Mediastinum on imaging
Mediastinum on the chest radiograph and CT: divisions, borders, lines and masses
On a frontal chest film the mediastinum is a central shadow whose edges are made by structures touching aerated lung. CPSP asks the anatomical divisions and their contents, which structure forms each border, the normal lines and stripes, and how to place a mass in the right compartment from its shape and density.
Divisions and contents
- A plane from the sternal angle to the T4–T5 disc separates the superior from the inferior mediastinum; the inferior part is divided into anterior, middle and posterior by the pericardium.
| Compartment | Main contents | Typical masses |
|---|---|---|
| Superior | Thymus, brachiocephalic veins, upper SVC, aortic arch and its branches, trachea, oesophagus, thoracic duct, phrenic and vagus nerves, left recurrent laryngeal nerve | Retrosternal goitre, nodes |
| Anterior | Thymus remnant, fat, nodes, internal thoracic vessels | Thymoma, teratoma, lymphoma, thyroid |
| Middle | Heart and pericardium, ascending aorta, pulmonary trunk, lower SVC, main bronchi, phrenic nerves, nodes | Nodes, bronchogenic cyst (often subcarinal), pericardial cyst (right cardiophrenic angle) |
| Posterior | Oesophagus, descending aorta, azygos system, thoracic duct, vagus nerves, sympathetic chain | Neurogenic tumours, extramedullary haematopoiesis, oesophageal lesions |
- The thymus lies in the superior and anterior mediastinum. It is relatively largest in infancy, reaches its greatest absolute weight around puberty (still prominent at 10 years), then involutes and is replaced by fat.
- The pericardium and bronchogenic cysts belong to the middle mediastinum.
- CT reporting now often uses three compartments (prevascular, visceral and paravertebral), which map closely onto anterior, middle and posterior.
- Infection reaches the mediastinum from the neck: a tracheostomy or pretracheal infection spreads into the superior mediastinum; a retropharyngeal abscess spreads through the danger space into the posterior mediastinum.
Mediastinal borders on a frontal film
| Border | Formed by (top to bottom) |
|---|---|
| Right | Right brachiocephalic vein, SVC (above the right atrium), right atrium (IVC at the base). In elderly hypertensives the unfolded ascending aorta may form the upper right border |
| Left | Left subclavian artery, aortic knuckle, aortopulmonary window, pulmonary trunk, left atrial appendage, left ventricle |
- The left hilum is 1–2 cm higher than the right because the left pulmonary artery arches over the left main bronchus.
- A lung opacity that removes the aortic knuckle outline lies in contact with it: the apicoposterior segment of the left upper lobe.
Lines and stripes
| Line | Made by | Clinical point |
|---|---|---|
| Anterior junction line | Two lungs meeting behind the sternum | Does not extend above the clavicles |
| Posterior junction line | Two lungs meeting behind the oesophagus | Extends above the clavicles to the top of the aortic arch, seen through the tracheal air |
| Right paratracheal stripe | Right tracheal wall against lung | Normally up to about 4 mm; widened by paratracheal nodes (tuberculosis, lymphoma) or fluid |
| Azygo-oesophageal line | Right lower lobe against the azygos vein and oesophagus, below the azygos arch | Becomes convex to the right with a subcarinal mass, enlarged left atrium or hiatus hernia |
| Paraspinal lines | Lung against the paravertebral tissues | Smooth bulge with a vertebral fracture haematoma, abscess or extramedullary haematopoiesis |
| Aortopulmonary window | Concavity between the aortic knuckle and pulmonary trunk | Becomes flat or convex with nodes |
Locating a mass
- Cervicothoracic sign: a mass whose edge stays sharply outlined by lung above the clavicles must be posterior, because the anterior mediastinum ends at the clavicles where it meets the neck soft tissues.
- Hilum overlay sign: if the hilar vessels are still seen through a mass, the mass is in front of or behind the hilum, not hilar.
- Silhouette principle: a mass that blurs the heart border is anterior or middle; one that leaves it sharp but blurs the descending aorta or paraspinal line is posterior.
| Mass | Clue |
|---|---|
| Normal infant thymus | Triangular "sail" from the upper mediastinum with a wavy edge from rib indentation; no airway compression |
| Mature teratoma | Anterior; fat (about −90 HU), fluid and calcification, even a tooth |
| Retrosternal goitre | Continuous with a thyroid lobe, denser than muscle on plain CT (iodine), displaces the trachea |
| Thymoma | Anterior soft-tissue mass in an adult; association with myasthenia gravis |
| Bronchogenic cyst | Thin-walled, water or protein density (about 10 HU), no enhancement, often subcarinal |
| Pericardial cyst | Water density at the right cardiophrenic angle, touching the pericardium |
| Extramedullary haematopoiesis | Bilateral lobulated paravertebral masses with widened ribs in thalassaemia |
| Neurogenic tumour | Posterior, beside the spine; may widen a foramen or erode ribs |
Nerves, thoracic duct and oesophagus
- Vagus nerves: the right vagus enters the thorax by crossing in front of the right subclavian artery (giving the right recurrent laryngeal nerve, which loops under that artery), then runs down on the right side of the trachea. The left recurrent laryngeal nerve loops under the aortic arch at the ligamentum arteriosum, so an arch aneurysm or left hilar nodes cause hoarseness. Both vagi form the oesophageal plexus and pass through the diaphragm with the oesophagus at T10.
