The Pulmonary Hila

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.

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.

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.

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.

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.

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