How do you interpret a chest x-ray (CXR)?

How to Interpret a Chest X-ray

A systematic approach to chest X-ray interpretation should follow a structured sequence examining the trachea, mediastinum, cardiac silhouette, lung fields, pleura, bones, and soft tissues to avoid missing critical findings. 1

Systematic Review Approach

Airway and Mediastinum

• Examine tracheal position and patency first, looking for deviation that may indicate mass effect, pneumothorax, or pleural effusion 1
• Assess mediastinal contours and width, with widening potentially indicating aortic injury, dissection, or lymphadenopathy 2, 1
• Check for pneumomediastinum, identified by air tracking along mediastinal structures 1

Cardiac Assessment

• Evaluate cardiac size using the cardiothoracic ratio, measured on a PA film; a ratio >0.5 indicates cardiomegaly 1
• Note that AP films magnify the cardiac silhouette and should not be used for definitive cardiac size assessment 1

Lung Parenchyma

• Systematically examine lung fields using side-to-side comparison to identify asymmetry 1
• Look for focal opacities or consolidation, which appear as areas of increased density obscuring vascular markings 1
• Evaluate interstitial patterns including reticular (linear), nodular (small rounded opacities), or reticulonodular patterns 2, 1

Common pitfall: Dependent atelectasis in supine or semi-erect patients can mimic pathology; correlation with patient position is essential 2

Pleural Assessment

• Evaluate for pneumothorax by identifying a visible pleural line with absence of lung markings peripherally 1
• Assess for pleural effusion by looking for meniscus sign and blunting of costophrenic angles 2, 1
• Check for pleural thickening or plaques, which may indicate prior asbestos exposure or chronic inflammation 1

Important limitation: Chest X-rays have only 28% sensitivity for detecting pulmonary metastases compared to CT 2, and up to 50% of pneumothoraces may be missed on standard AP radiographs 1

Osseous and Soft Tissue Structures

• Examine ribs, clavicles, scapulae, and vertebrae for fractures, lytic or blastic lesions 1
• Evaluate diaphragm contours and position bilaterally; elevation may indicate phrenic nerve injury or subdiaphragmatic pathology 1
• Check soft tissues for subcutaneous emphysema, masses, or asymmetry 1

Context-Specific Considerations

Trauma Patients

• Pay particular attention to widened mediastinum (>8 cm on upright PA or >6 cm on supine AP), which may indicate aortic injury 2, 1
• Identify rib fractures, especially first and second ribs which indicate high-energy trauma 1
• Look for pneumothorax or hemothorax, which are critical immediate findings 1

Suspected Pulmonary Embolism

• Recognize that chest X-rays are often normal in PE; a normal CXR in a patient with acute dyspnea and hypoxemia increases suspicion 1
• Look for Westermark sign (focal oligemia) and Hampton's hump (peripheral wedge-shaped opacity), though these are uncommon 1

Pediatric and Neonatal Patients

• Ultrasound findings in neonates mirror adult patterns but are context-specific 2
• Lung ultrasound is as accurate as chest radiography for diagnosing respiratory distress syndrome and transient tachypnea of the newborn in neonates 2

Quality and Technical Factors

Image Quality Assessment

• Verify adequate penetration: thoracic vertebrae should be faintly visible through the cardiac silhouette 1
• Check for rotation: medial clavicular heads should be equidistant from the spinous processes 1
• Assess inspiratory effort: 8-10 posterior ribs should be visible above the diaphragm on a good inspiratory film 1

Critical consideration: Reducing source-to-image distance from 180cm to 100cm increases entrance skin dose by 121.5% and decreases diagnostic quality 3

When to Advance to CT

• CT is mandatory when CXR findings are equivocal or when higher sensitivity is required for detecting pulmonary nodules, metastases, or subtle pneumothorax 2, 1
• CT provides superior spatial resolution and contrast, detecting small pulmonary nodules that are invisible on radiography 2
• For suspected interstitial lung disease in children, thin-section CT with controlled ventilation technique is recommended despite requiring sedation 2

Emerging Technologies

Artificial intelligence tools for CXR interpretation demonstrate pooled sensitivity of 98.57% and specificity of 98.05% for tuberculosis detection, though clinical validation in real-world settings remains limited 2

Human expert interpretation remains the gold standard, with AI serving as a potential adjunct rather than replacement 2