Mosaic Attenuation on CT Chest: Diagnostic Significance and Causes
Mosaic attenuation on CT chest indicates a patchwork of regions with varying lung density that can reflect small airway disease, pulmonary vascular disease, or infiltrative lung disease, with hypersensitivity pneumonitis being a common specific diagnosis when the characteristic "three-density pattern" is present. 1
Definition and Pathophysiology
- Mosaic attenuation refers to coexisting areas of varying attenuation within the lung parenchyma on inspiratory CT images, appearing as a combination of areas with low and high attenuation 2
- This term should only be used for findings on inspiratory CT images, not to be confused with air trapping which is an expiratory finding 1
- Mosaic attenuation is never a normal finding and always indicates underlying pathology 3
Three Main Causes of Mosaic Attenuation
1. Small Airway Disease
- Characterized by areas of decreased attenuation and decreased vascularity due to air trapping 1
- Examples include respiratory bronchiolitis, constrictive bronchiolitis, and small airway involvement in hypersensitivity pneumonitis 4
- Pulmonary vessels within the lucent regions of lung appear small relative to vessels in more opaque lung regions 5
- Expiratory CT confirms air trapping in affected areas 2
2. Pulmonary Vascular Disease
- Shows inhomogeneous perfusion with hyperperfused and hypoperfused areas 1
- Common in chronic thromboembolic pulmonary hypertension (CTEPH) and pulmonary arterial hypertension 4
- Mosaic pattern is seen significantly more often in patients with pulmonary hypertension due to vascular disease (74%) compared to cardiac (12%) or lung disease (5%) 6
- Associated with enlarged main pulmonary artery and increased pulmonary artery to aorta ratio 7
3. Infiltrative Lung Disease
- Presents with areas of ground-glass opacity interspersed with normal lung 1
- Vessels are more uniform in size throughout different regions of lung attenuation 5
- Can occur in acute (infection, pulmonary edema), subacute (organizing pneumonia), or chronic (fibrotic diseases) settings 4
Specific Patterns and Diagnostic Significance
The "Three-Density Pattern" in Hypersensitivity Pneumonitis
- Highly specific for fibrotic hypersensitivity pneumonitis 1
- Combines three different lung densities in close proximity: ground-glass opacification, lobules of decreased attenuation, and normal-appearing lung parenchyma 2
- Also referred to as the "headcheese sign" in radiological literature 2
- This pattern emphasizes the diagnostic value of lobules with decreased attenuation and vascularity on inspiratory CT 2
Quantitative Assessment
- Five or more lobules of mosaic attenuation in each of three or more lobes bilaterally has high specificity for fibrotic hypersensitivity pneumonitis 2
- This threshold helps differentiate fibrotic hypersensitivity pneumonitis from idiopathic pulmonary fibrosis 2
Optimal CT Technique for Evaluation
- Non-contrast examination with volumetric acquisition using submillimetric collimation 1
- Both inspiratory and expiratory acquisitions are essential for evaluating air trapping 1
- Thin-section CT images (<1.5 mm) using a high-spatial-frequency algorithm 1
- Paired inspiratory/expiratory CT scans are required to distinguish small airways disease from primary vascular disease 5
Clinical Implications and Pitfalls
- CT findings alone are not sufficient for definitive diagnosis and must be integrated with clinical information 1
- The term "mosaic perfusion" should be avoided as it implies pulmonary vascular pathology, when the cause could be airway disease or infiltrative disease 5
- Multidisciplinary discussion is recommended for definitive diagnosis, especially in cases of suspected hypersensitivity pneumonitis 1
- Mosaic pattern is not specific as an isolated finding for distinguishing the subtype of pulmonary hypertension 7
Distribution Patterns in Hypersensitivity Pneumonitis
- In fibrotic hypersensitivity pneumonitis, lung fibrosis is typically most severe in the mid or mid and lower lung zones 2
- Mid or upper lung predominance favors hypersensitivity pneumonitis compared with idiopathic pulmonary fibrosis 2
- Relative basal sparing helps differentiate fibrotic hypersensitivity pneumonitis from idiopathic pulmonary fibrosis 8
- On axial images, there is often no central or peripheral predominance of lung fibrosis in hypersensitivity pneumonitis 2