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Reflecting Upon The Left Superior Mediastinum

McComb, Barbara L. MD

Cardiac Imaging, Part II: Original Articles
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Summary Pathology developing within the confines of the superior mediastinum mayproduce changes that appear quite subtle on conventional radiographs. The zone of contact of lung with the left superior mediastinum is represented radiographically as a series of interrelated pleural reflections. These reflections may be altered by mediastinal disease, and an understanding of this anatomy can facilitate detection of abnormalities.

From the Department of Radiology, Mayo Clinic, Jacksonville, FL.

Address correspondence and reprint requests to Dr. Barbara L. McComb, Department of Radiology, Mayo Clinic, 4500 San Pablo Rd, Jacksonville, FL, 32224.

Important interfaces are depicted on radiographs where mediastinal pleura and contiguous pleura of the left lung approximate the surfaces of protruding structures of the left superior mediastinum. Three pleural reflections are related anatomically and project in close proximity on the posteroanterior (PA) radiograph, yet represent different coronal planes in the thorax. The appearance of each reflection is influenced by anatomic variation, radiographic projection, and adjacent pathology. A thorough understanding of the anatomy that produces the aortic-pulmonary, aortic-pulmonic window, and preaortic recess reflections can facilitate a three-dimensional appreciation of normal and pathologic contours represented on the two-dimensional chest radiograph.

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AORTIC-PULMONARY REFLECTION

Blank and Castellino discussed the variable appearance of reflection “A” in their article on pleural reflections of the left superior mediastinum (1). This discussion encompassed a particular interface reported shortly thereafter by Keats, who ascribed the term “aortic-pulmonary stripe” to a shadow that he deduced represented both mediastinal pleura and visceral pleura of the left lung reflecting along the main pulmonary artery and aorta. (2). The length of this reflection and its continuation over these arteries (Fig. 1) distinguish it on the PA radiograph from the shorter and more posterior aortic-pulmonic window reflection, with which it may be confused. Computed tomography (CT) has shown that the interface is visible largely because of the presence of mediastinal fat deposited anterolateral to the left pulmonary artery and anterior and to the left of the transverse portion of the aortic arch. The region of contact of lung with mediastinum must be both sufficiently long from anterior to posterior and tangential to the x-ray beam in order for the reflection to be appreciated on the PA radiograph (3) (Fig 2A, Fig 3). It is typically represented as an edge demarcating the transition between more opaque mediastinum medially and contiguous less opaque lung laterally. On occasion, however, it is identified as a line or slightly thicker stripe, rather than an edge, with both medial and lateral sides outlined.

FIG. 1.

FIG. 1.

FIG. 2.

FIG. 2.

Figure 2

Figure 2

FIG. 3.

FIG. 3.

Figure 3

Figure 3

The normal aortic-pulmonary reflection generally appears either straight (Fig. 1) or projects slightly convex laterally (Fig. 4) along the main pulmonary artery on the PA radiograph. It variably continues cephalad, and then typically appears either straight or slightly concave when it projects over the aortic arch, although occasionally a convex lateral configuration (Fig. 4) is produced by mediastinal fat at the level of the aorta. The appearance of the normal reflection may also vary with positional and inspiratory differences on radiographs performed on the same patient. The ability to detect early pathologic deviation of the reflection in the face of such considerable variability in curvature and length may be enhanced by comparing sequential radiographs (1,2). When comparison studies are not available, a convex configuration over the aorta or projection of the reflection lateral to the aortic arch should be viewed with suspicion (3,4), and may necessitate cross-sectional imaging.

FIG. 4.

FIG. 4.

The aortic-pulmonary reflection is normally inconspicuous on the lateral chest radiograph, where it is largely imaged en face (Fig. 3, Fig. 5). A poorly marginated region of increased opacity may become visible as pathology develops and enlarges. A discrete margin will be identified once a border is oriented in tangent to the x-ray beam (Fig. 6A-C).

FIG. 5.

FIG. 5.

FIG. 6.

FIG. 6.

