Pedriatic radiology

Respiratory system

 

Modalities

Chest radiography remains the "foundation" examination for the evaluation of any thoracic abnormality. Both AP or PA and lateral views of the chest should be obtained for accuracy of interpretation. Evaluation of the upper airway requires AP and lateral radiographs. Abnormalities identified by conventional chest radiographs may require additional plain films for clarification. Potential supplementary chest radiography includes oblique views, high-kilovoltage techniques, inspiration-expiration films, and lateral decubitus views. Other modalities which may be helpful for evaluation of abnormalities of the respiratory system include fluoroscopy, esophagography, angiography, bronchography, US, CT, and MRI.

Chest radiographs comprise up to 1/3 of all X -ray examinations in children. Conventional X-ray technology demonstrates most respiratory tract disease. All children, who can stand, should be examined in a chest unit. Special equipment, where the patient is restrained, is used as little as possible. Supine radiographs are appropriate in younger infants or for portable examinations. Frontal and lateral views with good positioning, deep inspiration, and adequate exposure is critical (Fig. 1).

Neonatal chest X-ray examinations are performed for respiratory problems. Many of these children are treated in incubators and with breathing assistance. They must be examined in the intensive care unit with portable X-ray equipment. To avoid heat loss, exposures are made with the patient in the incubator. Electric cables and electrodes are not removed from the patient. However, an electrode situated so that it will obscure the carina or trachea should be permanently moved since it can obscure location of an endotracheal tube. It is important that the frontal film is not oblique. Only AP films are required for follow-up examination of the newborn; lateral films are helpful for initial diagnostic evaluation or clarification of unexplained abnormalities on the AP radiograph.

Airway abnormalities

Choanal Atresia
Choanal atresia is a congenital obstruction of the posterior nasopharynx that can be membranous or bony, unilateral or bilateral, complete or incomplete. Choanal atresia is the most common congenital anomaly of the nasal cavity. Since the newborn is an obligate no se breather, bilateral choanal atresia causes severe respiratory distress, especially during feeding. The diagnosis is suspected clinically by failure to pass an enteric tube via the nasal route. The diagnosis can be verified by injecting water-soluble, non-ionic contrast into the nasal cavity (Fig. 2 a). CT is also able to confirm the diagnosis by demonstrating the anatomic abnormality (Fig. 2 b).

Tracheomalacia
Tracheomalacia is a weakness in the wall of the trachea, either local or generalized, causing collapse during breathing. It may be due to intrinsic weakness of the tracheomembranous cartilage but more commonly is a result of extrinsic factors. The diagnosis is verified by fluoroscopy. Contrast in the esophagus aids in the diagnostic evaluation. Some of the secondary vascular causes of tracheomalacia include innominate artery compression, double aortic arch, right aortic arch with anomalous left subclavian artery, and pulmonary sling. Initial diagnostic study should be a contrast examination of the esophagus in both frontal and lateral projections.

a

Figure 2. Choanal atresia. a) Bilateral choanal atresia. Newborn male with respiratory distress during feeding. Water-soluble contrast material fills the nasal cavity (arrow) but does not enter the pharynx. b) Unilateral choanal atresia. 7 year-old male with nasal congestion. Axial CT section demonstrates bony plate (arrow) on the left. [From Kirks DR. Practical Pediatric lmaging: Diagnostic Radiology of lnfants and Children. 2nd Ed. Boston: Little. Brown and Company, 1991. With permission of editor and publisher.]

