hip prostheses and in a variety of neurologic disorders, especially paraplegia. Venous insufficiency and thermal injuries may lead to soft tissue ossification in the extremities. Myositis ossificans progressiva (fibrodysplasia ossificans progressiva) is a rare cause of soft tissue ossification associated with anomalies and hypoplasias of the great toes and thumbs, exostoses, and progressive fusion of primarily the axial skeleton. Soft tissue ossifications are summarized in Table 11.5.
The hallmark of necrotizing fasciitis (Fig. 11.10) is the presence of soft tissue gas, but this finding is not universally present. Gas in this life-threatening condition is commonly produced by a mixture of both aerobic and anaerobic bacteria. CT features include, besides soft tissue gas, marked edematous thickening of both the superficial and deep fasciae often associated with fluid collections, whereas the edematous changes in the muscle and adipose tissue are often less severe. Cellulitis, a streptococcal or, less commonly, a staphylococcal infection, can be differentiated from necrotizing fasciitis by the predominant involvement of the subcutaneous fat and superficial fascial tissues and the absence of soft tissue gas. Pyomyositis typically affects otherwise healthy children and young adults, especially in tropical regions, but it is also recognized with increasing frequency in malnourished and immunodeficient patients. Gas bubbles are occasionally seen in this intramuscular abscess caused by a Staphylococcus aureus infection in about 90% and by a streptococcal infection in the remaining cases.
Soft tissue contamination with gas gangrene occurs in devitalized tissues in which the arterial blood supply has been compromised. Predisposing factors for this clostridial infection include contaminated wounds, burns, decubitus ulcers, and diabetes mellitus. Gas in the subcutaneous tissue presents in this condition typically as linear or netlike lucent areas, whereas in the muscles, it characteristically produces circular collections of varying sizes. Conditions associated with soft tissue gas are summarized in Table 11.6.
Differentiation between a benign and malignant soft tissue lesion is not always possible either. Features for a benign process are small size; smooth, well-defined borders; and the absence of invasion of the adjacent muscles and bones. Criteria for a malignant process include a large size, poor definition, inhomogeneous density, blurring of the adjacent fat, and invasion of the adjacent muscles and bone.
The differential diagnosis of soft tissue lesions is discussed in Table 11.7.
Skin defects (e.g., decubitus ulcer and surgical defects)
Sinus tracts and fistulas
Penetrating trauma (e.g., gunshot wound)
Perforation of gas-containing structures (e.g., rib fracture with lung injury, fractured trachea)
Pyomyositis and abscess formation
Gangrene (gas phlegmon)