Pectus excavatum is a congenital anomaly of the chest in which abnormal growth of the ribs and sternum results in a concave appearance of the chest wall (14,15). It usually occurs as an isolated condition, but it can be associated with scoliosis, Marfan syndrome, Noonan syndrome, Ehlers–Danlos syndrome, and prune belly syndrome. Patients come to medical attention because of severe chest wall deformity or associated symptoms, such as decreased exercise tolerance or chest pain.

CT is used to compute the Haller index, which is a measurement of the depth of the pectus (16). Axial scans are obtained through the area of greatest sternal depression. The index is obtained by dividing the maximum internal transverse diameter (from side to side inside the rib cage) by the minimum anteroposterior depth of the chest at the same level (distance from sternum to vertebral body)
(Fig. 12.2). Measurements are taken at the deepest point of the pectus deformity. The normal Haller index is 2 to 3. When the index is >3.2, patients often require surgical correction (16,17). CT with sagittal multiplanar and 3D reconstructions of the chest wall is useful for planning the surgical approach and also for evaluating the degree of cardiac, pulmonary, and airway compression and displacement (17).

Developmental dysplasia of the hip (DDH), formerly called congenital dysplasia of the hip, is a spectrum of abnormalities ranging from mild acetabular dysplasia and reducible subluxation to irreducible subluxation and dislocation of the femoral head. It is believed to be due to abnormal ligamentous laxity (rather than a structural abnormality). The cause of this condition is multifactorial and includes excessive levels of circulating maternal estrogens, heritable factors, faulty intrauterine position, family history of this condition, and breech delivery. These factors result in an abnormal position of the femoral head in the acetabulum, which in turn results in abnormal growth of both the proximal femur and acetabulum (18,19,20,21). In most cases, the femoral head is displaced superior, posterior, and lateral to its normal location. DDH is more common in girls than in boys and occurs in approximately 1 in 1,000 live births.

The diagnosis of DDH is typically suspected clinically when physical examination reveals asymmetric skin folds, limited abduction of the hip, or an abnormal Ortolani maneuver (i.e., a palpable “clunk” when the hip reduces into the acetabulum). Sonography is the study of choice in the neonate for evaluating clinically suspected hip dysplasia (22,23). In the neonate, radiographs are not very sensitive because the proximal femoral epiphyses and acetabular cartilage are unossified, so it is difficult to assess the relationship of the femoral head to the acetabulum. When the femoral head ossifies, usually between 3 and 8 months of age, the relationship of the proximal femur to the acetabulum and the extent of acetabular coverage are easier to assess by plain radiography.

Early diagnosis and treatment of DDH are critical to prevent long-term complications, such as degenerative changes or limb shortening. In early infancy (<6 months), treatment is a flexion/abduction external-rotation splint (Pavlik harness). In later infancy (6 to 18 months), closed reduction under general anesthesia and patient immobilization with a spica cast may be necessary. In older children, open reduction with femoral and/or pelvic osteotomy may be needed
for treatment (24). Successful intervention before 4 years of age results in a normal relationship of the acetabulum and the femur in approximately 95% of patients (18,19,20,21). CT is performed preoperatively in older children to plan surgical repair. It is used following closed or open reduction to define the success of the reduction (1,25,26,27,28,29).

When the child is in a splint or Pavlik harness, serial sonography is useful to demonstrate the position of the femoral head relative to the acetabulum. After closed reduction and cast immobilization, CT plays an important role in the evaluation of concentricity of the femoral head, even when the femoral head is unossified. The post-reduction CT requires a limited number of scans. A preliminary digital topogram is obtained through the pelvis and axial images (<1-mm collimation, 1.0 to 1.5 pitch) are acquired from the acetabular roofs superiorly to the tops of the greater trochanters inferiorly (see Chapter 1 for more details). Low dose scans can provide sufficient diagnostic scans, even in small infants with nonossified femoral heads.

Although CT cannot differentiate cartilage (i.e., the unossified femoral head) from other soft tissue structures of the hip joint, certain indirect signs allow recognition of the femoral head and its relationship to the acetabulum. When the patient is in a cast, the femurs are abducted and flexed. With concentric reduction, the femoral metaphyses are directed into the acetabulum and are equidistant from the triradiate cartilage of the acetabula. In lateral displacement, the proximal femur is directed toward the acetabulum, but it is not completely seated within the socket of the acetabulum. The distance between the femoral metaphyses and acetabula is greater on the affected side than on the normal side, unless there is a contralateral abnormality. In posterior displacement, the femoral metaphysis is directed toward the posterior lip of the acetabulum (Fig. 12.3). In addition, the fat plane anterior to the gluteus maximus is deformed or displaced posteriorly.

