Anterior Middle and Posterior Mediastinum

These regions are superimposed on the frontal view. The major structure is the heart. For all practical purposes the pericardium will be inseparable from the heart on plain film views. Review the heart for overall size and shape. A rough yardstick for size on the frontal film is the ratio of the widest diameter of the heart to the widest width of the thoracic cage as measured from inner aspect of rib to rib. This cardiac-thoracic ratio should be less than 50% (see inset for a graphic illustration of ratio measurements). Look carefully for calcifications, pneumopericardium, pneumomediastinum, sutures, prosthetic valves etc., that you may have overlooked on the general survey of the entire mediastinum. 


Follow the outline on both frontal and lateral views for specific chamber enlargement.


Look at the CT and MR to reinforce heart chamber relationships. See how these relationships correlate to chamber borders seen on theplain film.


Try tracking it from the root to distal descending aorta. In the young adult the ascending aorta usually is hidden in the mediastinum, in older people it may swing to the right enough to cast a soft tissue bulge. The arch should always be seen, make sure it is to the left of the distal trachea and actually pushes the distal trachea slightly to the right. Check for aortic calcifications and size. The left lateral border of the descending aorta abuts the left lung (column of dots on the pt's. left, on the annotated image). [The other column of dots is not the right side of the aorta, but instead is the paraesophageal line - see below.] On the lateral view the aorta is usually not seen. 


On the frontal view, the only part of the main pulmonary artery seen is the left lateral border where it meets the left lung. It can be relatively straight or convex (most commonly in young females). When convex, it forms a "middle mogul" just above the heart. The upper "mogul" is the aortic knob, the lower mogul is the left ventricle. The left pulmonary artery is directly behind the main pulmonary artery, and is visible on frontal films as a branching structure.


This is seen on the forntal view only and is formed by the right lower lobe where it meets the portion of the mediastinum containing the esophagus and the azygous vein. It usually overlies the thoracic spine, at or near the midline, and is usually fairly straight, vertically. If it bulges convex toward the lung, be suspicious of a mediastinal mass, usually subcarinal lymph nodes or an enlarged left atrium.


Use the CT image to understand exactly what structures form the border of the esophageal line. Air in the esophagus changes this edge into a line.


On the frontal view, the left pulmonary artery is the soft tissue density behind the main pulmonary artery, branching into the lung. The proximal right pulmonary artery is buried in the mediastinum, and is not seen on the frontal view until it branches as the right hilum.


The right pulmonary artery is seen on the lateral view as an ovoid branching structure, just anterior to the air column of the trachea and main bronchi. The left pulmonary artery is never seen as clearly as the right, unless it is markedly enlarged. It is a curved shadow, similar in shape to the aorta, just behind the air column.


This is another area radiologists double check for subtle mediastinal masses. It is seen on the frontal view (line of white dots) and is formed by a portion of the upper lobe sitting in the space immediately lateral to the area between the aortic arch and left pulmonary artery (remember ligamentum arteriosum and left recurrent laryngeal nerve?). The AP window should have a concave or straight border. If there is a mediastinal mass in the AP window region, the lung will be pushed laterally and the border becomes convex.


Sometimes on the frontal view, the plural edge is seen as a vertical density running parallel to the lateral margins of the vertebral bodies. If visible, this edge should be only a few millimeters beyond the vertebral bodies, and should not be lumpy or bulging. (The paraspinal edges are not visible on this image.)


On the lateral view, the anterior mediastinum cephalad to the heart in the adult should be lung-air density, not soft tissue density. In infants and young children, thymus fills this area. Also check the posterior sternal margin for small masses that might represent internal thoracic lymph node enlargement.