ct signs and patterns of lung disease

HIGH-RESOLUTION CT OF THE LUNG 0033-8389101 $15.00 + .OO

CT SIGNS AND PATTERNS OF LUNG DISEASE

Jannette Collins, MD, MEd

A "sign" on CT scanning of the lung refers to a radiologic finding that suggests a specific disease process. Understanding the meaning of a sign implies an understanding of the findings on the CT scan. Knowing the name of the sign is not as important as recognizing the radiologic findings, and understanding the meaning of the findings, but is useful when communicating with clinicians and ra- diologists who may use the term. A CT "pattern" refers to a nonspecific finding or collection of findings suggesting one or more specific disease processes. This article reviews some of the more common and useful CT signs and patterns of focal and diffuse lung disease.

RETICULAR PATTERN

A reticular pattern consists of interlacing line shadows appearing as a mesh or net, and is associated with a heterogeneous group of interstitial lung diseases (Table 1). These include idiopathic pneumonias (usual interstitial pneumonia [UIP], desquamative interstitial pneumonia [DIP], acute interstitial pneumonia [AIP], and nonspecific interstitial pneumonia [NSIP]); idiopathic pulmonary

fibrosis (IPF); collagen vascular diseases; drug-induced lung disease; radiation pneumonitis and fibrosis; and asbestosis.

The idiopathic interstitial pneumonias are a heterogeneous group of lesions that have no well-defined cause? UIP is the most common type and is characterized histologically by a patchy heterogeneous pattern with foci of normal lung, interstitial inflammation, fibro- blastic proliferation, interstitial fibrosis, and honeycombing. Temporal heterogeneity is an important histologic feature and helps to distinguish UIP from DIP. UIP can result from dust exposure (e.g., asbestosis); drugs ( eg , bleomycin); radiation; or collagen vascular diseases. If no cause is found it is classified as an idiopathic interstitial pneumonia and considered synonymous with IPF. In general, the term LIIP refers to the histologic abnormality and IPF to the disease that results in the histologic abnormality. The diagnosis of IPF is limited to patients who have histologic findings of UIP. Patients who have histologic findings of DIP, AIP, or NSIP and in whom no cause is found are considered to have idiopathic DIP, AIP, or NSIP and not IPE2 The typical high-resolution CT (HRCT) features of UIP are honeycombing and reticulation. Ground-glass opacity (GGO) and consolida-

From the Department of Radiology, Graduate Medicine Education, University of Wisconsin Medical School and Hospital and Clinics, Madison, Wisconsin

RADIOLOGIC CLINICS OF NORTH AMERICA

VOLUME 39 NUMBER 6 NOVEMBER 2001 1115

1116 COLLINS

Table 1. DIFFERENTIAL DIAGNOSIS OF PATERNS OF DISEASE ON HRCT OF THE LUNGS

Reticular Idiopathic pneumonias Idiopathic pulmonary fibrosis Collagen vascular diseases Drug-induced lung disease Radiation pneumonitis and fibrosis Asbestosis Honeycomb Usual interstitial pneumonitis Idiopathic pulmonary fibrosis Collagen vascular diseases Asbestosis Chronic hypersensitivity pneumonitis Drug-related fibrosis Interlobular septa1 thickening Smooth

Pulmonary edema Pulmonary hemorrhage Lymphangitic spread of carcinoma Lymphoma and leukemia Amyloidosis Infectious pneumonia

Lymphangitic spread of carcinoma Lymphoma Sarcoidosis Silicosis and coal workers' pneumoconiosis Lymphocytic interstitial pneumonitis Amyloidosis

Interstitial pulmonary fibrosis

Beaded

Irregular

Cystic Langerhans' cell histiocytosis Lymphangioleiomyomatosis Sarcoidosis Lymphocytic interstitial pneumonitis Pneurnocystis carinii pneumonia Honeycombing Centrilobular emphysema Nodular Perilymphatic

Sarcoidosis Random

Silicosis and coal workers' pneumoconiosis Tuberculosis and fungal infection Metastases Langerhans' cell histiocytosis

Centrilobular Subacute hypersensitivity pneumonitis Respiratory bronchiolitis

Nodular (continued) Bronchovascular

Lymphoproliferative disorders Leukemia Kaposi's sarcoma

Metastases Wegener 's granulomatosis Septic emboli Tuberculosis or fungal infection

Cavitating

Ground-glass opncity Infectious pneumonia Pulmonary edema Pulmonary hemorrhage Acute or subacute hypersensitivity pneumonitis Desquamative interstitial pneumonitis Pulmonary alveolar proteinosis Consolidation Acute

Pulmonary hemorrhage Pulmonary edema Pulmonary alveolar proteinosis Infectious pneumonia Acute eosinophilic pneumonia Bronchiolitis obliterans with organizing

Acute hypersensitivity pneumonitis Radiation pneumonitis Pulmonary infarction

Bronchioloalveolar cell carcinoma Lymphoma Lipoid pneumonia Bronchiolitis obliterans with organizing

Chronic eosinophilic pneumonia Sarcoidosis

pneumonia

Chronic

pneumonia

Mosaic lung attentuation Infiltrative lung processes Small airways disease Pulmonary vascular disease Tree-in-bud Infection Allergic bronchopulmonary aspergillosis Cystic fibrosis Aspiration Diffuse panbronchiolitis Obliterative bronchiolitis Asthma

tion can be seen, but are not dominant features.

