RADIOLOGICAL SIGNS

A sign in chest radiology refers to a radiographic and/or computed tomographic scan finding that implies a specific pathologic process. Understanding the meaning of a sign indicates comprehension of an important concept related to the radiologic findings. Knowing the name of the sign is not as important as recognizing and understanding the meaning of the radiologic findings, but it will help in communicating with clinicians and radiologists who use the “sign” terminology. A CT “pattern” refers to a nonspecific radiologic finding or collection of findings suggesting one or more specific disease processes. The material that follows is not an all-inclusive list but represents a collection of the more common and useful signs and patterns of focal and diffuse lung disease.
Air Bronchogram Sign

This sign refers to a branching, linear, tubular lucency representing a bronchus or bronchiole passing through airless lung parenchyma . This sign does not differentiate nonobstructive atelectasis from other abnormal parenchymal opacities such as pneumonia. An air bronchogram indicates that the underlying opacity must be parenchymal rather than pleural or mediastinal in location. Although cancers tend to be solid masses, air bronchograms are a characteristic feature of lymphoma and bronchoalveolar cell carcinoma.

Air Crescent Sign
A mass growing within a pre-existing cavity, or an area of pneumonia that undergoes necrosis and cavitates, may form a peripheral crescent of air between the intracavitary mass and the cavity wall, resulting in the air crescent sign . Intracavitary masses are most often caused by mycetomas. In immunocompromised 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 response to the infection.

Bulging Fissure Sign
Historically, the bulging fissure sign was seen as a result of pne-umonia caused by Klebsiella pneumoniae involving the right upper lobe . Also called Friedländer pneumonia, the disease is often confined to one lobe, with consolidation spreading rapidly, causing lobar expansion and bulging of the adjacent fissure inferiorly . Because of timely antibiotic treatment, pneumonia rarely progresses to this state.
Continuous Diaphragm Sign
This sign is seen as a continuous lucency outlining the base of the heart, representing pneumomediastinum . Air in the mediastinum tracks extrapleurally, between the heart and diaphragm . Pneumopericardium can have a similar appearance but will show air circumferentially outlining the heart.

CT Angiogram Sign
This sign refers to the identification of vessels within an airless portion of lung on contrast-enhanced CT . The vessels are prominently seen against a background of low-attenuation material . This sign has been associated with bronchoalveolar cell carcinoma and lymphoma, but it can also be seen with other processes, including many infectious pneumonias.
Deep Sulcus Sign
This sign refers to a deep, sometimes fingerlike collection of intrapleural air or pneumothorax in the costophrenic sulcus as seen on the supine chest radiograph (5). In the supine patient, air rises to the nondependent anteromedial basilar pleural space and may not cause displacement of the visceral pleural line laterally or at the apex, as seen on upright chest radiographs . When present, this sign may represent a pneumothorax that is much larger than initially expected.


Fallen Lung Sign
This sign refers to the appearance of the collapsed lung occurring with a fractured bronchus . The bronchial fracture results in the lung “falling” away from the hilum, either inferiorly and laterally in an upright patient or posteriorly, as seen on CT in a supine patient. Normally, a pneumothorax causes a lung to collapse inward toward the hilum.
Flat Waist Sign
This sign refers to flattening of the contours of the aortic knob and adjacent main pulmonary artery . It is seen in severe collapse of the left lower lobe and is caused by leftward displacement and rotation of the heart .
Finger-in-Glove Sign
In allergic bronchopulmonary aspergillosis, a clinical disorder secondary to Aspergillus hypersensitivity, the bronchi become impacted with mucus, cellular debris, eosinophils, and fungal hyphae. The impacted bronchi appear radiographically as opacities with distinctive shapes , variously described as “gloved finger,” “Y,” “V,” “inverted V,” “toothpaste,” and so forth .

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 relevant fissure is concave toward the lung peripherally but convex centrally, and the shape of the fissure resembles an S or a reverse S . This sign is important because it signifies the presence of a central obstructing mass that, in an adult, may represent bronchogenic carcinoma.
Halo Sign
This sign refers to ground-glass attenuation on CT scanning that surrounds, or forms a halo around, a denser nodule or area of consolidation . Although most hemorrhagic pulmonary nodules produce this sign , when seen in patients with acute leukemia, the halo sign suggests early invasive pulmonary aspergillosis .

Hampton Hump Sign
Pulmonary infarction secondary to pulmonary embolism produces an abnormal area of opacification on the chest radiograph, which is always in contact with the pleural surface . The opacification may assume a variety of shapes. When the central margin is rounded, a “hump” is produced, as described by Hampton and Castleman .

Juxtaphrenic Peak Sign
This sign refers to a small triangular shadow that obscures the dome of the diaphragm , secondary to upper lobe atelectasis. The shadow is caused by traction on the lower end of the major fissure, the inferior accessory fissure, or the inferior pulmonary ligament.