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Croup and Epiglottitis

Croup is a common cause of upper-airway obstruction in temperate zones and is almost invariably due to viral infection. Although the entire airway (laryngotracheobronchitis) is infected, the critical area of airway narrowing is located l cm below the larynx. Croup is a disease of infants and young children, with the age range being between 6 months and 3 years; the peak age for occurrence is approximately 12 months. The child presents with a mild barky cough and intermittent respiratory stridor. AP radiographs show loss of the normal lateral convexities ("shoulders") of the subglottic trachea (Fig. 3 a). Lateral radiographs demonstrate hypopharyngeal overdistention during inspiration, normal epiglottis and aryepiglottic folds, loss of definition of the tracheal lumen just below the level of the vocal cords, and narrowing of the subglottic portion of the trachea (Fig. 3 b).

a	Figure 3. Croup. 2-year-old fem ale with inspiratory stridor. a) AP view of the upper airway shows subglottic narrowing well below the level of the pyriform sinuses, producing a steeple appearance (arrows). [From Kirks.] b) Lateral view of the upper airway shows mild subglottic tracheal narrowing (arrow).
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Figure 4. Epiglottitis. 2-year-old male with inspiratory stridor and drooling. Lateral view of the upper airway shows moderate enlargement of the epiglottis with marked thickening of the aryepiglottic folds. [From Kirks.]

Epiglottitis is less common but much more dangerous than croup. It affects older children than does croup, with the peak incidence between 3 and 6 years of age. The usual bacterial etiology is Hemophilus influenzae. A patient with acute epiglottitis is in mortal danger; no physical or radiologic examination should be performed that may further compromise the airway. The lateral radiograph shows marked enlargement of the epiglottis and thickening of the aryepiglottic folds (Fig. 4). The development of an effective vaccine has decreased the incidence of both
 

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Figure 5.Wet lung disease. a) Term 2-hour-old female with increased respiratory rate but no cyanosis or fever. The heart is at the upper limits of normal in its transverse diameter. There is increase in vascularity with hazy margins and patchy interstitial and acinar opacities that obscure the heart margins. Note the right pleural effusion. b) Follow-up chest radiograph at 24 hours of age. The heart and vascularity are now normal. Pleural fluid no longer identified.

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epiglottitis and H influenzae pneumonia.

Respiratory distress in the newborn

Respiratory difficulties occur in up to 3 % of all newborn infants. At least half of all newborn infants weighing 2kg or less will have some respiratory distress. Infants weighing 1 kg or less have a 95% incidence of respiratory difficulties.

Medical Disease

Wet-lung disease
Wet-lung disease (pulmonary adaption syndrome; transient tachypnea of the newborn), due to delay in normal clearing of lung fluid, is one of the most common causes of respiratory distress in the newborn. Typically, infants with wet-lung disease are full-term. There is an increased frequency with cesarean section, prematurity, or maternal sedation. Tachypnea develops during the first few hours of life but pH and pCO2 are normal. Chest radiographs demonstrate the pathophysiologic abnormalities due to a delayed resorption of fluid. The findings include indistinct pulmonary vessels indicating vascular congestion, fluid in the fissures, bilateral pleural effusions, and patchy parenchymal opacities (Fig. 5 a). The changes are usually symmetric and the general aeration

a	Figure 6.Respiratory distress syndrome. a) Severe RDS. There is diffuse opacity with marked underaeration of both lungs. b) Mild RDS. There are bilateral reticulogranular densities. The general aeration of the lungs is only slightly decreased.
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of the lungs is normal. In severe cases, it may be difficult, if not impossible, to distinguish wet-lung disease from neonatal pneumonia or meconium aspiration. The clinical appearance as well as the chest radiograph are usually normal by 24-48 hours (Fig. 5 b). In fact, the chest radiograph usually shows marked improvement by 12 hours of age.

Respiratory distress syndrome
Respiratory distress syndrome (RDS), also called hyaline membrane disease or idiopathic respiratory distress syndrome, is due to a deficiency of surfactant. Deficiency of this lipoprotein decreases alveolar surface tension and causes acinar atelectasis. The diffuse collapse of alveoli is associated with interstitial edema, damage to alveolar epithelium, uneven expansion, and generalized lung underaeration. The disease is most common in premature infants but can occasionally occur in full-term newborns.

