The Pulmonary Hila





CT is helpful in the diagnosis of endobronchial lesions, hilar and parahilar masses, and hilar vascular lesions.


Technique


In most patients the hila are adequately assessed with spiral CT with a 5-mm slice thickness (it takes about 15 contiguous 5-mm slices to image the hila), but thinner slices are optimal in identifying some findings such as bronchial abnormalities, small lymph nodes, and hilar vessels. Scans with a 1.25-mm thickness are routinely obtained for most chest CT studies and are used in this chapter to illustrate normal anatomy. Contrast medium infusion is optimal for imaging the hila.


Scans are usually viewed with a mean window level of –600 to –700 Hounsfield units (HU) and a window width of 1000 or 1500 HU (lung window) for accurate assessment of hilar contours and bronchial anatomy. Scans are also viewed at a mean window level of 0 to 50 HU and a window width of 400 to 500 HU (soft-tissue or mediastinal window) to obtain information about hilar vessels, lymph nodes, and masses. Both views are necessary.




Diagnosis of Hilar Mass and Lymphadenopathy


A detailed understanding of cross-sectional hilar anatomy is needed to detect and accurately localize hilar abnormalities on CT. Contrast enhancement simplifies the identification of hilar masses and lymph node enlargement.


Lobar and segmental bronchi ( Fig. 5.1 ) are consistently seen on CT and reliably localize successive hilar levels; their identification is key to interpretation of hilar CT. In general, hilar anatomy and contours, at the same bronchial levels, are relatively consistent from one patient to another. Bronchial anatomy and branching is less variable than the branching patterns of arteries or veins.




FIG. 5.1


Normal bronchial tree.

All the bronchi shown are visible on CT in most patients. Those bronchi that appear horizontal (such as the right upper lobe) or nearly vertical are usually seen better than those that have an oblique course relative to the scan plane (such as the right middle lobe or lingular bronchi). LL , Lower lobe; seg , segment.


In some locations, normal hilar silhouettes, visible with a lung window, are consistent enough that a diagnosis of hilar adenopathy or mass can be suggested on the basis of hilar contour alone. In other locations, hilar contours vary according to the size and position of the pulmonary arteries and veins, and contrast opacification of pulmonary vessels is essential for accurate diagnosis.


A hilar mass or lymph node enlargement may be suggested by a local or generalized alteration in hilar contour; a visible mass or lymph node enlargement; bronchial narrowing, obstruction, or displacement; or thickening or obliteration of the walls of bronchi that normally contact the lungs.


As a general rule, any nonvascular (unenhancing) hilar structure larger than 5 to 10 mm (short-axis) should be regarded with suspicion and may represent an enlarged lymph node. However, normal amounts of soft tissue larger than this and representing fat and normal nodes are visible in some hilar regions. Mild lymph node enlargement is commonly present in patients with inflammatory lung disease (e.g., pneumonia), and such lymph node enlargement should not be of great concern. In patients with lung cancer, a lymph node larger than 1 cm should be considered enlarged.




Normal and Abnormal Hilar Anatomy


There are two ways to read hilar CT. The first way is to look at each hilum separately, identifying each important structure, and the second is to compare one side with the other at successive scan levels, looking for points of similarity and difference. It is a good idea to do both.


I suggest that as you read the next section, you first learn about right hilar anatomy, skipping what is written about the left hilum. When you finish, and are somewhat oriented, you should start over, reading about both hila, comparing their anatomy, noting what is symmetric and what is not, and learning how the left hilum differs from the right. Also, you should learn to trace each lobar bronchus from its origin to its segmental branches, because this should be done during interpretation of CT.


Although the hila are not symmetric, they have a number of similarities, and identifying these can be of value. These similarities are emphasized in the following descriptions. To reinforce the normal appearances and their significance, and expected alterations in anatomy occurring because of mass or node enlargement, abnormal findings are discussed for each hilar level described.


Some variation exists among patients in the relative levels of the right and left hila; therefore there is some variation in the levels at which specific right and left hilar structures are visible on CT. The right-to-left relations illustrated in Fig. 5.1 and described in the following text may not be present in individual cases, although side-to-side variation will usually be minor (1 or 2 cm).


