From the Guidelines
Mosaic attenuation on a chest CT with areas of lucencies indicative of small airway disease typically indicates air trapping due to bronchiolar obstruction, as seen in conditions such as asthma, bronchiolitis, and hypersensitivity pneumonitis 1. This pattern is characterized by a patchwork appearance of different lung densities, with areas of lucency (darker, less dense regions) suggesting small airway disease. The affected areas appear darker (more lucent) because trapped air has lower density than normally ventilated lung tissue. Common causes of this finding include:
- Asthma
- Bronchiolitis
- Bronchiolitis obliterans
- Hypersensitivity pneumonitis
- Certain infections The presence of mosaic attenuation with small airway disease lucencies should prompt further clinical correlation with pulmonary function tests, particularly those measuring expiratory flow 1. Treatment depends on the underlying cause but often includes bronchodilators, inhaled corticosteroids, or other anti-inflammatory medications. This finding is significant as it helps differentiate small airway disease from other lung pathologies and guides appropriate therapeutic interventions. It is essential to note that the diagnosis of hypersensitivity pneumonitis, for example, can be supported by the presence of specific HRCT features, including centrilobular nodules, air trapping, and lobular areas with decreased attenuation and vascularity 1. In the context of fibrotic hypersensitivity pneumonitis, mosaic attenuation is often described as "extensive" and "marked," and the presence of a "three-density pattern" on HRCT images can be highly suggestive of this condition 1. Overall, the identification of mosaic attenuation on a chest CT with areas of lucencies indicative of small airway disease is crucial for the diagnosis and management of various lung conditions.
From the Research
Definition of Mosaic Attenuation
Mosaic attenuation on a CT chest refers to a pattern of variable lung attenuation in a lobular or multilobular distribution, which can be indicative of small airway disease, vascular lung disease, or infiltrative disease 2, 3.
Causes of Mosaic Attenuation
The causes of mosaic attenuation can be categorized into three main groups:
- Small-airway disease: characterized by areas of low attenuation due to air trapping or bronchiectasis 2, 3
- Vascular lung disease: characterized by areas of low attenuation due to reduced blood flow 2, 3
- Infiltrative disease: characterized by areas of high attenuation due to interstitial or air-space infiltrates 2, 3
Differentiation of Causes
It is sometimes possible to distinguish among these categories by using additional CT findings, such as:
- Size of pulmonary vessels within the lucent regions of lung: small vessels are more indicative of small airways disease or pulmonary vascular disease, while uniform vessel size is more indicative of infiltrative disease 3
- Use of paired inspiratory/expiratory CT scans: can help distinguish small airways disease from primary vascular disease 3
Diagnosis of Small Airway Disease
Small airway disease can be diagnosed correctly on CT scans in a high percentage of cases, with one study reporting correct diagnosis in 95% of cases by one observer and 86% of cases by another observer 4.
Key Features of Mosaic Attenuation
Key features of mosaic attenuation include:
- Areas of variable lung attenuation in a lobular or multilobular distribution
- Presence of lucencies indicative of small airway disease
- Size of pulmonary vessels within the lucent regions of lung
- Use of paired inspiratory/expiratory CT scans to distinguish small airways disease from primary vascular disease 2, 3, 4