Figure 7. Pulmonary hemorrhage. 2-day-old premature male with RDS. Sudden clinical deterioration with blood suctioned from the trachea. Chest radiograph shows severe bilateral parenchymal opacities with decreased general aeration.

The chest radiograph in severe cases demonstrates nearly airless lung; only air in bronchi ("air bronchograms") are seen (Fig. 6 a). In mild cases, there are fine reticulogranular densities in both lungs (Fig. 6 b). These radiographic findings are due to the superimposition of multiple acinar nodules caused by atelectatic alveoli contrasting against adjacent enlarged bronchi and air-containing parenchyma.

The primary factor predisposing to RDS is prematurity. Dyspnea is manifest by inspiratory retraction, tachypnea, nasal flaring, expiratory grunting, and progressive cyanosis. The lungs in patients with RDS are stiff with decreased compliance. Due to strongly negative intrapleural pressures associated with laboured breathing, the esophagus is trequently air-filled. Radiologic examination is essential to characterize the disease and exclude other causes of respiratory distress. Newborns with RDS are treated by respirators; this increases inspiratory pressure and maintains end-expiratory pressure. During the first week of life, there is a continued increase in surfactant production. This permits the alveoli to expand. RDS may also be treated by the tracheal instillation of exogenous surfactant.

RDS is frequently complicated by iatrogenic effects of artificial ventilation. Air-block complications include pulmonary interstitial emphysema, pneumomediastinum, and pneumothorax. Differential diagnostic considerations in newborns with RDS are wet-lung disease, meconium aspiration, and neonatal pneumonia. Without clinical correlation and sequential radiographs, it is extremely difficult to distinguish RDS from wet-lung disease or neonatal pneumonia.

Figure 8. Meconium aspiration. Full-term male with respiratory distress requiring ventilator therapy. There are patchy pulmonary parenchymal opacities with associated regions of atelectasis. Note the marked hyperaeration.

Pulmonary haemorrhage
Pulmonary hemorrhage is usually a complication of RDS or neonatal pneumonia. Clinically, blood is present in the trachea. After a sudden deterioration, the chest radiograph shows a diffuse increase in lung density bilaterally with air bronchograms. In fact, the lungs can look completely airless (Fig. 7).

Meconium aspiration
Severe in utero hypoxemia or asphyxia may cause fetal defecation and gasping; this leads to aspiration of meconium in amniotic fluid into the tracheobronchial tree below the level of the vocal cords. Meconium aspiration causes both mechanical obstruction of the larger airways and an inflammatory reaction peripherally in the bronchioles. Ventilation disturbance is greater in meconium aspiration than in wet-lung disease.
There are patchy, bilateral, asymmetric areas of opacity. There is associated hyperinflation of the lungs with flattening of the domes of the hemidiaphragms (Fig. 8). Air-block complications occur in approximately 25 % of patients with proven meconium aspiration. Differential diagnostic considerations include RDS, neonatal pneumonia, wet-lung disease, and pulmonary hemorrhage.

Neonatal pneumonia
Neonatal pneumonia may be acquired in utero, during delivery, or after birth. Etiologic organisms include viruses, bacteria, protozoans, and fungi.

Figure 9. Bronchopulmonary dysplasia. This 5-week-old female required ventilator therapy at birth for RDS. Circular lucencies and curvilinear densities produce a honeycomb appearance of the lungs; this is Stage IV BPD.

Hemolytic streptococcal infection is a common neonatal acquired pneumonia; the radiologic appearance may be identical to that of RDS, with reticulogranular densities frequently associated with pleural fluid or fluid in the fissures. Organisms causing neonatal pneumonia acquired through the ascending route or during delivery are normal inhabitants of the vaginal flora. With the exception of ß-hemolytic streptococcal pneumonia, most neonatal pneumonias are characterized by patchy, asymmetric pulmonary opacities with associated hyperaeration.

