Firstly, the importance of recognizing the impact of suboptimal technical factors (such as poor inspiration and rotation) in degrading the quality of the chest x-ray is illustrated, and how it may lead to misinterpretations. Then, an explanation of how to interpret a normal pediatric chest x-ray, comprising an analysis of ten structures, is demonstrated. Finally, an exhibit of the common variants and the unconventional appearance of normal structures on a pediatric chest x-ray are emphasized, which is especially relevant for those who are accustomed to reading adult chest x-rays and are new to pediatric chest x-rays.

The interpretation of pediatric chest x-rays is a taught skill that consists of a multistep approach, as shown in (scheme 1)

Table 1: Scheme 1: Summary of Pediatric Chest X-ray Interpretation.

Preliminary steps before looking for abnormalities:

1. The clinical history indicated on the request should be checked as it will be interrelated with the CXR findings in the future.
2. The patient's name and the study date should be confirmed with the information labeled on the CXR. Of particular importance is the patient's age, which is correlated with the presence or absence of the thymus and the expected signs of bony maturity.
3. Technical factors are scrutinized, as poor inspiratory and rotated films may lead to misinterpretations:

Technical factors:

Good inspiration: (Figure 1-4)

Fig 1: Criteria for optimal inspiratory film in (A) Erect and (B) Supine CXR: 1. The medial endpoint of the first rib medially (highlighted in orange). 2. On the erect film, six, while on the supine film, five right anterior ribs intersect with the copula of the diaphragm. 3. The entire cardiac contour is positioned above the diaphragm (arrows). 4. The base of the heart (curved line) forms an acute angle with the diaphragm (angles).

Fig 2: Poor inspiratory films in the Erect position: (A) depicts 1. Four right anterior ribs 2. A tent-shaped heart (trapezoid shape) 3. The base of the heart forms an obtuse angle with the diaphragm (angle). (B) Despite having 6 right anterior ribs, this is still considered a poor inspiratory film because: 1. A large portion of the left heart is visible below the diaphragm (arrows) 2. The base of the heart forms an obtuse angle with the diaphragm (angle). (C) The first rib points downward instead of medially, and the left cardiac contour is visible below the diaphragm.

Fig 3: Poor inspiratory films in the Supine position: In both cases, four right anterior ribs intersect with the copula of the diaphragm. Additionally, in (A), a large portion of the left heart is visible below the diaphragm (arrows). (B) Highlights the pitfalls of poor inspiration: false cardiomegaly and diffuse opacification of the lungs.

Fig 4: Pitfalls of poor inspiration. Frontal supine chest X-rays of the same patient captured just moments apart to confirm the position of an adjusted NGT. (A) Represents an ideal inspiratory film, showcasing the visibility of the first rib's medial endpoint (highlighted in orange) and five right anterior ribs intersecting with the copula of the diaphragm (denoted by numbers). In contrast, (B) illustrates a poor inspiratory film with only 4 right anterior ribs (numbers) and the first rib pointing downward (curved lines). Note: False positive cardiomegaly and perihilar and retrocardiac infiltrates.

(Figure 5-8)

Fig 5: Central positioning. (A) Displays a central supine CXR of an 8-month-old baby, where the clavicles (arrows) exhibit a symmetrical shape, and the trachea (dashed line) is centrally positioned between the right and left pedicles. Please note that the spinous processes are not yet ossified. (B) Features a central erect CXR of a 5-year-old child, with clavicles (arrows) displaying a symmetrical shape, and their medial ends (curved lines) positioned equidistantly from the trachea. The trachea (dashed line) overlays the ossified spinous processes.

Fig 6: Rotation. (A) Depicts subtle rotation: the trachea (broken line) overlays the right thoracic pedicles, showing an asymmetric appearance of the clavicles (curved arrows). Additionally, the chin is tilted to the right (solid arrow). (B) Illustrates severe rotation to the right, where not only the trachea but also the entire mediastinum is observed overlying the right lung, accompanied by costal span asymmetry (dashed lines).

