The Thymus

The normal thymus ( Fig. 10.1 ) is a frequent cause of widening of the anterior mediastinum during the first years of life. The lateral margin often shows an undulation, the thymic wave, which corresponds to the indentations of the ribs on the inner surface of the thoracic cage. Particularly on the right, the thymus may have a triangular, sail-like configuration. The thymus may involute in times of stress, and steroids can induce a decrease in size. At times, the differentiation of a physiologic thymus from pathology in the anterior mediastinum can be difficult. Ultrasound examination will usually differentiate cystic lesions from the homogeneous normal thymic tissue (see ultrasound section later in this chapter). Occasionally, the normal thymus can act as a significant space-occupying structure in the superior mediastinum, and in such cases, differentiation may be helped by either ultrasound or magnetic resonance imaging (MRI), which shows a homogeneous echogenicity/signal within a normal thymus. On MRI, a normal thymus has an intermediate signal on T2-weighted images (similar to the spleen and lymph nodes) and shows minimal uniform enhancement on T1-weighted images after intravenous contrast medium injection ( Fig. 10.2 ).

Chest radiograph demonstrating slight widening of the superior mediastinum. Corresponding ultrasound demonstrates normal thymic tissue.

(Image courtesy of P Tomà.)

Chest radiograph (not shown) in a 3-month-old child showed an unusual upper mediastinal contour. Magnetic resonance imaging (T1-weighted spin echo after intravenous injection of gadolinium chelate) shows a normal signal and no abnormal contrast enhancement from a normal but large thymus, extending posterior to the right brachiocephalic vein (arrowhead). There is no sign of vascular or airway compression.

The Cardiothoracic Ratio

In toddlers, the cardiothoracic ratio can at times exceed 0.5, and care should be exercised in overdiagnosis of cardiomegaly, particularly if the film may be expiratory.

Kink of the Trachea to the Right

Kinking of the trachea to the right is a frequent feature of a chest film taken at less than full inspiration. This is a physiologic buckling and does not suggest a mass lesion.

Soft Tissue

Soft tissue may be prominent in children, and the anterior axillary fold that crosses the chest wall can mimic pneumothorax. Similarly, skin folds can cast confusing shadows at times. Braids (plaits) of hair over the upper chest can mimic pulmonary infiltrations in the upper lobes.

Pleural Fluid

Whereas in adults an early sign of pleural effusion is blunting of the costophrenic angles, in children it is more common to see lateral separation of the lung from the chest wall, with reasonable preservation of the clarity of the costophrenic angles and accentuation of the interlobar fissures. In the supine position, an apical rim of soft tissue density is seen, and if a moderate to large unilateral effusion is present, the affected hemithorax has a diffuse increase in density, with preservation of vascular markings simulating ground-glass parenchymal opacification. This is due to pleural fluid collecting in the dorsal (dependent) pleural space.

General Degree of Lung Inflation

Flattening of the diaphragm or diaphragmatic domes below the level of the eighth posterior ribs, elongation of the mediastinum, and widening of the intercostal spaces are all signs of pulmonary overinflation. Intercostal bulging of the pleura or lung parenchyma may be a sign of excessive ventilator pressures in an intubated child.

Generalized pulmonary underinflation is usually due to radiographic exposure during expiration, but it may be a real finding confirming small lung volumes (as in cases of respiratory distress syndrome [RDS] of the newborn, in which the lung parenchyma is noncompliant, or in bilateral pulmonary hypoplasia) or associated with lobar collapse.

One should also consider elevation of the diaphragm, with consequential lung compression due to bowel distention, pneumoperitoneum, or the presence of a large abdominal mass. Hence, the periphery of the radiograph (e.g., the area under the diaphragm) should always be carefully and routinely inspected as part of a systematic review.

Asymmetrical Lung Volume

In the absence of a pneumothorax, mediastinal shift toward a lung with uniformly increased density compared with the contralateral lung is a sign of differential inflation of the two lungs. This may be caused by overinflation of the more lucent lung (e.g., due to a ball-valve mechanism in the central airways), in which case the ipsilateral hemidiaphragm would be flattened ( Fig. 10.3 ).

