What does a mosaic pattern on a computed tomography (CT) scan indicate and how is it managed?

Mosaic Pattern on CT: Diagnosis and Management

A mosaic pattern on CT scan indicates areas of varying lung density creating a patchwork appearance, most commonly associated with small airways disease, vascular disease, or infiltrative lung disease, with specific management determined by the underlying cause. The diagnostic approach should focus on identifying the specific etiology through a combination of clinical, radiological, and sometimes histological findings, as the mosaic pattern itself is not a diagnosis but rather a radiographic finding requiring further evaluation.

Understanding Mosaic Attenuation

Definition and Appearance

• Mosaic attenuation refers to a patchwork of regions with varying lung density on inspiratory CT images 1
• Three main patterns:
  • Areas of ground-glass opacity (high attenuation) and normal lung (low attenuation)
  • Areas of normal lung (high attenuation) and decreased attenuation (low attenuation)
  • "Three-density pattern": combination of normal-appearing lung, ground-glass opacity, and lucent lung with decreased attenuation and vascularity 2

Common Causes of Mosaic Attenuation

1. Small Airways Disease

   • Bronchiolitis obliterans
   • Asthma
   • Hypersensitivity pneumonitis (HP)

2. Vascular Disease

   • Chronic thromboembolic pulmonary hypertension (CTEPH)
   • Pulmonary arterial hypertension

3. Infiltrative Lung Disease

   • Hypersensitivity pneumonitis
   • Drug-related pneumonitis
   • Interstitial lung disease

Diagnostic Approach

Key CT Features to Evaluate

• Distribution pattern: Upper/mid lung predominance suggests HP; basal predominance may suggest other etiologies 2
• Three-density pattern: Highly specific for hypersensitivity pneumonitis, especially fibrotic HP 2
• Expiratory imaging: Essential to detect air trapping, which helps differentiate small airways disease from vascular causes 1
• Associated findings:
  • Centrilobular nodules: Suggest HP or bronchiolitis
  • Fibrosis: May indicate fibrotic HP
  • Enlarged pulmonary arteries: Suggest pulmonary hypertension

Distinguishing Features Between Causes

1. Hypersensitivity Pneumonitis

   • Three-density pattern (most specific finding)
   • Poorly defined centrilobular nodules
   • Mid or upper lung predominance
   • Air trapping on expiratory images 2

2. CTEPH

   • Mosaic attenuation with enlarged central pulmonary arteries
   • Variation in size of segmental arteries (smaller in affected segments)
   • Organized thrombus lining pulmonary vessels 2

3. Drug-Related Pneumonitis

   • Mosaic pattern with ground-glass opacities
   • May show organizing pneumonia pattern
   • Often bilateral and symmetric 2

Management Approach

General Approach

1. Identify the underlying cause through:

   • Detailed exposure history (occupational, environmental, medications)
   • Pulmonary function tests
   • Laboratory testing (specific IgG antibodies for suspected HP)
   • Additional imaging (V/Q scan for suspected CTEPH)
   • Bronchoscopy with bronchoalveolar lavage
   • Lung biopsy when diagnosis remains unclear

2. Disease-specific management:

Management of Hypersensitivity Pneumonitis

• Identify and remove the causative antigen exposure
• Pharmacological treatment:
  • Corticosteroids for acute/subacute phases
  • Immunosuppressants for fibrotic HP 1
• Supportive care:
  • Oxygen therapy for hypoxemia
  • Pulmonary rehabilitation

Management of CTEPH

• Pulmonary endarterectomy for operable cases
• Balloon pulmonary angioplasty or medical therapy for inoperable cases
• Long-term anticoagulation 2, 1

Management of Drug-Related Pneumonitis

• Discontinuation of the offending drug
• Corticosteroids for significant symptoms or extensive involvement
• Monitoring for resolution 2

Clinical Pearls and Pitfalls

Pearls

• The "three-density pattern" (previously called "headcheese sign") is highly specific for HP 2
• Expiratory imaging is crucial to differentiate small airways disease from vascular causes 1
• A relatively normal CT angiogram can be observed in CTEPH despite significant abnormalities on ventilation-perfusion scintigraphy 2

Pitfalls

• Mosaic attenuation alone is not diagnostic of any specific condition and requires clinical correlation 1
• Vascular causes of mosaic attenuation are often misinterpreted as infiltrative lung disease or airway disease 3
• The combination of bilateral pulmonary nodules and mosaic attenuation is not specific for any single diagnosis 4

Remember that the mosaic pattern is a radiographic finding that requires clinical correlation and often additional testing to determine the specific etiology and appropriate management.