Desquamative interstitial pneumonia is characterized histologically by a relatively uniform pattern of macrophages within the alveoli. Most patients are smokers and it is likely that DIP represents a reaction to cigarette smoke. The histologic features of DIP

are similar to those of respiratory bronchiolitis-interstitial lung disease (a condition seen exclusively in smokers), although the distribution of DIP is diffuse and respiratory bronchiolitis-interstitial lung disease has a predominantly bronchiolocentric distribution. The typical HRCT feature of DIP is diffuse GGO. Reticulation is uncommonly seen.

CT SIGNS AND PATTERNS OF LUNG DISEASE 1117

Figure 1. Nonspecific interstitial pneumonitis. High-resolution CT (HRCT) (1 -mm collimation) of a 74-year-old woman with a 1 0-day history of a viral- like illness shows bilateral ground-glass opacification, consolidation, and reticulation.

Acute interstitial pneumonia is characterized histologically by hyaline membranes within the alveoli and diffuse, active interstitial fibrosis. Patients with AIP present with respiratory failure developing over days or weeks. No etiologic agent is identified. Typi- cal HRCT features of acute-stage AIP are GGO and consolidation. Late-stage features are honeycombing and reticulation.

Nonspecific interstitial pneumonia is characterized histologically by interstitial inflammation and fibrosis without any specific features to allow a diagnosis of UIP, DIP, or AIP. It is a diagnosis of exclusion. The typical HRCT feature is GGO, although reticulation and consolidation are also commonly seen (Fig. 1). Honeycombing is uncommon.

HONEYCOMB PAlTERN

pleural surface, distinguishing them from paraseptal emphysema.

Honeycombing produces a characteristic appearance on HRCT that allows a confident diagnosis of lung fibrosis (Fig. 2).17 On HRCT, the cystic spaces usually average 1 cm in diameter, although they can range from several millimeters to several centimeters in size. They have clearly definable walls 1 to 3 mm in thickness, are air-filled, and appear lucent in comparison with normal lung parenchyma. Honeycombing is usually associated with other findings of lung fibrosis, such as archi- tectural distortion, intralobular interstitial thickening, traction bronchiectasis, and irregular linear opacities. Honeycombing on HRCT usually represents UIP (with the most common cause being IPF); collagen vascular disease; asbestosis; chronic hypersensitivity pneumonitis; or drug-related fibrosis (see Table 1).

Honeycombing is a process characterized by the presence of cystic airspaces, having thick, clearly definable fibrous walls lined by bronchiolar epithelium. It results from destruction of alveoli and loss of acinar architec- ture and is associated with pulmonary fibrosis. The cysts are typically layered along the

SEPTAL THICKENING

An interlobular septum marginates part of a secondary pulmonary lobule and contains pulmonary veins and lymphatics. These septa measure approximately 0.1 mm in thickness

1118 COLLINS

Figure 2. Idiopathic pulmonary fibrosis. HRCT (1-mm collimation) of a 62-year- old woman shows bilateral peripheral honeycombing and traction bronchiectasis (arrow). The peripheral areas of ground-glass opacification represent fibrosis and nontreatable lung disease.

and are occasionally seen on normal HRCT surface. They represent the HRCT counterpart scans. Abnormal thickening of interlobular of Kerley’s B lines seen on chest radiographs. septa results from fibrosis, edema, or infiltra- Interlobular septa1 thickening can be tion by cells or other material. Within the pe- smooth, nodular, or irregular (see Table l ) . I 4

ripheral lung, thickened septa 1 to 2 cm in Smooth thickening is seen in patients with length may outline part or all of a secondary pulmonary edema or hemorrhage, lymphan- pulmonary lobule, perpendicular to the pleural gitic spread of carcinoma (Fig. 3), lymphoma,

Figure 3. Lymphangitic spread of carcinoma. HRCT (1.5-mm collimation) of a 65-year-old man with adenocarcinoma of the stomach shows smooth thickening of the interlobular septa (arrow).


leukemia, interstitial infiltration associated with amyloidosis, and some pneumonias. Nodular or beaded thickening occurs in lymphangitic spread of carcinoma or lymphoma, sarcoidosis, silicosis or coal workers’ pneumoconiosis, lymphocytic interstitial pneumonia, and amyloidosis. Irregular thickening is seen in patients who have interstitial fibrosis.

The intralobular interstitium refers to the interstitial network supporting the structures of the pulmonary lobules, excluding the interlobular septa. It is not normally visible on HRCT. The term refers primarily to the fine network of very thin connective tissue fibers within the alveolar walls or the parenchymal interstitium. Thickening of the intralobular septa is an early sign of fibrosis in a number of lung diseases and can also be seen with pulmonary edema, hemorrhage, or other infiltrative lung diseases. HRCT features include thickening of the distal peribronchovas- cular interstitial tissues, resulting in a fine network of lines within visible lobules. This abnormality contributes to the appearance of irregular interfaces (the interface sign) at the edges of arteries and bronchi.

CYSTIC PATTERN

The term cyst is nonspecific and refers to a thin-walled (usually less than 3 mm thick),

well-defined and circumscribed, air- or fluid- containing lesion, 1 cm or more in diameter, that has an epithelial or fibrous wall.25 A cystic pattern results from a heterogeneous group of diseases, all having in common the presence of focal, multifocal, or diffuse parenchymal lucencies and lung destruction (see Table 1). Pulmonary Langerhans’ cell histiocytosis, lymphangioleiomyomatosis, sarcoidosis, lymphocytic interstitial pneumonitis, Pneumocystis carinii pneumonia, honeycombing, and centrilobular emphysema can manifest a cystic pattern on HRCT.