Luftsichel Sign
In left upper lobe collapse, the superior segment of the left lower lobe, which is positioned between the aortic arch and the collapsed left upper lobe, is hyperinflated. This aerated segment of left lower lobe is hyperlucent and shaped like a sickle, where it outlines the aortic arch on the frontal chest radiograph . This peri-aortic lucency has been termed the luftsichel sign, derived from the German words luft or air and sichel or sicklel. Although this sign can also be seen on the right, it is more common on the left because of the difference in anatomy and the presence of a minor fissure on the right. This sign and associated findings of upper lobe collapse signify the probable diagnosis of bronchogenic carcinoma in an adult.

Melting Ice Cube Sign
This sign refers to the appearance of a resolving pulmonary infarct on a chest radiograph or CT scan, which looks like an ice cube that is melting peripherally to internally. This is distinguished from the pattern of resolving pneumonia, where the opacification disappears in a patchy fashion .

Ring Around the Artery Sign
This sign refers to a well-defined lucency encircling the right pulmonary artery , as seen on frontal and lateral chest radiographs, representing pneumomediastinum .

Silhouette Sign
Felson and Felson popularized the term silhouette sign to indicate an obliteration of the borders of the heart, other mediastinal structures, or diaphragm by an adjacent opacity of similar density. An intrathoracic lesion not anatomically contiguous with a border of one of these structures will not obliterate that border. Parenchymal processes involving the medial segment of the right middle lobe obliterate the right heart border . If the lingula is involved, the left heart border is obliterated . Lower lobe processes involving one or more basilar segments result in obliteration of all or a part of the border of the diaphragm.


Split Pleura Sign
Normally, the thin visceral and parietal pleura cannot be distinguished as two separate structures on CT scanning. With an exudative pleural effusion, such as empyema , the fluid separates or splits the thickened and enhancing pleural layers .

Westermark Sign
This sign refers to oligemia of the lung beyond an occluded vessel in a patient with pulmonary embolism.

Spine Sign
Lower lobe pneumonia may be poorly visualized on a posteroanterior chest radiograph. In such cases, the lateral view is often helpful when it shows the spine sign, which is an interruption in the progressive increase in lucency of the vertebral bodies from superior to inferior .

Patterns
The following patterns are not always isolated findings on chest radiographs or CT scans. They commonly occur in combination with other patterns and findings and may or may not represent the predominant imaging feature.

Honeycomb Pattern
Honeycombing is characterized by the presence of cystic airspaces with thick, clearly definable fibrous walls lined by bronchiolar epithelium. It results from destruction of alveoli and loss of acinar architecture and is associated with pulmonary fibrosis. The cysts are typically layered along the pleural surface, helping to distinguish them from the nonlayered subpleural lucencies seen with paraseptal emphysema.
Honeycombing produces a characteristic appearance on CT that allows a confident diagnosis of lung fibrosis . On CT, 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 that are 1 to 3 mm thick, they are air-filled, and they appear lucent in comparison to normal lung parenchyma. Honeycombing is usually associated with other findings of lung fibrosis, such as architectural distortion, intralobular interstitial thickening, traction bronchiectasis, and irregular linear opacities. Honeycombing on CT usually represents idiopathic pulmonary fibrosis,


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 and are occasionally seen on normal thin-section CT scans. Abnormal thickening of interlobular septa is caused by fibrosis, edema, or infiltration by cells or other material. Within the peripheral lung, thickened septa 1 to 2 cm in length may outline part or all of a secondary pulmonary lobule, perpendicular to the pleural surface. They represent the CT counterpart of Kerley B lines seen on chest radiographs.
Interlobular septal thickening can be smooth or nodular . Smooth thickening is seen in patients with pulmonary edema or hemorrhage, lymphangitic spread of carcinoma, lymphoma, 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 worker's pneumoconiosis, lymphocytic interstitial pneumonia, and amyloidosis.

Cystic Pattern
The term “cyst” is nonspecific and refers to a thin-walled , usually less than 3 mm thick, well-defined, well-circumscribed, air- or fluid-containing lesion, 1 cm or more in diameter, that has an epithelial or fibrous wall. A cystic pattern results from a heterogeneous group of diseases that have in common the presence of focal, multifocal, or diffuse parenchymal lucencies and lung destruction (Table 2-1). Pulmonary Langerhan cell histiocytosis, lymphangioleiomyomatosis, sarcoidosis, lymphocytic interstitial pneumonitis, collagen vascular diseases, Pneumocystis pneumonia, and honeycombing can manifest a cystic pattern on CT. Although they do not represent true cystic disease, centrilobular emphysema and cystic bronchiectasis mimic cystic disease on chest CT scans.
In cases of Langerhan 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. 2-26). The intervening lung parenchyma is typically normal, without evidence of fibrosis or septal 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 (Fig. 2-27), and nodules are not a common feature. The “cystic” spaces seen with centrilobular emphysema often contain a small nodular opacity representing the centrilobular artery . This finding is helpful in distinguishing emphysema from lymphangioleiomyomatosis and Langerhan cell histiocytosis.

