REVIEW. Radiographic Features of Mediastinal Anatomy* RADIOGRAPHIC FEATURES OF MEDIAmNAl ANATOMY 609

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1 REVIEW Radiographic Features of Mediastinal Anatomy* Eric A.Hyson, M.D., andcarl E. Ravin, M.D., F.C.C.P. The increasing use of higher kilovoltage techniques and shorter exposure times in chest radiography has improved visualization of mediastinal structures to the point that many structures previously not imaged are now routinely demonstrated. This permits earlier and more accurate diagnosis of pathologic conditions, but also creates confusion when normal structures are not recognized as such. It is the purpose of this communication to review the radiographic anatomy of the mediastinal and hilar areas as seen on routine posteroanterior (PA) and lateral chest radiographs. Familiarity with these structures will facilitate accurate interpretation of chest radiographs in both normal and pathologic conditions. other similar process in the anterior mediastinum." The position and orientation of the line is changed by herniation of one lung across the midline into the opposite hemithorax. Posterior Junction Line Posterior to the trachea and anterior to the spine, the two lungs again approach one another near the ANATOMY ON THE POSTEROANTERIOR CHEST FILM Anterior Junction Line The anterior junction line, located in the retrosternal area anterior to the ascending aorta and heart, is created as the anteromedial portions of the lungs approach one another in the midline (Fig 1 and 2). The line visualized on the radiograph represents the very thin tissue of the anterior mediastinum plus the adjacent parietal and visceral pleural coverings of the right and left lungs. It is seen in percent of normal adult PA chest films, originating at the level of the sterno-manubrial joint and extending downward and slightly to the left for several centimeters.l'' Although absence of the anterior junction line is not a reliable s i of~ abnormality, c l e ~ t i 0~ I ol afnorffilli line isili<mevidence against any s i W f i e a n. L a n t e rmass. i q u Tee : n ~ d i a s t i n a l be thickened orobliterated by a-prominent ascending aorta, lymphadenopathy, tumor, hemorrhage, or From the Department of Diagnostic Radiology, Yale University School of Medicine, New Haven. Reprint requests: Dr. Ravin, Department of Radiology, Duke University Medical Center, Durham FIGURE 1. PA radiograph demonstrating the normal thin anterior junction line (smau asrtnoheads) beginning approximately at the level of the sterno-manubrial joint and following a normal course downward and slightly to the left. The normal right paratracheal stripe (a"owa) is also clearly seen with the small bulge inferiorly (large arrowheads) representing the arch of the azygous vein seen on end. RADIOGRAPHIC FEATURES OF MEDIAmNAl ANATOMY 609

2 FIGURE 2. Computed tomographic scan of the chest indicating the normal cross-sectional anatomy of the mediastinum at a level just below the carina. The anterior mediastinum is quite thin accounting for the appearance of the anterior junctional line as seen on the PA chest radiograph. The medial invagination of the right lung into the mediastinum to form the azygo-esophageal recess is indicated. The proximity of the air-filled esophagus to the recess can be seen. The posterior wall of the bronchus intennedius is outlined between the intrabronchial air and the aerated right lung in the azygoesophageal recess. midline above the aortic arch. As the mediastinum may be quite thin at this point, a line of soft tissue density frequently is created. It projects vertically through the tracheal air column on the PA chest radiographvv" (Fig 3). The posterior junction line is readily distinguished f;:om the anterior junction line, for although both may be thin verticalbands, the anterior junction line never extends above the r- ~ -. ~. _ - - level of the manubrium, which defines the level of the thoracic Inletanteriorly," "Unlike the 'anterior junction line, which usually is very thin, the posterior junction line has a wide range of normal thicknesses. In a given patient, however, progressively increasing thickness of the line may be secondary to infection, hemorrhage, esophageal dilatation, lymphadenopathy, neoplasm, or other mass. 7.8 Herniation of either lung through the posterior superior portion of the mediastinum may result in unusual bowing or displacement of the line. An interesting variation in the appearance of the posterior junction line is seen when the upper esophagus contains a small amount of air, as it often does because of negative intrathoracic pressure. The "esophageal-pleural stripe" can then be identified as a soft tissue line between the air in the right lung and the air in the esophageal lumenv" (Fig 3). The band of soft tissue consists of the esophageal wall plus adjacent mediastinal tissue and pleura. Although inflammation or neoplasm might theoretically cause thickening of the stripe, in most cases it is important to recognize it simply as a normal variant. 