Lung Disease

Major Fissures

The major fissures separate the lower lobes posteriorly from the upper lobe on the left and the middle and upper lobes on the right. They are not clearly visible on CT with 5-mm collimation but are easily seen as thin white lines on CT with thin slices ( Fig. 6.1 ). The lung parenchyma immediately adjacent to the fissures, being peripheral, contains few visible vessels and appears relatively avascular. The major fissures are incomplete in many patients (i.e., they do not completely separate the lobes).

Within the lower thorax, the major fissures angle anterolaterally from the mediastinum, contacting the anterior third of the hemidiaphragms. In the upper thorax, the major fissures angle posterolaterally. Above the aortic arch, they contact the posterior chest wall.

Minor Fissure

The minor fissure separates the right middle lobe from the right upper lobe. Although it parallels the scan plane, and is more difficult to see than the major fissure, it is usually visible on thin slices as a white line of varying sharpness and thickness. The relatively avascular lung on both sides of the fissure is visible on CT in most patients. In some patients the minor fissure mimics the appearance of the major fissure but is seen anterior to it.

Because the minor fissure often angles caudally, the right lower, middle, and upper lobes may all be seen on a single scan. If the minor fissure is concave caudally, it can sometimes be seen in two locations or can appear ring-shaped ( Fig. 6.2 ), with the middle lobe between the fissure lines or in the center of the ring and the upper lobe anterior to the most anterior part of the fissure.

Accessory Fissures

In patients with an azygos lobe, the four layers of the mesoazygos, or azygos fissure, are invariably visible above the level of the intrapulmonary azygos vein. The azygos fissure is C-shaped and convex laterally, beginning anteriorly at the right brachiocephalic vein and ending posteriorly at the right anterolateral surface of the vertebral body ( Fig. 3.21 ). Other accessory fissures, most commonly the inferior accessory fissure (separating the medial basal segment of either lower lobe from the other lower lobe segments), are occasionally seen on CT. They are not generally of diagnostic significance.

Obstructive Atelectasis

Obstructive atelectasis often occurs because of lung cancer or other bronchial tumors, and the airways should be examined closely. Lobar atelectasis commonly results; typically, the affected lobe is partially or completely consolidated ( Fig. 6.4 ). Air bronchograms are usually, but not always, absent. Mucus bronchograms (low-density fluid or mucus within obstructed bronchi) can sometimes be seen on CT. The air- or mucus-filled bronchi can be dilated in the presence of atelectasis, simulating bronchiectasis. If contrast medium infusion is used, opacified vessels are often visible within the consolidated and collapsed lobe (a collapsed lung is not simply airless; the alveoli are usually filled with fluid). If little volume loss is present, the term obstructive pneumonia is often used ( Fig. 6.5 ).

On CT, atelectasis can be diagnosed when displacement of fissures is seen. Typical patterns of lobar collapse can be identified ( Figs. 6.6 and 6.7 ).

Passive Atelectasis

In the presence of pleural effusion the lung tends to retract or collapse toward the hilum, and fluid entering the fissures allows the lobes to separate. With injection of contrast medium, the collapsed lung opacifies and is clearly distinguishable from surrounding fluid. Air bronchograms may be seen within collapsed lobes. Rounded atelectasis is a form of passive atelectasis. Linear, disk, or platelike atelectasis at the lung bases may be seen as a result of limited diaphragmatic motion.

Rounded Atelectasis

Rounded atelectasis represents focal, collapsed, and often folded lung. It is common and almost always occurs in association with pleural thickening or pleural effusion.

Rounded atelectasis is most common in the posterior paravertebral regions and may be bilateral in patients with bilateral pleural abnormalities. Areas of rounded atelectasis, which appear as a mass or mass-like consolidation, are usually several centimeters in diameter. Bending or bowing of adjacent bronchi and arteries toward the edge of the area of rounded atelectasis, because of volume loss or folding of lung, is characteristic ( Fig. 6.8 ); this finding has been termed the comet-tail sign . Air bronchograms can sometimes be seen within the mass. Rounded atelectasis opacifies after contrast medium infusion.