Brochopulmonary dysplasia
Bronchopulmonary dysplasia (BPD) is a common and significant complication of newborns that have undergone ventilatortherapy, usually for RDS. BPD is a distinct pulmonary disease affecting all the tissues of the developing lung related to prolonged oxygen and/or respiratory therapy.

During the course of the disease, mucosal necrosis, interstitial edema, and interstitial fibrosis develop. Initially, there is alveolar exudation and inflammatory reaction with decreased lung aeration. After 10-20 days of age, there is the development of a bubbly radiologic appearance of the lung; this is due to local areas of hyperventilation intermixed with areas of atelectasis and interstitial thickening. Stage IV BPD develops after one month of age; a honeycomb appearance of the lung is due to fibrosis. BPD can heal spontaneously, but in most cases the changes are chronic. Advanced cases with hyperaeration and severe chronic lung disease (Fig. 9) may lead to cor pulmonae and even death.

 

Figure 10. Diaphragmatic hernia. The stomach (identified by enteric tube) and small bowel are in the left chest; mass effect displaces the mediastinum from left to right and causes compressive atelectasis of the right lung.

Surgical Disease

Diaphragmatic hernia
Bochdalek hernia is due to a defect in the posterior pleuroperitoneal foramen. The left pleuroperitoneal foramen is most commonly involved; initial radiographic appearance may be that of a large intrathoracic soft-tissue density mass. After several hours, multiple loops of air-filled bowel are identified in the chest (Fig. 10). There is almost always ipsilateral lung hypoplasia; large diaphragmatic hernias will also cause contralateral lung hypoplasia.

Clinically, the abdomen is scaphoid and the thorax is asymmetric. If the bowel loops are fluid-filled, placement of an enteric tube and/or injection of a small amount of air will confirm the diagnosis. The prognosis of patients with diaphragmatic hernia is dependent on the degree of associated lung hypoplasia.

Cystic lung disease
Cystic intrathoracic masses in the newborn include cystic adenomatoid malformation, lobar emphysema, sequestration, and bronchogenic cyst.

Congenital Lobar Emphysema
Congenital lobar emphysema is a condition characterized by progressive overdistention of a pulmonary lobe or, rarely, multiple lobes. Location, in decreasing frequency, of lobar emphysema is left upper lobe, right

Figure 11. Cystic adenomatoid malformation. Neonate with mild respiratory distress. Clinically, the abdomen is normal. There are cystic and solid components of the mass in the left chest; the cystic adenomatoid malformation displaces the heart and mediastinum to the right. The left hemidiaphragm is well visualized.

middle lobe, and right upper lobe.

During the first few days of life, lung fluid may be trapped in the involved lobe, producing a soft-tissue mass. Subsequently, the classic radiologic appearance of an emphysematous lobe with diffuse radiolucency develops. Treatment for lobar emphysema is surgical resection; the prognosis is excellent.

Cystic Adenomatoid Malformation
Cystic adenomatoid malformation (CAM) is a developmental, hamartomatous malformation of the lung. Radiographically, there is a soft-tissue mass in the chest which usually displaces heart and mediastinal structures. This chest mass may be solid or cystic; the cysts may be small or large. As opposed to diaphragmatic hernia, the bowel gas pattern is normal and the diaphragm is intact (Fig. 11).

Air-block complications
Mild forms of air-block phenomena occur in 2 % of all newborns. This usually results in minimal respiratory symptoms. Resorption of gas occurs without difficulty.

The incidence of air-block complications increases by as much as 10% with mechanical ventilation. If there is alveolar rupture, air may localize and coalesce in the parenchyma to produce a pulmonary pseudocyst. More frequently, air accumulates in the perivascular spaces to produce pulmonary interstitial emphysema (PIE). The radiologic features are characteristic with multiple, tortuous linear lucencies radiating out from

a	Figure 12. Air-block complications. a) Tension pneumothorax. There is increased volume of the left hemithorax with associated lucency. Both lateral and anteromedial pleural air collections identified. The multiple linear lucencies in the left lung are pulmonary interstitial emphysema, b) Pneumomediastinum, The thymus is elevated and air is seen along the upper left mediastinum (arrow). c) Pneumopericardium. Air completely surrounds the heart with no cranial extension above the level of the great vessels. Note the associated left pneumothorax.
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the hilar regions (Fig. 12 a).

