What does a mosaic pattern on a CT scan indicate in a patient with ground glass opacities and suspected active tuberculosis (TB)?

Mosaic Pattern on CT in Suspected Active Tuberculosis with Ground Glass Opacities

In a patient with ground glass opacities and suspected active tuberculosis, a mosaic pattern on CT most commonly indicates small airways disease from bronchogenic spread of TB, though you must also consider hypersensitivity pneumonitis and chronic thromboembolic disease in the differential diagnosis. 1, 2

Primary Diagnostic Consideration: Active Tuberculosis

The mosaic pattern in active TB reflects heterogeneous lung involvement from bronchogenic spread, where caseation material fills bronchioles creating areas of varying attenuation alongside ground glass opacities. 2, 3

Key CT Features Supporting Active TB:

• Centrilobular nodules (2-4 mm) or branching linear structures represent caseation materials filling the bronchioles and are pathognomonic for bronchogenic spread of TB 2
• Poorly defined nodules (5-8 mm) correspond to centrilobular air-space consolidation with caseation necrosis 2
• Ground-glass pattern is a sign of active infection in TB and typically resolves after 2 months of appropriate treatment 3
• Lobular consolidation usually consists of central caseation necrosis with peripheral nonspecific inflammation 2

Critical Diagnostic Actions:

• Obtain sputum cultures, acid-fast bacilli smears, and nucleic acid amplification testing immediately - TB must be excluded first in any patient with this presentation 4, 5
• Ground-glass pattern persisting beyond 2 months of treatment suggests either inadequate therapy or superimposed infection 3

Alternative Diagnosis: Hypersensitivity Pneumonitis

If TB is excluded, hypersensitivity pneumonitis becomes the leading alternative diagnosis, particularly in nonsmokers with relevant exposure history. 6, 4

Distinguishing Features of HP:

• The "three-density pattern" (hypoattenuating, normal, and hyperattenuating lobules in close proximity on inspiratory imaging) is highly specific for fibrotic HP 6, 7, 4
• Profuse poorly defined centrilobular nodules of ground-glass opacity affecting all lung zones is highly suggestive of HP in nonsmokers 6, 4
• Inspiratory mosaic attenuation with air-trapping associated with centrilobular nodules supports HP diagnosis 6

Diagnostic Workup for HP:

• Obtain high-resolution CT with both inspiratory and expiratory views to assess for mosaic attenuation and air-trapping 4, 5
• Perform BAL with lymphocyte differential - greater than 20% lymphocytes supports HP diagnosis 4, 5
• Obtain detailed exposure history to organic antigens (birds, mold, hot tubs) as this is the cornerstone of HP diagnosis 4
• Consider transbronchial biopsy combined with BAL to increase diagnostic yield, particularly in fibrotic HP 6, 5

Vascular Causes: Chronic Thromboembolic Disease

Mosaic attenuation from vascular disease shows a distinct pattern where pulmonary vessels within lucent regions are smaller relative to vessels in more opaque lung. 1, 8

Features of CTEPH:

• Inhomogeneous perfusion with mosaic pattern reflects areas of hyperperfused (high attenuation) and hypoperfused (low attenuation) lung 6
• Organized thrombus lining pulmonary vessels eccentrically with enlargement of RV and central pulmonary arteries 6
• Variation in segmental artery size - relatively smaller in affected segments than uninvolved areas 6

Algorithmic Approach to Differentiation

Step 1: Vessel Size Assessment

• In small airways disease (TB, HP) and vascular disease, vessels within lucent regions are small relative to vessels in opaque lung 1, 8
• In infiltrative diseases, vessels are uniform in size throughout different attenuation regions 8

Step 2: Expiratory Imaging

• Obtain paired inspiratory/expiratory CT scans to distinguish small airways disease from primary vascular disease 8
• Air-trapping on expiratory images confirms small airways disease (TB or HP) rather than vascular pathology 6

Step 3: Associated Findings

• Centrilobular nodules or branching linear structures favor TB or HP over vascular disease 2, 3
• Cardiovascular abnormalities (RV enlargement, central PA dilation) favor CTEPH 6
• Hilar or mediastinal lymphadenopathy occurs in up to 43% of adult TB cases 9

Critical Pitfalls to Avoid

• Do not assume infectious etiology without considering HP, especially in nonsmokers with exposure history 4, 5
• Do not delay TB evaluation - obtain AFB smears and NAAT immediately regardless of perceived risk 4, 5
• Do not use the term "mosaic perfusion" when describing heterogeneous attenuation - use "mosaic pattern of lung attenuation" as "mosaic perfusion" incorrectly implies vascular pathology 8
• Do not rely on chest radiography alone - conventional radiographs can be normal in confirmed TB cases, particularly when CT shows ground-glass opacity, bronchial wall thickening, or centrilobular nodules 6

Prognostic Implications

• Ground-glass pattern and poorly marginated nodules indicate active TB infection and should resolve with appropriate treatment 3
• ARDS-like manifestations on CT carry the highest mortality rate (64.5%) among pulmonary TB presentations in critically ill patients 9
• Persistent ground-glass opacity after 2 months of TB treatment suggests treatment failure or superimposed infection requiring immediate reassessment 3