In cases of Langerhans’ cell histiocytosis, the cysts are often confluent, usually thin- walled, and often associated with pulmonary nodules 1 to 5 mm in diameter that may or may not be cavitary (Fig. 4). The intervening lung parenchyma is typically normal, without evidence of fibrosis or septa1 thickening. The distribution of findings is usually upper lungs, with sparing of the costophrenic sulci. The cysts are distributed diffusely throughout the lungs in lymphangioleiomyomatosis, and nodules are not a common feature. The cystic spaces seen with centrilobular emphysema contain a small nodular opacity representing the centrilobular artery (Fig. 5). This finding is helpful in distinguishing emphysema from lymphangioleiomyomatosis and Langerhans’ cell histiocytosis.

Figure 4. Langerhans’ cell histiocytosis. CT scan (5-mm collimation) of a 32- year-old man shows bilateral thin-walled cysts Qrge arrow) with bizarre shapes and nodules (small arrow).

1120 COLLINS

Figure 5. Centrilobular emphysema. HRCT (1 -mm collimation) of a 52-year-old woman shows bilateral “cystic” spaces, some containing a central nodular opacity (arrow) representing the centrilobular artery. Peripheral bullae represent paraseptal emphysema (P).

NODULAR PATTERN

A nodular pattern refers to multiple round opacities, generally ranging in diameter from 1 mm to 1 cm, which may be difficult to separate from one another as individual nodules on a chest radiograph, but which are accurately diagnosed on HRCT. Nodular opacities may be described as miliary (1 to 2 mm, the size of millet seeds), small, medium, or large, as the diameter of the opacity in- creases. Nodules can be further characterized according to nodule margins (e.g., smooth or irregular); presence or absence of cavitation; and distribution (e.g., centrilobular, perilymphatic, or random) (see Table l).,,

Multiple small smooth or irregularly mar- ginated nodules in a perilymphatic distribution are characteristic of sarcoidosis. The nodules represent coalescence of microscopic noncaseating granulomas, distributed along the bronchoarterial bundles, interlobular septa, and subpleural regions. A similar appearance can be seen with silicosis or coal workers’ pneumoconiosis, although with the latter, the distribution of nodules is random with predominant upper lung zone involvement. Within affected areas, the nodules of silicosis can show a predominantly posterior distribution. As disease progresses, coales-

cence of the silicotic nodules leads to progressive massive fibrosis. Numerous small nodules of GGO, in a centrilobular distribution, are characteristic of the subacute stage of extrinsic allergic alveolitis or respiratory bronchiolitis. The nodules are poorly defined and usually measure less than 3 mm in diameter. A random distribution of miliary nodules can be seen with hematogenous spread of tuberculosis (Fig. 6), fungal infection, or metastases from a variety of primary sources. When associated with thin-walled cysts, randomly distributed nodules suggest the diagnosis of Langerhans’ cell histiocytosis. Multiple cavitary nodules can be seen with metastases, usually of squamous cell histology; Wege- ner’s granulomatosis; rheumatoid lung disease; septic emboli; and multifocal infection, typically of fungal or mycobacterial cause. Multiple irregular nodules in a bronchovascular distribution are characteristic of lymphoproliferative disorders, lymphoma, leukemia, and Kaposi’s sarcoma.

GROUND-GLASS PATTERN

Ground-glass opacity is defined as3 . . . hazy increased attenuation of lung, with preservation of bronchial and vascular mar-


Figure 6. Miliary tuberculosis. HRCT (1-mm collimation) of a 63-year-old male alcoholic cigarette smoker with chills and night sweats shows numerous 1- to 2- mm nodules in a random pattern. Transbronchial biopsy showed necrotizing granulomas and “red snappers.” Culture was positive for acid-fast bacilli.

gins; caused by partial filling of air spaces, interstitial thickening, partial collapse of alveoli, normal expiration, or increased capil- lary blood volume; not to be confused with

consolidation, in which bronchovascular margins are obscured; may be associated

with an air bronchogram.

Ground-glass opacity is a common but nonspecific finding on HRCT, reflecting the presence of abnormalities below the limit of CT resolution (see Table 1). In one investigation of patients with chronic infiltrative lung disease in whom lung biopsy was performed in areas of GGO, the pattern was shown to be caused by predominantly interstitial diseases in 54% of cases, equal involvement of the interstitium and airspaces in 32%, and predominantly airspace disease in 14Y0.l~ GGO is an important finding. In certain clinical circumstances, it can suggest a specific diagnosis, indicate a potentially treatable disease, and guide a bronchoscopist or surgeon to an appropriate area for biopsy6

Acute lung diseases characteristically associated with GGO include pneumonia (Fig. 7), pulmonary hemorrhage, and pulmonary edema. In patients with AIDS, the presence of focal or diffuse GGO on CT is highly suggestive of P. carinii pneumonia. In patients with lung transplants, GGO is very sugges-

tive of cytomegalovirus pneumonia (Fig. 8) or acute rejection. When diffuse GGO is seen in the first month after bone marrow transplantation, both infection and diffuse alveolar hemorrhage (Fig. 9) should be considered.

Ground-glass opacity is frequently the main abnormality seen in the acute or subacute phase of extrinsic allergic alveolitis (Fig. 10). It is also the predominant finding in patients who have DIP, in which it reflects the presence of mild interstitial thickening and filling of the air spaces with macrophages. In pulmonary alveolar proteinosis, the areas of GGO usually have a patchy or geographic distribution. Although the abnormality consists mainly of filling of air spaces with pro- teinaceous material, interlobular septa1 thickening is frequently identified on CT in the areas of GGO, creating a ”crazy paving” pattern. Solitary small areas of GGO can signify early stage bronchioloalveolar carcinoma or bronchoalveolar adenoma.