Spontaneous pneumomediastinum is not uncommon in a newborn; most of these are small and clinically insignificant. A moderate-sized pneumomediastinum produces a radiolucent line that outlines the heart border and elevates the thymus (Fig. 12 b). Air can also dissect into the neck and into the abdominal cavity.

A large pneumothorax can be identified by transillumination of the chest in premature infants; chest X-ray remains the best method of diagnosis, particularly in larger and more mature infants. Chest radiography is also important in following up air-block complications. Pneumothorax in the newborn should be considered a tension pneumothorax. Tension is manifest by an increase in volume of the hemithorax with associated contralateral shift of the heart and mediastinum, depression of the diaphragm, and widening of the intercostal spaces (Fig.12 a). Tension pneumothorax is life-threatening and requires immediate tube drainage.

It may be difficult to distinguish a medial pneumothorax from pneumomediastinum or pneumopericardium. With pneumopericardium, air completely outlines the heart on both AP and lateral views (Fig. 12 c).

Pulmonary infection

Respiratory tract infection is the most common illness that occurs in humans. Viruses are the major cause of pulmonary infection in children, particularly in patients less than 5 years of age. Bacteria become an increasing important cause of pneumonia in children who are 5 years of age or older, have other diseases, and are hospitalized.

Pulmonary infection involves the peripheral air spaces, interstitium, or conducting airways. Infection may primarily involve the peripheral air-exchanging lung (consolidative pneumonia), the conducting airways and adjacent air spaces (bronchopneumonia), or the conducting airways alone (airways infection). Acute pulmonary infection in childhood can be divided into three pathologic types: those that primarily involve the acini or peripheral air spaces, those that primarily involve the airways, and those that involve both the airways and peripheral air spaces. Although this radiological localization is useful, it lacks specificity and requires correlation with both clinical information and laboratory data.
Air space disease (acinar disease) is characterized by lobar, segmental, or subsegmental coalescent opacities with discrete or irregular markings

a

Figure 13. Viral airways disease. a) 2-year-old male with viral bronchiolitis. The lungs are mildly hyperaerated, and there is a diffuse increase in linear markings in the parahilar regions with associated peribronchial cuffing (arrows). b) Microscopic lung section of a patient who died of adenovirus pneumonia. The alveoli (A) are normally aerated and contain no inflammatory exudate. There is marked inflammatory exudate within a bronchiole (B) with associated bronchiolar necrosis (arrows) and peribronchial interstitial extension of the inflammatory process (I). [From Kirks.]

b

and frequently air bronchograms. This pattern is present when infection involves the peripheral segment of lung, and it reflects local extension of inflammatory exudate into adjacent air spaces. This type of consolidative pneumonia is typically due to bacterial infection. However, viral inflammatory disease can occasionally cause this radiological appearance.

Airways disease causes bronchial wall thickening (peribronchial cuffing) beyond the inner 1/3 of the lung. Parahilar linear markings and ring shadows with the anatomic configuration and location of bronchi are identified. Streaky opacities frequently radiate from the hilar regions (parahilar opacities). Generalized hyperaeration with lung hyperlucency, flattened diaphragms, and increased lung volumes are also manifestations of airways disease. Irregular aeration with areas of hyperaeration and atelectasis is usually present. These findings are frequently bilateral and usually symmetrical; this radiological appearance is typical of infection involving the airways.

Pulmonary infection may produce a mixture of the above patterns with characteristics of both air space and airways disease. This type of bronchopneumonia is most frequently due to viral organisms but can also be seen in bacterial infection.

