What are Radiological Signs in the Chest that every doctor should know?

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identify pathological processes

Radiological signs allow you to identify pathological processes to guide diagnosis. That is why you must be able to identify and interpret them.

What is a “Radiological Sign”

“A radiological sign is an image in a radiological study that, duly interpreted by an expert professional, allows the diagnosis of a specific pathological process, the elaboration of a reduced list of differential diagnoses, the determination of a specific location, or the establishment of a reference of normality.”

At this point, I find it convenient to remember the difference with a radiological pattern. It can be defined as a set of findings lacking specificity. Therefore, it should not be confused with a radiological sign. This topic deserves its article that I will publish shortly.

Classification of Radiological Signs

A formal classification of radiological signs in general or specifically in the chest has not been described.

However, with a purely semiological criterion, some authors propose a classification based on the sign’s meaning. Following this line, I will transfer the type to the radiological signs in the thorax.

Radiological signs that indicate normality. These are images that determine the normality of a finding. They have value in ruling out pathology. To this group belong, for example, the sign of the shadow of the nipple and the sign of the thymic candle, among others.

Radiological signs indicating location. They correspond to images that specifically tell the site of a specific lesion. This is the case with the silhouette sign, cervicothoracic, and thoracoabdominal signs.

Radiological signs indicated pathology. They are the most abundant. They may suggest a specific diagnosis or a brief list of differential diagnoses—for example, the cam alone sign in the ruptured hydatid cyst.

The ten radiological signs in the chest

Many radiological signs have been described in the chest, which you can find in books, articles and web pages. In this article, I will present the ten radiological signs in the thorax that, due to their frequent appearance of clinical air importance.

  • silhouette sign
  • Cervicothoracic sign
  • Thoracoabdominal sign
  • hidden hilum sign
  • Hilar convergence sign
  • Air bronchogram sign
  • Sign of extrapulmonary injury
  • Signs of lung collapse
  • S sign for Golden
  • Continuous diaphragm sign
  1. Sign of the Silhouette

It is the best known and most classic of the radiological signs in the thorax and allows the location of a pulmonary lesion to be recognised. As you can deduce, it also applies to other regions since it applies the sum of densities and the site in the planes of the images.

A silhouette sign is described when any pulmonary opacity in contact with the cardiac border, aorta, or diaphragm blurs its outline.

Positive silhouette sign. Right pericardiac opacity with air bronchogram suggests an alveolar infiltrate. It erases the border of the heart, inferring its location in the middle lobe.

  1. Cervicothoracic sign

This sign is based on the anatomic knowledge that the structures of the anterior mediastinum do not exceed the level of the clavicles on the PA chest radiograph.

Therefore, if the cephalic portion of a well-defined mass exceeds this anatomical border, we can be sure that said mass is not in the anterior mediastinum.

  • Cervicothoracic sign
  • Thoracoabdominal sign
  1. Thoracoabdominal or iceberg sign

Remember this sign when you identify a mass at the thoracoabdominal crossroads on the PA chest radiograph. It will help you define in which of these cavities the lesion is located. 

If the mass is well defined and has converging edges in the form of parentheses on both sides of the spine, it is thoracic because it is drawn by the air surrounding it.

The cervicothoracic and thoracoabdominal signs are variants of the silhouette sign and are classified as localising radiological signs of the chest.

  1. Occult hilum sign

It is a variant of the silhouette sign, which allows you to determine if a mass projected in the pulmonary hilum originates in the hilum itself or the mediastinum. 

So if you can see the pulmonary vessels through the mass, it is not in contact with the pulmonary artery. Therefore, the lesion is in the mediastinum, in front of or behind the hilum.

On the contrary, when you cannot identify the outline of the hilar vessels within the image, the lesion is in contact with the pulmonary artery and therefore obliterates them. The lesion is located at the hilum.

  • hidden hilum sign
  • Hilar convergence sign
  1. Hilar convergence sign

This sign allows you to differentiate between hilar growth caused by increased vessel size from that caused by a juxtahilar mediastinal mass. 

The convergence of the vascular images towards the enlarged hilum indicates that it represents the enlarged pulmonary artery. 

  1. Air bronchogram sign

Normal bronchi are not usually seen on plain chest radiography. Remember that the lumen is filled with air (radiolucent), and the walls are skinny.

However, they become visible when the air in the alveoli is replaced by water, blood, pus, or cells. The bronchus, which has an atmosphere in its lumen, is seen as a radiolucent tubular structure surrounded by alveolar condensation.

  1. Sign of extrapulmonary injury

It allows us to differentiate a lung injury from a pleural or chest wall injury.

Extrapulmonary lesions of pleural or extrapleural origin have sharp borders at their interface with the lung because they are delimited by the pleura. They are convex towards the lung and form obtuse angles with it at their upper and lower ends.

Intrapulmonary lesions, in contrast, have imprecise borders with sharp edges towards the pleura.

  1. Signs of lung collapse

The term atelectasis or lung collapse describes a loss of volume in the lung parenchyma. 

Typically, although not always, it is associated with increased radiological density. Its differential sign is that the volume is reduced.

For the diagnosis and localisation of atelectasis, the first test you should request is a chest X-ray, in its two PA and lateral projections.

  • Direct Signs of Lung Collapse
  • There are two immediate radiological signs :

Indirect radiological signs of atelectasis are secondary data on the volume loss of a part of the lung. They are related to the mechanisms that compensate for the reduction in intrapleural pressure.

Displacement of parenchymal marks, e.g. granulomas, surgical clips.

Compensatory hyperinflation is a sign of chronic atelectasis. The average lung adjacent to the collapsed area may hyperextend to fill the space, called compensatory emphysema. It is identified radiologically as increased transparency, a sign of great diagnostic value. In case of massive collapses, hyperinflation occurs in the contralateral lung and can become so large that the overstretched lung can herniate through the mediastinal septum.

  1. Golden S sign

When a mass located in the hilum produces atelectasis of the upper lobe, it causes a displacement of the fissure, giving it the shape of an inverted “S”. The upper part, concave, represents the displaced fissure, the lower part convex, corresponds to the edge of the mass located at the hilum. 

  1. Sign of the Continuous Diaphragm

On the PA chest radiograph, the central part of the diaphragm is not generally seen because it is obliterated by the heart. If air is interposed between the heart and the diaphragm, an interface between these structures is observed, and the entire diaphragm can be seen.

This sign is characteristic of pneumomediastinum. Therefore, it is classified as a radiological chest sign indicating a specific pathology.