610 HYSON, RAYIN FIGURE 3. "Coned-down" views of the upper portion of two PA chest radiographs indicating the variation in appearance of the junction of the right lung with the mediastinum. The upper radiograph demonstrates the thin posterior junction line formed by the right and left lungs meeting in the midline. Note that the posterior junction line extends above the level of the clavicles and terminates at the level of the aortic arch. In the lower half of the figure, the typical appearance of the esophageal-pleural stripe is seen. The aerated right lung forms the right border of the stripe and a small amount of intra-esophageal air forms the left border. Azygo-esophageal Recess Superiorly, the right lung invaginates toward the midline to join the left lung in forming the posterior junction line. Below this level, the right lung is displaced laterally by the azygous vein as it arches forward across the right side of the mediastinum to join the superior vena cava. Below the level of the azygous arch, the right lung again intrudes quite medially behind the heart and anterior to the spine to form the azygo-esophageal recess 3 5,8 (Fig 4). The lung-mediastinal interface created by the azygoesophageal recess usually follows the esophagus closely, and is usually concave laterally, particularly in its upper portion as it CUrves outward to reach the arch of the azygous vein. Loss of the normal contours of the azygo-esophageal recess can be expected with esophageal dilatation or tumor, hiatal hernia, cardiomegaly (particularly left atrial en-

3 tissue stripe is seen between the aerated right lung and air in the tracheal lumen (Fig 1). This right paratracheal stripe is made up of three structures: the tracheal wall, surrounding mediastinal connective tissue, and pleura. u ta The normal stripe usually has a thickness of 1-4 rom, measured 2 cm above the azygous vein. In some normal patients with abundant mediastinal fat, however, the stripe may exceed 4 mm." Abnormal thickening of the paratracheal stripe may be due to processes involving any of its three soft tissue components: trachea (tumor, amyloid, polychondritis ); mediastinum (tumor, lymphadenopathy, hemorrhage, mediastinitis, intrathoracic goiter) ; or pleura (thickening, effusion). FiGURE 4. PA radiograph demonstrating the interface of the right lung with the mediastinum. The lung invaginates into the mediastinum above (closed arrows) and below (open arrows) the level of the azygous arch (arrowhead). The open arrows indicate the azygo-esophageal recess. largement), large pericardia! effusion, or subcarinal lymphadenopathy.vv'? Aortic-pulmonary Window The "aortic-pulmonary window" is a small space below the aortic arch and above the main pulmonary artery containing the ligamentum arteriosum, the recurrent laryngeal nerve, and the ductus lymph nodes As the mediastinal pleura reflects over the aortic arch and pulmonary artery, it forms the lateral border of the aortic-pulmonary window. The contour of this pleural reflection has proven to be a sensitive indicator of pathology in this area (Fig5). As a rule, the pleura! reflection over the window is concave laterally; a convex bulge, particularly if seen to develop on serial Blms, is most often indicative of lymphadenopathy. Other diagnostic possibilities include neurogenic tumors, aneurysm, hemorrhage, mediastinitis, cysts, lipomatosis, cardiomegaly, pericardial effusion, and left upper lobe volume 10ss.11 Right Paratracheal Stripe In over half of normal adult PA chest Blms,a soft Azygous Arch In over half the normal adult PA chest radiographs, the arch of the azygous vein is seen as a small bulge at the inferior end of the right paratracheal stripe (Fig 1). The azygous vein ascends from the level of the diaphragm just anterior to the spine and slightly to the right of midline. Upon reaching the T4 or T5 level, the vein arches anteriorly, crossing the right lateral aspect of the mediastinum, going over the right mainstem bronchus at its origin from the treachea, and entering the posterior aspect of the superior vena cava. On an upright PA chest BIm, the arch of the azygous vein averages about 5 mm in widest diameter (measuring perpendicular to the wall of the tracheobronchial tree and including the thickness of the wall). The upper limit of normal size is about 7 mm, FIGURE 5. PA radiograph demonstrating carcinoma (arrows) in the anterior segment of the left upper lobe. The small lateral bulge in the aortic-pulmonary window (arrowheads) reflects metastatic involvement of the mediastinal nodes in the ductus area. RADIOGRAPHIC FEATURES OF MEDIASTINAL ANATOMY 811