Failure to obtain reduction or to maintain stability of reduction may indicate the presence of hypertrophied pulvinar or an interposed iliopsoas tendon. The pulvinar appears as a collection of fibrofatty tissue located at the apex of the acetabulum. The iliopsoas tendon normally lies medially and anterior to the joint capsule. When the femoral head is laterally displaced, the tendon may migrate medially, interposing itself between the femoral head and the acetabulum. The hypertrophied iliopsoas tendon is seen as a soft tissue structure within the joint space.

The goals of surgical intervention are to improve acetabular coverage, decrease pressure loading on the femoral head, and increase efficiency of para-articular musculature. The role of CT is to provide information about the extent of femoral head displacement, the amount of coverage of the femoral head, the depth of the acetabulum, and the size of the anterior and posterior lips of the acetabulum.

CT protocols for surgical planning utilize axial images (<1-mm collimation, 1.0 to 1.5 pitch), which are acquired from the acetabular roofs superiorly to the tops of the greater trochanters inferiorly (see Chapter 1 for more details). Axial images are best for demonstrating posterolateral displacement of the femoral head, acetabular configuration, especially acetabular size and depth, and the presence or absence of femoral head deformity. Multiplanar and 3D images (2-mm thickness × 1-mm intervals) in coronal and sagittal planes are best to show the superolateral and anteroposterior coverage, respectively, of the femoral head.

Because the femoral head is ossified in older infants and children, it is easy to recognize on CT scans (Fig. 12.4). In uncorrected DDH, the femoral head is usually displaced superiorly, posteriorly, and laterally (Figs. 12.5 and 12.6). The acetabulum is usually underdeveloped and shallow, and the femoral head may be deformed. Following open reduction, the femoral head may be seated, subluxed or dislocated laterally, anteriorly or posteriorly.

Avascular necrosis is the most serious complication of surgically treated hip dysplasia. CT findings of avascular necrosis include epiphyseal fragmentation and sclerosis, subchondral collapse, coxa magna, and poor lateral coverage (see subsequent section on avascular necrosis). Multiplanar images and 3D volume rendering allow a more detailed display of these complications than do axial images.

Femoral torsion refers to the inclination of the axis of the femoral neck with reference to the transcondylar plane of the distal femur (30,31). Normally there is some anteversion of the proximal femur. The angle of anteversion ranges between 30 and 50 degrees in the neonate and then decreases with increasing patient age, averaging about 20 degrees in children 6 to 12 years of age and 10 to 15 degrees in older individuals (32). An increased angle of anteversion is thought to contribute to proximal femoral subluxation or dislocation. Retroversion may contribute to the development of slipped capital femoral epiphysis.

The examination is performed with the patient supine and the feet flat perpendicular to the scanning table. The contralateral extremity should be symmetrically positioned to allow comparison images. A scout CT is obtained from the hips through the knees. Scans are then obtained from the femoral heads through the lesser trochanters, and a second set of images is acquired from just above the distal femoral physes through the tops of the tibias. The images that best show the femoral neck and femoral condyles are selected for measurements.

Measurements are made in the axial plane; reformatting is not required. Two angles are measured. One is the femoral neck–horizontal (NH) angle and the other is the femoral condyle–horizontal (CH) angle. For the neck-horizontal angle, a line is drawn through the long axis of the femoral neck and a reference line is drawn horizontal to the table; the angle between the two lines is measured. For the condyle-horizontal angle, a line is drawn through the widest part of the transcondylar axis and a reference line is drawn horizontal to the table; the angle between these two lines is measured. When the femoral condyles are externally rotated, the lines will meet laterally. In these cases, the fermoral condyle–horizontal angle is subtracted from the femoral neck–horizontal angle, and the difference is the angle of anteversion (Å = NH - CH). When the femoral condyles are internally rotated, the lines meet medially. In these cases, the condyle-horizontal angle is added to the neck-horizontal angle (Å = NH + CH) (Fig. 12.7).

Tarsal coalition, which refers to an abnormal fusion of two or more tarsal bones, is a cause of hindfoot rigidity and pain (34). CT has been accepted as the cross-section imaging method of choice for delineating suspected tarsal coalitions (35). Prior to the introduction of isotropic CT data sets, CT for tarsal coalition required imaging in two separate planes. With newer technology, images can be acquired in only one plane and then the original data sets can be reconstructed in other planes as needed. In general, planes perpendicular to the coalition are best for displaying the coalition. Since high detail is needed, the scans should be acquired with 0.5- to 0.75-mm collimation and pitch of 1.