Because recognizing lobar and segmental bronchial anatomy is fundamental to interpreting hilar CT, it is reviewed briefly in Table 5.1 . Each of the segments listed is commonly, but not invariably, visible.



TABLE 5.1

Lobar and Segmental Bronchial Anatomy











































Right Lung Left Lung
Upper Lobe Middle Lobe Lower Lobe Upper Lobe Lower Lobe
Apical Medial Superior Apical-posterior Superior
Posterior Lateral Anterior Anterior Anteromedial
Anterior Medial Superior lingula Lateral
Lateral Inferior lingula Posterior
Posterior


Five levels are reviewed, each localized by the bronchi that are usually visible. These levels are:




  • upper hila and the right apical and left apical-posterior segments



  • right upper lobe bronchus and left upper lobe segments



  • right bronchus intermedius and left upper lobe bronchus



  • right middle lobe and left lower lobe bronchi



  • lower lobe bronchi and basal segmental branches



Upper Hila


Right Hilum


CT at the level of the distal trachea or carina shows the apical segmental bronchus of the right upper lobe in cross section, surrounded by several vessels of similar size ( Fig. 5.2A–B ). On either side a mass or lymphadenopathy is easily recognized. Anything larger than the expected pulmonary vessels is abnormal ( Figs. 5.3 and 5.4 ). Comparison with the opposite side at this level is helpful.




FIG. 5.2


Upper hilar level: normal anatomy.

(A) Approximate scan level shown in (B). (B) CT with lung window settings at a level slightly above the carina shows the apical segmental bronchus of the right upper lobe in cross section, with several adjacent vessels of similar size. On the left the apical-posterior segmental bronchus (Ap-post) of the upper lobe of the left lung and associated arteries and veins have a similar appearance. LL , Lower lobe; seg , segment.



FIG. 5.3


Abnormal upper hila in two patients.

(A) In a patient with sarcoidosis and bilateral hilar adenopathy, a contrast-enhanced scan through the upper hila shows lymph node enlargement (arrows) . On the right a large node is visible anterior to the apical segmental bronchus (Ap seg) of the right upper lobe lung. On the left an enlarged lymph node is visible lateral to pulmonary vessels. (B) In a patient with a carcinoma of the right upper lobe, lymph node enlargement (arrow) is visible anterior to the apical segmental bronchus of the right upper lobe.



FIG. 5.4


Abnormal upper right hilum in bronchogenic carcinoma.

(A) A large mass (large arrow) encompasses the region of the apical segmental bronchus of the right upper lobe. A thin linear opacity (small arrows) along the right mediastinum reflects collapse of the right upper lobe. (B) Below the level shown in (A) the mass results in obstruction of the right upper lobe bronchus. The mass (arrow) is also visible posterior to the right main bronchus.


Left Hilum


The apical-posterior segmental bronchus and associated arteries and veins have a similar appearance to the right side at this level ( Fig. 5.2B ), as does lymph node enlargement ( Fig. 5.3A ).


Right Upper Lobe Bronchus and Left Upper Lobe Segments


Right Hilum


Approximately 1 cm distal to the carina, the right upper lobe bronchus is usually visible along its length, with its anterior and posterior segmental branches both generally seen at the same level ( Fig. 5.5A–D ). The anterior segment, usually lying in or near the scan plane, is commonly seen over a length of 1 or 2 cm. The posterior segmental bronchus usually angles slightly cephalad, out of the scan plane, and may not be seen as well. If it is not seen at the level of the upper lobe bronchus, you should look for it at the next higher level. In some normal individuals the origin of the apical segment can be seen at this level as a round lucency, usually at the point of bifurcation (or, in this case, trifurcation) of the right upper lobe bronchus.




FIG. 5.5


Level of the right upper lobe bronchus and left upper lobe segments: normal anatomy.

