Causes of Small Airway Disease with Mosaic Attenuation on Lung Imaging
Mosaic attenuation on lung imaging is primarily caused by small airway disease resulting from bronchiolar obstruction, which leads to areas of decreased attenuation and vascularity due to air trapping. 1
Primary Causes of Mosaic Attenuation
- Small airway disease (bronchiolitis) is the most common cause, characterized by obstruction of bronchioles leading to air trapping and areas of decreased attenuation and vascularity 1, 2
- Hypersensitivity pneumonitis is a classic cause, presenting with the characteristic "three-density pattern" (also called "headcheese sign") showing three different lung densities (ground-glass opacities, decreased attenuation areas, and normal-appearing lung) 3
- Obliterative (constrictive) bronchiolitis causes mosaic attenuation through concentric fibrosis leading to luminal narrowing of small airways 2, 4
- Pulmonary vascular disease, such as chronic thromboembolic pulmonary hypertension, can cause mosaic attenuation through inhomogeneous perfusion with hyperperfused and hypoperfused areas 1, 5
- Infiltrative lung diseases can present with areas of ground-glass opacity interspersed with normal lung 1, 6
Pathophysiological Mechanisms
Small Airway Disease Mechanism
- Bronchiolar obstruction causes air trapping distal to the obstruction 3
- This creates areas of decreased attenuation and decreased vascularity on inspiratory CT images 5
- The pattern is exaggerated on expiratory imaging, confirming air trapping 3
Specific Disease Patterns
Hypersensitivity Pneumonitis
- In nonfibrotic HP: diffusely distributed ground-glass opacities and mosaic attenuation with ill-defined centrilobular nodules 3, 7
- In fibrotic HP: coexisting lung fibrosis and signs of bronchiolar obstruction with the characteristic three-density pattern 3, 7
- Five or more lobules of mosaic attenuation in each of three or more lobes bilaterally has high specificity for fibrotic HP 3
Obliterative Bronchiolitis
- Presents with mosaic attenuation without the three-density sign or characteristic centrilobular nodules 3
- Shows areas of decreased attenuation representing air trapping or hypoxic vasoconstriction in secondary pulmonary lobules 4
Connective Tissue Disease
- Particularly rheumatoid arthritis can present with a combination of lung fibrosis and mosaic attenuation 3
Diagnostic Approach
- Evaluate for presence of the "three-density pattern" which is highly specific for hypersensitivity pneumonitis 1
- Assess for air trapping on expiratory images, which confirms small airway involvement 3
- Look for associated findings such as centrilobular nodules (suggesting cellular bronchiolitis) or fibrosis (suggesting chronic disease) 8
- Consider distribution pattern - diffuse distribution in the axial plane and upper or mid-lung predominant fibrosis supports the diagnosis of HP 3
Clinical Implications
- Mosaic attenuation should prompt consideration of specific diagnoses, particularly hypersensitivity pneumonitis when the three-density pattern is present 1
- CT findings alone are not sufficient for definitive diagnosis and must be integrated with exposure history and clinical information 7
- Multidisciplinary discussion is recommended for definitive diagnosis, especially in cases of suspected hypersensitivity pneumonitis 7