CONSOLIDATION

In contrast to GGO, consolidation obscures underlying vascular structures and frequently is associated with air bronchograms. By defi-

1122 COLLINS

Figure 7. Ground-glass pattern. HRCT (1-mm collimation) of a 34-year- old man with bilateral lung transplants shows patchy areas of consolidation surrounded by ground-glass opacification in the right lower lobe. Biopsy revealed aspergillus pneumonia.

nition, diseases that produce consolidation are characterized by a replacement of alveolar air by fluid, cells, tissue, or some other sub- stance (see Table 1). In the setting of chronic diffuse infiltrative lung diseases, consolidation is seen in chronic eosinophilic pneumonia, bronchiolitis obliterans organizing pneumonia, bronchioloalveolar carcinoma, lipoid

pneumonia, sarcoidosis, and lymphoma. If acute, consolidation is most commonly caused by pulmonary edema (of both cardio- genic and noncardiogenic causes), pulmonary hemorrhage, acute eosinophilic pneumonia, acute extrinsic allergic alveolitis, radiation pneumonitis, pulmonary infarction, or infectious pneumonia (Fig. 11). Pulmonary alveo-

Figure 8. Ground-glass pattern. HRCT (1-mm collimation) of a 52-year-old woman with bilateral lung transplants and cytomegalovirus pneumonia shows a diffuse mosaic pattern of ground-glass opacification. In patients within a few months after lung transplantation, this appearance is suggestive of cytomegalovirus pneumonia or acute rejection.


Figure 9. Ground-glass pattern. HRCT (1-mm collimation) of a 19-year-old woman with acute myelogenous leukemia and bone marrow transplant shows diffuse bilateral ground-glass opacification. Bronchoalveolar lavage revealed diffuse alveolar hemorrhage. This CT finding in patients with recent bone marrow transplantation is suggestive of this diagnosis.

Figure 10. Ground-glass pattern. HRCT (1.5-mm collimation) of a 59-year- old woman with fever, chills, dyspnea on exertion, headache, nonproductive cough, fatigue, and hypoxemia shows bilateral ground-glass nodules (arrows) in a centrilobular distribution. The patient’s symptoms resolved while in the hospital receiving antibiotic treatment and reappeared after discharge to home. The patient had parakeets at home and her husband raised pigeons. Biopsy revealed noncaseating granulomas and lymphocytes consistent with the diagnosis of hypersensitivity pneumonitis. This CT appearance is suggestive of this diagnosis and should prompt a thorough clinical history of specific antigenic exposure.

1124 COLLINS

Figure 11. Pseudomonas pneumonia. HRCT (1-mm collimation) of a 37- year-old woman with acute myelogenous leukemia and elevated white blood cell count shows consolidation with air bronchograms in the lower lobes.

lar proteinosis can present as acute or recurrent pulmonary consolidation.

Chronic eosinophilic pneumonia and bronchiolitis obliterans organizing pneumonia typically result in subpleural consolidation. Sarcoidosis can also manifest as peripheral areas of consolidation on HRCT, although this is not the most common appearance of this disease. Pulmonary infarction is characterized by one or more wedge-shaped areas of consolidation along a pleural surface. When multiple and extensive, pulmonary infarcts can mimic other peripheral lung diseases. Pa- tients with eosinophilic pneumonia and bronchiolitis obliterans organizing pneumonia can rapidly respond to steroid therapy. Pulmo- nary alveolar proteinosis typically manifests as patchy or diffuse consolidation and GGO with geographic margins and septa1 thickening.

MOSAIC PATTERN OF LUNG ATTENUATION

In normal subjects, lung attenuation in- creases significantly during expiration. In the presence of airway obstruction and air trapping, lung remains lucent on expiration and

shows little change in cross-sectional area. Ar- eas of air trapping are seen as relatively low in attenuation on expiratory scans. Areas of air trapping can be patchy and nonanatomic; can correspond to individual secondary pulmonary lobules, segments, and lobes; or may involve an entire lung. Air trapping in a lobe or lung is usually associated with large airway or generalized small airway abnormalities, whereas lobular or segmental air trapping is associated with diseases that affect small airways. Pulmonary vessels within the low-attenuation areas of air trapping often appear small relative to vessels in the more opaque normal lung regions.23 This finding is also seen with vascular disease caused by decreased perfusion to affected areas of lung.

The presence of heterogeneous lung attenuation on inspiratory scans, so-called mosaic pattern of Iung attenuation, can result from infiltrative processes; airway obstruction and reflex vasoconstriction; mosaic perfusion resulting from vascular obstruction (e.g., chronic thromboembolic disease [Fig. 121); or a combination of these (see Table 1). In patients with GGO from infiltrative processes, expiratory HRCT shows a proportional in- crease in attenuation in areas of both increased and decreased opacity. In patients


Figure 12. Sickle cell disease. CT scan (2.5-mm collimation) of a 36- year-old man shows a mosaic pattern of lung attenuation, so-called “mosaic perfusion.” In the abnormal, relatively lucent areas of lung (L), the pulmonary vasculature is diminutive secondary to decreased perfusion from chronic microvascular occlusion.

with mosaic attenuation resulting from airway disease, attenuation differences are ac- centuated on expiration. In patients with mosaic perfusion resulting from vascular disease, expiratory HRCT findings mimic those seen in patients with infiltrative disease on inspiratory scanning. On expiratory scanning, air trapping is not a dominant feature of vascular disease.