(A) Approximate level of scans shown in (B)–(D). (B) Right hilum: CT with 2.5-mm slice thickness shows the right upper lobe bronchus (RUL) along its length, together with its anterior segmental branch (Ant seg) and posterior segmental branch (Post seg) arising from the right upper lobe bronchus in a Y-shaped pattern. Left hilum: On the left side, the apical-posterior segmental bronchus (Ap-post seg) and anterior segmental bronchus of the left upper lobe are both visible. The apical-posterior segment is seen in cross section, whereas the anterior segment is directed anteriorly. (C and D) Slices of 1.25-mm at this level in a patient different from the patient for the scan shown in (B). Right hilum: The right upper lobe bronchus (RUL) arises from the right main bronchus (RMB) just below the carina. The anterior segmental bronchus (Ant seg) and posterior segmental bronchus (Post seg) arise from the right upper lobe bronchus. The posterior wall of the right upper lobe bronchus contacts lung and is a few millimeters thick. The truncus anterior is anterior to the right upper lobe bronchus. An upper lobe vein branch (Post vein) lies in the angle between the anterior and posterior segmental branches. A superior vein branch (Sup vein) lies anteriorly. Left hilum: The left main bronchus (LMB) is within the mediastinum. In this patient the anterior segmental bronchus (LUL) is seen at the point at which it separates from the apical-posterior segment (A-P seg) . (D) The left upper lobe segmental bronchi lie lateral to the main branch of the left pulmonary artery (LPA) , which produces a convexity in the posterior hilum, and the superior pulmonary vein, which results in an anterior convexity. The artery supplying the anterior segment of the left upper lobe is seen medial to the anterior segmental bronchus and adjacent to the vein. LL , Lower lobe; PA , main pulmonary artery; seg , segment.


Anterior to the right upper lobe bronchus, the truncus anterior (pulmonary artery supplying most of the upper lobe) produces an oval opacity of variable size but often about the same size as the right main bronchus visible at the same level ( Fig. 5.5D ). An upper lobe vein branch (posterior vein), lying in the angle between anterior and posterior segmental branches, is present and is visible in almost all patients. The posterior wall of the right upper lobe bronchus is usually outlined by lung and appears smooth and 2 to 3 mm thick.


Within the anterior right hilum at this level, a mass or lymph node enlargement can be identified if a soft-tissue opacity larger than the expected size of the truncus anterior is visible ( Fig. 5.6 ). This, of course, could be confirmed by contrast medium injection. Laterally, in the angle between the anterior and posterior segmental bronchi, anything larger than the expected vein is abnormal ( Fig. 5.6 ). Posteriorly, thickening of the wall of the upper lobe bronchus or main bronchus ( Fig. 5.7 ) or a focal soft-tissue opacity behind it will almost always be abnormal. An anomalous pulmonary vein branch may sometimes be seen posterior to the bronchus; it is seen at multiple adjacent levels.




FIG. 5.6


Hilar adenopathy in three patients shown at the same level as in fig. 5.5C and D .

(A) In a patient with sarcoidosis, there is extensive adenopathy (arrows) at the level of the right upper lobe bronchus (RUL) and the apical-posterior segmental bronchus (A-P) of the left upper lobe. On the right, nodes are visible as unopacified structures anteriorly and laterally. The soft-tissue opacity seen in the position of the posterior vein on the right is too large to represent a vessel. On the left side, there are enlarged nodes (arrows) in both the lateral and the posterior hilum, which are distinguishable from the opacified left pulmonary artery. (B) CT at the level of the right upper lobe bronchus (RUL) and the apical-posterior segmental bronchus ( A-P) of the upper lobe of the left lung shows extensive lymph node calcification secondary to sarcoidosis. The calcified lymph nodes are similar in location to those shown in (A). (C) Lymph node enlargement ( arrows ) at the level of the right upper lobe bronchus (RUL) in the same patient as shown in Fig. 5.3B .



FIG. 5.7


Bronchogenic carcinoma with a right hilar mass.

(A) A large carcinoma causes narrowing of the right upper lobe bronchus and obstruction of the anterior and posterior segmental bronchi. The truncus anterior (small arrow) , anterior to the bronchus, is markedly narrowed and surrounded by tumor. The posterior walls of the right upper lobe bronchus (large arrow) and right main bronchus are thickened. (B) At a lower level the bronchus intermedius is narrowed and its posterior wall is thickened (arrow) . The mass also invades the mediastinum, surrounding and narrowing the right pulmonary artery.


