Chest X-ray Anatomy
Mediastinal contours

Key points

  • The heart is the main visible structure in the mediastinum
  • Important diseases change the appearance of the aortic knuckle, the descending aorta, the aortopulmonary window, and the right paratracheal stripe

The heart is located in the middle mediastinum. Other visible structures of the mediastinum should also be checked when viewing a chest X-ray. These include the aortic knuckle, descending aorta, aortopulmonary window, and right paratracheal stripe.

Some structures of the mediastinum are not visible on a chest X-ray, for example the oesophagus (posterior mediastinum), great vessels (superior mediastinum), and thymus (anterior mediastinum).

Normal aortic knuckle

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Normal aortic knuckle

  • The aortic knuckle represents the left lateral edge of the aorta as it arches backwards over the left main bronchus
  • The contour of the descending thoracic aorta can be seen in continuation from the aortic knuckle
  • Displacement or loss of definition of these contours can indicate diseases such as aortic aneurysm or adjacent lung consolidation

Normal aortopulmonary window

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Normal aortopulmonary window

  • The aortopulmonary window is located between the Aortic Knuckle (AK) and the Left Pulmonary Artery (LPA)
  • It is a space where abnormal enlargement of mediastinal lymph nodes can be seen on a chest X-ray
  • (AA) = Ascending Aorta
  • (DA) = Descending Aorta

Normal right paratracheal stripe

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Normal right paratracheal stripe

  • From the level of the clavicles to the azygos vein the right edge of the trachea is seen as a thin white line or 'stripe'
  • This stripe is created by air of low density (blacker) lying either side of the comparatively dense (whiter) tracheal wall
  • Thickening of the paratracheal stripe (normally less than 3 mm) may represent pathology such as a paratracheal mass or enlarged lymph nodes
  • The left side of the trachea is not so well defined because of the position of the aortic arch and great vessels

Assessing the mediastinal contours

Whenever you look at a chest X-ray it is well worth looking for abnormalities in the region of the aortic knuckle, the aortopulmonary window, and the right paratracheal stripe.

Appreciation of the range of normal appearances of these structures will come with viewing as many chest X-rays as you can.

Page author: Salisbury NHS Foundation Trust UK (Read bio)

Last reviewed: February 2020