Chest radiograph in a young child shows a semicircular left convex distortion of the left mediastinal outline due to a bronchogenic cyst and secondary overinflation of the left lower lobe.

Alternatively, it may be caused by volume loss in the denser lung, in which case the diaphragm of the denser hemithorax would be elevated. This may be a sign of unilateral pulmonary hypoplasia, aplasia, or agenesis ( Figs. 10.4–10.6 ), in which case the mediastinum is shifted toward the hemithorax containing the small lung, and ipsilateral elevation of the hemidiaphragm is seen. Combined overinflation of one lung and volume loss of the other can sometimes be seen secondary to mass lesions that affect the central airways ( Fig. 10.7 ).

In a 10-year-old girl who presented with shortness of breath, this chest radiograph shows a small right hemithorax (rib crowding, diaphragmatic elevation, compensatory large left lung), but no lung opacification. This was later diagnosed as an interrupted right pulmonary artery (see Figs. 10.45 and 10.70 ).

Chest radiograph shows a hypoplastic right lung with an abnormal vascular structure running toward the diaphragmatic level medially. This represents systemic venous drainage of the right lung. The shape of the abnormal vein resembles a Turkish scimitar (bowed sword); hence, the denotation scimitar syndrome (see Figs. 10.44 and 10.46 ).

The right hemithorax is opacified by the mediastinal structures that are shifted to the right. However, there is no overexpansion of the left lung or pleura. On computed tomography, the right lung was absent (see Fig. 10.47 ).

Chest radiograph of a neonate with respiratory distress syndrome shows an overexpanded left lung (inverted left hemidiaphragm, intercostal bulging of the left lung) as well as a small right hemithorax (elevated right hemidiaphragm, crowding of the right ribs). Both were secondary to a central bronchogenic cyst (see Fig. 10.43 ), although this is not apparent on the chest x-ray, but is seen subsequently on computed tomography scan.

Other causes of asymmetrical lung volumes include diaphragmatic paresis/paralysis and a large abdominal mass lesion that causes elevation of the ipsilateral hemidiaphragm. The diaphragm may also be apparently elevated secondary to a subpulmonic fluid collection. In congenital diaphragmatic hernia, the multicystic appearance of bowel contents may or may not be obvious within the thorax ( Fig. 10.8 ). If seen, it may sometimes be difficult to distinguish from congenital cystic adenomatoid malformation ( Table 10.1 ).

An infant with antenatally diagnosed left congenital diaphragmatic hernia. Chest radiograph shows disruption of the lateral left diaphragmatic outline, bowel in the left hemithorax, and mediastinal shift to the right. Note venous-arterial extracorporeal membrane oxygenation with a metal marker on the tip of the venous cannula (arrow). The endotracheal tube is too high.

Lobar Overinflation

Lobar overinflation may have a similar appearance to whole-lung overinflation. However, there is usually evidence of lobar confinement because one can identify the lobar outline as it herniates across the midline, and the remaining ipsilateral pulmonary lobes show compressive atelectasis or collapse ( Fig. 10.9 ). The left-upper, right-middle, or left-lower lobe is usually affected by congenital lobar overinflation (congenital lobar emphysema). A lateral radiograph may be helpful in deciding which lobe is involved, although computed tomography (CT) of the chest is required in most cases to clarify the anatomy and to identify a potential underlying causative abnormality. This could be an extrinsic lesion causing partial bronchial compression (e.g., a mediastinal bronchogenic cyst) or a mass within the bronchial lumen causing a ball-valve effect (e.g., endobronchial granuloma or adenoma).

The lateral view is not part of a routine chest radiograph. In this case, the anteroposterior view shows probable lobar overexpansion of the right lung (white arrowheads), but it is not clear which lobe is involved. The lateral view is helpful, showing depression of the posterior diaphragm (black arrowheads) and thereby clarifying right lower lobe involvement, which is uncommon in lobar overinflation (congenital lobar emphysema).

