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.