The mediastinum consists of potential spaces used to describe the location of disease processes
The middle mediastinum contains the heart
Important diseases change the appearance of the aortic knuckle, the aorto-pulmonary window and the right para-tracheal stripe
The mediastinum itself contains the heart and great vessels (middle mediastinum) and potential spaces in front of the heart (anterior mediastinum), behind the heart (posterior mediastinum) and above the heart (superior mediastinum). These potential spaces are not defined on a normal chest X-ray, but an awareness of their position can help in describing the location of disease processes.
There are several structures in the superior mediastinum that should always be checked. These include the aortic knuckle, the aorto-pulmonary window, and the right para-tracheal stripe.
The aortic knuckle (red line) represents the left lateral edge of the aorta as it arches backwards over the left main bronchus, and pulmonary vessels. The contour of the descending thoracic aorta (yellow line) can be seen in continuation from the aortic knuckle.
Displacement or loss of definition of these lines can indicate disease, such as aneurysm or adjacent lung consolidation.
The aorto-pulmonary window lies between the arch of the aorta and the pulmonary arteries. This is a potential space in the mediastinum where abnormal enlargement of lymph nodes can be seen on a chest X-ray.
In this chest X-ray, which is entirely normal, the curved arrow points towards the aorto-pulmonary window between the Aortic Knuckle (AK) and the Left Pulmonary Artery (LPA).
From the level of the clavicles to the azygos vein the right edge of the trachea is seen as a thin white stripe. This appearance is created by air of low density (blacker) lying either side of the comparatively dense (whiter) tracheal wall. If this stripe is thickened (normally less than 3mm) this may represent pathology such as a paratracheal mass or enlarged lymph node.
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 aorto-pulmonary window, and the right para-tracheal stripe.
Appreciation of the range of normal appearances of these structures will come with viewing as many chest X-rays as you can.