Mediastinal Distortion

Mediastinal distortion may occur secondary to a mediastinal mass. Therefore, the normal outline of the mediastinum should always be reviewed. On the left, this normally constitutes the thymus, aortic arch, pulmonary outflow tract, pulmonary hilum, and left heart border; on the right, it constitutes the thymus, azygos vein, hilum, and the right heart border. In young children, the outline of the superior structures may be obscured by a normal thymus (discussed earlier), but this should never obscure the posterior paraspinal lines as the thymus lies in the anterior mediastinum. Distortion of the airways (e.g., narrowing, deviation, or splaying of the main bronchi) suggests extrinsic mass effect or functional/structural abnormalities of the airways ( Fig. 10.10 ).

A 2-year-old boy presents with pyrexia. Chest radiograph shows displacement of the left main and lower lobe bronchi by a subcarinal mass (between arrowheads) and associated increased transradiency of the left lung due to air trapping. The mass was later proven to be lymphadenopathy caused by Mycobacterium avium-intracellulare.

Mass lesions disrupting the paraspinal lines or involving the apices of the chest are most likely localized in the posterior mediastinum, and the list of differential diagnostic possibilities includes congenital abnormalities (lateral meningocele, neurenteric cyst, duplication cyst), neoplasm (neurogenic tumor), and infection (spondylodiscitis). Rib or vertebral body erosion suggests an aggressive lesion, such as infection or malignancy (e.g., neuroblastoma).

Any abnormal appearance of the thymus (discussed earlier), such as inappropriate size or shape or evidence of associated airway compression, suggests an anterior mediastinal mass lesion. Diagnostic differentials include rebound thymic hyperplasia, germ cell tumor, T cell lymphoma, and thymoma.

Any other mass lesion in the mediastinum usually originates from the middle mediastinum. In the young child, the diagnosis is likely to be a congenital abnormality (e.g., bronchogenic cyst). In the older child, the diagnosis is likely to be enlarged lymph nodes, which may be reactive or due to malignant disease (lymphoproliferative disease, which is rarely metastatic); sarcoidosis (rare, usually paratracheal); or idiopathic hyperplasia.

Lung atelectasis (discussed earlier) should be considered when there is loss of the upper mediastinal outline, even without apparent lung opacification.

Hilar Expansion

Unlike in adults, where bronchogenic carcinoma is a common cause of hilar adenopathy, in children, hilar enlargement is often secondary to acute infection. However, prominent hila may also be seen due to enlargement of the pulmonary arteries and, in infants, due to a bronchogenic cyst (see Fig. 10.3 ). Distinguishing vascular from nodal enlargement can be difficult, but pulmonary arterial enlargement should result in a concave lateral hilar outline, whereas soft tissue masses are said to cause a convex lateral hilar margin, with a noticeable increase in soft tissue density at the enlarged hilum.

False impressions of hilar enlargement occur when the child is rotated on the film cassette: a hilum that is pointing away from the detector becomes more distinct from the heart shadow and consequently appears more prominent. A repeat exposure may be necessary in difficult inconclusive cases, and CT/MRI may be performed if there is doubt.

Peribronchial markings should not be prominent in children, unlike in adults. More distinct markings in the perihilar regions, particularly with coexisting general overinflation, often represent bronchial inflammation (e.g., asthma or infection). Other conditions included in the differential diagnosis will be discussed in the section on high-resolution computed tomography (HRCT). Patchy perihilar opacification, with air bronchograms, is usually due to radiographic summation of peribronchial thickening, but may be difficult to distinguish from airspace opacification (consolidation).

Lung Opacities

The plain radiograph usually allows distinction between atelectasis, which is defined as parenchymal opacification with loss of volume, and consolidation, which is opacification without volume loss and with the outline of gas-filled bronchi (air bronchograms) ( Fig. 10.11 ). Opacities in the lungs can be localized according to the neighboring structure, which is obscured (silhouette sign). Loss of the upper mediastinal outline is consequent on upper lobe opacification; loss of the heart borders is caused by right middle lobe or lingular opacification; and loss of diaphragmatic definition is caused by lower lobe pathology.

Consolidation. The lung parenchyma is opacified with obvious air bronchograms, but there are no vascular markings.

