The Common Vein

Opacity in the Lungs (Revised)

• What is it:
• Opacity in the lungs refers to:
  • • An area of increased density visible on imaging,
    • representing a reduction in the
      • normal air content of the lung, and
    • caused by a wide range of pathological processes.
  • It can represent:
    • Consolidation (airspace filling), or
    • An area of atelectasis (collapsed lung tissue),
      • which are more commonly well-defined opacities.
  • The term “opacity” is often used as a
    • generalized descriptor
      • when a more specific diagnosis or description cannot be made.
    • It has replaced the use of the term “infiltrate”,
      • providing a more neutral and descriptive alternative.
• Etymology:
  • Derived from the Latin word opacitas, meaning “shaded or dark,” referring to reduced transparency of the lung field on imaging.
• AKA:
  • Lung opacity, Pulmonary opacity, Airspace opacity.
• How does it appear on each relevant imaging modality:
  • Chest X-ray:
    • Appears as a region of increased whiteness compared to surrounding normal lung.
    • It may be well-defined or poorly defined
  • Chest CT:
    • Parts: Opacities can involve the lung parenchyma, pleura, or airways.
    • Size: May vary from small focal opacities (e.g., nodules) to large areas of consolidation or atelectasis.
    • Shape: Can be round (e.g., nodule), patchy (e.g., consolidation), or linear (e.g., atelectasis or interstitial thickening).
    • Position:
      • Can be localized (e.g., segmental or lobar) or diffuse.
      • Peripheral (e.g., organizing pneumonia) or central (e.g., pulmonary edema).
    • Character:
      • Solid, ground-glass, or mixed attenuation.
    • Time:
      • Opacities may resolve (e.g., infections), persist (e.g., fibrosis), or progress (e.g., malignancy).
  • PET-CT:
    • Recommended for lesions larger than 7 mm.
    • Identifies:
      • Active lesions with increased metabolic activity (e.g., malignancy, acute infection/inflammation).
      • Cold lesions with low metabolic activity (e.g., chronic infections, slow-growing neoplasms).
• These findings reflect:
  • Airspace processes:
    • Infection:
      • Bacterial pneumonia (e.g., lobar consolidation).
      • Tuberculosis (focal or cavitating opacities).
      • Fungal infections (e.g., aspergillosis).
    • Inflammation:
      • Organizing pneumonia.
      • Hypersensitivity pneumonitis.
  • Interstitial processes:
    • Pulmonary fibrosis.
    • Sarcoidosis (reticulonodular opacities).
  • Neoplastic processes:
    • Primary lung cancer (e.g., mass or irregular opacity).
    • Metastatic nodules (e.g., multiple opacities).
  • Circulatory processes:
    • Pulmonary edema (central opacities with batwing distribution).
    • Pulmonary hemorrhage.
• Differential diagnosis:
  • Infection:
    • Bacterial pneumonia (e.g., Streptococcus pneumoniae, Staphylococcus aureus).
    • Viral pneumonia (e.g., COVID-19, influenza).
    • Tuberculosis (cavitating or nodular opacities).
  • Inflammation:
    • Hypersensitivity pneumonitis (diffuse ground-glass opacities).
    • Organizing pneumonia (peripheral or patchy opacities).
  • Neoplasm:
    • Primary lung cancer (e.g., spiculated opacity or mass).
    • Metastatic disease (e.g., nodular opacities).
  • Circulatory:
    • Pulmonary edema (central, fluffy opacities).
    • Pulmonary hemorrhage (diffuse or patchy opacities).
  • Iatrogenic:
    • Radiation pneumonitis.
    • Drug-induced lung disease (e.g., amiodarone toxicity).
  • Immune-related:
    • Sarcoidosis (perilymphatic distribution of opacities).
    • Eosinophilic pneumonia.
  • Congenital or inherited:
    • Pulmonary sequestration (associated opacity).
    • Cystic fibrosis (mucus plugging with airspace opacities).
• Recommendations:
  • Correlate imaging findings with clinical history and physical examination.
  • Perform high-resolution CT to better characterize the morphology, distribution, and density of the opacity.
  • Use PET-CT for metabolic evaluation of opacities larger than 7 mm to distinguish active from cold lesions.
  • Biopsy or bronchoscopy may be necessary for suspected neoplastic, granulomatous, or inflammatory causes.
• Key points and pearls:
  • The term “opacity” is non-specific and encompasses a broad differential diagnosis, requiring correlation with clinical, laboratory, and imaging findings.
  • The distribution of opacities (e.g., focal, diffuse, central, or peripheral) is critical for narrowing the differential.
  • Consolidation and atelectasis are more commonly well-defined opacities, helping distinguish them from diffuse or poorly defined ground-glass opacities.
  • Time-dependent changes (e.g., resolution, persistence, or progression) provide important clues to the underlying cause.