Four findings must be present to make a confident diagnosis of rounded atelectasis on CT; if these are present, follow-up is usually unnecessary ( Fig. 6.8 ). However, if one or more of these findings is lacking, you should be cautious in making the diagnosis, and close follow-up or biopsy may be necessary. The four findings of rounded atelectasis are:

• ipsilateral pleural effusion or thickening

• significant contact between the lung lesion and the abnormal pleural surface

• the comet-tail sign

• volume loss in the lobe in which the opacity is seen

Rounded atelectasis may be associated with asbestos-related pleural thickening, occurring adjacent to regions of thickened pleura, but often has an atypical appearance. Areas of atelectasis or focal fibrosis in patients exposed to asbestos can be irregular, may not have extensive pleural contact, and may not be associated with the comet-tail sign.

Cicatrization Atelectasis

Cicatrization atelectasis occurs in the presence of pulmonary fibrosis and may be associated with tuberculosis (TB), radiation, or chronic bronchiectasis. In this condition, there is no evidence of bronchial obstruction. Rather, air bronchograms and bronchial dilatation (bronchiectasis) are usually visible within the area of collapse. The volume loss is often severe.

Bronchogenic Cyst

The appearance of a mediastinal bronchogenic cyst is described in Chapter 4 . Pulmonary bronchogenic cysts are typically well defined, round or oval, and of fluid or soft-tissue attenuation; previously infected cysts can contain air or an air-fluid level. When a cyst contains air, its wall appears very thin, although consolidation of surrounding lung may be present. Such cysts are most common in the lower lobes.

Pulmonary Arteriovenous Malformation

Pulmonary arteriovenous malformation (AVM) is supplied by the pulmonary artery(s) and drained by the pulmonary vein(s). Pulmonary AVM can be single (65%) or multiple (35%) and is often associated with Osler-Weber-Rendu syndrome (65%). On CT, pulmonary AVM can appear as a single dilated vascular sac or fistula, visible as a smooth, sharply defined round or oval nodule (most common), or a tangle of dilated tortuous vessels, appearing as a lobulated or serpiginous mass. In each type the feeding pulmonary artery (or arteries) and the draining pulmonary vein (or veins) are dilated and should be easily seen on CT ( Fig. 6.11 ).

An AVM with a single feeding artery and draining vein ( simple AVM ) is most common; a complex AVM with multiple supplying vessels is less frequent. In most cases the fistula is immediately subpleural in location. These characteristic findings are generally sufficient to make a specific diagnosis on scans without contrast medium infusion, even for AVMs only a few millimeters in diameter.

Although contrast medium is not usually needed for diagnosis, a pulmonary AVM shows rapid and dense opacification after bolus contrast medium injection, followed by rapid washout of the contrast ( Fig. 6.11 ). As would be expected, opacification occurs just after opacification of the right ventricle and pulmonary artery. For a fistula larger than 3 mm, catheter embolization is the treatment of choice. Catheter pulmonary angiography is generally performed in association with embolization.

Systemic Arteriovenous Malformation

Rarely, an anomalous systemic artery arising from the aorta or one of its branches supplies an area of otherwise normal lung. Drainage into pulmonary veins is via normal capillaries rather than through a dilated vascular sac or fistula. Systemic AVM is usually diagnosed incidentally, although hemoptysis may occur. In contrast to sequestration, described next, systemic AVM supplies normally aerated lung, containing normal bronchi (i.e., there is no sequestered lung).

Intralobar Sequestration

Intralobar sequestration is most common and is usually diagnosed in adults. It is contained within an otherwise normal lobe, usually the left lower lobe. Recurrent or chronic infection is common. Venous drainage is usually by means of pulmonary veins, although systemic (azygos) vein drainage can be seen. Intralobar sequestration typically contains air but can be quite variable in appearance. On CT, intralobar sequestration can appear as

• a region of hyperlucent lung ( Fig. 6.12A )

• a cystic or multicystic abnormality (sometimes with air-fluid levels)

• consolidated or collapsed lung ( Fig. 6.13 )

• a combination of these findings

Areas of lucent lung in association with or representing part of an intralobar sequestration are common ( Fig. 6.12A ). Normal bronchi are not seen in the sequestration, and if the sequestration is aerated, the vascular branching pattern within it may appear abnormal.

Extralobar Sequestration

Extralobar sequestration is relatively uncommon and is usually diagnosed in infants or children. Infection is rare. It has its own pleural envelope, rarely contains air, and almost always appears as a solid mass. Venous drainage is usually by systemic veins.