Left Hilum


On the left side, at or near this level, the apical-posterior and anterior segmental bronchi of the left upper lobe are usually visible ( Fig. 5.5A–C ). The apical-posterior segment is seen in cross section as a round lucency, whereas the anterior segment is directed anteriorly, roughly in the scan plane, at about the one o’clock position. In some individuals the anterior segmental bronchus is seen at a lower level. These bronchi lie lateral to the main branch of the left pulmonary artery, which produces a large convexity in the posterior hilum at this level, and the superior pulmonary vein, which results in an anterior convexity. In many normal individuals the artery supplying the anterior segment of the upper lobe is seen medial to the anterior segmental bronchus. Lymphadenopathy can be seen in relation to all these structures and is most easily recognized after contrast medium infusion ( Fig. 5.6A and B ).


Right Bronchus Intermedius and Left Upper Lobe Bronchus


Right Hilum


Below the level of the right upper lobe bronchus, the bronchus intermedius is visible as an oval lucency at several adjacent levels ( Fig. 5.8 ). Its posterior wall is sharply outlined by lung. Anterior and lateral to the bronchus, the hilar silhouette may differ in appearance, primarily because of variations in the sizes and positions of pulmonary veins. A collection of fat and normal-sized nodes, sometimes measuring more than 10 mm in diameter, is commonly seen at the level of the bifurcation of the right pulmonary artery, anterior and lateral to the bronchus intermedius ( Fig. 5.8 ). A mass involving the posterior hilum can be readily diagnosed without contrast medium injection because of thickening of the posterior bronchial wall ( Fig. 5.7 ); thickening of the posterior wall of the bronchus intermedius is a common finding in patients with a right hilar mass, particularly when it results from lung cancer.




FIG. 5.8


Normal bronchus intermedius and left upper lobe bronchus level.

(A) Approximate level of the scans shown in ( B) and ( C) . (B and C) The bronchus intermedius (BI) is visible as an oval lucency with its posterior wall sharply outlined by the lung. Anterior and lateral to the bronchus, the hilum is made up of the right pulmonary artery (RPA) and superior pulmonary veins (Sup veins) . Normal lymph nodes and fat are visible in the anterolateral hilum, between the opacified pulmonary artery and veins. On the left the left main bronchus (LMB) and left upper lobe bronchus (LUL) are visible. The left superior pulmonary vein is anterior to the bronchi, and the interlobar or descending branch of the left pulmonary artery (LPA) forms an oval soft-tissue opacity posterior to the left upper lobe bronchus. LL , Lower lobe; seg , segment.


Diagnosis of anterior or lateral hilar masses at this level generally requires contrast medium administration ( Figs. 5.9 and 5.10 ). Normal soft tissue and nodes ( Fig. 5.8 ) should not be mistaken for a hilar mass.




FIG. 5.9


Abnormal bronchus intermedius and left upper lobe bronchus level in two patients with sarcoidosis.

(A) On the right a scan at the level of the bronchus intermedius (BI) shows enlargement of the normal node group (arrows) shown in Fig. 5.8B , situated lateral to the pulmonary artery (a) . On the left a scan at the level of the left upper lobe bronchus (LUL) shows enlarged lymph nodes (arrows) in the anterior hilum and surrounding the opacified pulmonary artery (a) . Enlarged lymph nodes are situated posterior to the left upper lobe bronchus. (B) A scan at the level of the bronchus intermedius (BI) and left upper lobe bronchus (LUL) shows multiple calcified lymph nodes.



FIG. 5.10


Abnormal bronchus intermedius and left upper lobe bronchus level.

In a patient with non-Hodgkin lymphoma and bilateral hilar adenopathy (arrows) , enlarged lymph nodes are clearly distinguished from opacified pulmonary vessels.


Left Hilum


The left upper lobe bronchus is usually visible at the level of the bronchus intermedius on the right. It is typically seen along its axis, extending anteriorly and laterally from its origin at an angle of 10 to 30 degrees ( Fig. 5.8 ). The apical-posterior and anterior segmental bronchi of the left upper lobe usually arise from a common trunk that originates from the upper aspect of right upper lobe bronchus. The left superior pulmonary veins are anterior and medial to the left upper lobe bronchus at this level, and the descending branch of the left pulmonary artery forms an oval soft-tissue opacity posterior and lateral to it. Normal lymph nodes (<5 mm in diameter) are commonly visible medial to the artery and lateral to the bronchus. Because only the oval artery occupies the lateral hilum, lobulation of the lateral hilum (more than one convexity) indicates a mass or lymphadenopathy ( Figs. 5.9–5.11 ).