Coalitions can be osseous, fibrous, or cartilaginous. Osseous bridging is obvious and appears as a bony bar between two joint surfaces. Fibrous and cartilaginous unions are more difficult to diagnose, but CT features supporting the diagnosis are joint space narrowing, irregular or sclerotic articular surfaces, and subchondral cyst formation (34,35,36,37,38).

Talocalcaneal and calcaneonavicular coalitions are the most common types of coalition, accounting for approximately 90% of cases. Talonavicular, calcaneocuboid, cuboidonavicular, and navicular-cuneiform coalitions occur less commonly (34,35). Calcaneonavicular coalition is best seen on axial images (i.e., images parallel to the plantar surface) (Fig. 12.9). Talonavicular coalition may involve the middle or posterior parts of the subtalar joint and is best seen on coronal images (i.e., images perpendicular to the plantar surface of the feet) (Fig. 12.10). Images should be viewed in planes perpendicular to the
one that shows the coalition to search for other subtle coalitions. Because coalitions can be bilateral (50% frequency) the opposite hindfoot should be included in the examination.

The Salter–Harris type fracture, which involves the physeal plate, is the most common injury of the skeletally immature skeleton. The ankle and wrists are the most common sites for Salter–Harris type fractures. There are five basic types of growth plate fractures. Salter type I fracture is a fracture of the growth plate and does not involve bone. Salter II fractures involve the growth plate and metaphysis. Salter III fractures pass through the growth plate into the epiphysis and are intra-articular. Salter IV fractures involve the growth plate, metaphysis, and epiphysis and are also intra-articular. Salter V injury is a crush injury of the growth plate. CT is valuable in Salter III to V fractures to determine the severity of injury for treatment planning. These fractures may require open reduction and internal fixation, because malalignment of the fragments is associated with formation of a bony bridge and asymmetric bone growth. A posttraumatic bridge tethers the physis on one side and results in varus or valgus deformity. The risk is greater with Salter III, IV, and V fractures. Radiographs often show the physeal bridge, but CT is useful to confirm the presence of the bridge and determine its extent for surgical treatment (44,45).

CT also has a role in the evaluation and treatment of triplane and Tillaux fractures (46). These injuries occur in adolescent patients in whom the distal tibial physes are closing. The triplane fracture constitutes 6% to 10% of ankle fractures; mean patient age is 13 to 15 years. The fracture is characterized by three distinct fracture planes: a fracture through the metaphysis in the coronal plane, a fracture through the physis in the axial plane, and a fracture through the epiphysis in a sagittal plane (Fig. 12.11). A Tillaux fracture is a variant of the Salter–Harris type III fracture in which there is avulsion of the anterolateral corner of the distal tibial epiphysis by tension from the anterior tibiofibular ligament during external rotation. The fracture extends sagittally through the epiphysis and transversely through the lateral part of the physis (Fig. 12.12) (46). In either fracture, >2 mm separation of the fracture fragments in any plane is often regarded as unsatisfactory alignment and an indication for surgery to prevent premature degenerative changes. There is virtually no risk of growth arrest because the physis is partially closed and hence, has little growth potential.

The physeal injury and the integrity of the articular surface are best seen on coronal and sagittal images. Axial images provide information about the extent of fracture comminution and distraction.

Pelvic fractures are frequently complex, and the evaluation of the precise pathologic anatomy is often limited on plain radiographs (47,48,49). Acetabular fractures are nearly always complex and may involve a supporting column (i.e., the shorter posterior ilioischial column or the longer anterior iliopubic column), the acetabular rim, or the pubic ramus. CT evaluation can provide useful information about the alignment of the fracture fragments and the presence of intra-articular fragments. Intra-articular fragments, especially when small, may not be apparent on plain radiographs; their recognition is important because they need to be either reduced or removed from the joint.

Axial images often suffice for demonstrating acetabular fractures and the presence of free fragments (Fig. 12.13). Multiplanar and 3D reconstructions provide additional information about the extent of injury of the acetabular dome and weight-bearing columns as well as allowing a determination of any incongruity of the articular surface (50,51,52,53). Complex pelvic fractures have an association with concomitant vascular injury. The use of intravenous contrast agent allows the evaluation of vascular anatomy from the same CT data set obtained for evaluation of the fracture.

Fractures of the lower extremity in children are less common than fractures of the upper extremity. Dislocations of the proximal femur are more common than fractures of the proximal femur, although they are infrequent. They are more likely to occur in adolescents than in children.

Posterior hip dislocations account for most traumatic hip dislocations. The mechanism of injury is a force along the axis of the femoral shaft with the hip flexed. The dislocated hip is usually diagnosed on plain radiographs; it is reduced almost immediately to minimize the risk of avascular necrosis. The role of CT is to demonstrate associated acetabular fractures and intra-articular fragments. The presence of gas bubbles in the hip joint after trauma is a reliable finding for dislocation (54). Gas bubbles are seen within 4 hours of dislocation and are usually located anterior to the femoral neck. Patients with gas bubbles after hip dislocation usually also have evidence of joint effusion and surrounding soft tissue injury.

