How to Read a Chest X-Ray

• Tracheal position — should be midline. Slight deviation to the right at the level of the aortic arch is a normal variant. Pathological deviation: pushed away from the side of a tension pneumothorax, large pleural effusion, or mediastinal mass; pulled towards the side of collapse, fibrosis, or pneumonectomy.
• Carina angle — the angle subtended by the left and right main bronchi should not exceed 70°. Widening beyond 70° (splaying of the carina) is a sign of left atrial enlargement, most commonly from mitral stenosis or left ventricular failure.^[4]
• Main bronchi — follow each main bronchus from the carina. Abrupt cut-off suggests an endobronchial lesion or foreign body.
• Radio-opaque foreign bodies — always check the airway column for unexpected densities, particularly in children.

Compare upper, middle, and lower zones on each side. Assess each zone systematically before moving to the next.

• Consolidation — homogeneous opacity with air bronchograms (visible bronchi within the opacity). Occupies a lobe or segment. The silhouette sign localises it: right middle lobe consolidation obliterates the right heart border; lower lobe consolidation obliterates the hemidiaphragm; lingular consolidation obliterates the left heart border.^[4]
• Collapse / atelectasis — opacity with volume loss. Signs: displacement of fissures towards the collapse, mediastinal shift towards the affected side, ipsilateral hemidiaphragm elevation. A collapsed lobe may not be visible as a discrete opacity — indirect signs are often more reliable.^[2]
• Pneumothorax — a visible pleural line with absent lung markings peripheral to it. On an erect film the apex is affected first. Tension pneumothorax additionally shows contralateral mediastinal shift. On a supine film (ICU), air collects anteriorly — look for the deep sulcus sign (abnormally lucent costophrenic angle).
• Pleural effusion — on an erect film: blunting of the lateral costophrenic angle (≥200 mL), meniscus sign (concave upper border sweeping laterally), progressive opacification of the lower zone. On a supine film, effusions layer posteriorly producing diffuse hemithoracic haziness.
• Nodules and masses — characterise by size, margin (smooth = benign/metastasis; spiculated = primary malignancy), density (solid, ground-glass, cavitating), and distribution (unilateral vs bilateral; upper vs lower zone).
• Interstitial pattern — reticular (network of lines), nodular (discrete round densities), or reticulonodular. Distribution and zone predominance narrow the differential: upper zone reticulation suggests sarcoidosis or silicosis; basal-predominant reticulation suggests UIP/IPF or drug-induced fibrosis.^[5]
• Hila — the left hilum should be 0.5–1.5 cm higher than the right. Bilateral symmetrical enlargement suggests sarcoidosis or pulmonary arterial hypertension. Unilateral enlargement suggests malignancy or TB lymphadenopathy. A depressed hilum suggests ipsilateral lower lobe collapse.

• Cardiac size — the cardiothoracic ratio (CTR) is the ratio of the maximum transverse cardiac diameter to the maximum transverse thoracic diameter. A CTR above 0.5 on a PA film indicates cardiomegaly. CTR cannot be reliably applied to AP films due to cardiac magnification.^[6]
• Cardiac borders — right heart border: right atrium (lower), superior vena cava (upper). Left heart border: left ventricle (lower), left atrial appendage (middle), pulmonary artery trunk (upper), aortic knuckle (top). Loss of a border = silhouette sign from adjacent consolidation or collapse.
• Mediastinal width — the upper mediastinum should be less than 8 cm wide on a PA film. Widening raises concern for aortic dissection, traumatic aortic injury, mediastinal lymphadenopathy, or retrosternal thyroid. Widening on an AP film may be artefactual — correlation with clinical context is essential.
• Aortic knuckle — visible as a left upper mediastinal density. An unfolded, calcified, or dilated aorta suggests age-related change, hypertension, or aneurysm. A widened aortic arch may indicate dissection.
• Pulmonary vasculature — on an erect PA film, upper lobe vessels are normally smaller than lower lobe vessels. Upper lobe blood diversion (cephalization) is an early sign of elevated pulmonary venous pressure from cardiac failure. Peripheral vascular pruning indicates pulmonary arterial hypertension.