There are several important signs of volume loss. First, the entire hemithorax may appear shrunken, with ipsilateral mediastinal shift, diaphragmatic elevation, and crowding of the ribs. Second, adjacent noncollapsed lung may show compensatory overinflation and appear hyperlucent due to dilution of vascular shadows. Third, the hilum may be displaced, either cranially or caudally, toward the atelectasis ( Fig. 10.12 ).

The tip of the endotracheal tube is in the proximal right main stem bronchus (arrow). There is associated atelectasis of the right upper lobe (loss of definition of the right upper mediastinal border, opacification of the right upper hemithorax, elevation of the right hemidiaphragm, and crowding of the right ribs).

It is important to recognize these signs, even with no apparent lung opacity, particularly in upper lobe atelectasis, where the affected segments, or the whole lobe, may be collapsed against the mediastinum and therefore difficult to identify.

The clinical and radiographic history of atelectasis may give important clues as to the causative pathology. Acute atelectasis may be caused by a dislodged endotracheal tube, an aspirated foreign body, or mucous plugging ( Fig. 10.13 ) and may therefore require further nonradiologic investigation and intervention. Chronic atelectasis is more likely caused by extrinsic airway obstruction (e.g., bronchogenic cyst, mediastinal lymphadenopathy, neoplasms) or chronic infection (e.g., tuberculosis) and may therefore warrant CT/MRI. In a febrile infant with respiratory distress and multifocal segmental atelectasis that changes location over the course of hours, one may suspect infection with respiratory syncytial virus, causing bronchiolitis.

A neonate in the intensive care unit with increasing oxygen requirement. The chest radiograph (left) shows loss of the left cardiomediastinal and diaphragmatic outlines, opacification of the left hemithorax without air bronchograms, and mediastinal shift to the left. These findings strongly suggest left lung collapse, and, at bronchoscopy, a mucous plug was removed from the left main stem bronchus. Immediately afterward (right), there is considerably improved aeration of the left lower lobe (diaphragm now seen), but persistent collapse of the left upper lobe (persisting mediastinal blurring and shift).

Consolidation has many causes, as in adults, and is due to any process that replaces air in the terminal airspaces with fluid, mucus, or cellular material. The clinical history, the distribution of abnormality, and the presence of associated calcification, lymphadenopathy, or pleural effusion may assist in deduction of the specific cause ( Figs. 10.14 and 10.15 ). Typically, pulmonary edema causes bilateral patchy consolidation in a perihilar distribution as well as pleural fluid (discussed earlier), although there may be lateral predominance.

In a 1-year-old girl with a cough, the chest radiograph shows a calcified mass in the right upper and mid zones. Also seen is pleural fluid, which was caused by infection with Mycobacterium tuberculosis.

A 3-year-old boy who was treated in the intensive care unit after a traffic accident had increasing difficulty with oxygenation. A chest radiograph on admission (left) shows patchy areas of consolidation. Hemorrhagic fluids returned via the endotracheal tube. The second day (right), there was almost complete whiteout of both lungs, with air bronchograms and loss of the cardiomediastinal and diaphragmatic outlines. The findings confirm extensive pulmonary hemorrhage.

Ground-glass change, a description initially confined to HRCT, is also used now to describe radiographic lung opacification with partial preservation of vascular markings, with or without air bronchograms ( Fig. 10.16 ). This sign is nonspecific and may be caused by interstitial or partial airspace opacification processes. In the neonatal setting, it is often used in the description of RDS ( Fig. 10.17 ), and is usually combined with generally decreased lung volumes. Pleural fluid may give a similar appearance (discussed earlier). Where there is continued clinical doubt, an ultrasound can prove useful in differentiating pleural fluid from peripheral consolidation, demonstrating the presence of ultrasonographic air-bronchograms and normal arborization of pulmonary vessels in consolidated lung tissue as opposed to pleural collections (see ultrasound section later in this chapter).

Ground-glass change. There is moderately increased opacity of the lung with air bronchograms (arrowheads), but preservation of vascular markings.