TREE-IN-BUD PATTERN

The CT pattern of centrilobular nodular and branching linear opacities has been lik- ened to the appearance of a tree in bud. Many disorders can result in this pattern, the most common being infectious processes with endobronchial spread of disease (see Table l).I,

The common CT features of all processes pro- ducing the tree-in-bud pattern are bronchiolar dilatation, and impaction of bronchioles with mucus, pus, or other material. The CT findings are nonspecific, but a specific diagnosis can occasionally be suggested when the findings are correlated with patient history, clinical information, associated CT scan findings, and chronicity of disease.

The term tree-in-bud dates back to the bronchogram descriptions of normal respiratory bronchioles by Twining and Kerle~,2~ but has been more recently popularized by Im et all3 to describe the CT appearance of the endobronchial spread of Mycobacterium tubercuIosis.

There are numerous noninfectious disorders associated with the tree-in-bud pattern. In allergic bronchopulmonary aspergillosis, immunologic responses to the endobronchial growth of aspergillus species results in dam- age to the bronchial wall, central bronchiectasis, and the formation of mucous plugs that contain fungus and inflammatory cells. The tree-in-bud pattern is seen when the process extends to the bronchioles. In cystic fibrosis, an abnormally low water content of airway mucus is at least partially responsible for decreased mucous clearance, mucous plugging of small and large airways, and an increased incidence of bacterial airway infection. Bron- chial wall inflammation progresses to bronchiectasis and bronchiolar secretions result in a tree-in-bud pattern (Fig. 13). The tree-in- bud pattern can also be seen with aspiration of infected oral secretions or other irritant material, diffuse panbronchiolitis (Fig. 14), obliterative bronchiolitis, and asthma.

1126 COLLINS

Figure 13. Tree-in-bud pattern. HRCT (1-mm collimation) of a 15-year-old girl with cystic fibrosis shows small peripheral nodular and linear branching opacities bilaterally (arrows). Decreased mucous clearance in patients with cystic fibrosis leads to bacterial airway infection, bronchial wall inflammation, bronchiectasis, and bronchiolar secretions leading to this CT pattern.

AIR BRONCHOGRAM SIGN or branching tubular but tapering lucencies representing bronchi or bronchioles passing through densely opacified lung parenchyma (Fig. 15). An air bronchogram within an opacity confirms that the opacity is intrapulmo-

Originally described by Fleischner9 in 1927 and given its name by Felsons many years later, this sign refers to one or more linear

Figure 14. Tree-in-bud pattern. HRCT (1-mm collimation) of a 41-year-old man with progressive dyspnea, productive cough, and multiple episodes of pneumonia after bilateral lung transplantation shows bronchiolar opacities blilaterally (arrows). Biopsy revealed recurrent diffuse panbronchiolitis. (Courtesy of H. Page McAdams, Duke University Medical Center, Durham, NC.)


Figure 15. Air bronchogram sign. CT scan (8-mm collimation) of an 82- year-old man with a history of lymphoma in remission shows bilateral areas of consolidation in a predominantly bronchovascular distribution. Air bronchograms are prominently seen in the right middle (straight arrows) and left lower (curved arrows) lobes. There are also small peripheral nodules (arrowheads). Biopsy revealed bronchiolitis obliterans organizing pneumonia. Both air bronchograms and a bronchovascular distribution are commonly seen with this diagnosis.

nary, not pleural or mediastinal, in location. The most common causes of an air bronchogram are pneumonia and pulmonary edema. Other less common causes include normal expiratory radiograph, nonobstructive pulmonary atelectasis, interstitial fibrosis, and certain neoplasms. Although cancers tend to be solid masses, lymphoma and bronchioloalveolar cell carcinoma are characteristically associated with the air bronchogram sign.

AIR CRESCENT SIGN

A tumor or fungal mass growing within a pre-existing cavity, or an area of pneumonia that undergoes necrosis and cavitates, can form a peripheral crescent of air between the intracavitary mass and the cavity wall, resulting in the air crescent sign (Fig. 16). Intra- cavitary masses are most often caused by fungal mycetomas. In immune-compromised patients with invasive aspergillosis, the appearance of the air crescent sign, representing necrosis and cavitation, indicates recovery of the immune system and white blood cell re-

sponse to the infe~tion.~ Less common causes of this sign are complicated hydatid disease, hematoma, lung abscess, and necrotic neoplasm.

CT ANGIOGRAM SIGN

This sign refers to the visualization of pulmonary vessels within an airless, often "drowned" portion of lung, on contrast-en- hanced CT scanning. The vessels are prominently seen against a background of relatively low-attenuation material (Fig. 17). This sign was first described with bronchioloalveolar cell carcinoma but is seen in a variety of other conditions including lymphoma, obstructive pneumonitis, lipoid pneumonia, bacterial pneumonia, and pulmonary edema.Is

DROWNED LUNG SIGN

This sign refers to gas absorption and replacement of air by edema fluid distal to an acute bronchial obstruction. On CT, the bron-

1128 COLLINS

Figure 16. Air crescent sign. HRCT 1-mm collimation) of a 50-year-old woman with acute myelogenous leukemia and fever shows a wedge-shaped peripheral area of consolidation in the right lung. Early cavitation is mani- fested as a crescentic-shaped collection of air (arrow) within the consolidation anteriorly. Biopsy revealed invasive pulmonary aspergillosis. The air crescent sign, representing necrosis and cavitation, indicates recovery of the immune system and white blood cell response to the infection.

