Thoracic signs can be classified into three groups as shown in table 1. 

In the first group we consider those signs that are able to determine the localization of the opacity such as intrapulmonary, mediastinal or pleural; in the second group those signs that determine a variation of the pulmonary volumes in terms of increase or decrease, the last group consider signs that help us in the differential diagnosis.

LOCALIZE SIGN

Silhouette sign is the loss of an anatomic soft-tissue border. The silhouette sign results from the juxtaposition of structures of similar radiographic opacity; it helps to determine where the pathologic process is located. It is caused by consolidation and/or atelectasis of the adjacent lung by a tumor, or by pleural effusion. Three types of silhouette sign are described: - the superior heart silhouette that represent the loss of the 1st right or left cardiac arch (for pathological involvement of the right and left upper lobe respectively); - the right lower cardiac silhouette, whose loss is due to pathologies of the middle lobe (Fig. 1), pericardium and lower portion of the large fissure and on the contrary, its persistence is found in pathologies of the right lower lobe, posterior pleural cavity and posterior chest wall (Fig. 2); - the left lower cardiac silhouette, whose loss is due to pathologies of the lingula, pericardium and lower portion of the fissure and on the contrary, its persistence is found in pathologies of the left lower lobe, posterior pleural cavity and posterior chest wall (Fig. 3).

Cervicothoracic sign, is a variation of the silhouette sign. This sign is used to differentiate between an anterior and posterior mass in the superior mediastinum.Any mass when situated in the posterior mediastinum is completely surrounded by the lung tissue, and this leads to a well‐defined cephalic border seen above the clavicle (Fig. 4). In contrast to this, anterior mediastinal masses have ill‐defined cephalic margins due to their anatomical contact with the soft tissues of the neck (Fig. 5). 

Toraco-abdominal sign (Fig. 6).

Hylum convergence sign helps to distinguish a bulky hilum due to pulmonary artery dilatation from juxta-hilar mass/nodal enlargement (Fig. 7). 

Hilum overlay sign is useful in differentiating hilum enlargement determined by a mediastinal mass. When a mass arises from the hilum, the normal pulmonary vessels are in contact with the lesion and their silhouette is obscured. If the edges of the vessels are appreciated, this implies the mass is not in contact with the hilum and is, therefore, anterior or posterior to it (Fig. 8).

Incomplete border sign is often present in case of an extrapulmonary mass on chest X-ray. Extrapulmonary lesions have often a well-defined inner border because it is tangential to the X-ray beam and has good inherent contrast with the adjacent lung, whereas the outer margin is ill-defined (Fig. 9-10). Moreover an intrapulmonary mass has acute angles, the pleural or extrapleural mass has obtuse angles with the parietal wall (Fig. 11)

Air bronchogram is a pattern of air-filled (low-attenuation) bronchi on a background of opaque (high-attenuation) air-less lung (Fig 12). The sign implies patency of proximal airways and evacuation of alveolar air by means of absorption (atelectasis) or replacement (eg, pneumonia) or a combination of these processes. 

Finger in glove sign or gloved finger sign refers to airway filling by mucoid secretions with mucoid impaction, mucocele or bronchocele.
The secretions may be depicted on chest radiographs or CT images as tubular or branching opacities that radiate from the hilum toward the periphery of the lung (Fig. 13). 

LUNG VOLUME CHANGES

Luftsichel sign, it refers to the hyperinflation of the superior segment of left lower lobe interposing itself between the mediastinum and the collapsed left upper lobe (Fig. 14).

Golden S sign is named because it resembles a reverse S shape. It can be produced by a central mass such as metastasis, primary mediastinal tumor, or enlarged lymph nodes obstructing the upper lobe bronchus with atelectasis/collapse of the lung. Although typically seen in right upper lobe collapse (Fig. 15), the S-sign can also be present in the collapse of other lobes. 

Retrocardiac sail sign or “flat waist sign” represents the characteristic appearance of left lower lobe collapse on a frontal chest X-ray. The collapsed left lower lobe is represented by a triangular area of increased density with sharp margins superimposed on the heart shadow; the heart contours remain preserved while the contour of the left hemidiaphragm is partially effaced (Fig. 16). 

Juxtaphrenic peak sign, is a small triangular opacity based at the apex of the dome of a hemidiaphragm, associated with upper lobe volume loss of any cause. The peak is caused by upward retraction of the inferior accessory fissure or an intrapulmonary septum associated with the pulmonary ligament. It is most readily appreciated on a frontal chest radiograph (Fig 17). 

DIFFERENTIAL DIAGNOSES

Air crescent sign is a collection of air in a crescentic shape that separates the wall of a cavity from an inner mass (Fig 18). The air crescent sign is often considered characteristic of either Aspergillus colonization of preexisting cavities. However, the air crescent sign has also been reported in other processes that cause pulmonary necrosis.

Spinnaker sign (also known as the angel wing sign) is a sign of pneumomediastinum seen on neonatal chest radiographs and it refers to the thymus outlined by air and displaced laterally, appearing like spinnaker sails (Fig. 19). It is distinct from the sail sign appearance of the normal thymus that represents the triangular shape inferior margin of the thymus seen on neonatal chest radiograph (Fig. 19). 

The double-density sign), also known as the double right heart border represent left atrial enlargement, that appears as a curvilinear soft‐tissue density in the right retrocardiac region along with the right atrium opacity (Fig. 20).

The Deep sulcus sign, is seen in supine chest radiographs of patients with pneumothorax. This sign is visualized in supine position because air accumulates in the nondependent parts of the pleura, i.e. anterior and basal parts, in contrast to the upright position in which air accumulates in the apex of the pleura. Commonly air collects in the lateral costophrenic angle, which appears lucent and deep when compared to the other costophrenic angle (Fig.21). Once visualized, one should always look for other signs of pneumothorax in cases of major trauma, neonates, and ICU patients to avoid errors.

Continuous Diaphragmatic Sign is seen in pneumomediastinum in which air accumulates between the lower border of the heart and the superior part of the diaphragm, which results in complete visualization of the diaphragm in chest X‐ray (Fig. 22). Normally, the central part of the diaphragm is obscured by the heart, and hence is not seen on chest radiographs. Though this sign is commonly seen in pneumomediastinum, it can occasionally be also seen in pneumopericardium.

Doughnut sign occurs when mediastinal lymphadenomegaly occurs behind the bronchus intermedius in the subcarinal region. Lymphadenopathies are seen as lobulated densities on lateral chest x-ray, surrounding the bronchail central radiolucent area (Fig. 23).