Mediastinum General and Superior

An enormous amount of information about the mediastinum can be extracted from plain films; the key is a thorough knowledge of anatomical relationships and how structures are likely to project on a radiograph. Use of cross-sections from CT and MRI will supplement this section. Understand on plain films the mediastinum projects as a water density surrounded by the two air filled lungs and intersected by the air filled trachea and major bronchi. The interfaces of these air-soft tissue margins may be distorted by pathological processes, usually masses, that otherwise would be hidden in the mediastinum. 

MEDIASTINUM

At this time, look at the overall size and shape of the entire mediastinum on the frontal and lateral views and decide if it is normal for the patient's age. Also look for obvious masses and calcifications, double check for tubes, electrical leads, a pacemaker,or artificial valves. Check for evidence of mediastinal shift and if present, is the entire mediastinum shifted, or just a section of it. Look at the trachea and major bronchi for size, position, and presence of intraluminal masses. 

SUPERIOR MEDIASTINUM - LAT

Although there are several methods of dividing the mediastinum into regions, this program will continue with the system taught in gross anatomy. The superior mediastinum begins at the root of the neck and ends caudally at a line drawn between T-4 vertebrae and the sternomanubrial junction. Usually that line skims the top of the aortic arch. The area between this line and the diaphragm is further divided into three regions, anterior, middle, and posterior. Basically, the heart and pericardium form the middle section, everything anterior to the heart is the anterior region, and everything posterior to the heart back to the spine is the posterior mediastinum. 

SUPERIOR MEDIASTINUM - PA

First, check the overall width for normal size, again look for masses, calcifications, and free air. The rest of the superior mediastinum review is a detailed search for subtle distortion of several major plural mediastinal interfaces. Not all of the following structures are seen on every film, but try to find them.


MARGIN OF SUPERIOR VENA CAVA (SVC)

The SVC is seen on the frontal view only, and depending how laterally it projects, its right edge may cast a subtle line on the film. Sometimes the entire edge is seen, often only a portion, but it should not bulge into the lung with a convex border.




SUPERIOR VENA CAVA

Look at the CT and superior vena cavagram to understand how the edge of the SVC may be seen on the plain frontal film.





RIGHT PARATRACHEAL STRIPE

The normal width is less than 5 mm, usually it is only 2-3 mm. This is an important marker for otherwise subtle adenopathy. The distal end of the stripe is formed by the azygous vein, and if the vein is distended, that portion of the stripe may normally be up to 1 cm wide. The medial margin of the stripe is the air-soft tissue interface along the right mucosal surface of the trachea. The outer margin of the stripe begins around the level of the medial end of the clavicle and is formed by the plural surface of the right upper lobe (RUL) against the mediastinum. The only structures normally at that level to give soft tissue density between the air filled trachea and the RUL are the right wall of the trachea, nerves, some fat, lymph nodes, and pleura of the RUL. The stripe ends where the RUL bronchus sweeps under the azygous vein as the latter arches anteriorly to empty into the posterior surface of the SVC. 

PLAIN TOMOGRAM OF THE RIGHT PARATRACHEAL STRIPE

On this view, the azygous vein is distended giving a tear drop shape to the terminus of the stripe.





CT AT THE LEVEL OF THE MID TRACHEA

The two accompanying CTs demonstrate why the right paratracheal stripe changes in thickness at the azygous arch.





LEFT SUBCLAVIAN STRIPE

The normal width is 1.0-1.5 cm. Its inner margin is the air mucosal interface along the left mucosal surface of the trachea, and its outer margin is the interface of the medial aspect of the left upper lobe against the lateral margin of the left subclavian artery.You usually will pick up the outer edge of the stripe at the level of the clavicle and will be able to follow it down to the bulge of the aortic arch.


LEFT SUBCLAVIAN STRIPE -CT

The accompanying CTs demonstrate why the left subclavian stripe is so wide.





LATERAL VIEW OF TRACHEAL WALL

On the lateral view, the posterior tracheal wall, if visible, should measure no more than 4mm.