Fig 7: Pedicles-trachea as a reference for centrality assessment. (A) Shows a 23-month-old supine CXR where the baby is rotated to the right, and (B) Features a 3-year-old erect CXR where the child is rotated to the left. In both cases, the medial ends of the clavicles cannot be discerned separately, making them unsuitable as reference points for determining the centrality of the CXR. Conversely, by comparing the tracheal distance (dashed line) to the positions of the pedicles (circles), the direction of rotation can be easily determined.

Fig 8: Pitfalls of rotation: (A) Demonstrates rotation to the right (trachea overlying the right pedicles (arrow), resulting in false hyperlucency of the right lung compared to the left lung and pseudo left hilar plethora. (B) Illustrates mild rotation to the right, leading to a pseudo right paratracheal mass from the sternum manubrium (arrows). (C) Shows rotation to the left (trachea (arrow ) overlying left pedicles), resulting in a false positive impression of cardiomegaly.

Criteria for optimal inspiratory film on an erect CXR:

1. The medial endpoint of the first rib medially.
2. Six right anterior ribs intersect with the copula of the diaphragm.
3. The entire cardiac contour is positioned above the diaphragm.
4. The base of the heart forms an acute angle with the diaphragm.

On the supine film, the same criteria are followed except for five right anterior ribs intersect with the copula of the diaphragm.

Pitfalls of poor inspiration: false cardiomegaly and diffuse opacification of the lungs.

Central positioning:

Criteria for the central position:

1. Symmetrically shaped clavicles,
2. The trachea is centrally positioned between the right and left pedicles. Please note that the pedicles are considered a reference because spinous processes may not be ossified and the clavicular medial ends may be indistinguishable from the surrounding shadows.

Perfectly centralized CXR films are hard to acquire most of the time, so minimal rotation is accepted.

Pitfalls of rotation:

1. False hyperlucency of the lung ipsilateral to the side of rotation and pseudo-contralateral hilar plethora.
2. Pseudo right paratracheal mass from the sternum manubrium.
3. False positive impression of cardiomegaly.

Patient’s posture: Whether the x-ray is taken in a supine or erect position is considered mainly to assess the criteria of proper inspiration.

Exposure: In the era of digital radiology, exposure evaluation plays no role.

4. Developing a systematic approach:

There are several approaches that can be utilized when reading a pediatric CXR. Potential methods for the initiation of checklist assessment are the following:

1. From caudal to cranial direction, or vice versa,
2. From the peripheral to central structures or vice versa,
3. From the least important (chest wall) to the most important (blind spots) or vice versa,
4. The 3 shades (air, soft tissue, and bone)

Applying either of the aforementioned strategies would result in a proper evaluation of the 11 structures illustrated on CXR. It is essential to be aware of the normal appearance of these structures as well as their variants.

5. Comparison with previous CXR, chest CT and other relevant radiological studies from archives.

Normal Anatomy: (Scheme 2-3)

Table 2: Scheme 2: Interpretation of Chest X-rays (CXR): Checklist for Evaluation of Chest Wall Soft Tissue, Bones, and Abdomen.

Table 3: Scheme 3: Continuing CXR Interpretation: Checklist for Airway, Hila, Aorta and Pulmonary Trunk, Heart, Diaphragm, Lungs, Thymus, and Evaluation of Lines & Tubes (if present).

      1.Abdomen:

• Free air:Subdiaphragmatic pneumoperitoneum is noted if the CXR is acquired in the erect position.
• Stomach air bubble position: The status assessment is discussed at (9. Heart)
• Calcifications: Paravertebral calcifications may refer to neuroblastoma or neurogenic tumors. Calcifications in the projection of the liver may be due to hepatoblastoma. Gallbladder or kidney stones may  be picked up on CXR.
• Foreign body
• Bowel distension
• Bowel loops above the diaphragm

2. Diaphragm:

• Both domes are sharp with smooth contour. Usually both domes are at the same level, but the left dome maybe 1.5 cm lower than the right side.

3. Costophrenic angle:

• The lateral angles normally form sharp acute angles.
• Blunted or obliterated angle is due to pleural effusion or extension of lower lobe consolidation.
• Deep sulcus sign: Deep, lucent, costophrenic angle on supine chest radiograph, as a sign of pneumothorax.