FIG. 5.11


Left hilar adenopathy (left upper lobe bronchus level).

(A) Lymph node enlargement (arrow) is visible in the posterior hilum, behind the left upper lobe bronchus, and between the aorta and the left pulmonary artery. (B and C) The enlarged lymph node (arrows) lies posterior to the bronchus (i.e., in the region of the retrobronchial stripe) and prevents the lung from outlining its posterior wall.


Although the lung contacts and sharply outlines the posterior wall of the bronchus intermedius at several levels, the left posterior bronchial wall is usually outlined by the lung only at this level; that is, at the level of the left upper lobe bronchus. In approximately 90% of individuals the lung sharply outlines the posterior wall of the left main or upper lobe bronchus, medial to the descending pulmonary artery ( Figs. 5.8 and 5.12B and C ); this is termed the left retrobronchial stripe. As on the right, the bronchial wall should measure 2 to 3 mm in thickness. Thickening of this stripe, or a focal soft-tissue opacity behind it, indicates lymph node enlargement or bronchial wall thickening ( Figs. 5.9 and 5.11 ). In 10% of normal individuals, however, the lung does not contact the bronchial wall because the descending pulmonary artery is medially positioned against the aorta. This should not be misinterpreted as abnormal.


Usually the lingular bronchus is also seen on the left at the level of the bronchus intermedius ( Fig. 5.12 ). The lingular bronchus is usually visible at a level near the undersurface of the left upper lobe bronchus, from which it originates; its two segments (superior and inferior) can sometimes be seen ( Fig. 5.12 ). The superior segmental bronchus of the lower lobe is often visible at this level, arising posteriorly. The pulmonary artery and veins appear the same as at the level of the left upper lobe bronchus ( Fig. 5.8 ). Normal lymph nodes are commonly visible medial to the artery. At this level, significant lobulation of the lateral hilar contour indicates a mass or adenopathy ( Fig. 5.13 ).




FIG. 5.12


Normal bronchus intermedius and lingular bronchus level.

(A) Approximate level for scans shown in (B)–(D). (B) CT with 2.5-mm slice thickness. At a level below that in Fig. 5.8 , the bronchus intermedius (BI) is visible as an oval lucency on the right, with its posterior wall sharply outlined by lung tissue. On the left the left upper lobe bronchus (LUL) is visible, extending anteriorly and laterally from the left main bronchus (LMB) . The left posterior bronchial wall is outlined by the lung at this level. This is termed the left retrobronchial stripe. The lingular bronchus (Ling) arises from the lower edge of the left upper lobe bronchus and divides into two branches, the superior lingular segment (Sup Ling) and inferior lingular segment (Inf Ling) . (C and D) Slices of 1.25 mm at a level slightly below that shown in (B). Right hilum: On the right the bronchus intermedius (BI) is visible as an oval lucency, with its posterior wall sharply outlined by the lung. The interlobar pulmonary artery (IPA) and superior pulmonary veins (Sup vein) are anterior and lateral to the bronchus. The artery branch to the superior segment of the right lower lobe (Sup seg art) is directed posteriorly. Left hilum: Below the level of the left upper lobe bronchus, the lingular bronchus branches into the superior and inferior lingular segments. The proximal left lower lobe bronchus (LLL) is visible, along with its first branch, the superior segmental bronchus (Sup seg) . The left superior pulmonary vein is anterior and medial to the bronchus, and the descending branch of the left interlobar pulmonary artery is posterior to the lingular bronchus and lateral to the lower lobe bronchus. The lingular artery (Ling art) accompanies the lingular bronchus. Inf ling , Inferior lingular segment; LL , lower lobe; seg , segment; Sup ling , superior lingular segment.





FIG. 5.13


Abnormal bronchus intermedius and lingular bronchus level in a patient with sarcoidosis.

