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Densities in CXR



Pulmonary artery Liquid density
Vertebra Bone density
Lung Air density
Left atrium Liquid density

Air bronchogram

Bronchial Visibility

Bronchi are visible only up to 4th or 5th order of branching.

If the bronchi are recognizable beyond 5th order of branching it is abnormal. This can occur in

  1. Visibility of Lumen: Alveolar filling will provide the necessary contrast. Recognizable bronchi as branching air filled lucencies in a density is called air bronchogram and indicates alveolar disease.

  2. Visibility of wall: If the bronchi wall is thickened (Chronic bronchitis, Bronchiectasis) or there is cartilaginous calcification then you can recognize them also. There is no surrounding density in this instance.


Silhouette sign:

Helps in localization of disease in Lungs and Mediastinum

Example: Consolidation of RLL and RML will project in the right lower lung field. We can use Silhouette sign to localize. RML is adjacent to right heart margin and does not reach diaphragm. Hence if the right heart margin is indistinct with a sharp diaphragm the lesion has to be in the RML. Of course lateral view will easily resolve whether it is RML or RLL lesion.


 In order to use Silhouette sign you need to know the location of each Silhouette and the  lobe adjacent to it.


Adjacent structure

Right diaphragm Right Lower lobe, Pleura
Right Heart margin Middle lobe, Anterior Mediastinum
Ascending Aorta RUL, Anterior Mediastinum
Aortic Knob LUL, Posterior Mediastnum
Left Heart margin Lingula, Anterior Mediastinum
Left Diaphragm Left Lower lobe, Pleura

Pleural disease can obliterate all of the silhouettes.


Radiological Characteristics



Unilateral opacification

  1. Massive pleural effusion
  2. Complete atelectasis of Lung


Acute Diffuse alveolar Disease

Radiological Characteristics

Common Etiology


Calcified miliary Lung nodules


Tuberculosis Co-existing nodes
Histoplasmosis Co-existing nodes and Splenic calcification
Chicken pox No nodes No Splenic calcification

Calcified hilar nodes


Tuberculosis Homogenous
Histoplasmosis Multiple densities clumped together
Silicosis Egg shell/Calcification of rim


Segmental density




Radiological Characteristics





Fungous ball

  • Radiological characteristics

    • cavity

    • Ball inside

    • Crescent of air

  • Etiology

    • Aspergillus

Contents of Cavity
  • Air fluid level
    • Abscess
    • Hemorrhage
  • Mass
    • Fungous ball
  • Irregular density: stalagmites
    • Lung cancer

Unilateral Hyperlucency 


Rarely done nowadays. because

Note the carina, Right main and left main

1, 2, 3 are Posterior , Anterior and Apical segments of RUL

4 and 5 are Lateral and Medial segments of RML

6, 7, 8, 9, 10 are Superior Medial, Lateral, Anterior and Posterior segments of RLL

Try to appreciate the three dimensional view of these segments. Appreciation of this is necessary to understand why certain segments are prone for aspiration. You have to think of it also on the position of the patient and determine which segment would be the site for aspiration based on gravity.

Pleural effusion

Radiological Characteristics :

  1. Homogenous density
  2. in base in upright film
  3. obliterating costophrenic angle
  4. meniscus high in axilla (visual illusion)
  5. shift of Mediastinum to opposite side with large effusions
  6. Loss of diaphragmatic Silhouette

Common Etiology

Pleural masses

Radiological Characteristics



Metastatic lesions