Chest X-Ray (PA, Lateral, Decubitus)

The chest radiograph is the most requested imaging examination worldwide. It images the lungs, pleura, heart, mediastinum, diaphragm, chest wall, ribs, thoracic spine and shoulder girdle, plus the position of tubes, lines and devices.

The routine view is the erect posteroanterior (PA) projection at full inspiration, with the patient facing the detector and the scapulae rotated clear of the lungs, taken at 180 cm focus-to-film distance and 110 to 125 kV, which keeps the heart magnification low. A lateral view (left side against the detector) locates lesions in the retrocardiac and retrosternal spaces and in the lung fissures.

Anteroposterior (AP) supine or semi-erect films are used for patients who cannot stand or be moved (they magnify the heart, widen the mediastinum and reduce sensitivity for pleural fluid and pneumothorax).

A lateral decubitus view (patient lying on one side with a horizontal beam) shows free flowing pleural fluid (as little as 50 mL), differentiates fluid from pleural thickening and shows small pneumothoraces in patients who cannot sit up. Expiratory films show air trapping and small pneumothoraces, and apical lordotic views show the lung apices and the middle lobe.

Chest radiography is the first test for cough, dyspnoea, chest pain, fever, haemoptysis, trauma, suspected heart failure, tuberculosis, lung cancer, and device checks. It detects consolidation, pleural effusion, pneumothorax and cardiomegaly well, but is insensitive to small nodules (under 1 cm), early interstitial disease and mediastinal and hilar disease, for which CT is used.

The effective dose is about 0.02 mSv for a PA film and 0.04 to 0.06 mSv for a lateral, equal to a few days of natural background radiation. In tuberculosis screening and many resource limited health systems the chest radiograph, with or without computer aided detection software, remains the main tool for triage and diagnosis.

Indications. Respiratory symptoms (cough over 3 weeks, dyspnoea, haemoptysis, pleuritic chest pain, fever with respiratory signs). Suspected pneumonia, tuberculosis, lung cancer, heart failure, pneumothorax, pleural effusion, pulmonary embolism with infarction (to exclude other causes), aspiration, foreign body. Trauma to the chest.

Pre-operative assessment in selected patients (cardiopulmonary disease, malignancy). Follow up of known abnormalities (pneumonia in smokers over 50 to confirm resolution, pneumothorax after drainage). Position of endotracheal and tracheostomy tubes, central venous catheters, chest drains, nasogastric tubes, pacemakers, and pulmonary artery catheters.

Occupational lung disease surveillance (pneumoconiosis). Screening for tuberculosis in people at risk. Not for routine screening of healthy adults or for non-specific symptoms without clinical indication, and not for suspected pulmonary embolism as the main test.

Contraindications and precautions. No absolute contraindication. Pregnancy: the dose to the foetus is under 0.01 mGy with shielding of the abdomen unnecessary; imaging is done when clinically needed. Remove metal objects, ECG leads and clothing with fasteners on the field. Patients who cannot cooperate (infants, confusion) need immobilisation and shortest exposure; staff protection with a lead apron for portable studies and a distance of 2 m.

Preparation and technique. Remove clothing and jewellery from the chest. Erect PA: chin raised, shoulders rolled forward, deep inspiration held, exposure under 20 ms.

Check adequacy: full inspiration (6 anterior or 10 posterior ribs visible over the diaphragm), no rotation (medial ends of clavicles equidistant from the spinous processes), penetration (vertebrae faintly visible behind the heart), and the whole lung from the apices to the costophrenic angles in the field. AP portable films: label as AP and supine or erect.

Normal appearances. Both lungs equally translucent, with branching vessels diminishing toward the periphery. Trachea central. Heart with a cardiothoracic ratio under 0.5 on PA (up to 0.6 in neonates and on AP films). Smooth cardiac and aortic outlines. Hila with the left hilum slightly higher than the right.

Sharp costophrenic angles, with the right hemidiaphragm 1 to 2 cm higher than the left. Mediastinum under 8 cm wide on PA. Gastric air bubble under the left hemidiaphragm. Symmetric soft tissues and ribs.

Interpretation. A systematic review includes airway, breathing (lungs and pleura), circulation (heart and mediastinum), diaphragm and abdomen, and everything else (bones, soft tissue, tubes, lines).