Sternoclavicular joint injury results from blunt chest trauma, usually a motor vehicle accident or sports-related trauma. Posterior dislocation of the clavicular head at the sternoclavicular joint occurs as a result of direct force to the medial part of the clavicle. Posterior displaced fractures can impinge on the mediastinal great vessels or trachea. Anterior dislocation occurs from force applied to the lateral part of the clavicle. Complications are unusual.
CT, especially with coronal reformatting and 3D imaging, is superior to plain radiography to confirm the presence and extent of sternoclavicular joint dislocation (Fig. 12.14) as well the presence of associated clavicular shaft fractures and vascular or central airway injuries (55,56). In patients with posterior dislocations, the mediastinum should be imaged after administration of intravenous contrast medium.

Most fractures and dislocations of the shoulder, elbow, wrist, knee, and foot do not require CT evaluation for diagnosis. However, CT scanning can be useful to establish the diagnosis when plain radiographs are equivocal, assess fracture displacement and integrity of the joint surface, and delineate intra-articular fragments (1,57,58,59,60) (Figs. 12.15 and 12.16).

Carpal fractures are rare in the pediatric age group (61). Among carpal bones, scaphoid fractures are the most common, with peak incidence occurring between the ages of 12 and 15 years (59,61). When they occur, CT has a role in evaluating fracture displacement (Fig. 12.17) and assessing complications, such as nonunion or avascular necrosis. Homogeneous ossification between the scaphoid fragments indicates fracture healing. Increased density of part or all of the scaphoid or collapse suggests osteonecrosis (Fig. 12.18). Multiplanar reconstructions in planes parallel to the long axis of the scaphoid along with images in the axial plane are most helpful for evaluating the extent of the fracture.

The earliest finding of acute osteomyelitis is an increase in the attenuation value of the marrow cavity, representing
replacement of the hematopoietic components by purulent material, blood, and debris. When areas of fatty marrow are infected, the marrow attenuation increases to that of water. When areas of red marrow are infected, the attenuation value becomes closer to that of soft tissue. Comparing the affected side with the contralateral normal extremity is usually of more value than the measurement of absolute Hounsfield units. Of note, marrow edema secondary to infection is nonspecific and indistinguishable from that seen in trauma or neoplastic replacement. Subsequently, periosteal elevation, cortical destruction (Fig. 12.22), intraosseous gas, and soft tissue abscesses (74) may be seen.

The periosteum is loosely attached to the bone in children, so that intraosseous pus can penetrate the cortex and accumulate under the periosteum. Subperiosteal abscess is seen as a fluid collection with enhancing walls beneath the periosteal layer.

A Brodie abscess is a localized subacute bone infection. It may develop de novo without evolving through an acute phase or it may develop in the site of a prior acute osteomyelitis. Pathologically, it is an avascular cavity, filled with fluid but not usually pus and lined by granulation tissue. Typically, it occurs in the metaphyses or diaphysis of long bones, particularly the tibia or femur. The CT appearance is that of a small, well-defined, low-attenuation lesion surrounded by a rim of reactive bone formation that may blend imperceptibly with surrounding cortex (Fig. 12.23).

Chronic osteomyelitis is a low-grade or recurrent infection characterized by necrotic bone and reactive sclerosis. CT typically demonstrates increased bone density with encroachment on the medullary cavity and bone deformity (Fig. 12.24) (73).

A sequestrum occurs when an area of devitalized bone is surrounded by a rind of granulation tissue or by a rind of thick periosteal new bone formation (termed involucrum). At CT, the sequestrum appears as a high-attenuation focus within a region of marrow infection, a Brodie abscess, or a sinus tract (Fig. 12.25). A sinus tract or cloaca appears as a channel extending from the marrow cavity through the cortex (Fig. 12.26). Other findings associated with chronic infection include soft tissue abscesses and foreign bodies.

Late sequelae of osteomyelitis are a bony bridge producing early physeal fusion and limb deformity. The CT appearance of a postinfectious bridge is identical to that seen with a posttraumatic bridge (see above).

Cellulitis is an infection of the skin and subcutaneous tissue superficial to the deep fascia. Clinical findings are cutaneous edema, warmth, erythema, and tenderness. CT findings are skin thickening, linear bands of increased attenuation in the subcutaneous tissues, loss of tissue planes between subcutaneous fat and muscle, and enhancement after intravenous administration of contrast medium (Fig. 12.28) (79). In lymphedema, there is thickening of the subcutaneous tissues, which have a striated or honeycomb appearance. Enhancement is absent, and the subfascial tissues are usually normal.

