What is the approach to managing mosaic attenuation on high-resolution computed tomography (HRCT) scans?

Approach to Mosaic Attenuation on HRCT

The diagnostic approach to mosaic attenuation on HRCT requires identification of the underlying cause through systematic evaluation of imaging characteristics, with expiratory imaging being essential to differentiate between airway, vascular, and infiltrative lung diseases. 1

Understanding Mosaic Attenuation

Mosaic attenuation is defined as a sharply defined geographic patchwork of regions with differing attenuation on inspiratory HRCT images. This pattern is never a normal finding and represents one of three main underlying pathologies:

1. Small airway disease (bronchiolitis)
2. Vascular lung disease (pulmonary vascular occlusion)
3. Infiltrative lung disease (interstitial or air-space disease)

Diagnostic Algorithm

Step 1: Characterize the Mosaic Pattern

• Identify the specific type of mosaic attenuation:
  • Three-density sign (most specific for hypersensitivity pneumonitis): Combination of lung lobules with preserved density, surrounded by patchy ground-glass opacification, and interspersed with lobules of decreased density and vessel size 1
  • Other forms of mosaic attenuation: Lobular decreased attenuation interspersed with normal lung 1

Step 2: Obtain Expiratory Imaging

• Always obtain paired inspiratory and expiratory scans when mosaic attenuation is identified
• Expiratory scans significantly improve diagnostic accuracy (from 79% to 92%) and confidence level (from 45% to 92%) 2
• Look for lobular air-trapping on expiration, which is identified by sharply demarcated areas that fail to increase in density with expiration 1

Step 3: Assess Vessel Size

• In small airways disease and vascular disease: Vessels within lucent regions are smaller compared to vessels in more opaque lung regions
• In infiltrative disease: Vessels are more uniform in size throughout different regions 3

Step 4: Evaluate Distribution and Associated Findings

• Distribution patterns:

  • Mid or upper lung predominance favors hypersensitivity pneumonitis over IPF 1
  • Diffuse involvement in axial plane or peribronchovascular component suggests hypersensitivity pneumonitis 1

• Associated findings:

  • Centrilobular nodules (poorly defined, <3mm, ground-glass attenuation) suggest hypersensitivity pneumonitis 1
  • Fibrosis (reticular pattern, traction bronchiectasis, honeycombing) indicates chronic disease 1

Differential Diagnosis Based on Imaging Features

1. Small Airway Disease

• Key findings: Air trapping on expiratory CT, bronchial wall thickening
• Examples: Hypersensitivity pneumonitis, bronchiolitis obliterans, asthma
• Diagnostic clue: Air trapping is significantly more pronounced on expiratory imaging 2

2. Vascular Disease

• Key findings: Decreased vessel size in lucent areas, no significant air trapping on expiration
• Examples: Chronic thromboembolic pulmonary hypertension, pulmonary arterial hypertension
• Diagnostic clue: Most difficult to identify correctly (accuracy only 27-36%) 4
• Special technique: Consider using CTMosaic window settings (window level -800 HU, window width 200 HU) to better visualize perfusion defects 5

3. Infiltrative Disease

• Key findings: Ground-glass opacities, uniform vessel size throughout
• Examples: Hypersensitivity pneumonitis (inflammatory phase), nonspecific interstitial pneumonia
• Diagnostic clue: Vessels are uniform in size throughout regions of varying attenuation 3

Pitfalls and Caveats

1. Terminology confusion: Avoid using terms like "mosaic perfusion" or "mosaic oligemia" as they imply vascular pathology; "mosaic attenuation" is the preferred term 3

2. Misdiagnosis of vascular disease: Vascular causes are frequently misinterpreted as infiltrative or airway disease (accuracy only 27-36%) 4

3. Incomplete imaging: Failure to obtain expiratory scans significantly reduces diagnostic accuracy 2

4. Overlapping patterns: In hypersensitivity pneumonitis, both infiltrative and airway disease components may coexist, creating complex patterns 1

5. Lesion size challenges: For lesions >8mm in diameter, multiple image sets with at least 10% overlap are needed for complete evaluation 1

By following this systematic approach and paying careful attention to the specific characteristics of mosaic attenuation, clinicians can more accurately determine the underlying cause and guide appropriate management to improve patient outcomes.