4 although pregnant women may have upright azygous measurements as high as 15 mm. 14 Azygous vein enlargement occurs with systemic venous congestion or when the vein acts as a collateral pathway for blood flow (superior and inferior vena cava and portal vein obstruction, congenital venous anomalies). Lymphadenopathy may mimic a prominent azygous vein. DiHerentiation can be made by observing that the azygous vein changes in size with changes in position (erect to supine) or in intrathoracic pressure. ANATOMY ON THE LATERAL CHEST FILM Because it provides a right angle view to that obtained with the PA radiograph, the lateral view is extremely helpful in analysis of mediastinal, as well as of pulmonary parenchymal, abnormalities. Understanding the anatomy as it is projected in this view is essential to interpretation. Hilar Structures The major landmark in the hilar region is the tracheal air column, which is always clearly visible on well-exposed lateral radiographs. The column appears to end in a rounded radiolucency which represents the origin of the left upper lobe bronchus and is, from a radiographic standpoint, the center of lateral hilar anatomyv" (Fig 6). The right pulmonary artery, seen on end as it courses across the mediastinum to supply the right lung, is projected as a circular soft tissue density anterior to the left upper lobe bronchus. The left pulmonary artery is seen as a soft tissue density arching over the left upper lobe bronchus. Approximately 1 ern above the left upper lobe bronchus, another rounded lucencythe right upper lobe bronchus-can occasionally be visualized. A thin vertical white line representing the posterior wall of the bronchus intermedius can usually be identified originating from the back wall of the right upper lobe bronchus, and coursing inferiorly.3.6 It is seen as a soft tissue line between the air in the lumen of the bronchus intermedius anteriorly, and the aerated right lung in the azygoesophageal recess posteriorly (Fig 2). Thickening of the posterior wall of the bronchus intermedius is analogous to thickening of the right paratracheal stripe, and occurs most commonly with lymphadenopathy. The area beneath the left upper lobe bronchus should be relatively clear. Enlargement of nodes in the hilar and subcarinal areas, however, increases the density in this area. As a result, the lateral view can be particularly helpful in analyzing whether a "prominent hilum" is due to enlargement of pulmonary arteries or to lymphadenopathy. Dilatation of 812 HYSON,RAVIN FIGURE 6. Lateral chest radiograph demonstrating normal hilar anatomy. The left upper lobe bronchus (black arrowheads) is projected as a central lucency abutted by the soft tissue density of the right pulmonary artery anteriorly and the left pulmonary artery superiorly (white arrows). The rounded lucency of the right upper lobe bronchus is especially well seen in this case (black arrows). The posterior wall of the bronchus intermedius (small white arrowheads) is represented by the thin white line extending downward from the posterior aspect of the right upper lobe bronchus. Note that the area beneath the left upper lobe bronchus is relatively free of soft tissue density. This area fills in when hilar adenopathy occurs. The posterior tracheal stripe (large white arrowheads) has a normal thickness of 2-3 mm. central pulmonary arteries will leave the area directly below the left upper lobe bronchus relatively clear while adenopathy, if generalized, will tend to fill this area. Posterior Tracheal Stripe In most good lateral chest radiographs, a thin band of soft tissue density can be seen forming the posterior margin of the tracheal air shadow (Fig 6). The soft tissue is composed of posterior tracheal wall, adjacent mediastinal connective tissue, plus pleural covering, interposed between air in the tracheal lumen and air in the right lung invaginating into the mediastinum behind the trachea Occasionally, the air posterior to the stripe is intraesophageal, in which case the esophageal wall contributes to the thickness of the stripe." The stripe may not be seen in some normal subjects because the right lung does not always invaginate far enough behind the trachea to create a tangential interlace. Normal thickness of the posterior trachea stripe is

