Diffuse Mosaic Attenuation: Diagnostic Workup and Management
When you encounter diffuse mosaic attenuation on chest CT, immediately obtain expiratory images to differentiate small airway disease from vascular or infiltrative causes, and specifically look for the "three-density pattern" which is pathognomonic for hypersensitivity pneumonitis and demands urgent investigation for antigen exposure. 1, 2, 3
Understanding the Pattern
Mosaic attenuation represents a patchwork of regions with varying lung density on inspiratory CT, combining areas of low and high attenuation that correspond to anatomic boundaries of secondary pulmonary lobules. 2, 4 This finding reflects one of three underlying pathological processes:
- Small airway disease shows decreased attenuation and decreased vascularity due to air trapping 2
- Pulmonary vascular disease (particularly chronic thromboembolic pulmonary hypertension) demonstrates inhomogeneous perfusion with hyperperfused and hypoperfused areas 2, 5
- Infiltrative lung disease (especially hypersensitivity pneumonitis) presents with areas of ground-glass opacity interspersed with normal lung 2
Critical Diagnostic Algorithm
Step 1: Confirm with Expiratory Imaging
- Obtain both inspiratory and expiratory CT acquisitions using volumetric acquisition with submillimetric collimation and thin-section images (<1.5 mm) 3, 6
- Air trapping that persists or worsens on expiratory images confirms small airway disease as the mechanism 2, 3, 7
- Areas that fail to increase in density on expiration indicate bronchiolar obstruction 3
Step 2: Assess Vessel Caliber
- In small airway and vascular disease, pulmonary vessels within lucent regions are smaller relative to vessels in more opaque lung 7, 8
- In infiltrative disease, vessels remain uniform in size throughout different regions of attenuation 7, 8
- This distinction has 83-92% accuracy for identifying infiltrative lung disease 8
Step 3: Identify the Three-Density Pattern
The three-density pattern is the single most important diagnostic feature to recognize. 1, 2, 6 This pattern shows three distinct lung densities sharply demarcated from each other:
- Hypoattenuating lobules (lucent lung from air trapping)
- Normal-appearing lung
- Hyperattenuating lobules (ground-glass opacities from active inflammation)
When present, this pattern is pathognomonic for fibrotic hypersensitivity pneumonitis and has 100% positive predictive value when combined with centrilobular nodules and air trapping. 1, 6
Step 4: Look for Associated Features
Features Suggesting Hypersensitivity Pneumonitis:
- Centrilobular nodules (ill-defined, <5 mm) on inspiratory images 1, 6
- Five or more lobules of mosaic attenuation in each of three or more lobes bilaterally has high specificity for fibrotic HP 2, 6
- Mid or upper lung predominance with relative basal sparing 1, 6
- Peribronchovascular distribution or diffuse axial involvement 1
- Coexisting fibrosis (irregular reticulation, traction bronchiectasis, septal thickening) 6
Features Suggesting Vascular Disease:
- Mosaic attenuation is seen in 74% of patients with pulmonary artery hypertension due to vascular disease, compared to only 5% with lung disease and 12% with cardiac disease 5
- Absence of centrilobular nodules 2
- Enlarged central pulmonary arteries 5
Essential Clinical Integration
CT findings alone are never sufficient for definitive diagnosis—you must integrate imaging with exposure history and clinical information through multidisciplinary discussion. 1, 3, 6
For Suspected Hypersensitivity Pneumonitis:
- Obtain detailed exposure history for potential antigens (birds, mold, hot tubs, occupational exposures) 1, 3
- Assess for symptoms: dyspnea, cough, systemic symptoms with exposure 1
- Consider pulmonary function tests showing restrictive or mixed pattern 1
- Multidisciplinary discussion is mandatory for definitive diagnosis 6
For Suspected Vascular Disease:
- Evaluate for risk factors for chronic thromboembolic disease 2
- Consider echocardiography or right heart catheterization 5
- V/Q scanning may be indicated 5
For Suspected Small Airway Disease:
- Assess for history of transplantation, connective tissue disease, or inhalational injury 2, 3
- Pulmonary function tests showing obstructive pattern 3
Optimal CT Technique
To avoid missing critical findings, never use ultra-low-dose CT (<1 mSv) as subtle centrilobular nodules and air trapping may be missed. 6 The recommended protocol includes:
- Non-contrast examination 6
- Volumetric acquisition with submillimetric collimation 3, 6
- Both inspiratory AND expiratory acquisitions (mandatory) 3, 6
- Thin-section images ≤1.5 mm with high-spatial-frequency reconstruction 3, 6
- Recommended radiation dose: 1-3 mSv 6
Common Pitfalls to Avoid
- Missing small airway disease by omitting expiratory images: Always obtain expiratory CT, as air trapping may be the only clue to bronchiolar involvement 6
- Confusing vascular and infiltrative causes: The presence of centrilobular nodules together with mosaic attenuation strongly favors HP over vascular disease 6
- Misclassifying chronic fibrotic HP as idiopathic pulmonary fibrosis: Key distinguishing features include the three-density pattern, mid-lung predominance, relative basal sparing, and evidence of small airway disease 6
- Using inadequate technical protocols: Inspiratory-only scans or ultra-low-dose protocols miss critical diagnostic features 6
Management Based on Etiology
If Hypersensitivity Pneumonitis is Diagnosed:
- Antigen avoidance is the cornerstone of treatment when the causative agent is identified 6
- Corticosteroids are recommended for symptomatic disease, particularly in acute and subacute forms 6
- Immunosuppressive agents may be considered in chronic, progressive disease 6
- Lung transplantation evaluation for end-stage disease 6
If Vascular Disease is Identified:
- Anticoagulation for chronic thromboembolic disease 2
- Consider pulmonary endarterectomy or balloon pulmonary angioplasty 2