Figure 17. CT angiogram sign. CT scan (3-mm collimation) of a 66-year-old woman with oxygen-dependent emphysema shows prominent vessels (arrows) against a background of relatively low attenuation material. In this case, the consolidation was caused by streptococcal pneumonia.


Figure 18. Drowned lung sign. A, CT scan (5-mm collimation) of an 18-year-old woman with fever, shortness of breath, cough, and wheezing shows collapse of the right lower lobe. Within the collapsed lobe are low-attenuation tubular structures (arrows) representing mucus and edema fluid within bronchi. This appearance was secondary to an obstructing carcinoid tumor within the bronchus intermedius. Lung air has been absorbed and replaced by edema fluid distal to the obstruction. 6, CT image of the same patient at a level inferior to A shows expansion of the right lower and middle lobes secondary to a buildup of secretions and infection behind the obstructing lesion. Note prominent vessels (arrows) against a background of low-attenuation secretions, creating the CT angiogram sign.

chi within the lobe are prominently visualized because they are filled with low-attenuation material, yielding a mucus bronchogram (Fig. 18). This sign is similar to, and may be seen with, the CT angiogram and air bronchogram signs. If the obstruction persists and the ob- s h c t e d lobe remains sterile, excess edema fluid and blood within the drowned lobe are gradually reabsorbed. The affected lobe en- larges when there is a buildup of secretions

and infection behind the obstructing lesion, often seen in postobstructive pneumonia related to endobronchial carcinoma.

FALLEN LUNG SIGN

The fallen lung refers to a diagnostic but uncommon CT sign of complete or near complete bronchial transection, in which the col-

1130 COLLINS

Figure 19. Fallen lung sign. CT scan (10-mm collimation) of a young woman involved in a motor vehicle accident shows a collapsed right lung (large arrows), which has fallen posteriorly and laterally away from the hilum. Note a large pneumothorax with a chest tube (arrowhead) in place. The pneumothorax is present because there is free communication between the fractured bronchus intermedius and the pleural space. There is also a small pneumomediastinum (small arrows), which is commonly seen with this type of injury.

lapsed lung falls away from the hilum toward the lateral and posterior chest wall or dia- phragm with supine positioning (Fig. 19).19 Normally, the lung collapses inward toward the hilum. The finding is usually associated with a large pneumothorax, which because of the free communication between the fractured airway and the pleural space, is persis- tent despite thoracostomy tube drainage. Pneumomediastinum is also commonly seen. CT may directly show the bronchial tear as discontinuity or focal narrowing of the affected bronchus.

GLOVED FINGER SIGN

When bronchiectatic segments become filled with mucus or pus, they appear on CT as opacities with distinctive shapes, variously described as gloved finger, finger-in-glove, Y, V, inverted V, toothpaste, and so forth.21 This sign is characteristically seen when central bronchi are involved with allergic bronchopulmonary aspergillosis, a clinical disorder secondary to aspergillus hypersensitivity (Fig. 20). In this case, the bronchi become impacted

with mucus, cellular debris, eosinophils, and fungal hyphae. This sign can also be seen in cystic fibrosis, another disease in which central bronchiectasis predominates.

GOLDEN S SIGN

When a lobe collapses around a large central mass, the peripheral lung collapses and the central portion of lung is prevented from collapsing by the presence of the mass. The affected fissure has a central convexity because of the mass itself and distal concavity as a result of atelectasis, resembling an S or a reverse S shape.lo Although initially described for right upper lobe atelectasis, the sign is applicable to atelectasis of any lobe. The im- portance of this sign is to realize the presence of central obstruction. In an adult, an underlying bronchogenic carcinoma is usually the cause of obstruction.

HALO SIGN

This CT scan sign refers to GGO surrounding or forming a halo around a denser


Figure 20. Gloved finger sign. HRCT (1-mm collimation) of a 28-year-old woman with allergic bronchopulmonary aspergillosis shows a dilated central bronchus (B) filled with mucus, cellular debris, and fungal hyphae (arrows). The airless, impacted portion of the dilated bronchus can create a gloved finger, Y, V, inverted V, or toothpaste appearance on CT scan.

nodule or focal area of consolidation (see Fig. 7). Many hemorrhagic pulmonary nodules, such as those seen with Kaposi’s sarcoma metastases, metastatic angiosarcoma, Wegener’s granulomatosis, and infection from Candida species, Mucorales species, cytomegalovirus, and herpes simplex can produce this s i p z o When seen in patients with acute leukemia, the halo sign is highly suggestive of early angioinvasive pulmonary aspergill~sis.’~ In this case, the halo is related to the presence of hemorrhage surrounding the central necrotic nodule. With time, these lesions may develop air crescents and progress to frank cavitation.

ies, usually 3 to 5 cm in diameter but reaching 10 cm. Air bronchograms are rarely seen. When necrosis occurs, radiologic clearing may take weeks. The pattern of resolution is a helpful indicator of pulmonary infarction, which gradually diminishes while main- taining its homogeneity and original shape (the so-called melting ice cube sign [Fig. 211). This is distinguished from the pattern of resolution of pneumonia, where the opacification disappears in a patchy fashion.27 Residual findings from pulmonary infarction are common and include linear scars, pleurodi- aphragmatic adhesions, and localized pleural thickening.