5 apposition, but they are seen only because of adjacent radiolucent fat. The line separates fat overlying the heart (accompanying the left anterior descending coronary artery) and fat outside the pericardial sac in the retrostemal area. The normal pericardialline may be up to 2 mm in width. Pericardial thickening or effusion can be detected on the lateral chest mm if widening of this line is noted. H the line is not visualized, an effusion cannot be ruled out, but a normal, thin pericardial line effectively excludes significant pericardial effusion. FIGURE 7. Lateral chest radiograph illustrating a prominent but normal cardiac incisura of the left lung (large arrowheads). In addition, the normal pericardium (small arrowheads) can be seen outlined between the fat overlying the left anterior descending coronary artery and the fat outside the pericardial sac in the mediastinum. 2-3 mm, rarely as much as 4 mm. IB 17 Thickening of the stripe has been described as a sign of midesophageal tumor, but may also be seen with esophageal dilatation, intratracheal mass, tracheitis, lymphadenopathy, mediastinitis, hemorrhage, vascular anomaly (eg, aberrant right subclavian artery), posterior intrathoracic goiter, or pleural thickening. Cardiac Incisura Frequently, a triangular soft tissue density simulating a mass is seen in the anterior cardiophrenic angle on the lateral chest radiograph. The posterior margin of the density represents the normal cardiac incisura of the left lung3,18 (Fig 7). Because the cardiac apex projects into the left hemithorax, the anterior inferior margin of the left lung is often displaced away from the anterior-most portion of the chest wall. Interposed cardiac and mediastinal tissue anterior to the left lung may then simulate a mass. The characteristic triangular configuration and absence of a mass on the PA view allow one to distinguish this normal variantfrom a true mass. Pericardial Stripe A thin vertical white line representing the pericardium can be identified anterior to the cardiac silhouette in as many as 40 percent of normal lateral chest radiographs3,19.20 (Fig 7). The line is formed by the epicardial and parietal pericardial layers in 1 Cimmino CV: The anterior mediastinal line on chest roentgenograms. Radiology 82: , Berne AS, Gerle RD, Mitchell GE: The mediastinum: Normal roentgen anatomy and radiologic technics. Semin Roentgen 4:3-21, Heitzman ER: The Mediastinum. St. Louis, C.V. Mosby, Figley MM: Mediastinal minutiae. Semin Roentgen 4:22 32, Proto AV. Lane EJ : Air in the esophagus: A frequent radiographic finding. Am J RoentgenoI129: , Heitzman ER, Scrivani JV, Martino J, et al: The azygous vein and its pleural reflections. I. Normal anatomy. Radiology 101: , Cimmino CV, Snead LO: The posterior mediastinal line on chest roentgenograms. Radiology 84: , Lane EJ, Heitzman ER, Dinn WM : The radiology of the superior intercostal veins. Radiology 120: DeGinder WL : Pleuro-esophageal line in normal chest roentgenograms. JAMA 167: , Heitzman ER, Scrivani JV, Martino J, et al: The azygous vein and its pleural reflections. II. Applications in the radiological diagnosis of mediastinal abnormality. Radiology 101: , Schwartz, MI, Marmorstein BL: A radiographic sign of left-sided mediastinal lymph node enlargement. Chest 68: , Heitzman ER, Lane EJ, Hammack DB, et al: Radiological evaluation of the aortic pulmonary window. Radiology 116: , Savoca CJ, Austin JHM, Goldberg HI : The right paratracheal stripe. Radiology 122: , Keats TE, Lipscomb GE, Betts, CS: Mensuration of the arch of the azygous vein and its application to the study of cardiopulmonary disease. Radiology 90: , Vix VA, Klatte EC: The lateral chest radiograph in the diagnosis of hilar and mediastinal masses. Radiology 96: , Putman CE, Curtis AM, Westfriend M, et al : Thickening of the posterior tracheal stripe : A sign of squamous cell carcinoma of the esophagus. Radiology 121: , Bachman AL, Teixidor HS: The posterior tracheal band: A reflector of local superior mediastinal abnormality. Br J RadioI48: , Whalen JP, Meyers, MA, Oliphant M, et al: The retrostemalline. Am J Roentgenol 117: , Lane EJ Jr, Carsky EW: Epicardial fat: Lateral plain film analysis in normals and in pericardial effusion. Radiology 91:1-5, Baron MG: Pericardial effusion. Circ 44: , 1971 RADIOGRAPHIC FEATURES OF MEDIASTINAL ANATOMY 813