Figure 6

Figure 6

Figure 6

Figure 6

Figure 6

Figure 6

The coronal plane represented by the aortic-pulmonary reflection is anterior to that of the aortic-pulmonic window reflection (Fig. 5), and the ability to distinguish between pathologic involvement at the two different planes can have diagnostic significance during the interpretation of conventional radiographs. For instance, certain prevascular masses, such as those arising from the thymus or thyroid gland, might be considered in the differential diagnosis when the aortic-pulmonary reflection is displaced, yet the aortic-pulmonic window reflection is unaffected. The anatomy is interrelated, however. Continuity may not seem obvious on conventional radiographs, but can be readily appreciated by tracing the mediastinal-lung interfaces on cross-sectional images. (Figs. 6C-D, Fig 7). It should not come as a surprise that the reflections might be deviated in similar situations. Enlarged paraaortic (5,6) nodes displacing the aortic-pulmonary reflection commonly accompany subaortic adenopathy displacing the aortic-pulmonic window reflection. A persistent left superior vena cava may alter either reflection (3,7). Pathology may extend from one region into the other.

FIG. 7.

FIG. 7.

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AORTIC-PULMONIC WINDOW REFLECTION

The origin of the term aortic-pulmonic window is not entirely clear, although Parkinson and Bedford (8) reported that a similar term, “aortic window”, was commonly used in Vienna, Austria by the year l936. These authors considered the window the mediastinal space encompassed by the aortic arch above and the left atrium below, divided by the left pulmonary artery. Heitzman preferred to consider the left pulmonary artery the inferior boundary since the view into the mediastinum provided by the aortic-pulmonic window lies above this level (3,9). The left pulmonary artery was also cited as the inferior boundary in the definition of aortic-pulmonary window recommended by the Nomenclature Committee of the Fleischner Society (10).

The aortic-pulmonic (aortopulmonary, aorticopulmonary, “AP”) window (Fig 2B, Fig 5) is bordered on its medial side by the left lateral wall of the trachea and the left main bronchus anteriorly, and by the esophagus posteriorly (3,9). It is continuous medially with the inferior pretracheal space (11). The posterior surface of the ascending aorta forms the anterior boundary. The lateral border is appreciated on the PA radiograph as the aortic-pulmonic window reflection (Fig. 1), and is formed by pleura that subtends from the aortic arch inferiorly onto the left pulmonary artery. Anterolaterally, reflecting pleura courses over the main pulmonary artery and is continuous with pleura that produces the more anterior aortic-pulmonary reflection. The window also communicates alongside the aorta with the prevascular space anteriorly. Closure is accomplished posteriorly as the left pulmonary artery and descending aorta approximate one another. When left lung protrudes in front of the proximal descending aorta, the upper portion of the preaortic recess reflection closes the posterolateral aspect of the window (Fig. 5, Fig. 7).

The aortic-pulmonic window is filled with fat and areolar tissue, and is traversed by the ligamentum arteriosum and left bronchial arteries. Subaortic (aortopulmonary) nodes (5,6) can be found lateral to the ligamentum arteriosum, while left paratracheal lymph nodes may extend into the region medially. The vagus nerve runs alongside the aorta at the posterior aspect of the window. The left recurrent laryngeal nerve arises from the vagus nerve, courses behind the ligamentum arteriosum, and then ascends beside the trachea in front of the esophagus. The phrenic nerve passes just anterior to the window. This anatomy (Fig. 5) clarifies why masses in the aortic-pulmonic window may produce left vocal cord or left diaphragmatic paralysis.

The aortic-pulmonic window may be recognized on PA, lateral, and oblique radiographs as a relatively lucent region between the aortic arch and left pulmonary artery (Fig. 8, Fig. 9A). Its visibility and relationship to several significant anatomic structures solidify its position of importance during interpretation of the conventional chest radiograph. The aortic-pulmonic window reflection (Fig. 1) is identified frequently (4,12) and denotes the midportion of the window from anterior to posterior, in addition to defining its lateral border. The anterior extent of the window is uncommonly appreciated on the PA radiograph as a finger-like projection directed inferomedially (3,9). The posterior extent of the window may be inferred when the preaortic recess reflection is identified. An understanding of this anatomy adds a three-dimensional perspective to radiographic analysis of the window.

FIG. 8.

FIG. 8.

FIG. 9.

FIG. 9.

