Chest X-ray Systematic Approach
Locating abnormalities

Key points

  • The silhouette sign can help locate abnormalities in 3D space within the chest

'Silhouettes' and their adjacent tissues

  • Left heart border (left ventricle) - Lingula
  • Right heart border (right atrium) - Right middle lobe
  • Left hemidiaphragm - Left lower lobe
  • Right hemidiaphragm - Right lower lobe
  • Aortic knuckle - Left upper lobe/middle mediastinum
  • Descending aorta - Left lower lobe
  • Right paratracheal stripe - Right upper lobe/anterior mediastinum
  • Paraspinal lines - Medial lung/Posterior mediastinum

As well as determining the side and distribution of an abnormality it is often helpful to consider its anterior-posterior position. A lateral view may help, but 3D location may also be possible on a posterior-anterior (PA) view if you have a knowledge of chest X-ray anatomy and an understanding of the silhouette sign.

The silhouette sign

Normal adjacent anatomical structures of differing densities form a crisp contour or 'silhouette'. Loss of a specific contour can help determine the position of a disease process. This phenomenon is known as the silhouette sign.

The silhouette sign is a misnomer - it should really be called the 'loss of silhouette' sign.

For example, the heart (a soft tissue density structure - near white) lies adjacent to lung tissue (near air density - near black). A crisp contour or 'silhouette' is formed at the interface of these two tissue densities. Loss of clarity of the right heart contour (formed by the right atrium) implies disease of the right middle lobe which lies next to the right atrium. Loss of distinction of the left heart contour indicates an abnormality of the lingula (part of the left upper lobe which wraps over the left ventricle).

Simulated silhouette signs 1

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Simulated silhouette signs 1

  • 1 - Left heart border - Lingula disease
  • 2 - Hemidiaphragm - Lower lobe lung disease
  • 3 - Paratracheal stripe - Paratracheal disease
  • 4 - Chest wall - Lung, pleural or rib disease

Note

  • If you find a 'loss of silhouette' it may not be possible to determine the tissue from which the abnormality arises. Often the lung is the origin of disease but abnormalities of other structures such as the pleura, mediastinum, vessels or heart may mimic lung disease.

Simulated silhouette signs 2

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Simulated silhouette signs 2

  • 5 - Aortic knuckle - Anterior mediastinal or left upper lobe disease
  • 6 - Paraspinal line - Posterior thorax disease
  • 7 - Right heart border - Middle lobe disease
  • 8 - Density above horizontal fissure - Anterior segment of the right upper lobe (due to formation of a silhouette rather than loss of a silhouette)

Page author: Salisbury NHS Foundation Trust UK (Read bio)

Last reviewed: September 2016