000 Mosaic Attenuation Caused by Air Trapping

  • Etymology
    “Mosaic” derives from the Greek word mousaikos, meaning “pertaining to the muses” and refers to the patchy or varied pattern seen on imaging.
  • AKA and abbreviation
    Mosaic attenuation (MA). Air trapping (AT).
  • What is it?
    Mosaic attenuation refers to a patchwork appearance of differing lung attenuation on high-resolution computed tomography (HRCT), often due to varying ventilation.
    Air trapping is defined as the retention of air in parts of the lung due to airway obstruction or abnormal expiratory mechanics.
  • Characterized by
  • Heterogeneous attenuation of lung parenchyma, visible on CT, especially in expiratory views.
  • Areas of hypodense lung representing trapped air juxtaposed with denser lung regions.
  • Sharp demarcation between high- and low-attenuation regions.
  • Head cheese sign: A combination of patchy areas of air trapping, ground-glass opacities, and normal lung tissue, commonly seen in hypersensitivity pneumonitis and indicative of mixed pathological processes.

Can appear as:

  • Expiratory air trapping: Hypodense areas do not change density during expiration.
  • Caused by
    • Most Common Cause(s): Chronic obstructive pulmonary disease (COPD), asthma.
    • Other Causes Include:
      • Infection: Viral bronchiolitis, post-infectious bronchiolitis.
      • Inflammation/Immune: Hypersensitivity pneumonitis (head cheese sign), obliterative bronchiolitis.
      • Neoplasm: Obstructing tumors causing localized air trapping.
      • Mechanical Trauma: Post-radiation changes, inhalational injury.
      • Metabolic: None commonly associated.
      • Inherited: Cystic fibrosis.
      • Congenital: Congenital lobar overinflation, Swyer-James syndrome (post-infectious bronchiolitis with unilateral air trapping).
  • Resulting in:
  • Impaired ventilation and gas exchange.
  • Dyspnea, wheezing, and other respiratory symptoms.
  • Structural changes:
  • Narrowing or complete obstruction of small airways.
  • Loss of elastic recoil in surrounding parenchyma.
  • Pathophysiology:
    Air trapping arises from partial or complete obstruction of airways, leading to overinflation of certain lung regions during expiration. Mosaic attenuation reflects heterogeneity in lung ventilation. The subtending vessel of the segment of air trapping may show decreased diameter due to ventilation-perfusion physiology.
  • Pathology:
  • Thickened airway walls, peribronchiolar fibrosis, or granulomas.
  • Diagnosis:
    Clinical symptoms, imaging (HRCT, expiratory phase CT), and pulmonary function testing.
  • Clinical:
    Symptoms include chronic cough, dyspnea, wheezing, and recurrent infections.
  • Radiology Detail:
  • CXR
    • Findings: Not typically diagnostic, but may suggest hyperinflation or regional oligemia.
    • Associated Findings: Flattened diaphragm, increased retrosternal airspace.
  • CT
    • Parts: Both lungs affected diffusely, segmentally, subsegmentally, or involving only a few secondary lobules.
    • Size: Varies with underlying etiology.
    • Shape: Patchy or lobular.
    • Position: Distribution depends on the underlying disease and can involve upper or lower lobes, focal or multicentric patterns, or be diffuse. It may affect lobar, segmental, subsegmental regions, or even only a few secondary lobules.
    • Character: Hypodense, non-enhancing regions that persist on expiration.
    • Time: Chronic conditions show stable findings; acute causes may resolve.
    • Associated Findings: Bronchial wall thickening, mucus plugging.
    • Normal mosaic attenuation: Seen in studies with poor inspiratory effort, characterized by globally low lung attenuation without focal abnormalities. This appearance is more commonly seen in basal sections, particularly peripherally and subpleurally.
  • Other relevant Imaging Modalities
    • MRI: Rarely used but may show areas of ventilation defects.
    • PET CT: Occasionally used to evaluate associated malignancies.
    • VQ Scan: Useful in evaluating ventilation-perfusion mismatches to confirm suspected air trapping and mosaic attenuation patterns.
    • Angio: Helps assess secondary changes in perfusion mismatch.
  • Pulmonary function tests (PFTs)
    Reduced forced expiratory volume (FEV1) and increased residual volume (RV) indicative of obstructive pathology.
  • Recommendations
  • Evaluate with expiratory HRCT for better characterization.
  • Manage underlying cause, such as corticosteroids for hypersensitivity pneumonitis or bronchodilators for obstructive diseases.
  • Key Points and Pearls
  • Best visualized on expiratory CT.
  • Differentiation of mosaic attenuation due to air trapping versus other causes is crucial for diagnosis.
  • Mosaic attenuation caused by vascular abnormalities and interstitial lung disease is discussed separately.
  • Mosaic attenuation can also mimic ground-glass opacities in certain clinical scenarios.