Anterior Middle and Posterior Mediastinum
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
CT AT THE LEVEL OF THE HEART
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.
AZYGOESOPHAGEAL LINE OR PARAESOPHAGEAL LINE
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.
CT OF THE AZYGOESOPHAGEAL LINE
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.
RIGHT AND LEFT PULMONARY ATERIES
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.
PULMONARY ARTERIES, Lateral View
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.
AORTICOPULMONARY WINDOW (AP WINDOW)
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
PARASPINAL EDGES (Stripes)
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