In a 3-week-old infant born at 32 weeks’ gestation, on ventilator support, there is bilateral diffuse ground-glass change (opacification, blurring of the cardiomediastinal and diaphragmatic outlines, preserved vascular markings), in keeping with respiratory distress syndrome. Note the lucency overlying the liver, outlining the right hemidiaphragm as well as the bowel wall in the upper left quadrant. This is diagnostic of pneumoperitoneum. The child had bowel perforation secondary to necrotizing enterocolitis.

Focal and Multifocal Lung Densities

Focal and multifocal lung densities often require additional CT/MRI for their definitive underlying cause to be elucidated, except in cases with clear clinical evidence of infectious pneumonia. In children, pneumonic consolidation often has a more distinct, rounded appearance (round pneumonia), which should be recognized and followed with plain radiographs only. In equivocal cases, CT/MRI is necessary for further characterization of the lesion. Differential diagnosis of a solitary parenchymal lesion includes congenital malformation, such as sequestration (usually in the posterobasal left lower lobe), microcystic congenital cystic adenomatoid malformation, and vascular malformation. A lung abscess may have no apparent gas-fluid level.

Multifocal lesions may represent infectious processes (e.g., fungus, tuberculosis, papillomatosis, or septic emboli) ( Fig. 10.18 ), granulomatous disease, Langerhans cell histiocytosis, other inflammatory disease ( Figs. 10.19 and 10.20 ), diffuse interstitial lung disease ( Fig. 10.21 ), or metastases (e.g., nephroblastoma, hepatoblastoma, malignant germ cell tumor, sarcoma).

In a 13-year-old boy with a tracheostomy tube because of laryngeal papillomatosis, the chest radiograph shows multiple nodular processes, some of which are cavitating, caused by parenchymal dissemination of the papillomatosis ( arrows; see Fig. 10.48 ).

Radiograph of a male neonate shows patchy consolidation in the right upper zone and behind the heart on the left, with overinflated lungs. This picture is commonly seen in meconium aspiration.

A 6-year-old boy presented with difficulty breathing. Inflammatory markers were increased. The chest radiograph shows areas of opacification in the right upper and mid zones, and the left mid zone, which represent vasculitic lesions (see Fig. 10.60 ). On plain film, this is indistinguishable from multifocal pneumonia.

A 4-year-old boy undergoing chemotherapy had acute respiratory failure. Chest radiograph shows globally increased density of both lungs in a granular pattern, with preservation of vascular markings. This was due to diffuse interstitial pneumonitis caused by bleomycin (see Fig. 10.52 ).

Some lung lesions tend to be predominantly cystic in radiographic terms (containing a gas-filled cavity). Pneumatoceles usually follow pneumonia, which is classically caused by infection with Staphylococcus aureus, and occasionally by Klebsiella.

Multifocal, multicystic parenchymal lesions may be congenital cystic adenomatoid malformation, Langerhans cell histiocytosis nodules at the cavitating stage, granulomatosis with polyangiitis, disseminated laryngeal papillomatosis (see Fig. 10.18 ), or necrotizing vasculitis (see Fig. 10.20 ).

Pulmonary Interstitial Emphysema

Pulmonary interstitial emphysema is a complication that occurs when high ventilator pressures are used to ventilate stiff lungs. It appears as lacelike lucencies in a linear pattern radiating from the pulmonary hilum to the surface of the lung, and it may be further complicated by pneumothorax or pneumomediastinum ( Fig. 10.22–10.24 ). In some cases, this may be difficult to distinguish radiologically from ventilator-induced central bronchial dilation. There are rare reported cases of apparent spontaneous pulmonary interstitial emphysema in term babies who have never been ventilated. The differential diagnosis includes congenital or acquired cystic lung disease.

Detail from a chest radiograph in an infant after long-standing ventilatory support shows monotonous tubular lucencies (white arrowhead) suggestive of pulmonary interstitial emphysema. The mediastinal border is seen very crisply, with a medial lung edge (black arrowheads) due to anterior pneumothorax.

A 6-week-old girl born at 30 weeks’ gestation was very difficult to ventilate. Chest radiograph shows overexpanded lungs that are seen bulging out intercostally, with the diaphragm flattened. Concurrently, there is increased opacification of the lungs, which is presumed to be due to respiratory distress syndrome. Linearly arranged bubbly lucencies can be seen radiating from the hila, suggesting pulmonary interstitial emphysema secondary to high-pressure ventilation. There is also a pneumothorax seen at the base of the right lung.