Viral pneumonia

Common etiologies of viral airways disease are respiratory syncytial virus, adenovirus, influenza and parainfluenzae. Viral infections usually cause bronchiolitis, bronchitis, or bronchopneumonia. Typical findings are hyperaeration, peribronchial cuffing, and increase in parahilar linear markings (Fig. 13 a). Characteristically, there are no areas of focal lung opacity since the infection involves the airways and not the airspaces (Fig. 13 b).

Bacterial pneumonia

Common etiologies of pediatric bacterial pulmonary infection are Streptococcus pneumoniae, Staphylococcus aureus, Streptococcus pyogenes, Hemophilus influenzae, and tuberculosis.

Bacterial infections usually cause bronchopneumonia or consolidative (segmental, subsegmental) pneumonia. The chest X-ray typically shows segmental or subsegmental parenchymal opacity (Fig. 14 a, b); there may or may not be associated atelectasis. This radiological appearance is due to the fact that inflammation involves primarily the airspaces (Fig. 14 b, c), as opposed to viral infection which primarily involves the airways.

Acute pneumonia in children may produce a spherical or rounded density on chest radiographs. This round pneumonia should not be confused with an intrathoracic neoplasm. Such round pneumonias are almost always bacterial in etiology, and most are pneumococcal in origin. The patient should be treated with appropriate antibiotics.

There is an increasing frequency of tuberculosis in the pediatric population. Initial pulmonary inflammatory exudate produces localized air space disease that may involve any lobe. Regional lymph node enlargement and pleural effusion are frequently also present.

Thoracic tumours

Chest masses in infants and children are uncommon but not rare abnormalities. Radiology plays a critical role in the detection, diagnosis, preoperative evaluation, treatment planning, and follow-up of pediatric

a	Figure 14.Bacterial pneumonia. a) AP chest radiograph of an 8-year-old male with cough and fever shows segmental consolidation in the left upper lobe. b) Lateral view confirms left upper lobe consolidation (arrows). c) Histologic section of patient with bacterial pneumonia. Alveoli (A) are completely filled with organisms and inflammatory exudate. [From Kirks.]
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thoracic tumors. The symptomatology is highly variable, depending on the patient age, size and location of the thoracic mass, and any compromise of adjacent anatomic structures.

Chest masses in the pediatric patient may be located in the chest wall, lung, or mediastinum. They may arise in a variety of tissues and organs. Pathologic abnormalities may be congenital, inflammatory, neoplastic, traumatic, or vascular in etiology. A normal variant, as well as normal variability in organ size and configuration may mimic thoracic masses. These pseudotumors, such as prominent thymus (Fig. 15), can be confusing to the unwary.

The role of radiology in pediatric chest masses should be not only to confirm the presence of an abnormality but also to characterize it. Current

a	Figure 15. Normal thymus. a) Prominent left lobe of thymus. The wavy contour is due to the soft thymus being indented by the anterior ribs. b) Prominent right lobe of thymus. The right lobe of the thymus mimics a mediastinal mass. Fluoroscopy verified that the mass was anterior in location and relatively flat. The child was asymptomatic. A follow-up film3 years later was normal.
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pulmonary imaging techniques allow precise determination of characteristics, extent, and associated abnormalities of a pediatric thoracic tumor. In addition to the chest radiograph, which remains the "foundation" examination for evaluation, this diagnostic approach may require supplementary plain films, fluoroscopy, sonography, nuclear scintigraphy, CT, or MRI.

Chest wall masses
Tumors of the chest wall are the rarest of all thoracic masses in infants and children. Bone destruction, not just bone erosion or rib splaying, indicates that the mass arises from the chest wall. Cross-sectional imaging (US, CT, MRI) are frequently helpful in determining whether a chest wall mass is soft tissue, bony, or extrapleural intrathoracic in location.

