The radiological pathologies that may emerge on a pediatric CXR are categorized into four main densities: air, soft tissue, bone and metallic densities.

Fig 1: Abnormalities on chest X-Ray can be categorized into four major densities (air, soft tissue, calcific/bone or metallic).

This poster addresses the entire spectrum of abnormalities manifesting as air densities, including Pneumothorax, pneumomediastinum, pneumopericardium and hyperlucencies. In addition to tackling the various causes of solely lung opacification among the abnormalities appearing as soft tissue densities.

While discussing the rest of the entities is beyond the scope of this poster.

The degree of increased lung lucency or opacification is affected by a combination of factors, as illustrated in figure 2.

Fig 2: The degree of lung lucency or opacification arises from a balance between air (lucency) and four factors which contribute to opacification: 1. Airless alveoli, 2. Denser materials that replace air, like fluid, debris, or inflammatory cells, and 3. Thickness of chest wall muscles. 4. Pulmonary vascularity. At times, it may be challenging to figure out the abnormal hemithoracic side, whether it is the brighter or the darker one! In this case, it is crucial to decide by correlating between clinical history, physical examination findings, previous radiological examinations and the supplementary radiological observations.

Air densities:

1. Air leaks: increased intra alveolar pressure may lead to alveolar rupture, air dissection along the bronchovascular bundle, and free air reaching the pleura, mediastinum or pericardium. Therefore, mechanical ventilation (barotrauma) is the most common cause of air leak syndrome in children.

A. Pneumothorax: maybe spontaneous (secondary to barotrauma) or traumatic.

Fig 3: Signs of Pneumothorax on supine chest X-Ray. All cases show deep sulcus sign (Green). A: Non tension PTX; Focal increased lucency at the left upper zone medially (Orange) with increased sharpness of left cardiac border (Blue). B: Small amount and C: larger amount of partial Tension pneumothorax: Diffusely increased left hemi-thoracic lucency, outer one-third devoid of vascular markings, pleural line sign (Pink), increased sharpness of the hemidiaphragm (Blue), depression of the ipsilateral hemidiaphragm and contralateral mediastinal shift. (trachea in red)

B. Pneumomediastinum: due to traumatic or non-traumatic events like forceful coughing, asthma, vomiting, Valsalva maneuver and playing musical wind instruments.

Fig 4: Signs of Pneumomediastinum on supine chest X-Ray. A: Thymic wing sign: Ill defined lucency (*) seen with elevated unilateral thymic lobe (Orange), a sign of small amount of pneumomediastinum. B: Moderate amount of pneumomediastinum: Extensive subcutaneous emphysema (yellow) with curvilinear lucency that elevates the thymus remnant (Blue), linear paratracheal, upper thoracic lucencies (*), and Ring around artery sign (Green) C: Large amount of pneumomediastinum: In addition to all the signs on case B, Continuous diaphragm sign also seen (Orange)

C. Pneumopericardium: may be due to trauma or post operative.

Fig 5: Spontaneous Pneumopericardium on supine chest X-Ray. A: Pneumothorax, pneumomediastinum and pneumopericardium: Air surrounding the heart contour (Orange), surgical emphysema (Red) and deep sulcus sign (Green) also seen. B: Pneumopericardium and pneumomediastinum: Continuous diaphragm sign (Orange) and angel wing sign (Green).

2. Hyperlucency:

The various presentations, etiologies and differential diagnosis are summerized in (Figure 6).

Fig 6: Differential diagnosis of Hyperlucencies.

The patient’s medical history, previous radiological and laboratory examinations must be reviewed, in order to prioritize the most probable diagnosis.

A. Focal Hyperlucency:

Fig 7: Focal lung hyperlucency (cystic lesions, dotted circle). A: Thin-walled well-defined focal lucency at the right upper lobe, in a case of CPAM type I. B: Thick-walled focal lucency within a rounded ill-defined perihilar opacity, with air-fluid level, in a case of round pneumonia with cavitation.

If the lesion was antenatally detected or seen at birth, think of a congenital abnormality, further follow up imaging with CXR or higher level imaging with chest CT is recommended.

A list of differential diagnoses is provided in the table (Figure 8) which describes each one's characteristics and how to arrive at a definitive diagnosis.

Fig 8: DDx of Focal Hyperlucency

B. Lobar hyperlucency and its differential diagnosis

Fig 9: Lobar lung hyperlucency and its differential diagnosis (dotted shape), (A) in the Left upper lobe and (B) in the Right middle lobe, in two different cases of congenital lobar overinflation.

C. Unilateral hemithoracic hyperlucency and its differential diagnosis

Fig 10: Diffuse unilateral hemi-thoracic hyperlucency (dotted shape). A: There is minimal intercostal space widening (green compared to blue) and no mediastinal shift, in a case of Swyer-James syndrome. B: There is widened intercostal spaces (green compared to blue) mainly basally with flattened right hemidiaphragm and contralateral mediastinal shift, all are signs of overinflation-air trapping, in a case of foreign body aspiration.

Fig 11: Differential diagnosis of Unilateral hemithoracic hyperlucency

D. Bilateral hemithoracic hyperlucency and its differential diagnosis

Fig 12: Diffuse bilateral hemi-thoracic hyperlucency (dotted shape) with its DDx. A: With symmetrical normal hilar vascular caliber (green oval), a known case of SMA spinomuscular atrophy, therefore generalized muscular atrophy. B: With symmetrical bilateral hilar oligemia (green oval) in a case of bronchiolitis obliterans.

Soft tissue densities:

Well-defined lung opacities: (

Fig 13: Differential diagnosis of well-defined lung opcification.