- Thoracic duct: ascends behind the oesophagus between the aorta and azygos vein, crosses to the left at about T5 and drains into the junction of the left internal jugular and left subclavian veins. Its injury or blockage (surgery, lymphoma) causes chylothorax.
- Large posterior mediastinal masses compress the oesophagus (dysphagia), sympathetic chain, thoracic duct and spinal nerves; a dilated aortic arch compresses the trachea, oesophagus and left recurrent laryngeal nerve.
- Oesophageal muscle: the upper third is skeletal (striated), the middle third mixed and the lower third smooth; the lining is non-keratinised stratified squamous throughout. Blood supply: inferior thyroid artery above, oesophageal branches of the descending thoracic aorta (and bronchial arteries) in the middle, left gastric below.
- Narrowings where foreign bodies lodge: cricopharyngeus (C6), the commonest site for a swallowed coin; the aortic arch and left main bronchus crossing (T4–T5); the diaphragmatic hiatus (T10).
- Oesophageal atresia with tracheo-oesophageal fistula: frothy saliva, choking and cyanosis with feeds; the nasogastric tube coils in the upper pouch on the radiograph. Gas in the stomach means a distal fistula: the commonest type (about 85%) is proximal atresia with a distal TOF; a gasless abdomen suggests pure atresia.
Air and blood in the mediastinum
- Pneumomediastinum (asthma, vomiting, trauma): a lucent line along the heart border and aortic knuckle, air under the heart joining both hemidiaphragms (continuous diaphragm sign), and air tracking into the neck as surgical emphysema.
- Widened mediastinum after deceleration injury suggests aortic rupture at the isthmus near the ligamentum arteriosum; look also for loss of the knuckle, a left apical cap, rightward displacement of the trachea or nasogastric tube and a depressed left main bronchus.
Remember Above the clavicles, only a posterior mass keeps a sharp edge.
Exam trap The posterior junction line rises above the clavicles; the anterior junction line does not.
Exam trap A wavy triangular shadow in a well infant is usually normal thymus, not a mass.
How FCPS asks about this
- Right mediastinal border above the right atrium: superior vena cava.
- Anterior mediastinal masses: thymoma, teratoma, lymphoma, thyroid.
- Bronchogenic cyst compartment: middle mediastinum, often subcarinal.
- Thoracic duct termination: junction of the left internal jugular and subclavian veins.
- Nerve looping round the aortic arch: left recurrent laryngeal nerve.
- Commonest type of tracheo-oesophageal fistula: proximal atresia with distal fistula.
- Mass above the clavicles with a sharp lung edge: posterior mediastinum (cervicothoracic sign).
Test yourself on this topic
5 of the 34 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 22-year-old man has an anterior mediastinal mass removed. It is a cyst containing sebaceous material and hair, with a tooth in its wall, and histology shows only well-differentiated skin, cartilage and gut-type epithelium. What is the diagnosis?
Answer: Mature teratoma
A mature teratoma is a germ cell tumour made of well-differentiated tissues from more than one germ layer, such as skin with adnexa, hair, teeth, cartilage and respiratory or gut epithelium. The anterior mediastinum is the commonest extragonadal site, reflecting the path of primordial germ cells along the midline. It is usually benign. Immature neural tissue would indicate an immature teratoma, and seminoma is a sheet of uniform germ cells with lymphocytes.
Mediastinal teratoma
- Young adult, anterior mediastinum
- Tissues from several germ layers
- Hair, sebum, teeth
- Mature: usually benign
Remember Hair and teeth in a front mediastinal cyst: teratoma.
Exam trap Immature neuroepithelium in a teratoma signals malignant potential.
Why the other options are wrong
- Invasive thymoma Epithelial thymic cells mixed with lymphocytes, no skin or teeth.
- Mediastinal seminoma Sheets of uniform germ cells with lymphocytic stroma.
- Hodgkin lymphoma Reed–Sternberg cells in a mixed inflammatory background.
- Retrosternal goitre Thyroid follicles with colloid, continuous with the neck.
A 20-year-old with acute severe asthma develops swelling of the neck with crackling under the skin and a crunching sound in time with the heartbeat. Air has escaped from ruptured alveoli and tracked along the bronchovascular sheaths. What is the diagnosis?
