Copyright © 2009 [Gerald R. Aben, MD MSU Department of Radiology]. All rights reserved.
Revised: April 08, 2009
Step-by-Step Approach


Evaluating the Chest Radiograph

Step 7:  The Mediastinum and Hila

The evaluation of the mediastinum and hila is one of the most difficult in Chest Radiograph analysis.   Considerable normal variation can be seen in the mediastinum depending on a patient's age, history of prior disease and positioning.The first step in evaluation of the mediastinum is the identification of normal structures including; aortic arch, trachea, bronchi,  pulmonary arteries and veins.The aortic arch is represented by the first prominent bulge along the left mediastinal border when evaluated from top to bottom.  It is usually possible to evaluate both ascending and descending aorta in most patients.   As individuals age, the aorta will increase in both diameter and length leading to some 'widening' of the mediastinum at the level of the arch and below.   However, pathologic processes including aneurysm can cause the aorta to increase in size.  This might be suggested by the chest radiograph.

Normal Chest unlabeled

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White line outlines aortic arch


The trachea should have an almost midline location.  It's caliber should be relatively constant from the thoracic inlet to the bifurcation.  Changes in caliber may be a direct or indirect sign of tracheal pathology.   Deviation in its course is usually a sign of a mass in the mediastinum pushing on the trachea.  The trachea normally is in close proximity to the right lung.  A thickening in the 'wall' of the trachea can be a sign of lymph nodes or tumor in the upper mediastinum.    


Frontal Chest Unlabeled

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White lines roughly outline trachea and proximal bronchi


The proximal portions of both main bronchi should be visible on the radiograph.  Note that the azygous vein will be the ovoid structure just above the right main stem bronchus at its origin.  The left pulmonary artery will set just above the left bronchus.


Labeled Azygous Vein and Left Pulmonary Artery

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Labeled Right and Left Bronchi




What is wrong in this image?

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Right aortic arch and clearly identified azygous vein (arrow)


The pulmonary outflow tract creates the second 'bulge' that is noted along the left mediastinal border when moving from top to bottom.   There is normally a slight concavity between the aortic knob and the pulmonary artery on the frontal study.   A loss of this normal concavity is frequently a sign of lymph nodes or tumor in the mediastinum.  The branching of the pulmonary arteries can be seen from the level of the hilum for a short distance  into the lungs.  This should be differentiated from the pulmonary veins which will be noted to have their branching directed to the area below the hila.


Frontal Exam 

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Pulmonary outflow tract outlined in white



Pulmonary vessels indicated by arrows



Arrows indicate mediastinal mass obscuring the normal left cardiac and mediastinal borders


Lymph nodes or tumors in the hilar regions will present as increased fullness or size to the hila.  This may be unilateral or bilateral.  At times distinct rounded densities may be seen in these areas. (see above images)