Lung Cancer

Adenocarcinoma is the most common cell type of lung cancer (50% of cancers) and is the most common cell type presenting as a lung nodule (60%–70%), but any lung cancer cell type can present in this manner. On CT, lung cancers may appear solid, of GGO, or of mixed GGO and solid.

Although a definite diagnosis of lung cancer cannot be made on CT, findings that suggest malignancy in a patient with a solitary nodule include:

• an irregular or spiculated nodule margin, usually caused by fibrosis or invasion of surrounding lung (90% of nodules with a spiculated edge are malignant; Fig. 6.15 );

  

• a lobulated contour ( Fig. 6.16 );

  

• air bronchograms ( Fig. 6.15 ) or cystic or “bubbly” air–containing regions within the nodule (seen in 65% of cancers but only 5% of benign lesions);

• a nodule exceeding 6 mm that is of GGO or of mixed GGO and solid attenuation (two-thirds are malignant; Fig. 6.17 );

  

• cavitation ( Fig. 6.16 ) with a nodular cavity wall or a wall exceeding 15 mm in greatest thickness (90% are cancers);

• an upper lobe nodule;

• a diameter exceeding 2 cm (95% are cancers);

• associated emphysema or lung fibrosis.

A spiculated edge and the presence of air bronchograms ( Fig 6.15 ) or cystic air-containing regions are particularly common with adenocarcinoma. Lobulation of the nodule also suggests the diagnosis of carcinoma but may also be seen with other lesions, particularly hamartoma.

Both primary lung carcinomas and metastases can cavitate. Typically, a cavitary carcinoma has a thick, irregular, and nodular wall ( Fig. 6.16 ), but some metastatic tumors, particularly sarcomas and those of squamous cell origin, can be thin walled. A cavitary nodule with a thin wall (<5 mm) is most likely (90%) benign.

CT Diagnosis of Nodule Calcification

CT is important in the diagnosis of nodule calcification, a finding that may be very helpful in determining that the nodule is benign. About 25% to 35% of benign nodules appearing uncalcified on chest radiographs show calcification on thin-slice CT. CT with thin slices is optimum for detecting calcium, but dense calcification can also be seen with 5-mm slices. Soft-tissue window settings are best for showing that calcium is present.

When you are using CT to diagnose “benign” calcification, you must be sure that the calcification is benign in appearance ( Fig. 6.18A ). To be reasonably sure of a benign diagnosis, one of the following patterns must be present:

• diffuse calcification ( Fig. 6.18B ), typical of a granuloma;

• dense, central (i.e., bull’s-eye ) calcification, most typical of histoplasmosis ( Fig. 6.18C );

• central and popcorn calcification, typical of hamartoma ( Fig. 6.20B );

• concentric rings of calcification ( target calcification; Fig. 6.18D ), typical of histoplasmosis.

A caveat is that sarcomas and carcinoid tumors may be associated with dense calcification that is either homogeneous or “chunky.”

About 5% to 10% of carcinomas contain some calcium, either as a result of tumor calcification or because the carcinoma has engulfed a preexisting granuloma. Calcification in malignancies is typically punctate or stippled or is eccentric within the nodule ( Fig. 6.19 ). Although these patterns of calcification may also be seen in benign lesions, they should be considered to be indeterminate or potentially associated with malignancy.

Hamartoma

CT can be valuable in diagnosing pulmonary hamartoma. Hamartoma usually appears as a smooth and sharply marginated nodule, rounded, or with a lobulated contour. By CT with thin slices, about two-thirds of hamartomas can be correctly diagnosed because of visible fat (60%; Fig. 6.20A ) that is either focal or diffuse, fat and calcification (30%), or calcification (10%). Fat is easily seen on thin-slice CT, with CT numbers ranging between –40 and –120 HU. Calcification may have a “popcorn” appearance because of calcification of nodules of cartilage ( Fig. 6.20B ) or may be diffuse.

Granulomas

Granulomas usually appear rounded and well defined. They may contain calcium, which may be dense and central, diffuse, or on occasion, ring-shaped ( Fig. 6.18 ). An infection or noninfectious granulomatous lesion (e.g., sarcoidosis) presenting as a lung nodule or mass may be associated with small nodules adjacent to or surrounding it (i.e., satellite nodules ; Fig. 6.21 ). In patients with sarcoidosis, this appearance has been referred to as the galaxy sign. Satellite nodules may also be seen with lung carcinoma, but they are evident in only a small percentage of cases.