Answer: Pneumomediastinum
In severe asthma, raised alveolar pressure ruptures alveoli and air tracks along the bronchovascular sheaths to the hilum and into the mediastinum (the Macklin effect). From there it spreads into the neck as surgical emphysema. Air around the heart causes a crunching sound with each beat (Hamman sign). On a radiograph air outlines the mediastinal structures and may join both domes beneath the heart (continuous diaphragm sign).
Pneumomediastinum
- Asthma: alveolar rupture
- Air along bronchovascular sheaths
- Neck surgical emphysema
- Hamman crunch with heartbeat
Remember Asthma, neck crepitus, crunching heart sound: pneumomediastinum.
Exam trap After vomiting, pneumomediastinum may be due to oesophageal rupture, which is far more dangerous.
Why the other options are wrong
- Pneumoperitoneum Air lies under the diaphragm, from a perforated viscus.
- Left pneumothorax Air in the pleural space; does not spread into the neck.
- Pneumopericardium Confined to the pericardium; does not reach the neck.
- Oesophageal perforation with empyema Follows vomiting, with sepsis and effusion.
A trainee notices that on a normal frontal chest radiograph the left hilum lies about 1–2 cm higher than the right. What is the anatomical reason?
Answer: The left pulmonary artery arches over the left main bronchus
The hilum seen on a radiograph is mostly the pulmonary arteries. On the left, the pulmonary artery passes up and over the left main bronchus before descending behind it, so the left hilar shadow sits higher. On the right, the pulmonary artery lies in front of and below the right upper lobe bronchus, so the right hilum is lower. In about 3% of normal people the hila are level, but a right hilum higher than the left suggests upper lobe volume loss.
Hilar height
- Left hilum higher than right
- Left PA arches over left bronchus
- Right PA lies below upper lobe bronchus
- Right higher than left: suspect volume loss
Remember Left hilum high: the artery rides over the bronchus.
Exam trap Hila at the same level can be normal, but a raised right hilum is a sign of right upper lobe collapse.
Why the other options are wrong
- The left main bronchus is shorter and steeper than the right The left main bronchus is longer and more horizontal than the right.
- The heart pushes the right hilum downward The heart does not set hilar height.
- The aortic arch lifts the left lung root The arch lies above the root but does not lift it.
- The left lung has no middle lobe Lobe number does not decide hilar height.
A 19-year-old student has a thin-walled cyst removed from just below the carina. It does not communicate with the airway, contains mucoid fluid and is lined by ciliated columnar epithelium, with cartilage and seromucous glands in its wall. Which developmental lesion is this?
Answer: Bronchogenic cyst
A bronchogenic cyst arises from abnormal budding of the ventral foregut (the embryonic tracheobronchial tree) and most often sits in the middle mediastinum near the carina. It is lined by respiratory (ciliated columnar) epithelium, and its wall may contain cartilage, smooth muscle and seromucous glands, like a bronchus. Pericardial cysts have a mesothelial lining, and oesophageal duplication cysts have two muscle layers and gut-type mucosa.
Bronchogenic cyst
- Ventral foregut budding defect
- Site: subcarinal, middle mediastinum
- Ciliated columnar lining
- Wall may contain cartilage
Remember Cyst under the carina with respiratory lining: bronchogenic.
Exam trap Mucus can make a bronchogenic cyst look solid on imaging, mimicking a lymph node.
Why the other options are wrong
- Pericardial cyst Lined by flat mesothelium; at the cardiophrenic angle.
- Thymic cyst Contains thymic tissue; in the anterior mediastinum.
- Neurenteric cyst Posterior, with gut or neural tissue and vertebral anomalies.
- Lymphangioma Lymphatic channels lined by endothelium, usually in the neck.
A 17-year-old with poorly transfused beta-thalassaemia major has smooth, lobulated masses on both sides of the lower thoracic spine, with widened ribs and intact vertebrae. Biopsy shows erythroid and myeloid precursors with megakaryocytes. What is the diagnosis?
Answer: Extramedullary haematopoiesis
In thalassaemia major, ineffective erythropoiesis and anaemia drive marrow expansion, widening the ribs and skull. When this is not enough, haematopoietic tissue grows outside the marrow, classically as smooth, bilateral paravertebral masses in the lower thoracic posterior mediastinum, containing all three marrow cell lines. Adequate transfusion suppresses it.
Extramedullary haematopoiesis
- Thalassaemia or chronic haemolysis
- Bilateral lower thoracic paravertebral masses
- Widened ribs, intact vertebrae
- Suppressed by transfusion
Remember Thalassaemia with paraspinal masses: marrow outside the marrow.
Exam trap Biopsy of these vascular masses can bleed badly; the history usually makes it unnecessary.
Why the other options are wrong
- Tuberculous paravertebral abscess Caseating granulomas with vertebral and disc destruction.
- Bilateral neurofibromas Spindle cells of nerve sheath origin.
- Paravertebral lymphoma Sheets of malignant lymphoid cells.
- Descending aortic aneurysm A dilated vessel, not marrow tissue.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Thorax, Mediastinum on the chest radiograph: divisions, lines and contents