Figure 9

Figure 9

The aortic-pulmonic window reflection has also been termed “reflection B”(1). This interface is typically directed slightly medially on the PA radiograph (Fig. 1), although there is a range of normal configurations. A straight margin may be considered normal unless the reflection appeared concave on previous radiographs, and therefore has become more convex. Pleura and accompanying lung may insert quite deeply into the window in the presence of a tortuous elongated aorta, emphysema, or an enlarged left pulmonary artery. This very concave configuration may also be appreciated in the rarer setting of congenital absence of the left pericardium. In this condition, parietal pleura is lacking and the window is essentially open along the left side, permitting lung to insert between the aortic arch and the left pulmonary artery. Lateral convexity identified on an initial PA radiograph should always raise suspicion for underlying pathology within the aortic-pulmonic window (Fig. 10), although it may be produced by variations of normal anatomy. The presence of normal mediastinal fat (Fig. 7), or occasionally a venous anomaly (7), is readily confirmed by cross-sectional imaging. Documentation of change in contour of the reflection from a concave to a more convex appearance upon comparison of sequential radiographs generally indicates interval development of pathologically enlarged lymph nodes.

FIG. 10.

FIG. 10.

Figure 10

Figure 10

Both the location and size of lesions within the aortic-pulmonic window have been reported to influence their visibility on the PA radiograph. Larger masses are more easily identified, as expected. Earlier detection is facilitated when pathology is situated laterally in the window and the reflection projects beyond the aortic arch (13), although the visibility of a lesion is reduced when it becomes associated with left upper lobe volume loss (14). Inferior displacement of the left pulmonary artery may be appreciated as an enlarging mass becomes restricted above by the aortic arch. A mass or adenopathy developing within the medial aspect of the window may eventually begin to displace the esophagus and trachea away from the aortic arch. A posterior lesion may deviate the preaortic recess reflection.

Like the aortic-pulmonary reflection, the aortic-pulmonic window reflection is essentially viewed en face on the lateral radiograph (Fig. 6D). Pathology may be perceived more quickly on the PA view because reflecting pleura is imaged tangentially to the x-ray beam, and subaortic nodes lie in close proximity to the reflection (15). An abnormal opacity will be identified on the lateral radiograph as a lesion grows and protrudes deeper into the left hemithorax, and an edge will become well-defined once it is presented in tangent to the x-ray beam. Early detection of pathology is facilitated by correlation with previous radiographs, along with careful evaluation for change in appearance of the inferior margin of the aortic arch and the superior margin of the left pulmonary artery bordering the window. A margin may be clearly defined (Fig. 9A) while lung inserts into the window, and then later become indistinct (Fig. 9B) as a mass or adenopathy prohibits contact with adjacent lung.

Once the question of an abnormality in the region of the aortic-pulmonic window is raised on a conventional radiograph, computed tomography should generally be the next study performed to help characterize it, and to define its extent and relationship to other mediastinal structures. CT should also be used to evaluate the aortic-pulmonic window when the conventional radiograph appears normal but there is high clinical suspicion for pathology. The height of the window may appear reduced on CT images performed with the patient supine (16). Care should be taken to differentiate nodes from volume averaging of adjacent vessels. Magnetic resonance (MRI) imaging is also of value in certain situations, such as when CT does not clarify findings, iodinated contrast is necessary but contraindicated, or the multiplanar capability of MRI is needed.

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PREAORTIC RECESS REFLECTION

The preaortic recess is present in individuals whose left lung is able to extend anterior to the descending aorta. It can be considered analogous to the extension of the right lower lobe in front of the azygous vein, which occurs more commonly and is identified as the azygoesophageal recess. The preaortic recess is more prominent in the presence of emphysema, kyphosis, or a tortuous aorta. The interface formed once left lung and visceral pleura extend medially and contact the mediastinum may be identified on the PA radiograph as the preaortic recess reflection.

The normal preaortic recess reflection typically parallels the descending thoracic aorta, projecting slightly to the right of its left lateral margin. It disappears inferiorly as the aorta courses toward the midline to enter the abdomen. The relevance of this reflection to a discussion of the left superior mediastinum relates specifically to its superior continuation (Fig. 5). It must terminate beneath the left pulmonary artery when posterior closure of the aortic-pulmonic window is accomplished by approximation of the pulmonary artery and the descending aorta. However, when able to continue superiorly toward the aortic arch, it then becomes responsible for closure of the posterolateral aspect of the window (Fig. 7). Subsequently, it either deviates laterally to merge with pleura along the aorta or it terminates directly beneath the arch. If it abuts the esophagus medially, it becomes synonymous with the left pleuroesophageal stripe (Fig. 11).