HAMPTON’S HUMP SIGN LUFTSICHEL SIGN

Pulmonary infarction secondary to pulmonary embolism produces an abnormal area of opacification on the chest radiograph or CT scan, which is always in contact with the pleural surface. The opacification usually re- sembles a truncated cone, so-called Humpton’s hump.12 The configuration of the opacity is that of homogeneous wedge-shaped consolidation in the lung periphery, with the base contiguous to a visceral pleural surface and the rounded, convex apex directed toward the hilum. The size of the consolidated area var-

In upper lobe collapse, there is hyperinfla- tion of the superior segment of the lower lobe. This hyperinflated lung is seen as a crescent- shaped lucency called a luffsichel, derived from the German words ”luft” (air) and ”si- chel” (~ickle).~ This sign is more often seen on the left side, in which the hyperinflated superior segment of the left lower lobe is positioned between the aortic arch and the collapsed left upper lobe. CT shows a V- shaped or peaked appearance of the postero-

1132 COLLINS

Figure 21. Hampton’s hump and melting ice cube signs. CT scan (7-mm collimation) of a 69-year-old man with acute pulmonary embolism diagnosed 6 weeks earlier shows bilateral peripheral opacities (arrows). These opacities have become pro- gressively smaller compared with the appearance on a CT scan at the time of diagnosis. The central portion of the opacities remains consolidated, while the total size decreases, as occurs with a melting ice cube. This pattern of resolution distin- guishes pulmonary infarction from pneumonia, the latter resolving both centrally and peripherally at the same time.

medial fissural surface between the atelectatic upper lobe and the overinflated lower lobe segment (Fig. 22). This sign, and associated findings of upper lobe collapse, signify the diagnosis of an endobronchial mass, such as endobronchial carcinoma.

SPLIT PLEURA SIGN

Normally, the thin visceral and parietal pleura cannot be distinguished as two separate structures on CT scanning. With empyema, the fluid separates or splits the thickened and contrast-enhancing pleural layers (Fig. 23).22 The split pleura sign allows confident distinction of loculated empyema from a peripheral lung abscess. Other processes that lead to pleuritis, such as hemothorax and treatment of malignant pleural effusion with talc pleurodesis, can also result in this appearance on CT.

WESTERMARK SIGN

This sign refers to focal oligemia of the lung distal to an occluded pulmonary artery. Oligemia distal to an obstructing embolus, so- called Westermark sign, is a relatively uncommon findingz6 Because bronchial obstruction is not a feature, there is no overinflation and lung volume may be reduced. A similar CT appearance of lung oligemia may be pro- duced by neoplastic obstruction of a pulmonary artery secondary to either invasion by a contiguous carcinoma or intravascular growth of a primary sarcoma.

SUMMARY

The ground-glass pattern is a common but nonspecific finding on CT. In certain clinical circumstances, it can suggest a specific diagnosis, indicate a potentially treatable disease, and guide a clinician to an appropriate area for biopsy. A pattern of centrilobular ground-


Figure 22. Luftsichel sign. CT scan (8-mm collimation) of a 67-year-old man with squamous cell carcinoma of the left upper lobe causing left upper lobe collapse shows a hyperexpanded, hyperlucent superior segment of the left lower lobe (L) situated between the aortic arch (A) and collapsed left upper lobe (U). CT shows a V-shaped appearance of the posterornedial fissural surface between the atelectatic upper lobe and the overinflated lower lobe segment. The crescentic-shaped, hyperlucent left lower lobe represents the luftsichel. In an adult, this sign, indicative of left upper lobe collapse, signals an underlying endobronchial obstructing lesion.

glass nodules is fairly specific for the diagnosis of hypersensitivity pneumonitis with the appropriate clinical history. The tree-in-bud pattern indicates disease affecting the small airways. The differential diagnosis is lengthy;

however, the most common process leading to this CT appearance is infection. Although commonly associated with M . tuberculosis, many infectious organisms can produce this pattern. When honeycombing is seen on

Figure 23. Split pleura sign. CT scan (10-rnrn collimation) shows separation of the thickened and enhancing visceral (V) and parietal (P) pleural layers. The fluid in the pleural space is infected with Staphylococcus aureus organisms, diagnostic of an ernpyerna. The air-fluid level (arrowhead) within the pleural space is a result of thoracentesis.

1134 COLLINS

HRCT, a confident diagnosis of lung fibrosis can be made. The most common causes of interlobular septal thickening on HRCT are pulmonary edema, pulmonary hemorrhage, and lymphangitic spread of cancer, and smooth thickening is characteristic of all three. Diffuse lung cysts in patients who are not immunocompromised generally signify Langerhans’ cell histiocytosis, lymphangioleiomyomatosis, or centrilobular emphysema. Centrilobular emphysema can be diagnosed when the centrilobular artery is seen as a small nodular opacity in the center of the cyst. Langerhans’ cell histiocytosis is often associated with parenchymal nodules, help- ing to distinguish it from lymphangioleiomyomatosis. When a nodular pattern is seen on HRCT, the differential diagnosis is very long, but can be narrowed by noting whether the nodules are random, centrilobular, or perilymphatic in distribution. A mosaic pattern of lung attenuation can represent an infiltrative, small airway, or vascular process. The distinction can often be made by noting the size of the pulmonary vessels in the abnormal areas of lung, and whether air trapping is present on expiratory scanning.

