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Imaging

• Chest x-ray
  • Diagnostic test of choice for pneumonia
  • May show lobar consolidation, interstitial infiltrates, cavitation, associated pleural fluid, etc.
  • Occasionally, an etiologic diagnosis is suggested by chest radiography findings.
    
    • A cavitating upper-lobe lesion raises the likelihood of tuberculosis.
    • Pneumatoceles suggest S. aureus pneumonia.
    • An air-fluid level suggests a pulmonary abscess, which often is polymicrobial.
    • In the immunocompromised host, a crescent (meniscus) sign suggests aspergillosis.
  • In most instances, no etiologic inference can be made from radiographic abnormalities, despite the traditional teaching that a lobar vs. interstitial appearance may be more suggestive of "typical" bacterial vs. "atypical" bacterial or nonbacterial etiologies.
  • If pneumonia is strongly suspected on clinical grounds and no opacity is seen on the initial chest radiograph, it is useful to repeat the radiograph in 24–48 hours or to perform CT.
    • Correction of dehydration may lead to development of chest film infiltrates.
  • Opacity visible on the chest radiograph may not be due to pneumonia; many other disease processes can result in opacities (see Differential Diagnosis).
• High-resolution CT
  • Occasionally detects pulmonary opacities in patients with symptoms and signs suggestive of pneumonia and negative chest x-ray
  • More likely than chest radiography to show bilateral involvement, pleural fluid/empyema, adenopathy, etc.
    

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