An abscess is a localized suppurative fluid collection that is walled off by vascular connective tissue (79). At CT, an abscess appears as an elliptical or spherical mass with a thick, enhancing rim and a lower-attenuation center (Fig. 12.29). A surrounding rim of soft tissue edema is common. The abscess may contain gas, which in the absence of a history of biopsy or aspiration is specific for the diagnosis of abscess. Associated findings include obliteration of adjacent fascial planes and edema of contiguous muscle groups.

Necrotizing fasciitis is a rare infection of the superficial fascia, sparing the deep fascia and muscle. The abdominal wall, extremities, and perineum are the most common sites of involvement. Early clinical findings include cutaneous erythema and edema, which rapidly progress to frank gangrene. CT findings include gas and streaky fluid collections in the subcutaneous soft tissues along with fascial thickening. Areas of inflammation enhance after contrast injection; areas of necrosis do not enhance.

Pyomyositis is a bacterial infection of skeletal muscle, usually involving the large muscle groups of the thighs, calves, and buttocks (71). It commonly is associated with a predisposing condition, such as local trauma or immunosuppression. At CT, the affected muscle group is enlarged and heterogeneous and the normal internal architecture is absent (Fig. 12.30) (81,82). The involved muscle may contain fluid collections with septations and enhancing walls. The fat planes between muscle groups are obliterated. Intramuscular gas may be seen if the infection is caused by a gas-producing organism. Associated findings of cellulitis are nearly always present.

An osseous pseudotumor is a hematoma with a thick fibrous capsule. It is a rare complication of repetitive intraosseous hemorrhage (92). It is most often seen in patients with hemophilia, although it can also be seen in patients with bleeding diatheses who sustain trauma. Clinically, it presents as a slowly expanding, painless mass. CT findings are those of a well-defined, central or eccentric, masslike, expansile lesion with bony struts (92). Fluid–fluid levels can be present.

CT is particularly valuable for evaluation of cartilaginous and osseous lesions. It is able to identify the mineralization pattern of the matrix and subtle areas of calcification and cortical destruction that may not be recognized on conventional radiographs, thus helping to characterize the tumor. Chondroblastoma, enchondroma, and osteochondroma are the common cartilaginous lesions in children.

Chondroblastoma consists of chondroid tissue mixed with cellular tissue. It characteristically is found in the epiphyses of long bones, but it may extend into the metaphysis. Most are found in the lower extremities, usually around the knee. Common CT features are a round or ovoid, low-attenuation, epiphyseal lesion with a central or eccentric location (Fig. 12.33). The margins are well defined and may be sclerotic. Some are expansile, and about one third contain central irregular calcifications (popcorn appearance) (93).

Enchondroma is a benign neoplasm composed of mature hyaline cartilage. It is most common in the phalanges of the hand, although it may be encountered in
the long tubular or flat bones. At CT, it appears as an expansile, intramedullary lesion with a soft tissue–attenuation matrix (Fig. 12.34 and 12.35). There may be matrix mineralization, consisting of speckled or coarse calcifications.

Osteochondroma, also known as osteocartilaginous exostosis, is the result of overgrowth of histologically normal but aberrant foci of cartilage on the bony surface. Common sites of involvement are the femur, proximal tibia, and proximal humerus. CT demonstrates an exostotic lesion with smooth, sharply defined margins. The medullary cavity and cortex are continuous with that of the bone from which they arise (Fig. 12.36). The cartilaginous cap is rarely seen as a discrete structure on CT. The cap is more easily seen on MRI.

Osteoid osteoma is a benign osteoblastic tumor consisting of a central nodule of vascular osteoid tissue (called the nidus)
and a surrounding rim of reactive sclerotic bone. The nidus may be cortical, medullary, or subperiosteal in location. This lesion has a limited growth potential and is rarely larger than 1.5 cm. It most often occurs in the diametaphyseal areas of the femur and tibia. Patients classically present with pain that is worse at night and is relieved by aspirin (94).

CT can help to define the location of the nidus, especially in areas of complex anatomy, which is helpful information for guiding removal or curettage of the nidus (95,96). CT findings include a well-defined, low-attenuation lesion (the nidus) with a margin of reactive sclerosis (Fig. 12.37) (94,97). Sclerosis is more extensive in the cortical type of osteoid osteoma than in the medullary and subperiosteal forms, which produce minimal if any reactive sclerosis (98). The nidus may contain central calcification, and because it is vascular, it often enhances following the administration of intravenous contrast medium. The presence of enhancement is useful to separate osteoid osteoma from a Brodie abscess, which has similar CT features but is avascular and does not enhance.

