The child's chest X-ray: thymus, foreign body, croup and epiglottitis
- Finding
A child's X-ray has its own normal: a big thymus near the heart. It also helps when a child chokes on something or has noisy breathing.
What it is
A child's chest is not a small adult's. In infants the thymus is large and may fill the upper anterior mediastinum, looking like a mass, and the heart looks relatively large on AP films. Imaging is used less and with care: croup is a clinical diagnosis and needs no X-ray in a typical case, and a child with suspected epiglottitis must not be sent for X-ray before the airway is safe. In foreign-body aspiration most objects are not radio-opaque, so the film shows the effect rather than the object: one lung that stays inflated when it should empty. When the story is strong, a normal X-ray does not exclude a foreign body.
What it looks like
- Normal thymus: a smooth soft-tissue shadow in the upper anterior mediastinum, often triangular like a sail, with a wavy edge where the ribs press on it; it does not compress the trachea.
- The thymus is largest relative to the chest in infancy and becomes less visible through early childhood.
- Croup: on the AP neck view the subglottic airway narrows into a pointed, steeple-like shape.
- Epiglottitis: on the lateral neck view the epiglottis is swollen and rounded like a thumb, with thick aryepiglottic folds.
- Aspirated foreign body: the affected lung stays hyperlucent and does not lose volume on expiration, and the mediastinum shifts away from it.
- Round metal object in the oesophagus: seen face-on as a disc on the AP view. A button battery shows a double ring and is an emergency.
Why it looks that way
- A foreign body in a bronchus can act as a one-way valve: air enters on inspiration but cannot leave, so that lung stays big and dark.
- The thymus is soft and moulds around neighbouring structures, so it does not displace the airway or vessels.
- Viral inflammation in croup swells the subglottis, the narrowest part of the child's airway; in epiglottitis the swelling is above, at the epiglottis.
Differential and pitfalls
- A large normal thymus is often mistaken for a mass or cardiomegaly; a mass compresses or displaces the trachea, the thymus does not.
- Most foreign bodies (nuts, food) are radiolucent: the X-ray is often normal, especially soon after the event.
- Children who cannot follow breathing commands: lateral decubitus views; the side lying down should lose volume, and a lung that stays inflated suggests air trapping.
- Examining the throat or lying a child with suspected epiglottitis flat can trigger complete obstruction.
- A coin seen edge-on on the AP view suggests it is in the trachea rather than the oesophagus; confirm with the lateral view.
Next step
- Suspected epiglottitis: keep the child calm and upright, no X-ray or throat exam; call anaesthesia and ENT to secure the airway.
- Typical croup: no imaging; X-ray only if the course is atypical, severe or not improving.
- Strong story of choking: rigid bronchoscopy even if the X-ray is normal; discuss with paediatric surgery or ENT.
- Swallowed object: X-ray from neck to abdomen; a button battery in the oesophagus needs urgent removal.
- Mediastinal shadow that compresses the trachea or looks unusual: ultrasound of the thymus or discuss with the paediatric radiologist before a CT.
Test yourself
Think of the answer, then tap the question to see it.
How do you tell a normal thymus from a mediastinal mass in an infant?
The thymus is smooth, may have a sail shape or wavy edge and does not compress or displace the trachea or vessels; a mass does.
What is the steeple sign and what does it suggest?
Narrowing of the subglottic airway into a pointed shape on the AP neck X-ray; it suggests croup, which is still diagnosed clinically.
Why can the chest X-ray be normal after foreign-body aspiration?
Most aspirated objects such as food and nuts are radiolucent, and air trapping may be absent early; a strong history still needs bronchoscopy.
What does the affected lung do on an expiratory film when a foreign body blocks its bronchus?
It stays inflated and dark (air trapping) instead of losing volume, and the mediastinum shifts toward the other side.
Elsewhere in DurgiPedia
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Croup (StatPearls (NCBI Bookshelf), 2025); Epiglottitis (StatPearls (NCBI Bookshelf), 2024); Foreign Body Airway Obstruction (StatPearls (NCBI Bookshelf), 2024); Anatomy, Head and Neck, Thymus (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.