Chapter 15 The Mediastinum
The mediastinum is an anatomic region bounded laterally by the two lungs, anteriorly by the sternum, posteriorly by the vertebrae, superiorly by the thoracic inlet, and inferiorly by the diaphragm. Many focal and diffuse abnormalities occur in the mediastinum. Computed tomography (CT) and magnetic resonance imaging (MRI) have improved our ability to detect, define, and characterize these abnormalities. In certain cases, the imaging features of a mediastinal mass allow a specific diagnosis to be made on the basis of imaging findings.
The classic radiologic differential diagnosis of an anterior mediastinal mass (i.e., the four Ts) is one of the best known approaches among medical students and first-year radiology residents. However, before arriving at this seemingly simple differential diagnosis, several important observations and deductions must be made, including the four Ds of mediastinal masses: detection of an abnormality, description of the abnormality, placement of the abnormality into the appropriate anatomic division of the mediastinum, and generation of a limited differential diagnosis based on the descriptive features and anatomic location.
To detect a mediastinal abnormality, the radiologist must be thoroughly familiar with the normal radiographic anatomy and with the characteristic changes produced by abnormalities within various portions of the mediastinum. The radiographic landmarks of normal mediastinal anatomy are the lines, stripes, and interfaces produced when the x-ray beam passes tangential to an edge formed between tissues of different attenuations (Box 15-1).
Box 15-1 Detection: Mediastinal Landmarks
A line is a longitudinal opacity no greater than 2 mm wide. Examples include the anterior and posterior junction lines, which are formed by the close apposition of the visceral and parietal layers of pleura of both lungs as they approximate anteriorly and posteriorly to the mediastinum (Fig. 15-1). The anterior portion of the thorax begins at the thoracic inlet. The anterior junction line begins at the undersurface of the clavicles; it courses inferiorly toward the left in an oblique orientation to the level of the heart. Because the posterior portion of the thorax extends more superiorly than the anterior portion, the posterior junction line can be seen as it extends above the level of the clavicles. It frequently appears as a straight line and often projects through the tracheal air column. The anterior and posterior junction lines are not seen on all radiographic examinations. However, detection of a displaced junction line allows identification of a mediastinal abnormality and localization as anterior or posterior.
Figure 15-1 Mediastinal lines, stripes, and interfaces. A, Frontal chest radiograph shows the anterior junction line (long arrows), right paratracheal stripe (short arrows), and azygoesophageal interface (open arrows). B, Anteroposterior tomogram shows the anterior junction line (arrows). Notice that the anterior junction line begins below the level of the clavicles. C, Coned-down frontal chest radiograph shows the posterior junction line (arrows). Notice that the posterior junction line extends superiorly above the level of the clavicles. D, Axial, high-resolution CT image shows the anterior (open arrows) and posterior (curved arrow) junction lines. E, Anterior coronal CT reformation image shows the anterior junction line (arrows). F, Axial CT image shows the azygoesophageal interface, formed by the juxtaposition of aerated lung and the lateral walls of the azygous vein (long arrow) and esophagus (short arrow).
Two additional mediastinal lines are the right and left paraspinal lines, which are each about 1 mm wide. They are best seen on anteroposterior thoracic spine films. The left paraspinal line extends superiorly from the level of the aortic arch and inferiorly to the level of the diaphragm and parallels the lateral margin of the vertebral bodies. An important relationship is the one between the left paraspinal line and the descending aortic interface (see “Interfaces”). The left paraspinal line normally lies medial to the descending aortic interface. Displacement of the left paraspinal line lateral to the descending aortic interface signals the presence of a posterior mediastinal abnormality (Fig. 15-2). The right paraspinal line is less frequently visualized; when seen, it is often identified only over a portion of its course, usually between the 8th and 12th thoracic vertebral levels. Both paraspinal lines are normally straight and maintain a constant relationship with the adjacent vertebral bodies, except when displaced laterally by osteophytes. An ectatic aorta may displace the left paraspinal line laterally.
Figure 15-2 The anteroposterior radiograph from an aortographic study shows contrast opacification of the aorta and bilateral lateral displacement of the paraspinal lines (two white arrows) in a patient with posterior mediastinal lymphadenopathy from lymphoma. Notice that the left paraspinal line (three white arrows) is displaced lateral to the descending aortic interface (black arrows).
A stripe is a longitudinal composite opacity that is 2 to 5 mm wide. The right paratracheal stripe is formed by the apposition of the right upper lobe pleura and the right lateral tracheal wall. It can be identified on most chest radiographs. The normal right paratracheal stripe is identified as a smooth stripe adjacent to the right lateral border of the tracheal air column, extending inferiorly to the level of the azygous vein (see Fig. 15-1). In normal individuals, it is seen as a smooth stripe of uniform width (≤3 mm). Widening of the right paratracheal stripe is a sign of middle mediastinal pathology, such as right paratracheal lymphadenopathy (Fig. 15-3).
An interface is the common boundary between the shadows of two juxtaposed tissues of different opacities, such as between the lungs and heart. Two interfaces that are important landmarks of normal mediastinal anatomy are the azygoesophageal interface and the descending aortic interface.
The azygoesophageal interface is formed by the juxtaposition of aerated lung within the right lower lobe or the soft tissue opacity of the right lateral margin of the azygous vein or esophagus, or both (see Fig. 15-1