Soft-tissue masses of the thorax may arise from cutaneous or subcutaneous tissues. Benign tumors are more common than malignant neoplasms. Clinical examination is extremely helpful in characterizing the les ion and its extent. Tangential radiography or fluoroscopy may be helpful; CT or MRI precisely characterize the internal features and extent of the soft-tissue mass.

Chest wall masses may arise from the soft-tissues, extrapleural-intrathoracic region, or bony thorax. Soft-tissue tumors are usually benign. The most common include lymphangioma, cystic hygroma, lipoma, and hemangioma. Extrapleural-intrathoracic tumors are most commonly malignant; rhabdomyosareoma is the most common mass arising in this location. Bony thoracic tumors may be part of a generalized bone disease (neurofibromatosis; histiocytosis; multiple exostoses) or a primary skeletal mass. Benign primary skeletal tumors include fibrous dysplasia, bony exostosis, and enchondroma. Malignant bony thoracic tumors may be either metastatic (neuroblastoma; leukemia) or primary. The most common primary skeletal malignant tumor of childhood is Ewing sarcoma and related round-cell tumors (Askin tumor; primitive neuroectodermal tumor).

Mediastinal masses

Mediastinal masses in infants and children may be located in the anterior, middle, or posterior compartments. The posterior mediastinum is behind a line drawn tangential to the ventral margins of the vertebral bodies. The anterior mediastinum is in front of a line drawn from the most cephalad portion of the manubrium to the diaphragm and paralleling the previously described posterior line. The middle mediastinum is between the anterior and posterior compartments; this places the trachea and esophagus in the middle of the middle mediastinum. These 3 mediastinal compartments may be extrapolated from the lateral chest radiograph to CT or MR images.

Anterior mediastinum
Approximately 30% of pediatric mediastinal tumors are located in the anterior compartment. They usually arise from either the thymus or lymph nodes. The radiologic differential diagnosis includes the four "Ts": Teratoma (germ-cell tumor), Thymic tumor, Thyroid tumor, and "Terrible" lymphoma/leukemia. Since tumors of the thymus and thyroid gland are unusual in infants and children, the primary differential diagnostic considerations of a pediatric anterior mediastinal mass are germ-cell tumor and lymphoma (Fig. 16).

Figure 16. Hodgkin disease. 12-year-oldfemale with cough and shortness of breath. There is a lobular mediastinal mass that extends to the left. CT verified the presence of a mass involving the anterior and middle mediastinum.

Middle mediastinum
Approximately 30% of pediatric mediastinal tumors are located in the middle compartment. Although the diagnostic considerations are extensive, the primary differential diagnosis is remembered by the letters AB. The masses usually arise from lymph nod (Adenopathy) or primitive foregut (Bronchopulmonary foregut malformation). Common abnormalities include infectious adenopathy (bacterial, granulomatous), neoplastic adenopathy (lymphoma/leukemia, metastatic disease), and bronchopulmonary foregut malformations (bronchogenic cyst, enteric duplication, enteric cyst, sequestration). The esophagus is the "roadmap" of the mediastinum; it serves as an important anatomic landmark. The esophagus may be displaced by or communicate with a mediastinal mass. Moreover, a mediastinal mass is occasionally esophageal in origin (ex: hiatus hernia) so that the esophagogram may be diagnostic.

Posterior mediastinum
Approximately 40 % of pediatric mediastinal tumors are in the posterior compartment. As many as 95 % of these pediatric posterior mediastinal masses are neurogenic in origin. These tumors are usually derived from sympathetic ganglion cells; there is a spectrum of such tumors from the most malignant neuroblastoma to ganglioneuroblastoma to benign ganglioneuroma. Posterior mediastinal masses have a propensity for extradural extension (Fig. 17).