• Consolidation: homogeneous opacity with air bronchograms, borders obscured with adjacent structures (silhouette sign: right heart border for right middle lobe, left heart border for lingula, diaphragm for lower lobes). Lobar pattern suggests pneumococcal pneumonia; patchy bronchopneumonia; multiple cavities or an upper lobe cavity suggests tuberculosis, abscess or necrotising infection.
• Collapse (atelectasis): increased density with volume loss (shift of fissures, trachea, hilum, diaphragm), with a "sail" sign in left lower lobe collapse behind the heart and a golden S sign when a central mass is present.
• Pleural effusion: homogeneous basal opacity with a meniscus, blunting of the costophrenic angle (about 200 mL on PA, 50 mL on lateral decubitus), and mediastinal shift away from large effusions. A "white out" with shift toward the opacity suggests collapse or pneumonectomy.
• Pneumothorax: visceral pleural line with absent lung markings beyond it. Tension pneumothorax shows a large lucent hemithorax with mediastinal shift, flattened hemidiaphragm and depressed diaphragm. On supine films, a deep sulcus sign and a clearly outlined heart border or hemidiaphragm are clues.
• Heart failure: cardiomegaly, upper lobe vessel diversion, interstitial oedema (Kerley B lines, peribronchial cuffing, fissure thickening), alveolar oedema ("bat wing" perihilar opacities), and bilateral effusions.
• Lung mass or nodule: a spherical opacity, with spiculation, cavitation, or calcification patterns. Solitary nodule over 8 mm needs CT. Hilar enlargement indicates nodes, mass or pulmonary arterial hypertension. Bilateral hilar lymphadenopathy suggests sarcoidosis, lymphoma or tuberculosis.
• Miliary pattern: numerous 1 to 3 mm nodules throughout the lungs (miliary tuberculosis, metastases, fungal infection).
• Interstitial disease: reticular or reticulonodular pattern, honeycombing, volume loss in fibrosis. Emphysema: hyperinflation with flattened diaphragms and a narrow heart, bullae.
• Mediastinal widening (over 8 cm on PA): aortic injury or dissection, lymphadenopathy, thymoma, retrosternal goitre, teratoma. Aortic knuckle and aortic contour changes need CT angiography.
• Free air under the diaphragm (pneumoperitoneum) and subphrenic collections.
• Bones and soft tissue: rib fractures, lytic or sclerotic lesions, mastectomy, surgical emphysema, scoliosis.
• Lines and tubes: endotracheal tube tip 3 to 5 cm from the carina with the head in neutral position; central venous catheter tip at the cavo atrial junction (lower superior vena cava), not in the right atrium or in a vessel branch; nasogastric tube tip under the diaphragm in the stomach; chest drain within the pleural space; pacemaker leads in the right atrium and ventricle.

Critical values and findings needing immediate action.

• Tension pneumothorax (mediastinal shift with a large pneumothorax in a compromised patient): immediate needle decompression and chest drain, without waiting for any further imaging.
• Free air under the diaphragm: surgical review for perforated viscus.
• Misplaced endotracheal tube (in a main bronchus or oesophagus), nasogastric tube in the bronchial tree, or central line in an artery or outside the vein: correct immediately before use.
• Widened mediastinum after trauma or with chest pain: urgent CT angiography for aortic injury or dissection.
• Large pleural effusion or white out causing respiratory failure: drainage.
• Acute pulmonary oedema or large pneumonia with sepsis: start treatment as the clinical picture requires; the radiologist telephones the clinician.
• New cavitating lung lesion or miliary pattern: isolate for suspected tuberculosis until sputum results return.
• Foreign body in the airway or ingested sharp object or battery: urgent endoscopic retrieval.

Limitations and artefacts.

• A normal film does not exclude disease: sensitivity is low for small nodules (under 1 cm), early interstitial lung disease, pulmonary embolism, small pneumothorax on supine films, hilar and mediastinal masses and lesions hidden behind the heart, diaphragm, clavicles or ribs. Review the "hidden areas" (lung apices, retrocardiac region, under the diaphragms, hila, and behind the sternum).
• Technical factors change the appearance: rotation mimics mediastinal shift or hilar enlargement, poor inspiration makes the heart look large and the lung bases look opaque, under penetration mimics consolidation, and over penetration hides small lesions.
• AP projection and supine position enlarge the heart and mediastinum and underestimate effusions and pneumothorax.
• Overlapping structures (nipples, skin folds, hair braids, ECG leads, external devices, healing rib fractures) mimic nodules. A repeat film or a different view differentiates them.
• Radiographic findings in pneumonia lag behind the clinical picture (up to 24 to 48 hours to appear, 6 to 12 weeks or longer to clear, particularly in older people and smokers).
• Interobserver variation is substantial, and clinical correlation is required. Computer aided detection helps with tuberculosis triage but needs local validation.
• Radiation risk is low; repeated films for tube position in intensive care add up, and need clinical justification.

Special groups.

• Children: use the shortest exposure and a collimated beam; the thymus gives the sail sign and a wide mediastinum in infants; infant films are taken supine or in a restraint device, and lateral views are used selectively.
• Pregnancy: foetal dose is negligible; image when needed.
• Elderly and bed bound patients: portable AP films with limited quality; ultrasound adds information about effusions.
• Tuberculosis and HIV: radiographic patterns are atypical in advanced HIV (lower lobe disease, effusions, adenopathy without cavities); a normal film is possible with positive sputum.
• Occupational exposure: pneumoconiosis is graded by a film classification applied by trained readers.
• Regions: chest radiography with digital systems and remote reading is the cornerstone of tuberculosis, pneumonia and heart disease diagnosis in district hospitals across Africa and Asia, where CT is scarce.

Follow up tests. Lateral or decubitus film for effusion, expiratory film for small pneumothorax, ultrasound for effusion, loculation and image directed drainage, CT chest for nodules over 8 mm, suspected mass, interstitial disease, mediastinal widening and bronchiectasis, CT pulmonary angiography for suspected embolism, echocardiography for heart failure,

sputum microscopy and molecular tests for tuberculosis, bronchoscopy for central lesions, natriuretic peptide for dyspnoea, and repeat chest radiograph 6 weeks after pneumonia in patients at risk of cancer.