Fibrous dysplasia and benign fibro-osseous lesions, either fibrous cortical defect or nonossifying fibroma, are the common fibrous lesions of bone. Fibrous dysplasia is an anomaly in which there is replacement of normal marrow by benign fibroblasts and primitive woven bone. It is more often monostotic than polyostotic. The monostotic form usually involves the ribs, proximal femurs, and craniofacial bones, whereas the polyostotic form is more common in the femur, tibia, pelvis, and foot (99). The lesions are primarily medullary, but they may involve both cancellous and cortical bone. The most common location in long bone is the diaphysis. The CT features are variable, but they include an intramedullary location, soft tissue attenuation with variable degrees of mineralization (calcification or ossification), sharply circumscribed borders, and surrounding reactive bone formation (Fig. 12.38) (99). Cortical thinning and bone expansion can occur.

Fibrous cortical defect is usually asymptomatic and an incidental finding on imaging studies. It occurs predominantly in the metaphyses of the long tubular bones, particularly the proximal tibia and the distal femur, and is found close to the physeal plate. It typically is situated in the cortex along one side of the bone. CT features are a small, nonexpansile, sharply marginated, intracortical lesion with soft tissue attenuation and sclerotic margins (Fig. 12.31). Nonossifying fibroma is a larger version of the fibrous cortical defect. Except for size, the CT features of nonossifying fibroma are similar to those of the fibrocortical defect (Fig. 12.39).

Osteofibrous dysplasia, also known as ossifying fibroma, is a benign tumor produced by a proliferation of fibrous tissue (100,101). The pathologic appearance is similar to fibrous dysplasia, but the location suggests the diagnosis. The lesion is virtually always isolated to the anterior cortex of the midtibia. Affected patients characteristically present in the first decade of life with pain and bowing of the affected bone. Initially, osteofibrous dysplasia appears as a unilocular, soft tissue–attenuation cortical
lesion. Later it develops the more classic appearance of a multilocular cortical mass. The cortex anteriorly is usually thin or focally absent, whereas the posterior rim along the border with the medullary cavity may be densely sclerotic (Fig. 12.40) (100). Soft tissue mass and periosteal reaction are uncommon findings.

Similar CT features may be seen in adamantinoma (102). Adamantinoma is a rare slow-growing malignant tumor with a location in the anterior cortex of the middiaphysis of the tibia. It is the age of the patient, rather than imaging features, that helps in the preoperative differentiation of the
two lesions. Adamantinoma characteristically is seen in patients older than 10 years of age.

Osteosarcoma is the most common primary malignant bone tumor in adolescents and young adults; it is typically seen in the second and third decades (103,104,105). Clinical complaints are pain and swelling at the site of the tumor. The typical sites of involvement are the distal femur, proximal tibia, and proximal humerus. The appearance of osteosarcoma varies depending on its predominant histologic composition (osteoblastic, chondroblastic, fibroblastic) and location within the bone (intramedullary, juxtacortical, or intracortical).

The central or intramedullary form of osteosarcoma, which is usually osteoblastic, constitutes >90% of osteosarcomas. It arises within the cancellous portion of a long bone, usually in the metaphysis, and eventually penetrates the cortex and invades the adjacent soft tissues. CT findings are a mixed lytic-sclerotic lesion with poorly defined margins replacing the normal low-attenuation fatty marrow, matrix calcifications, cortical destruction, new bone formation, and invasion of adjacent soft tissues (Fig. 12.41) (104,105,106).

Telangiectatic osteosarcoma is a rare form of intramedullary osteosarcoma representing 2.5% to 12% of all osteosarcomas (87). It is an expansile, destructive lesion that contains hemorrhage and necrosis and very little osteoid. At CT, it therefore appears as a soft tissue mass with attenuation equal to or less than that of muscle. Fluid–fluid levels occur in about 75% of cases (Fig. 12.42). Bone reaction and matrix calcification are minimal. Cortical destruction with an associated soft tissue mass is invariably present.

Treatment for osteosarcoma is either local resection with limb-salvage surgery or amputation. The absence of skip metastases and the preservation of the neurovascular bundle are mandatory for limb sparing to be feasible.

Three commonly recognized variants of osteosarcoma arise on the cortical surface: parosteal, periosteal, and high-grade surface osteosarcomas (107,108,109,110,111). They account for 4% to 10% of all osteosarcomas (108). Periosteal osteosarcomas are chondroblastic and moderately differentiated, whereas periosteal osteosarcomas are fibroblastic and well differentiated. High-grade surface osteosarcomas are very poorly differentiated. Like intramedullary osteosarcomas, these tumors have a predilection for long bones.