Nodular Pattern
A nodular pattern refers to multiple round opacities, generally ranging in diameter from 1 mm to 1 cm, that may be very difficult to separate from one another as individual nodules on a

chest radiograph because of superimposition but which are accurately diagnosed on CT. 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 increases. Nodules can be further characterized according to their margins , smooth or irregular, presence or absence of cavitation, attenuation characteristics ,such as ground-glass opacity or calcification, and distribution (e.g., centrilobular, perilymphatic, or random.
Multiple small smooth or irregularly marginated nodules in a perilymphatic distribution are characteristic of sarcoidosis . The nodules represent the 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 worker's 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, coalescence of the silicotic nodules leads to progressive massive fibrosis. Numerous small nodules of GGO in a centrilobular distribution are characteristic of the acute or subacute stage of extrinsic allergic alveolitis (Fig. 2-30) 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, fungal infection, or metastases from a variety of primary sources. When associated with irregularly shaped, thin-walled cysts, randomly distributed nodules suggest Langerhan cell histiocytosis. Multiple cavitary nodules can be seen with metastases , usually of squamous cell histology, Wegener granulomatosis, rheumatoid lung disease, septic emboli, and multifocal infection (typically of fungal or mycobacterial etiology). Multiple irregular nodules in a bronchovascular distribution are characteristic of benign lymphoproliferative disorders , lymphoma, leukemia, and Kaposi sarcoma.

Ground-Glass Pattern
GGO is defined as “hazy increased attenuation of lung, with preservation of bronchial and vascular margins; caused by partial filling of airspaces, interstitial thickening, partial collapse of alveoli, normal expiration, or increased capillary blood volume; not to be confused with consolidation, in which bronchovascular margins are obscured; may be associated with an air bronchogram” . GGO is a common but nonspecific finding on CT that reflects the presence of abnormalities below the limit of CT resolution (Table 2-1). 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 14% (25). 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 biopsy .

Acute lung diseases characteristically associated with diffuse GGO include pneumonia , pulmonary hemorrhage, and pulmonary edema. In patients with acquired immunodeficiency syndrome, the presence of focal or diffuse GGO on CT is highly suggestive of Pneumocystis pneumonia. In patients with lung transplants, GGO is very suggestive of Cytomegalovirus pneumonia or acute rejection. When diffuse GGO is seen in the first month after bone marrow transplantation, both infection and diffuse alveolar hemorrhage should be considered.
Diffuse or patchy GGO is frequently the main abnormality seen in the acute or subacute phase of extrinsic allergic alveolitis. It is also the predominant finding in patients with desquamative interstitial pneumonia, in which it reflects the presence of mild interstitial thickening and filling of the airspaces 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 airspaces with proteinaceous material, interlobular septal 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 atypical adenomatous hyperplasia (AAH).

Mosaic Pattern of Lung Attenuation
Lung attenuation normally increases during exhalation. In the presence of airway obstruction and air trapping, lung remains lucent on exhalation and shows little change in cross-sectional area; this is best appreciated when patchy and compared to normal lung. Areas of air trapping are seen as relatively low in attenuation on expiratory CT 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. Bronchiolectasis is a common associated finding. Pulmonary vessels within the low-attenuation areas of air trapping often appear small relative to vessels in the more opaque normal lung regions . This finding is also seen with vascular disease, such as chronic thromboembolic disease, as a result of decreased perfusion to affected areas of lung.
The presence of heterogeneous lung attenuation on inspiratory scans—the so-called mosaic pattern of lung attenuation—can result from infiltrative processes, airway obstruction and reflex vasoconstriction, mosaic perfusion resulting from vascular obstruction (e.g., chronic thromboembolic disease; or a combination of these . In patients with GGO from infiltrative processes, expiratory CT shows a proportional increase in attenuation in areas of both increased and decreased opacity. In patients with mosaic attenuation resulting from airway disease, such as obliterative bronchiolitis or asthma, attenuation differences are accentuated or seen only on expiration (Fig. 2-36). In patients with mosaic perfusion caused by vascular disease, air trapping can be seen but is not a dominant feature on expiratory CT.

Tree-in-Bud Pattern
The CT pattern of centrilobular nodular and branching linear opacities has been likened to the appearance of a budding tree. Many disorders can result in this pattern, the most common being infectious processes with endobronchial spread of disease . The common CT features of all processes producing the tree-in-bud pattern are bronchiolar

dilatation and (b) 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 Kerley but has been more recently popularized by Im et al (31) to describe the CT appearance of the endobronchial spread of Mycobacterium tuberculosis.
Numerous noninfectious disorders are associated with the tree-in-bud pattern. In allergic bronchopulmonary aspergillosis, immunologic responses to the endobronchial growth of Aspergillus sp result in damage 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. Bronchial wall inflammation progresses to bronchiectasis, and bronchiolar secretions result in a tree-in-bud pattern (Fig. 2-38). The tree-in-bud pattern can also be seen with aspiration of infected oral secretions or other irritant material , diffuse panbronchiolitis , obliterative bronchiolitis, and asthma.