a	Figure 17. Apical neuroblastoma. 4-year-old male with left Horner syndrome. a) Cone-down view of chest radiograph shows a left apical soft-tissue mass (m). b) CT myelogram demonstrates extradural extension of the left apical mass (m) encroaching on the subarachnoid space and displacing the spinal cord (e). [From Kirks.]
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a	Figure 18.Bronchopulmonary sequestration. 10-year-old male with recurrent right lower lobe pneumonia. a) PA chest radiograph shows opacity in the right lower lobe. b) Lateral film confirms this is in the posterior basilar segment of the right lower lobe. Thoracic aortography demonstrated a systemic vessel arising from the aorta and supplying the sequestration.
b

Lung tumors
Lung tumors in infants and children may involve the pleura or parenchyma. Metastatic lesions are the most common lung tumors in children. Most primary parenchymal tumors in children are benign. In contrast to the adult, primary malignant tumors in children are rare. Most common pleural masses in children are metastatic disease. Pleural metastases are usually small, round-cell tumors; common etiologies are lymphoma/leukemia, neuroblastoma, Ewing sarcoma, and PNET.

Malignancies which commonly metastasize to lung include Wilms tumor and osteogenic sarcoma. Common benign primary lung tumors are sequestration (Fig. 18) and parenchymal bronchogenic cyst.

Integrated imaging
As previously noted, the chest radiograph remains the "foundation" examination for evaluation of pediatric chest masses. P A and lateral chest radiographs permit localization of a chest mass to the chest wall (oblique views may be required), mediastinum, or lung parenchyma. However, accurate characterization, as well as precise location and extent of the mass, requires CT or MRI.

Because of the long examination time and expense of MRI, CT currently remains the modality of choice for evaluating bony chest-wall masses, anterior mediastinal masses, middle mediastinal masses, and pulmonary parenchymal lesions. MRI is the method of choice for evaluating soft-tissue masses of the chest wall and posterior mediastinal masses; the latter is due to a propensity for extradural tumor extension.

Chest trauma

Lower-airway foreign bodies
Foreign bodies of the lower airway are a serious clinical problem in children, and radiology plays an important role in diagnosis. Patients with aspirated foreign bodies range in age from 6 months to 15 years, with the peak incidence in children of 1-2 years. Three-fifths of foreign bodies are located in the right bronchial tree, 1/3 in the left bronchial tree, and the remainder in the larynx, trachea, or both bronchi. Only 10% of lower-airway foreign bodies are opaque. Radiological findings include hyperinflation of the lungs, atelectasis, consolidation, and air trapping.

Since normal inspiratory chest radiographs may be seen in children with aspirated foreign bodies, at least one supplementary diagnostic manoeuvre (expiration film, fluoroscopy, decubitus view) should always be performed if this diagnosis is suspected (Fig. 19). The possibility of

a	Figure 19.Airway foreign body. 2-year-old male with cough and fever. a) Inspiration view shows lucency of the left lung and hyperaeration of the left hemithorax. b) Expiration view confirms air trapping on the left; the mediastinum shifts to the right. At bronchoscopy, a peanut was found in the left mainstem bronchus.
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a foreign body in the lower airway must be considered in any child less than 3 years of age with clinically suspected or radiographically confirmed pneumonia. The radiologist must be alert to direct or indirect signs of air trapping in a pediatric patient with possible pneumonia.
Pneumomediastinum or pneumothorax may be associated with foreign body aspiration. The presence of pneumonia associated with either of these air-block complications in a child less than 3 years of age should suggest the possibility of foreign body in the lower airway.

Near-drowning
Near-drowning is a form of aspiration. The extent and severity of radiographic findings relate to the amount of water ingested rather than the type of water ingested. Moreover, many of the radiological findings of near-drowning are due to hypoxic lung injury.
The chest radiograph usually shows patchy parahilar acinar densities of pulmonary edema with a normal-sized heart. Clinical assessment and serial blood gas determinations are much more important than chest radiographs for following the clinical course and assessing the prognosis of the patient.

 

Donald R. Kirks and Sven Laurin