A 6-week-old girl was ventilated with high-pressure settings (chest radiographs show a flattened diaphragm and splayed ribs). After an acute exacerbation (left), the radiograph showed collapse of the left upper lobe (opacification without air bronchograms, increased interlobar fissure, and elevated diaphragm). The next day (right), the left upper lobe had reexpanded, but a left pneumothorax is seen. Linear bubbly lucencies can be seen from the hila to the lung edges, suggesting pulmonary interstitial emphysema.

Lung Abscess

A lung abscess is a cavitated lesion that normally contains both fluid and gas. The consequent gas-fluid level is easily recognized, but it may be missed if the x-ray beam is not tangential (i.e., horizontal) to the gas-fluid interface. With a diverging beam, the fluid level appears more blurred and meniscoid ( Fig. 10.25 ).

Two chest radiographs obtained in the same patient on the same day. The right is a true erect exposure showing the gas-fluid level of an abscess within the right lung, whereas the left is semierect and does not show the gas-fluid level of the abscess.

Diffuse Interstitial Lung Disease

We will discuss diffuse interstitial lung disease in more detail later in the chapter in the section on HRCT . More advanced interstitial processes may be appreciated radiographically as peribronchial thickening, ground-glass change, septal lines, or interstitial nodules ( Fig. 10.26 ; see also Fig. 10.21 ).

Chest radiograph of a 9-year-old girl with gastroesophageal reflux and chronic aspiration shows bilateral perihilar bronchial wall thickening, particularly in the upper zones (see Fig. 10.59 ). The apparent rotation is caused by scoliosis.

Pneumothorax

In young children, the appearance of a pneumothorax differs from the typical adult appearance. The variation is due to differences in lung parenchymal elasticity. In children, there is often no peripheral lucent zone on supine radiographs, which is the typical appearance in adults, because in the child, gas collects anteriorly in the anterior pleural reflection ( Figs. 10.27 and 10.28 ). Increased clarity of the cardiac outline may be the only finding, and it should be assessed carefully. If there is clinically significant doubt, a lateral shoot-through or decubitus x-ray should be performed. These are more sensitive for detecting small-volume pneumothoraces.

An anterior pneumothorax as seen typically in infants. The white arrowheads show the lateral boundary of the lucency. The gas adjacent to the right heart border causes a crisp outline (black arrowhead).

Chest radiograph of a 12-year-old girl shows a hyperlucent left hemothorax, inversion of the left hemidiaphragm, and a very crisp cardiodiaphragmatic outline. This is suggestive of anterior tension pneumothorax. Scoliosis is noted.

Skeletal Abnormalities Associated With Respiratory Disorders

On conventional radiographs, undermineralization of the skeleton can be diagnosed confidently only in severe cases. Associated with prematurity, this metabolic bone condition is commonly seen in infants with idiopathic RDS. Bone mineral loss is also a feature of a multitude of constitutional disorders and may be seen secondary to systemic corticosteroid therapy.

Scoliosis may be caused by vertebral abnormalities, which may be part of the VATER (Vertebral, Anorectal, Tracheo-Esophageal, Renal/Radial malformations) or VACTERL (VATER with the addition of cardiovascular malformations) sequences, or more commonly, may be caused by neuromuscular disorders. In addition, it may be secondary to chest conditions, such as hypoplastic lung, atelectasis, or empyema, in which case the resultant spinal curvature is concave toward the side of the abnormality.

Both focal and multifocal osseous lesions may be associated findings in conditions that also involve the lungs. Well-defined lytic (“punched out”) lesions in the ribs or scapulae and the collapse of vertebral bodies are features of Langerhans cell histiocytosis. More ill-defined lesions are seen in primary neoplasms (e.g., primitive neuroectodermal tumors [PNET], previously known as Ewing/Askin tumor), lymphoma, infection (e.g., tuberculosis), or infection associated with chronic granulomatous disease. Erosion of posterior rib elements, with splaying, is seen with thoracic paraspinal neuroblastoma.