ABC of CXR Interpretation

Of the myriad of alternative CXR interpretation techniques, we highlight some of the key points relating to the ABC of CXR interpretation.

Additional reading from Normal CXRs; Eric Strong Interpretation series; the DRABCDE approach; CXR for the OSCE and of course the Top 150 CXR to try your luck!

CXR Views

Posteroanterior (PA)

  • Erect inspiratory posteroanterior (PA) is the preferred CXR view. The patient stands in front of a radiographic plate, hands on hips, with the X-ray source 2 m behind.

Lateral CXR

  • Left lateral CXR is performed with the patient standing. It is used in combination with a PA CXR to further delineate and localise masses, lesions or consolidation, particularly those obscured by the heart or diaphragms

Anteroposterior (AP)

  • An AP CXR is performed in bed-bound patients unable to stand for a PA view, or those who require a portable CXR when it is unsafe to move them to the radiology department.
  • The X-ray cassette is placed behind the patient and the X-ray taken from the front. AP films magnify the size of the heart and mediastinum, and are more commonly associated with rotational artefact.


  • Supine AP, lateral decubitus and expiratory films: occasionally films taken in full expiration are helpful in defining a small pneumothorax, or to determine gas trapping with inhaled foreign bodies

Interpretation of Chest X-Ray

A systematic approach to CXR interpretation is essential to avoid missing significant pathological changes.

Determine the ownership, adequacy and technical quality of the film

  • Name and date of birth of the patient and date radiograph was performed.
  • Projection (e.g. PA, AP, lateral).
  • Posture (e.g. supine or erect).
  • Adequacy of exposure, as determined by easily visible mid-thoracic intervertebral spaces.
  • Degree of inspiration. Check if film is taken in full inspiration, with the diaphragm at the level of the 10th or 11th ribs posteriorly, and the 6th costal cartilage anteriorly.
  • Degree of rotation. A non-rotated film will have the spinous processes of the upper thoracic vertebrae located centrally and equidistant from the medial ends of the clavicles.

Identify the key components of the CXR


  • Should be central, with slight deviation to the right as it crosses the aortic arch.
  • Can be pushed away from an abnormal lung affected by a large pleural effusion, large simple pneumothorax, tension pneumothorax, aortic aneurysm or mediastinal mass.
  • The trachea can be pulled towards an abnormal lung affected by extensive collapse, consolidation, pulmonary fibrosis, lobectomy or pneumonectomy.

Superior mediastinum:

  • Should have a width <8 cm on a PA CXR.
  • A widened mediastinum can be associated with:
    • AP CXR view, which magnifies the heart and mediastinal structures
    • Unfolded aortic arch (not pathological) or a thoracic aortic aneurysm
    • Mediastinal lymphadenopathy, retrosternal thyroid, thymoma (can be particularly massive in children)
    • Paravertebral mass, oesophageal dilatation
    • Ruptured aorta in deceleration trauma from vehicle crash or fall from a height.
  • Look for evidence of mediastinal emphysema (abnormal air) secondary to:
    • Penetrating wound ± lacerated lung
    • Perforation of oesophagus or trachea
    • Asthma and whooping cough (pneumomediastinum).


  • level with the T6–7 intervertebral space on either side of the mediastinum, and are made up of the pulmonary arteries and veins.
  • The left hilum is usually higher (2cm) and squarer than the V-shaped right hilum.
  • Unilateral or bilateral hilar enlargement can be caused by
    • Enlarged hilar lymph nodes (e.g. sarcoidosis or infection)
    • Hilar malignancy (e.g. small-cell carcinoma)
    • Vascular disease (e.g. pulmonary hypertension or proximal pulmonary artery aneurysms).


  • Usually positioned with one-third of its diameter to the right, and two-thirds to the left of the thoracic vertebrae spinous processes.
  • The right atrium makes up the right heart border and the left ventricle the left heart border.
  • Poor distinction of the right heart border suggests consolidation of the right middle lobe.
  • Poor distinction of the left heart border suggests lingular consolidation.

Cardiothoracic ratio (CTR):

  • Compares the transverse diameter of the heart to the internal thoracic diameter (inner aspect of the ribs) at its widest point.
  • Should be less than 0.5 (50%) on a PA CXR, but may appear magnified on AP films.
  • Abnormally increased CTR occurs with ventricular dilatation (usually left), cardiac failure and a pericardial effusion.


  • The right is usually higher than the left by 1–3 cm.
  • Pleural effusions will blunt the costophrenic angles. Loss of diaphragmatic outline indicates fluid, consolidation or collapse of adjacent lung (i.e. of the right or left lower lobe).
  • Both hemidiaphragms are flat in chronic obstructive limitation disease such as emphysema.
  • Free gas under a diaphragm on an erect film indicates rupture of an abdominal hollow viscus, such as the duodenum or small or large intestine. It also occurs after laparoscopy with the deliberate introduction of a pneumoperitoneum.

Lung outlines :

  • Trace the outline of the left and right lungs, looking for evidence of pneumothorax, bullae, collapse, consolidation, effusion, masses or pleural changes such as fibrosis.
  • Both lung fields should be equally translucent, and on the lateral view the lung lucency should increase towards the diaphragms.
  • An abnormal increase in lucency occurs with vessel loss, as in emphysema or pneumothorax, and a decrease results from alveolar or interstitial fluid, effusion or consolidation.

Pulmonary nodules may be solitary or diffuse.

  • Neoplasia—metastases, primary lung tumour, adenoma , lymphangitis carcinomatosa
  • Infection— miliary TB, varicella (chicken pox) pneumonia, fibrotic lung disease, histoplasmosis
  • Vascular—arteriovenous malformation, hamartoma, pulmonary embolus.
  • Cavitating nodular lung disease

Silhouette Sign

  • When examining the lung fields of a normal CXR, the outline (silhouette) of the heart borders; the ascending and descending aorta; the aortic knuckle and the hemidiaphragms should be clearly visible.
  • All of these silhouettes, or structures, are in contact with a specific portion of the lung.
  • Obliteration of any of these silhouettes by a water density e.g. infection in the lung, blood, pus, etc
  • Obliteration of this normal air-soft tissue interface is known as the silhouette sign (of Felson).
  • By determining exactly which silhouette/structure is obliterated, you can determine where the lung pathology is located.
StructureContact with Lung
Ascending aorta and
Upper right heart border
Right Upper Lobe (RUL)
Right heart borderRight Middle Lobe (RML)
Upper left heart borderLeft Upper Lobe (LUL) – Anterior
Aortic KnuckleLeft Upper Lobe (LUL) – Apical portion
Left heart borderLingula of the left lung
Anterior hemidiaphragmsLower lobes (anterior)

Further Reading

Radiology Library

CT, X-ray, ultrasound

BA MA (Oxon) MBChB (Edin) FACEM FFSEM. Emergency physician, Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

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