1.
Anterior Junction Line
Anatomy:
Anterior junction line results from the anterior apposition of the lungs,
which lies behind the upper two-thirds of the sternum.
The line consists of four layers of pleura and a variable amount of intervening fat.
Normal radiographic appearance:
Anterior junction line appears as a thin oblique line projecting over the superior two-thirds of the sternum and runs from the upper right to the lower left.
It does not extend above the level of clavicles.
(Fig. 2).
Abnormal radiographic appearance:
It may be absent when its course is not tangential to the X-ray beam.
It can be obscured by other structures such as the heart,
great vessels or the thoracic spine.
Obliteration or abnormal convexity suggests anterior mediastinal disease (Fig. 3),
although it is usually the preservation of more posterior lines at radiography,
that helps identify the location of an anterior mediastinal mass.
Displacement of the line may be due to hyperinflation or volume loss of the surrounding lung (Fig. 4).
2.
Posterior Junction Line
Anatomy:
Posterior junction line results from the apposition of the lungs posterior to the esophagus and anterior to the 3rd to 5th thoracic vertebrae.
Similarly to anterior junction line,
it is formed by four layers of pleura.
Normal radiographic appearance:
Posterior junction line appears as a thin straight line projecting through the trachea.
Unlike anterior junction line,
it can be seen above the clavicles (Fig. 5).
Abnormal radiographic appearance:
Abnormal bulging,
convexity or obliteration of this line suggests a posterior mediastinal abnormality.
Further clues to the location of a mass can be inferred from the lateral margins of the mass above the clavicles (Fig. 6).
3.
Right Paratracheal Stripe
Anatomy:
The right paratracheal stripe is formed by the tracheal wall,
mediastinal connective tissue and paratracheal pleura.
Air within the trachea and the aerated right upper lobe outline the intervening soft tissues.
Normal radiographic appearance:
The right paratracheal stripe begins at the level of the clavicles and extends inferiorly,
projecting through the superior vena cava,
to the right tracheobronchial angle at the level of azygos arch (Fig. 7).
This stripe has a maximum normal thickness of 4mm and it should be uniform in width.
Abnormal radiographic appearance:
An abnormal contour,
widening or obliteration of this stripe may be due to abnormality of any of its components,
from the tracheal mucosa to the pleural space,
such as tracheal carcinoma,
paratracheal masses (most commonly lymphadenopathy) and pleural effusion or thickening (Fig. 8).
4.
Left Paratracheal Stripe
Anatomy:
Similarly to the right paratracheal stripe,
the left paratracheal stripe is formed by tracheal wall,
variable amount of mediastinal fat and paratracheal pleura.
Air within the trachea and the aerated left upper lobe outline the intervening soft tissues.
Normal radiographic appearance:
The left paratracheal stripe extends superiorly from the aortic arch to join with the reflection from the left subclavian artery.
It may be obscured by contact between the left upper lobe and either the proximal left common carotid artery anteriorly or the left subclavian artery posteriorly (i.e.
by the pararterial line) (Fig. 7).
Abnormal radiographic appearance:
An abnormal contour or widening may be caused by pleural effusions or thickening,
paratracheal lymphadenopathy,
hematomas or neoplasm (Fig. 8.).
5.
Aortic-Pulmonary Window Reflection
Anatomy:
AP window represents a mediastinal region bounded anteriorly by the ascending aorta,
posteriorly by the descending aorta,
superiorly by the aortic arch,
and inferiorly by the left pulmonary artery.
The medial border is formed by the ligamentum arteriosum,
whereas the lateral aspect forms the interface between the left lung and the mediastinum known as the aortic-pulmonary window reflection.
Normal radiographic appearance:
On a frontal radiograph,
the AP window reflection extends from the aortic knob to the left pulmonary artery (Fig. 9).
This should have a concave or straight border,
however,
a straight contour should be considered abnormal if previous studies demonstrated a concave border.
A convex shape of the AP window reflection is considered abnormal.
Abnormal radiographic appearance:
The abnormal convexity of the AP window reflection may be due to middle-mediastinum abnormalities such as lymphadenopathy (Fig. 10),
bronchial artery aneurysms,
nerve sheath tumors,
bronchopulmonary-foregut malformations or prominent mediastinal fat.
6.
Aortic-Pulmonary Stripe
Anatomy:
The aortic pulmonary stripe represents the interface between the left lung and the mediastinum along the main pulmonary artery toward the aortic arch.
This edge runs anteriorly to the AP window reflection.
Normal radiographic appearance:
The aortic pulmonary stripe extends from the aortic arch to the level of the left main bronchus,
where it usually continues as the border of the left side of the heart.
It delineates the anterior margin of the AP window and is normally straight or slightly convex (Fig. 11).
Abnormal radiographic appearance:
The normal appearance of this interface may be altered by anterior mediastinal disease such as thyroid masses,
thymic tumors and lymphadenopathy (Fig. 12).
7.
Azygoesophageal Recess
Anatomy:
The azygoesophageal recess represents a mediastinal recess into which the edge of the right lower lobe extends.
This space lies anteriorly to the spine,
it is limited superiorly by the azygos arch and it extends to the level of the aortic hiatus inferiorly.
It is bordered anteriorly by the left atrium and medially by the esophagus and the azygos vein (Fig. 13).
The interface caused by the right lower lobe outlining the medial limit of the azygoesophageal recess can be appreciated on frontal chest radiographs.
Normal radiographic appearance:
The azygoesophageal recess is seen on a frontal radiograph as a vertically oriented interface.
• In its upper third it may be straight or show mild leftward convexity.
Right superior convexity may be seen in children and younger adults.
• The middle third of the recess may be the most variable in the appearance but typically is straight edged or demonstrates mild leftward convexity.
• In its lower third it usually appears as a straight edge.
Abnormal radiographic appearance:
Abnormal contour,
convexity or disappearance of this interface suggests disease affecting the middle and posterior mediastinal compartments such as lymphadenopathy,
bronchopulmonary-foregut malformations,
pleural abnormalities,
left atrial enlargement,
esophageal disease and hiatal hernia (Fig. 14,
Fig. 15).
8.
Paraspinal Lines
Anatomy:
The paraspinal lines are the interfaces between the lungs and the paraspinous fat and soft tissues.
Normal radiographic appearance:
• The right paraspinal line appears straight and runs from the 8th through the 12th thoracic vertebral levels.
• The left paraspinal line runs vertically from the aortic arch to the diaphragm and lies medial to the paraortic line,
although sometimes it can lie lateral to the paraortic line (Fig. 16).
Abnormal radiographic appearance:
The paraspinal lines may be displaced laterally by prominent mediastinal fat or osteophytes.
Abnormal contour or displacement may also suggest posterior mediastinal disease such as mediastinal hematoma,
mass or extramedullary hematopoiesis (Fig. 17).
9.
Paraortic line
Anatomy:
The paraortic line represents the interface formed by the contact of the lateral wall of descending thoracic aorta with the left lower lobe.
Normal radiographic appearance:
The paraortic line appears as a straight vertical interface,
which lies posterior to the cardiac shadow and lateral to the left paraspinal line.
In its lower third it comes nearer to the vertebral bodies at the level of the 11th thoracic vertebra,
overlapping the left paraspinal line (Fig. 18).
Abnormal radiographic appearance:
An abnormal contour may be the result of different conditions affecting the lung and pleura or the middle and posterior mediastinum,
such as lymphadenopathy,
aortal aneurysm,
gastroesophageal varices and lung neoplasms (Fig. 17).