Imaging of the Lungs

Chest radiography is the most commonly performed imaging investigation in clinical medicine. Systematic interpretation prevents important findings from being overlooked.

A structured approach to chest X-ray interpretation uses the ABCDE framework. A — Airway: confirm the trachea is central and check the carina angle (normally less than 70°, widened by left atrial enlargement). B — Breathing: assess the lung fields for symmetry, consolidation, collapse, pleural fluid, and pneumothorax. C — Cardiac: assess cardiac size (cardiothoracic ratio should be less than 0.5 on a PA film) and mediastinal contours. D — Diaphragm: both hemidiaphragms should be visible; the right is normally slightly higher than the left due to the liver; loss of diaphragm clarity suggests adjacent consolidation or effusion. E — Everything else: bones, soft tissues, lines, and tubes.

The silhouette sign is a fundamental radiological principle: two structures of similar radiodensity that are in contact with each other will obliterate each other's borders on the X-ray. Loss of the right heart border indicates right middle lobe pathology, since the right heart border is normally silhouetted by aerated right middle lobe. Loss of the left heart border indicates lingula pathology. Loss of a hemidiaphragm outline indicates pathology in the adjacent lower lobe. This sign helps localise disease even when its exact position is not obvious.

Consolidation describes the replacement of air in the alveoli with a denser material (pus, fluid, blood, or cells). Radiologically, consolidation appears as opacification of the lung with preservation of the air bronchogram sign (visible air-filled bronchi within the opacity), which confirms that the airways are patent and the consolidation is within the alveoli. Lobar or segmental distribution suggests pneumonia or infarction.

Computed tomography (CT) of the chest provides cross-sectional images with superior contrast resolution compared to plain film. It uses X-rays at multiple angles with computational reconstruction. CT can detect small nodules, characterise mediastinal masses, guide biopsy, and diagnose pulmonary embolism using CT pulmonary angiography (CTPA). High-resolution CT (HRCT) provides detailed images of interstitial lung disease.