A. Focal opacities and its differential diagnosis

Fig 14: Three cases of Round pneumonia. It appears as a rounded ill-defined opacity with irregular margins which could be (A) solitary, (B) Bifocal or (C) giant. Table for differential diagnosis of focal opacities

B. Lobar opacities:

Wedge shaped (lobar) opacities are either due to atelectasis or consolidation. 

Fig 15: Comparison between lobar atelectasis and consolidation.

Fig 16: Characteristics of Atelectasis. 1. It usually recovers with proper treatment within 24 hours. A and B frontal chest X-Ray of the same patient, two days apart. A: left hemi-thoracic white out with ipsilateral mediastinal shift, therefore total left lung collapse. B: Follow up X-Ray two days later shows partial re-aeration of the left upper lobe. 2. Chronic atelectasis (*) is associated with compensatory hyperinflation (#) of the surrounding lobes, as seen in case C.

Fig 17: How to distinguish right upper lobe atelectasis from consolidation depending on the position of the transverse fissure? (Blue) All cases show wedge shaped opacity (*) in the right upper zone with (A): Minor fissure pulled up towards the opacity. (B) The fissure is neutral and in (C) it is pushed downward (away from the opacity, i.e. due to an expansile lesion). Acute atelectasis is solely seen as A, on the other hand, consolidation may manifest in either form of the three. Clinical data usually help to exclude a consolidation/ collapse in case A.

Fig 18: Differential diagnosis and potential etiologies of atelectasis.

Fig 19: Foreign body aspiration manifestations depend on the size of the FB: 1. If FB is small (i.e. doesn’t cause total occlusion of the airway), lobar or whole lung hyperinflation seen, due to ball-valve effect. Maybe accompanied with contralateral mediastinal shift and ipsilateral lung herniation to the retrosternal region. 2. If FB. Is large enough to totally obstruct the airway, ipsilateral lung collapse, ipsilateral mediastinal shift and compensatory hyperinflation of the contralateral lung are noted.

Airbronchogram presence is not specific to consolidation, it denotes that the opacity is pulmonary in origin.

It is crutial to compare CXR with previous ones, as recurrent or persistent consolidation/ atelectasis raises the suspicion for an underlying congenital anomaly or immune deficiency.

Consolidation is typically accompanied by pleural effusion, which manifests in a variety of forms depending on the quantity and the composition of the pleural effusion.

Fig 20: Consolidation is typically accompanied by pleural effusion. A: Right middle lobar consolidation (*) with mild amount of pleural effusion seen along the lower chest wall (Blue). B: Left lower lobe consolidation (*) with mild amount of pleural effusion (meniscus sign, blue line). C: Another case of LLL consolidation (*) with a pleural apical cap and pleural effusion extending along the lateral chest wall (moderate amount, Blue)

Fig 21: Changes in lateral costophrenic angle appearance according to the amount of pleural effusion. Sharp (normal). B. Blunted & C. Obliterated (minimal amount), D. Meniscus sign (mild amount), E. Fluid seen along the lateral chest wall with veiling (moderate amount) and F. Total hemi-thoracic white out (large amount). In case of E and F, US chest is indicated.

C. Veiling: 

A subtle homogenous opacity that doesn’t totally obscure the pulmonary vessels and overlapping normal lung. Its differential diagnosis are listed in the table.

Fig 22: Veiling. A and B: Two different cases of right sided hemi thoracic moderate pleural effusion, appear as hazy opacity (*) with pleural effusion seen along the lateral chest wall (Blue). C: Left upper lobe lung collapse: hazy opacity (* & Blue) with elevated left hilum (+) and left hemidiaphragm (<), the aortic arch is well defined by air i.e. Luftsichel sign (:) due to hyperinflated superior segment of left lower lobe interposing itself between the mediastinum and the collapsed left upper lobe.

D. Total hemithoracic white-out: 

A dense homogenous opacity that totally obscures the whole or part of the hemithorax. Its differential diagnosis list is provided in the table.

Fig 23: Total hemithoracic white out (*) and its DDx. Trachea in Blue. A: Left hemi-thoracic white-out with ipsilateral mediastinal shift in a case of left lung collapse. B: Left hemi-thoracic white-out with contra-lateral mediastinal shift in a case of pneumonia with empyema. C: Almost total right hemi-thoracic white out with central trachea in a case of pneumonia.

Patchy ill-defined lung opacities: 

Usually the term “infiltrates (European term) or alveolar /air space shadowing (American synonym)” are used to label these changes. Their differential diagnosis list are listed in the table.

Fig 24: Differential diagnosis of patchy and ill-defined lung opacification.

Fig 25: Unilateral and bilateral patchy and ill-defined lung opacification with its DDx. A: Right lower zonal infiltrates (circle) in a case of early bacterial bronchopneumonia. B: Bilateral perihilar infiltrates (8) and peri-bronchial thickening, lung hyperinflation (flattened diaphragm and > 5 anterior ribs seen) in a case of viral bronchiolitis.

Localization of the lung pathology: 

Determining the location or lobe of lung abnormaly is essential, as certain conditions exhibit a greater tendency for specific locations; for instance, sequestration and aspiration pneumonia typically present as lower lobar opacities. While congenital lobar overinflation is more prevalent in the upper lobes, and foreign body aspiration is more frequently observed in the lower lobes.

Fig 26: Localization of lobar consolidation. A. Right upper lobe (right upper zonal opacity above the transverse fissure), B: Right middle lobe (right lower zonal medial opacity obscuring the right cardiac border), C: Right lower lobe (right lower zonal opacity with sharp right cardiac contour), D: Right middle and lower lobe (right lower zonal opacity obscuring the right cardiac border and extending laterally), E: left upper lobe (left upper zonal opacity obscuring the left cardiac contour), F: Left lower lobe (left lower zonal opacity not obscuring the left cardiac contour) consolidations.