Computed tomographic signs can be useful indicators of a specific disease process. For instance, the air bronchogram sign indicates that an opacity is intrapulmonary in location, and signals the possibility of two types of neoplasm: lymphoma and bronchioloalveolar cell carcinoma. An air crescent sign indicates recovery of the immune system in an immunocompromised patient with invasive pulmonary aspergillosis. The fallen lung sign is diagnostic of a bronchial transection in the correct clinical setting. The gloved finger sign is very suggestive of allergic bronchopulmonary aspergillosis. The halo sign is highly suggestive of early angioinvasive pulmonary aspergillosis in patients with acute leukemia. When a split pleura sign is seen, the diagnosis is often empyema, although other causes of pleuritis can lead to a similar CT appearance.

References

1. Aquino SL, Gamsu G, Webb WR, et al: Tree-in-bud pattern: Frequency and significance on thin section CT. J Comput Assist Tomogr 20:594-599, 1996

2. ATS/ERS International Multidisciplinary Consensus Classification of Idiopathic Interstitial Pneumonias: General Principles and Guidelines. Toronto, Ameri- can Thoracic Society, 2000

3. Austin JHM, Miiller NL, Friedman PJ, et al: Glossary of terms for CT of the lungs: Recommendations of the nomenclature committee of the Fleischner Society. Radiology 200:327, 1996

4. Burgel E, Oleck HG: Ueber die rechtsseitige para- mediastinale Luftsichel bei Oberlappenschrumpfung. Rofo Fortschr Geb Rontgenstr Neuen Bildgeb Verfahr

5. Collins J, Blankenbaker D, Stem EJ: CT patterns of bronchiolar disease: What is ”tree-in-bud”? AJR Am J Roentgenol 171:365-370, 1998

6. Collins J, Stem EJ: Ground-glass opacity at CT The ABCs. AJR Am J Roentgenol 169:355-367, 1997

7. Curtis AM, Smith GJW, Ravin CE: Air crescent sign of invasive aspergillosis. Radiology 133:17, 1979

8. Felson B: Chest Roentgenology. Philadelphia, WB Saunders, 1973

9. Fleischner FG: Der sichtbare Bronchialbaum, ein differential diagnostisches Symptom im Rontgenbild der Pneumonie. Fortschr Roentgenstr 36:319, 1927

10. Golden R The effect of bronchostenosis upon the roentgen-ray shadows in carcinoma of the bronchus. AJR Am J Roentgenol 13:21-30, 1925

11. Gruden JF, Webb WR, Naidich DP, et al: Multinodular disease: Anatomic localization at thin-section C T

93~160-163, 1960

Multireader evaluation of a simple algorithm. Radiol- ogy 210:711-720, 1999

12. G m p t o n AO, Castleman B: Correlations of post mortem chest teleroentgenograms with autopsy findings with special reference to pulmonary embolism and infarction. AJR Am J Roentgenol 43:305- 326, 1940

13. Im JG, Itoh H, Shim YS, et al: Pulmonary tuberculosis: CT findings-early active disease and sequential change with antituberculous therapy. Radiology 186:653-660, 1993

14. Kang EY, Grenier P, Laurent F, et al: Interlobular septal thickening: Patterns at high-resolution computed tomography. J Thorac Imaging 11:260-264,1996

15. Kuhlman JE, Fishman EK, Siegelman SS Invasive pulmonary aspergillosis in acute leukemia: Charac- teristic findings on CT, the CT halo sign, and the role of CT in early diagnosis. Radiology 157611-614,1985

16. Leung AN, Miller RR, Miiller N L Parenchymal opacification in chronic infiltrative lung diseases: CT- pathologic correlation. Radiology 188:209, 1993

17. Miiller NL, Miller RR, Webb WR, et al: Fibrosing alveolitis: CT-pathologic correlation. Radiology

18. Murayama S, Onitsuka H, Murakami J, et al: CT angiogram sign in obstructive pneumonitis and pneumonia. J Comput Assist Tomogr 17:609-612, 1993

19. Oh KS, Fleischner FG, Wyman SM: Characteristic pulmonary findings in traumatic complete transection of a main stem bronchus. Radiology 92371- 372, 1969

20. Primack SL, Hartman TE, Lee KS, et al: Pulmonary nodules and the CT halo sign. Radiology 190:513- 515, 1994

21. Simon G: Principles of Chest X-ray Diagnosis, ed 3. London, Butterworth, 1971

22. Stark DD, Federle MP, Goodman PC, et al: Differenti- ating lung abscess and empyema: Radiography and computed tomography. AJR Am J Roentgenol 141:163-167, 1983

1601585-588, 1986


23. Stem EJ, Webb WR: Dynamic imaging of lung mor- the lung: A proposed glossary. J Thorac Imaging

26. Westermark N: On the roentgen diagnosis of lung embolism. Acta Radio1 19:357-372, 1938

27. Woesner ME, Sanders I, White GW The melting sign in resolving transient pulmonary infarction. AJR Am J Roentgen01 111:782-790, 1971

phology with ultrafast high-resolution computed to- 8:167-185, 1993 mography. J Thorac Imaging 8:273-282, 1993

24. Twining E, Kerley P: Textbook of X-Ray Diagnosis, ed 2. London, Lewis, 1951

25. Webb WR, Miiller NL, Naidich DP: Standardized terms for high-resolution computed tomography of

Address reprint requests to

Jannette Collins, MD, MEd Department of Radiology

University of Wisconsin Hospital and Clinics E3/311 Clinical Science Center

600 Highland Avenue Madison, WI 53792-3252

e-mail: jcollin48facstaff.wisc.edu

ct signs and patterns of lung disease

Documents