What is nonspecific attenuation of the lung?

Nonspecific Attenuation of the Lung

Nonspecific attenuation of the lung refers to a mosaic pattern of varying lung densities on CT imaging that can result from three main causes: small airways disease, vascular disease, or infiltrative lung disease, requiring further evaluation to determine the specific underlying etiology. 1

Understanding Mosaic Attenuation

Mosaic attenuation is a radiological term used to describe a patchwork of regions with different lung densities visible on inspiratory CT images. This pattern is characterized by:

• Areas of varying density that can present in two main configurations:

  • Areas of ground-glass opacity ("high" density) and normal lung ("low" density)
  • Areas of normal lung ("high" density) and decreased attenuation ("low" density) 2

• The term "mosaic attenuation" is preferred over "mosaic perfusion" or "mosaic oligemia" because it doesn't imply a specific pathological process 3

Three Main Causes of Mosaic Attenuation

1. Small Airways Disease

• Characterized by air trapping distal to obstructed airways
• Features:
  • Smaller pulmonary vessels within lucent (low-density) regions
  • Air trapping visible on expiratory CT scans
  • Often associated with ill-defined centrilobular nodules 2, 1

2. Vascular Disease

• Caused by regional differences in lung perfusion
• Features:
  • Decreased vascular sections within areas of low attenuation
  • No significant change in attenuation gradient between areas on expiratory scans
  • Often seen in conditions like chronic thromboembolic pulmonary hypertension 1, 4

3. Infiltrative Lung Disease

• Caused by interstitial or airspace infiltrates
• Features:
  • More uniform vessel size throughout regions of different attenuation
  • Ground-glass opacities representing areas of active inflammation or fibrosis 3, 4

The "Three-Density Pattern"

A specific type of mosaic attenuation called the "three-density pattern" is highly specific for hypersensitivity pneumonitis (HP), especially fibrotic HP. This pattern includes:

1. Normal-appearing lung
2. High attenuation areas (ground-glass opacity)
3. Lucent lung (decreased attenuation and decreased vascular sections)

These three densities are sharply demarcated from each other 2, 1

Diagnostic Approach

To differentiate between the various causes of mosaic attenuation:

1. Compare vessel sizes: In small airways disease and vascular disease, vessels in lucent regions are smaller than in opaque regions. In infiltrative disease, vessel sizes are more uniform 3

2. Obtain paired inspiratory/expiratory CT scans: Essential to distinguish small airways disease from vascular disease. In small airways disease, the attenuation differences are accentuated on expiration due to air trapping 2, 1

3. Evaluate distribution patterns: Upper/mid lung predominance suggests HP, while basal predominance may suggest other etiologies 1

4. Look for associated findings: Centrilobular nodules suggest HP or bronchiolitis; fibrosis may indicate fibrotic HP; enlarged pulmonary arteries suggest pulmonary hypertension 1

Clinical Implications

Understanding the cause of mosaic attenuation is crucial for proper management:

• In hypersensitivity pneumonitis: Remove causative antigen, consider corticosteroids or immunosuppressants
• In vascular disease (e.g., CTEPH): Consider pulmonary endarterectomy, balloon angioplasty, or medical therapy
• In small airways disease: Use bronchodilators, inhaled corticosteroids, and treat underlying cause 1

Important Considerations

• Mosaic attenuation is never a normal finding on CT scans 4
• Lung volume significantly affects density measurements - differences between dependent and non-dependent lung regions are smallest near total lung capacity 5
• The presence of mosaic attenuation should prompt further investigation to determine the specific underlying cause, as treatment approaches differ significantly 1