4. Chest wall soft tissue:

• Swelling:Can be due to masses (focal swelling) or due to cellulitis (diffuse swelling)
• Calcifications: the most common cause is BCG-oma.
• Foreign body
• Subcutaneous emphysema:Due to rib fractures with air leak or pneumomediastinum.
• Artifacts: from external shadows like a skin fold, hair braid, bandage, buttons, or clothes.

5. Bones:

• Vertebra:Vertebral bodies are normally rectangular with rounded pedicles.
• Ribs,
• Clavicles,
• Scapula,
• Humeri:Normally the humeral head epiphysis is ossified at the age of 4 months. It is an important landmark to assess the bony maturity of the patient. Its delayed ossification maybe due to prematurity, failure to thrive or hypothyroidism.

Detailed bony chest wall abnormalities are discussed in another poster (Hack the bones on pediatric CXR)

6. Airways: (figure 9-10)
   

   Fig 9: Airway Assessment - The normal tracheo-bronchial branching pattern should be looked for.
   

   Fig 10: Hilar Vascular Assessment - Zoomed images of the right hilum, (A) CXR, and (B) the reversed image of the same patient. The diameter of the artery (arrow) is typically nearly equivalent to that of the accompanying bronchus (dashed arrow).

• Trachea should be visible on all normal CXR, in the midline or occasionally with a slight buckle to right at the thoracic inlet.
• Bronchial branching pattern is pursued to help determine the situs besides the use of cardiac apical and gastric air-bubble positions.

7. Hila: (figure 11)
   

   Fig 11: Aorta, descending aorta, pulmonary trunk, and situs assessment. (A) Represents an erect CXR in a 5-year-old child: Aortic knob (curved line) and descending aorta (arrows), pulmonary trunk (dashed line), cardiac apex (*), and the gastric air bubble (#) are on the left side, indicating situs solitus. (B) Features a supine CXR in a 4-month-old baby: The aortic knob and the pulmonary trunk cannot be appreciated due to the overlying thymic shadow. The cardiac apex (*) is on the left. The gastric shadow cannot be differentiated from overlying gassy bowel (%), making it challenging to accurately determine situs.

• The diameter of the pulmonary arteries in the hilum is compared to the accompanying bronchus and it is normally identical in size.
• Hilar plethora is the term used to describe vascular dilatation (i.e. the artery’s diameter is double that of the bronchus,) on the contrary, hilar oligemia is the reverse.
• Vessels extend to the mid lung third, tapering gradually and no vessels are seen in the outer one third.
• Bilateral hilar vascular diameter should be compared (to rule out pulmonary artery hypoplasia)

8. Aorta and pulmonary trunk: (figure 11)
   

   Fig 11: Aorta, descending aorta, pulmonary trunk, and situs assessment. (A) Represents an erect CXR in a 5-year-old child: Aortic knob (curved line) and descending aorta (arrows), pulmonary trunk (dashed line), cardiac apex (*), and the gastric air bubble (#) are on the left side, indicating situs solitus. (B) Features a supine CXR in a 4-month-old baby: The aortic knob and the pulmonary trunk cannot be appreciated due to the overlying thymic shadow. The cardiac apex (*) is on the left. The gastric shadow cannot be differentiated from overlying gassy bowel (%), making it challenging to accurately determine situs.

• The aortic knob should always be looked for on the left side. If not seen, an indentation on the right wall of the trachea should be excluded to rule out a right sided aortic arch. Otherwise, mediastinal lesions must be ruled out.
• The pulmonary trunk contour is seen below the aortic knob, must form a straight contour (not a bulge)
• The descending aorta is usually seen as a straight line in the left paravertebral region.

9. Heart: (figure 11 & 12)
   

   Fig 11: Aorta, descending aorta, pulmonary trunk, and situs assessment. (A) Represents an erect CXR in a 5-year-old child: Aortic knob (curved line) and descending aorta (arrows), pulmonary trunk (dashed line), cardiac apex (*), and the gastric air bubble (#) are on the left side, indicating situs solitus. (B) Features a supine CXR in a 4-month-old baby: The aortic knob and the pulmonary trunk cannot be appreciated due to the overlying thymic shadow. The cardiac apex (*) is on the left. The gastric shadow cannot be differentiated from overlying gassy bowel (%), making it challenging to accurately determine situs.
   

