Main junctional lines and stripes
The visibility of mediastinal contours is determined by the contact of anatomical structures with lung tissue.
- Anterior junction line (anterior junction line). Formed where the pleura of the right and left lungs meet anterior to the great vessels. On a frontal chest radiograph, it appears as a thin oblique stripe against the background of the superior mediastinal shadow.
- Posterior junction line (posterior junction line). Divided into two segments. The supra-aortic part is located posterior to the esophagus and trachea. The lower posterior part is formed where the lungs converge posterior to the heart (between the spine and descending aorta).
- Pleuro-oesophageal line (pleuro-oesophageal line). Occurs when the lung extends behind the trachea and contacts the right wall of the esophagus. This border is particularly well seen on a frontal radiograph if air is present within the esophageal lumen. In the right mediastinum above the azygos vein, three key borders can potentially be differentiated: the contour of the superior vena cava, the right tracheal wall, and the right esophageal wall.
Topography of the superior mediastinum and aortopulmonary zone
- Left mediastinum above the aortic arch. The low-density shadow in this region is formed by the left common carotid and left subclavian arteries, as well as the left brachiocephalic and jugular veins. The smoothly curving outer contour is usually formed by the left subclavian artery (a. subclavia sinistra). The outer edge of the left tracheal wall is barely discernible here because the lung tissue is displaced by the great vessels.
- Aortopulmonary window. The lateral border of this zone is the aortic-pulmonary mediastinal stripe (aortic-pulmonary mediastinal stripe), a pleural fold between the aortic arch and the pulmonary artery trunk. This is an extremely sensitive area for detecting lymphadenopathy.
- "Nipple sign" ('nipple'). A small bulge projecting laterally from the edge of the aortic arch. This is a normal variant representing the cross-section of the left superior intercostal vein, which arches over the aorta and drains into the left brachiocephalic vein. This shadow should not be mistaken for an enlarged lymph node.
Evaluation of structures on lateral radiographs
The lateral projection requires special attention to the retrotracheal and retrosternal spaces.
- Trachea. The air column of the trachea descends obliquely downward and posteriorly. Normally, it is straight or slightly curved anteriorly (e.g., due to age-related aortic unfolding). The posterior tracheal wall is well visualized, forming the posterior tracheal stripe with a thickness of 2–3 mm. If the esophagus is collapsed and separates the lung from the trachea, a soft-tissue stripe of 10 mm or more is formed. The bifurcation (carina) itself is not identified on a lateral radiograph.
- Retrosternal line. Represents a stripe-like opacification along the lower third of the anterior chest wall. This is completely normal, caused by mediastinal fat and the lack of contact between the left lung and the anterior chest wall (the space is occupied by the heart).
- Supra-aortic structures. Among the major branches of the aorta, the brachiocephalic trunk (truncus brachiocephalicus) is most frequently recognized on a lateral radiograph, arising anterior to the tracheal air column and forming a smooth S-shaped line.
Posterior mediastinum and diaphragm
- Azygo-oesophageal line (azygo-oesophageal line). Below the arch of the azygos vein, the right lung extends into the azygo-oesophageal recess, contacting the esophagus and vein. Important clinical rule: the upper few centimeters of this line must be straight or concave toward the lung. A convex contour is a sign of pathology (subcarinal mass).
- Paravertebral lines. Exactly mirror the undulating contours of the lateral spinal ligaments. In obese individuals, due to abundant adipose tissue, the contours become smoothed, and the thickness of the left paravertebral space may exceed 10 mm.
- Diaphragm. The right dome is normally located at the level of the anterior segment of the 6th rib. The esophageal hiatus is located anterior to the aortic hiatus (which is bounded by the crura of the diaphragm and the median arcuate ligament). The inferior vena cava hiatus is located even further anteriorly—within the central tendon directly beneath the right atrium.
Briefly: answers to topical questions
What structures pass through the esophageal hiatus of the diaphragm?
The esophagus and its accompanying vagus nerves pass through the esophageal hiatus of the diaphragm.
- Esophagus (oesophagus) — a fibro-muscular tube passing through the opening at the T10 level.
- Vagus nerves (nn. vagi) — represented by the anterior (left) and posterior (right) nerves.
The hiatus itself (hiatus oesophageus / hiatus esophageus) is formed by muscular bundles of the diaphragmatic crura (usually the right one) that cross each other and form a physiological sphincter. During diaphragmatic contraction, these bundles compress the esophagus, preventing gastroesophageal reflux.
Which lymph node groups are located in the aortopulmonary window?
Subaortic lymph nodes, belonging to station 5 according to the IASLC classification, are located in the aortopulmonary window.
- Subaortic lymph nodes (station 5) — located lateral to the ligamentum arteriosum. Their upper border runs along the inferior margin of the aortic arch, and their lower border corresponds to the superior margin of the left main pulmonary artery.
Enlargement of lymph nodes in this location (lymphadenopathy) can lead to alteration of the pleural reflection shape and the appearance of a bulge in the aortopulmonary window region on imaging studies.
Into which compartments is the mediastinum divided on a lateral radiograph?
Traditionally, the mediastinum is divided into superior, anterior, middle, and posterior compartments. The exact boundaries of these compartments vary and do not act as barriers to the spread of disease. In the three-compartment classification, the anterior, middle, and posterior mediastinum are distinguished.
Which veins drain into the left brachiocephalic vein?
Several venous vessels drain directly into the left brachiocephalic vein, and the vein itself is formed by the convergence of major neck veins.
- Left internal jugular vein (v. jugularis interna) and left subclavian vein (v. subclavia) — merge posterior to the sternoclavicular joint, forming the left brachiocephalic vein itself.
- Left superior intercostal vein (left superior intercostal vein) — collects blood from the 1st through 4th left intercostal veins, arches anteriorly around the aorta, and drains into the left brachiocephalic vein.
- Lowest thyroid vein (v. thyroidea ima) — is also a direct tributary of the left brachiocephalic vein.
Topography of the diaphragmatic apertures (anterior to posterior): Vein — Esophagus — Aorta (VEA). The inferior vena cava hiatus is the most anterior, the esophageal hiatus is located behind it, and the aortic hiatus is the most posterior.
Class and exam questions
Why is a mass sometimes visualized behind the sternum on a lateral radiograph?
In healthy individuals, this can be an extrapleural soft-tissue bulge of the left brachiocephalic vein. This shadow should not be mistaken for an anterior mediastinal mass.
Is the tracheal bifurcation always identifiable on a lateral view?
No, the carina is not identified on a lateral radiograph. Students and doctors often mistakenly identify the contour of the right main bronchus as the carina.
What does an altered shape of the azygo-esophageal line indicate?
Normally, it is straight or concave toward the lung. If the contour becomes convex, it indicates pathology, such as left atrial enlargement or the presence of subcarinal masses.
What else to study on this topic
- The effect of age-related aortic unfolding on mediastinal structures
- Radiation anatomy of the aortopulmonary window and detection of lymphadenopathy
- Topography and radiographic contours of the diaphragmatic crura
- Evaluation of costophrenic angles: normal and athletic variants (flattening during inspiration)
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