Among the juxtacortical subtypes, parosteal osteosarcoma is most common (107,108,110). It affects slightly older patients than intramedullary osteosarcoma, usually presenting in the second to fifth decades of life (108). Parosteal osteosarcoma usually arises in the metaphysis of the femur. Typical CT findings are a densely ossified juxtacortical mass with a broad, sessile base and associated soft tissue mass (107,108) (Fig. 12.43). The density of the tumor is greatest where it is adjacent to bone. Involvement of the medullary cavity is rare. Parosteal osteosarcoma has a better prognosis then the conventional high-grade medullary osteosarcoma. Local resection is the usual treatment (108).

Periosteal osteosarcoma occurs in the same age group as intramedullary osteosarcoma (second and third decades). It most often involves the diaphysis of the femur or tibia. The CT appearance is that of a broad-based mass on the cortical surface of a long bone with spicules of calcification or periosteal new bone extending away from the cortex into the soft tissues (Fig. 12.44). The periphery of the lesion has less ossification then the base. The underlying cortex may be thickened; the medullary cavity is usually spared (105,106,107,108,109,110,111,112,113). Periosteal osteosarcoma has a prognosis better than that of high-grade intramedullary osteosarcoma but worse than that of parosteal osteosarcoma. Treatment is wide excision.

High-grade surface osteosarcoma is the least common form of osteosarcoma (114). It arises in the cortex, is broad based, and shows varying amounts of mineralization. Cortical and medullary invasion are more common than the other surface osteosarcomas (114,115).

The Ewing sarcoma family of tumors includes Ewing sarcoma, peripheral primitive neuroectodermal tumors, and desmoplastic round cell tumors (116,117). Ewing sarcoma is the second most common primary malignant
bone tumor in children. It is one of the pediatric cancers characterized by small, round blue cells. Most patients are between 10 and 15 years of age. Bone pain and local swelling are the most frequent clinical complaints. It commonly presents in the metaphyses of long bones, usually in the femur, tibia. or fibula, or in flat bones. CT findings include permeative bone destruction, replacement of fatty marrow by soft tissue–attenuation tumor, periosteal new bone formation, and a large associated soft tissue mass (Fig. 12.45). Sclerotic reactive bone may be seen at the periphery of the lesion but not in the central tumor matrix or in the soft tissue mass.

Primitive neuroectodermal tumor (PNET) has clinical and imaging features indistinguishable from that of Ewing sarcoma. Diagnosis requires tissue sampling.

Other less common primary malignant bone tumors, such as chondrosarcoma, fibrosarcoma, and malignant fibrous histiocytoma, can also be clearly depicted by CT. The role of CT is much the same as for osteosarcoma and Ewing sarcoma, e.g., to show cortical destruction, new bone production, and soft tissue calcifications for staging and therapy planning.

Langerhans cell histiocytosis (LCH), previously known as histiocytosis X, is a disorder of unknown cause characterized by granuloma formation secondary to proliferation of histiocytes of the Langerhans cell type (118,119,120,121). It is subdivided into three major subgroups: a localized form, a chronic recurring form, and a fulminant form (120). The localized or monostotic form is most common, accounting for approximately 70% of cases. Patients present with pain occasionally associated with fever. Approximately 75% of monostotic lesions are found in the skull, spine, pelvis, and ribs; the remaining lesions arise in the long bones, usually the femur, humerus, or tibia.

The chronic recurring form of LCH accounts for 20% of cases. Symptoms include bone pain, diabetes insipidus, draining ears, and dermatitis. Bone lesions are often limited to the calvarium. Affected patients are usually between 1 and 5 years of age. The fulminant form of LCH accounts for 10% of all cases and involves soft tissue structures and skeleton. Most patients are between the ages of several weeks and 2 years. Clinical findings include hepatomegaly, splenomegaly, lymphadenopathy, and anemia. The osseous lesions are often disseminated (121).

CT usually is not needed for the diagnosis of LCH, but it can be useful in confirming the presence of a bone lesion as well as defining the extent of cortical destruction and soft tissue involvement (121). CT findings include osteolysis, expansion of the medullary cavity, cortical thinning, periosteal reaction, and a nonenhancing soft tissue mass (Fig. 12.46). The transition zone between normal and abnormal bone is poorly defined. Reactive sclerosis is absent.

Hemangioma is a slow-growing lesion containing large vascular spaces and often nonvascular elements such as
fat, fibrous tissue, and smooth muscle. It can arise within superficial or deep soft tissues. On unenhanced CT, the blood-filled spaces are isoattenuating to muscle and become hyperattenuating after administration of intravenous contrast medium. Internal heterogeneity may be seen related to hemosiderin deposits, fibrosis, fat, calcification, or thrombosis. A dominant feeding artery or draining vein may be noted adjacent to the mass.