On the right, enlarged lymph nodes (large arrows) are visible adjacent to the bronchus intermedius (BI) and pulmonary artery. On the left, enlarged lymph nodes (large arrows) are visible medial and lateral to the interlobular pulmonary artery and posterior to the lingular bronchus (Ling) . Ling art , Lingular artery; SS , superior segment of the left lower lobe bronchus; SS art , superior segment of the right lower lobe artery.


Right Middle Lobe Bronchus and Left Lower Lobe Bronchus


Right Hilum


On the right, at the level of the lower bronchus intermedius, the middle lobe bronchus arises anteriorly and extends anteriorly, laterally, and inferiorly at an angle of about 30 to 45 degrees ( Fig. 5.14 ). Because of its obliquity, only a short segment of its lumen is visible at each level on CT, and this appearance should not be misinterpreted as bronchial obstruction. Often the superior segmental bronchus of the lower lobe arises posterolaterally at this level ( Fig. 5.14 ).




FIG. 5.14


Normal right middle lobe and left lower lobe bronchus level.

(A) Approximate level for scans shown in (B)–(E). (B) CT with 2.5-mm slice thickness through the right middle lobe bronchus (RML) shows its division into its medial segmental branch (Med seg RML) and lateral segmental branch (Lat seg RML) . The middle lobe bronchus extends anteriorly and laterally at an angle of about 45 degrees. The right lower lobe bronchus (RLL) is also visible at this level, giving rise to its superior segmental branch (Sup seg RLL) posterolaterally. The interlobar pulmonary artery lies lateral to the bronchi. On the left the left lower lobe bronchus (LLL) is visible along with a short segment of its superior segmental branch (Sup seg LLL) . The lower lobe artery is lateral to the left lower lobe bronchus. (C and D) A 1.25-mm slice in a different patient. Right hilum: The right middle lobe bronchus (RML) is visible, but because it angles caudad, only a short segment of its lumen is visible. The right lower lobe bronchus (RLL) and its superior segment (Sup seg) are also visible, as in (B). The superior pulmonary vein (Sup vein) lies anterior and medial to the right lower lobe bronchus, whereas the oval descending (interlobar) branch of the right pulmonary artery (IPA) lies beside and behind it. The right middle lobe pulmonary artery (RML PA) accompanies the right middle lobe bronchus. The appearance of the right hilum at this level is quite similar to that of the left hilum at the levels of the left upper lobe and lingular bronchi. Left hilum: The left lower lobe bronchus (LLL) is visible below the takeoff of the superior segment. It has a double-barreled appearance as it begins to divide into the basal segments of the LLL. As on the right, the superior pulmonary vein (Sup vein) is anterior. The basal segmental branches (LLL seg arteries) of the pulmonary artery are lateral. (E) Slightly below (C) the right middle lobe medial segmental bronchus (Med seg RML) and lateral segmental bronchus (Lat seg RML) are visible. LL , Lower lobe; seg , segment.






At the level of the origin of the middle lobe bronchus, the superior pulmonary veins lie anterior and medial to the bronchus, whereas the descending (interlobar) branch of the right pulmonary artery lies beside and behind it ( Fig. 5.12 ). Normal lymph nodes (<5 mm in diameter) are commonly visible lateral to the artery and bronchus. Because of this separation of the artery and veins, the lateral hilum at this level (representing the artery) is oval, without prominent lobulation. Any lobulation of significant size suggests hilar adenopathy or a mass ( Figs. 5.15–5.17 ).




FIG. 5.15


Abnormal right middle lobe and left lower lobe bronchus level in a patient with sarcoidosis.

On the right, enlarged lymph nodes (large arrows) are seen anterior and posterior to the interlobar pulmonary artery (IPA) and are situated lateral to the right middle lobe bronchus (RML) , right lower lobe bronchus (RLL) , and superior segmental bronchus (SS) . Subcarinal lymph node enlargement is also present. On the left, enlarged nodes (large arrow) are lateral to the lower lobe bronchus (LLL) and surround the interlobar pulmonary artery (IPA) .

Mar 19, 2020 | Posted by in GENERAL RADIOLOGY | Comments Off on The Pulmonary Hila
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