FIG. 11.

FIG. 11.

Understanding the preaortic recess reflection enhances appreciation of the interrelated anatomy of the left superior mediastinum. The aortic-pulmonic window not only relates anterolaterally to the paraaortic region and aortic-pulmonary reflection, but also (potentially) posterolaterally to the left lower lobe and preaortic recess reflection. A mass displacing the preaortic recess reflection on a conventional radiograph may subsequently be shown in cross-section to arise directly from the aortic-pulmonic window. The lateral margin of a left lower lobe mass tucked within the preaortic recess might also simulate a displaced reflection. Indeed, radiographic analysis is enriched through developing a three-dimensional perspective of the relationships of normal mediastinal structures, along with an understanding of the nuances of anatomy and the often subtle changes produced by disease. Our ability to extract information from conventional radiographs can be refined by taking advantage of opportunities that promote correlation with cross-sectional images.

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ACKNOWLEDGMENTS:

The author thanks Drs. Jon Dalton, Marvin Chasen, Mark Kransdorf, Thomas Berquist, and Stephen Carmichael for their assistance, Laura Newman for secretarial support, and Irma Lyndsey-Parker and Daniel Hubert for support in graphics and photography.

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REFERENCES

1. Blank N, Castellino RA. Patterns of pleural reflections of the left superior mediastinum– normal anatomy and distortions produced by adenopathy. Radiology 1972; 102:585–89.
2. Keats, TE. The aortic-pulmonary mediastinal stripe. Am J Roentgenol 1972; 116:107–9.
3. Heitzman ER. The Mediastinum–Radiologic Correlations with Anatomy and Pathology. Heidelberg: Springer-Verlag, l988.
4. Proto, AV. Mediastinal anatomy: emphasis on conventional images with anatomic and computed tomographic correlations. J Thor Imaging 1987; 2 (1):1–48.
5. Mountain CF, Dressler CM. Regional lymph node classification for lung cancer staging. Chest 1977; 111:1718–23.
6. Mountain CF, Lipshitz HI, Hermes KE. Lung cancer: a handbook for staging, imaging, and lymph node classification—Revised international system for staging and regional lymph node classification. Bristol-Myers Squibb, l999.
7. Shin MS, Jolles PR. Computed tomography of the aortopulmonary window–normal anatomy and pathology. J Computed Tomography 1987; 11:111–22.
8. Parkinson J, Bedford E. The aortic triangle–a radiological landmark in the left (or II) oblique position. Lancet 1936; 2:909–11.
9. Heitzman ER, Lane EJ, Hammack DB, et al. Radiological evaluation of the aortic-pulmonic window. Radiology 1975; 116:513–18.
10. Tuddenham, WJ. Glossary of terms for thoracic radiology–recommendations of the nomenclature committee of the Fleischner Society. AJR 1984; 143:509–17.
11. Naidich DP, Webb WR, Muller NL, et al. Computed Tomography and Magnetic Resonance of the Thorax. Philadelphia: Lippincott Williams and Wilkins, l999.
12. Woodring JH, Daniel TL. Mediastinal analysis emphasizing plain radiographs and computed tomograms. Medical Radiography and Photography 1986; 1:62.
13. Jolles PR, Shin MS, Jones WP. Aortopulmonary window lesions–detection with chest radiography. Radiology 1986; 159:647–51.
14. Muller NL, Nichols DM. Accuracy of the plain radiograph in the detection of aortopulmonary lymphadenopathy. J Can Assoc Radiol l987;38:82–6
15. Blank N, Castellino RA. Mediastinal lymphadenopathy. Semin Roentgenol l977;12(3):215–23.
16. Moss AA, Gamsu G, Genant HK. Computed Tomography of the Body—with Magnetic Resonance Imaging. Philadelphia: WB Saunders, 1992.

Section Description

Editor: Jeffrey S. Klein

Associate Editors: Ann Leung, MD

David Lynch, MD, Jung-Gi Im, MD

Michio Kono, MD, Charles White, MD

Guest Editor: William Stanford, M.D.

Keywords:

Mediastinum; Pleural reflections; Mediastinal lines; Anatomy; Radiography

© 2001 Lippincott Williams & Wilkins, Inc.