• Right hemidiaphragm height — the right hemidiaphragm is normally 1.5–2.5 cm higher than the left due to the liver. An elevated right hemidiaphragm may indicate right lower lobe collapse, phrenic nerve palsy, or subphrenic pathology.
• Diaphragm contour — both hemidiaphragms should have a sharp, convex upper border. Loss of the diaphragm silhouette indicates adjacent consolidation or effusion (silhouette sign). Scalloping may be a normal variant.
• Costophrenic angles — should be acute and clear. Blunting indicates pleural fluid (≥200 mL on erect film) or pleural thickening.
• Cardiophrenic angles — should be clear. A rounded opacity in the right cardiophrenic angle may represent a pericardial fat pad or Morgagni hernia.
• Free subdiaphragmatic gas — on an erect or left lateral decubitus film, free air collects under the right hemidiaphragm as a crescentic lucency between the liver and diaphragm. This is pneumoperitoneum until proven otherwise — a surgical emergency. Chilaiditi sign (colonic interposition) mimics this: haustral folds within the lucency confirm bowel rather than free air.
• Flat diaphragms — bilateral diaphragmatic flattening below the level of the anterior sixth ribs indicates air trapping, most commonly from COPD or severe acute asthma.
• Gastric bubble — the gas shadow of the gastric fundus should lie under the left hemidiaphragm. Its position helps confirm normal situs and may reveal hiatal hernia (gas bubble behind the heart).

Bones

• Ribs — trace each rib systematically. Fractures may be subtle, especially posteriorly. Look for associated soft tissue swelling. Multiple posterior rib fractures may indicate non-accidental injury or significant blunt trauma. Lytic or sclerotic lesions suggest metastatic disease or primary bone tumour.
• Clavicles and scapulae — check for fractures and lytic lesions. The scapulae project over the upper lung fields on AP films — do not mistake them for apical pathology.
• Vertebral column — check for compression fractures (loss of vertebral height), lytic destruction, or increased sclerotic density (metastases, myeloma, Paget's disease).

Soft tissues

• Subcutaneous emphysema — linear lucencies tracking through the chest wall soft tissues. Implies disruption of the pleura, trachea, or oesophagus, or follows recent instrumentation.
• Breast shadows — in women, the lower lung fields may show soft tissue density from breast tissue. Unilateral absence suggests previous mastectomy.
• Implanted devices — pacemakers, defibrillators, port-a-caths. Note position of leads and check for pneumothorax at the insertion site.
• Artefacts — necklaces, clothing, monitoring leads. Remove all jewellery before formal diagnostic imaging.

Review areas — the four most commonly missed locations

• Lung apices — site of reactivation tuberculosis, Pancoast tumour (apical mass with rib destruction), and apical pleural thickening. Do not dismiss apical shadowing as an artefact.
• Behind the heart — left lower lobe collapse produces a triangular opacity behind the cardiac shadow (sail sign) with loss of the left hemidiaphragm silhouette. Hiatal hernia presents as a retrocardiac mass with an air-fluid level.
• Costophrenic angles — small pleural effusions (200–300 mL) blunt only the lateral angle. Look for asymmetry between the two sides even if neither is frankly blunted.
• Below the diaphragm — check for free subdiaphragmatic gas, bowel gas pattern abnormalities, and any soft tissue mass beneath the diaphragm.

Lines and tubes (hospital and ICU films)

• Endotracheal tube (ETT) — tip should lie 3–7 cm above the carina on an AP film (carina is approximately at the level of the aortic knuckle, T4–T5). Right mainstem intubation causes right lung hyperinflation and left lung collapse. ETT position changes with head flexion and extension — always note current head position.
• Nasogastric / orogastric tube — must cross the midline and descend below the level of the left hemidiaphragm to confirm gastric placement. A tube that does not cross the midline has not left the oesophagus. Aspirate pH <5.5 on testing confirms gastric placement — do not rely on CXR alone as the sole safety check.
• Central venous catheter — tip should lie at the SVC–right atrial junction (approximately the lower border of the right main bronchus). Tip in the right atrium or right ventricle risks arrhythmia and perforation. Check for procedural pneumothorax.
• Chest drains — confirm position within the pleural space. Note the last side hole (marked by a radio-opaque interruption in the drain stripe) should be within the pleural cavity, not the chest wall or lung parenchyma.