   Fig 12: Cardiac size assessment: The left contour (>) should not exceed the left copula, and the right contour (<) should not exceed the medial third of the right hemithorax. (A) Represents an erect CXR in a 5-year-old child (B) Features a supine CXR in a 4-month-old baby:

Correlation with the gastric air bubble position with the site of cardiac apex is used to determine the situs.

1. If both are on the left side à situs solitus.
2. If both are on the right side à situs inversus.
3. If each is on the opposite side of the other à situs ambiguous.

Criteria for cardiac size assessment:

• The left cardiac contour should not exceed the left copula, and the right cardiac contour should not exceed the medial third of the right hemithorax. These criteria can be followed only on a good inspiratory and central CXR.

10. Lungs: (figure 13)
    

    Fig 13: Lung assessment. A: The three lung zones: upper zone: anterior first 2 ribs(blue), mid zone: Rib 2-4 anteriorly (green) and lower zone: below the 4th anterior rib (pink) B: Checklist for lung bind spots highlighted: Apices (in blue), Retrocardiac region (in red), Paravertebral region (in green).

• Divided into three zones, symmetrical zonal aeration and intercostal spacing is looked for. Blind spot areas are those areas where pathologies are most missed.

11. Thymus: (figure 14-15)
    

    Fig 14: Normal Thymus: (A): The most common appearance of thymus, having quadrangular “widened-mediastinum like appearance”. (B) Thymic notch sign.
    

    Fig 15: Normal Thymus. (A) Thymic Wave Sign (arrows). (B)Thymic Sail Sign.

• A soft organ with similar density as the other mediastinal structures, located in the superior anterior mediastinum. Its rectangular form winds seamlessly into the cardiac silhouette. Usually seen until 3years of age.
• The most common appearance of thymus, having quadrangular “widened-mediastinum like appearance” with straight contour and soft tissue density that is indiscernible from aortic knob, pulmonary trunk or heart silhouette.
• Thymic notch sign: notch at the inferior border of the normal thymus at the point where it interrupts with the border of the cardiac silhouette.
• Thymic Wave Sign: the left wavy contour of the thymus refers to the impression of the anterior portion of the ribs on the soft thymus gland Thymic Sail Sign: triangular lateral extension of thymus into the right hemithorax, against the transverse fissure.

Normal variants are displaied for further comprehension (figure 16-20)

Fig 16: Normal variants (A) Tracheal buckle: rightward deviation of the trachea (arrow). (B) Situs inversus totalis: both the cardiac apex (red arrow) and the gastric bubble (circle) are observed on the right side with reversal of bronchial branching pattern, i.e. long left main bronchus is situated on the right side (green arrow) and the steeper right bronchus situated on the left side (yellow arrow).

Fig 17: (A) Bulky anterior costo-chondral junctions is a normal variant till the age of 6 months. Therefore, to exclude rickets at this age, humeral metaphysis (arrows) must be evaluated for cupping, fraying, widening and irregularity. (B) Right second bifid Rib. Note normal costochondral junction shape in 13-month-old child.

Fig 18: Bone-in-Bone appearance (the appearance of a smaller version of a bone contained within its larger version, the vertebral bodies) of the thoracic and proximal lumbar vertebra seen in neonates. (A) Frontal CXR of a neonate (B) zoomed-in spine of a different neonate

Fig 19: (A) Linear lucencies overlying the thoracic vertebra (arrows) are the non-ossified spinous processes, a normal variant potentially confused with spinal dysraphism. (B) Bilateral symmetrical curvilinear air lucency seen in the projection of the shoulder joint space (arrows) due to vacuum phenomenon, a common finding in CXR taken for infants with abducted arms, it is thought to be related to air pushed from the sinusoids in the humeral head cartilaginous epiphysis.

Fig 20: Right Lateral Border of the Left Atrium (arrows) on (A) Erect CXR of a 13-year-old child and (B) the supine CXR of a 28-month-old baby. This finding may be visualized in up to 2.5% of pediatric CXRs and can potentially be overlooked as a mediastinal lesion.