Tuberculosis, Atypical Mycobacterial, and Fungal Infection

In most patients with TB exposure and a positive PPD skin test result, CT findings are normal. A small lung nodule, calcified or uncalcified and representing the original site of infection, is sometimes visible; associated calcified lymph nodes may be present. Small scattered nodules in the lung apex reflect the presence of hematogenous spread at the time of the original infection. In most patients, these heal without progression.

In a small percentage of individuals, usually children and immunosuppressed patients, primary exposure to TB results in a focal pneumonia (which may involve any lobe). In half of these cases, associated hilar or mediastinal node enlargement is present. Nodes often appear to be low in attenuation on enhanced CT.

Progression of primary TB (often in patients with immune compromise) or after primary reactivation of an earlier infection (usually in the lung apices) can result in cavitation ( Fig. 6.22 ). Cavities are usually irregular in shape and have irregular walls and multiple septations. Air-fluid levels are typically absent, although hemorrhage or superinfection may result in this finding. Cavities may be multiple and associated with satellite nodules.

The spread of infected material through the airways can result in findings of bronchopneumonia (i.e., patchy consolidation, tree-in-bud or centrilobular nodules ), often in regions near the cavity or in the lower lobes ( Fig. 6.22 ). Tree-in-bud and centrilobular nodules are described in the part of this chapter devoted to high-resolution CT (HRCT).

Infection with pathogenic atypical mycobacteria can result in identical findings. Fungal infections can result in findings indistinguishable from TB. If TB is considered in the differential diagnosis, it is generally a good idea to consider fungal infection as well.

The Halo Sign and Angioinvasive Aspergillosis

The halo sign, visible on CT, was first described in immunosuppressed patients with angioinvasive aspergillosis ( Fig. 6.23 ). As indicated in the previous discussion, the term halo sign is used to describe a nodule with a soft-tissue attenuation center surrounded by a less dense halo of GGO.

In patients who are severely immunocompromised, particularly those with treated leukemia and neutropenia, this appearance is typical and highly suggestive of angioinvasive aspergillosis; often treatment is begun without confirmation of the diagnosis. In patients with angioinvasive aspergillosis, the halo sign reflects the presence of a septic infarction (the dense center) surrounded by hemorrhage (the halo).

Although suggestive of angioinvasive aspergillosis in the proper clinical setting, the halo sign is nonspecific and can be seen with other infections (e.g., TB, Legionella infection, nocardiosis, and cytomegalovirus infection), with some tumors (particularly adenocarcinomas, lymphoma, and Kaposi sarcoma), and in patients with pulmonary infarction and granulomatosis with polyangiitis (GPA), formerly termed Wegener granulomatosis . The histologic correlate with the halo varies with the entity. It may represent hemorrhage (e.g., aspergillosis, infarction, Kaposi sarcoma, and GPA), heterogenous consolidation in pneumonia, or lepidic growth of tumor (adenocarcinoma).

The Air-Crescent Sign and Mycetoma

The presence of a lung mass capped by a crescent of air is termed the air-crescent sign (Figs. 6.24 and 6.25 ). It usually indicates the presence of a mass within a cavity. The air-crescent sign is most typical of mycetoma, but it may also be seen in the later stages of angioinvasive aspergillosis, some bacterial infections with lung infarction, cavitary tumors, a clot or neoplasm within a cavity, and echinococcal cysts.

In patients with a preexisting pulmonary cyst or cavity, a mycetoma or fungus ball can form as a result of saprophytic infection, usually by Aspergillus ( Fig. 6.25 ). On CT a round or oval mass (the fungus ball) can be seen in a dependent location within the cavity and is typically mobile. The mass is capped by a crescent of air. Thickening of the cavity wall is common. In patients with a developing mycetoma, the fungus ball can contain multiple air collections. The same appearance can be seen in semiinvasive (chronic necrotizing) aspergillosis, in which the fungus also invades the wall of the cyst or cavity. Hemorrhage and hemoptysis are common associations. With angioinvasive aspergillosis, septic infarction of the lung can result in an air-crescent sign as the patient recovers ( Fig. 6.24 ).