Lymphatic malformation (also termed lymphangioma) is a congenital lesion composed of dilated lymphatic channels. It usually presents as a painless, soft mass in the first year of life. The most common sites of involvement are the neck and axilla. Smaller lesions tend to be well marginated, whereas larger lesions often are infiltrative and ill defined. The characteristic CT appearance is that of a thin-walled, multilocular, predominantly low-attenuation mass (Fig. 12.48). Septal enhancement may be noted after intravenous contrast administration; the fluid-filled spaces do not enhance. Lymphatic lesions can be differentiated from hemangiomas based on their typical location and the absence of both feeding vessels and intense contrast enhancement.

Neurofibromas are the most common neural tumors in children. They arise within peripheral nerve fibers and may occur sporadically or in association with neurofibromatosis type 1 (NF-1). Malignant degeneration of neurofibromas occurs in 2% to 15% of patients with NF-1. Benign neurofibromas appear as homogeneous, well-defined, round or ovoid, soft tissue masses with attenuation less than or equal to skeletal muscle (Fig. 12.49). Distinction between benign and malignant tumors is difficult, but an irregular or infiltrating tumor border, internal heterogeneity, or asymmetrically large soft tissue masses should raise a suspicion of malignancy.

Fibromatosis is a histologically benign but locally aggressive lesion characterized by fibrous tissue proliferation, an invasive growth pattern, and a tendency to recur locally after surgical excision (124,125). Most occur after puberty, but they have been reported in infants and children. Fibromatosis appears as a poorly circumscribed, soft tissue mass on precontrast and postcontrast CT scans. The tumor matrix may be homogeneous or heterogeneous, containing areas of tumor necrosis or calcification. The CT features are nonspecific and mimic those of fibrosarcoma.

Almost all lipomatous tumors in children are lipomas or lipoblastomas (126,127,128,129). Lipomas contain mature fatty tissue and are most common in the subcutaneous tissues of the extremities. They typically are well-marginated, nonenhancing, homogeneous, fat-containing tumors. Occasionally they contain thin septations.

Lipoblastoma contains multiple lobules of immature fatty tissue separated by fibrous septa. It occurs almost exclusively in young children, usually under 3 years of age. It may appear as a soft tissue or fatty mass depending on the relative amount of fibrous and lipomatous tissue.
The margins may be well circumscribed or diffuse and infiltrative. Lipoblastoma and liposarcoma are indistinguishable on imaging studies, but the latter tumor is exceedingly rare in children, with an incidence of less than 1%.

Soft tissue hematomas occasionally present as masses (130). Acute hematomas typically appear as high-attenuation masses (Fig. 12.50). Subacute and chronic hematomas usually appear as low-attenuation masses. The subacute hematoma may have a heterogeneous matrix, whereas the chronic hematoma is usually homogeneous. The CT appearance of subacute and chronic hemorrhage is nonspecific and can mimic that of abscess. When gas is present in soft tissues or the abscess cavity, a specific diagnosis of abscess can be made. In the absence of gas, correlation with clinical history and in some cases percutaneous or surgical aspiration may be needed for confirmation.

Myositis ossificans refers to posttraumatic heterotopic ossification in skeletal muscle or soft tissues. It tends to occur in adolescents and young adults and is usually secondary to direct trauma, although a clear-cut history of trauma cannot always be elicited. Patients present with pain, tenderness, and soft tissue mass. The most common locations are the thigh, buttocks, and elbow. Myositis progresses over a period of weeks into an organized mass that begins to ossify. Bone formation proceeds in a centrifugal pattern, beginning peripherally and progressing centrally. The diagnosis is usually based on clinical and imaging findings, particularly plain radiographs and CT. Biopsy is less reliable and may result in an erroneous diagnosis of sarcoma.

Because the CT appearance is rather characteristic, CT is often the imaging procedure of choice if plain radiographs are not diagnostic. In the first 2 weeks after trauma, CT shows a low-attenuation mass without mineralization or calcification and edema of the surrounding soft tissues. After 4 to 6 weeks, the lesion shows curvilinear peripheral ossification (Fig. 12.51), and over the next several weeks and months as it matures more, it shows internal ossification (Fig. 12.52). Typically, myositis is separated from adjacent periosteum by a low-attenuation
zone. Myositis can mimic parosteal osteosarcoma, but the presence of this hypoattenuating zone supports the diagnosis of myositis ossificans. Parosteal osteosarcoma is contiguous with the underlying bone.