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E1280. A Step-by-Step Approach to Mosaic Attenuation Pattern on Thoracic CT
Authors
  1. Daniela Barros; Hospital de Braga
  2. Filipa Costeira; Hospital de Braga
  3. Ana Costa; Hospital de Braga
  4. Francisco Costa; Hospital de Braga
  5. Catarina Costa; Hospital de Braga
Background
The mosaic perfusion or mosaic attenuation pattern is a common pattern found on thoracic computed tomography (CT) and is defined as variable lung attenuation resulting in heterogeneous appearance of the parenchyma, representing a broad differential diagnosis. It may occur in diseases that affect small airways, pulmonary vasculature and pulmonary parenchyma (alveoli or intersticium).

Educational Goals / Teaching Points
To describe the characteristics of mosaic attenuation pattern on thoracic CT, to discuss the main differential diagnosis and to describe a step-by-step approach to help narrow the differential diagnosis.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
A step-by-step approach on thoracic CT to help narrow the differential diagnosis includes: 1) determine which portion of the parenchyma is abnormal; 2) look at the large airways; 3) look for direct signs of small airway injury; 4) assess the pulmonary vasculature; 5) look at the heart; 6) assess for interlobular septal thickening; 7) look for signs of fibrosis; 8) look for additional clues inside or outside the thorax and 9) perform expiratory imaging. Small airways disease can be a primer disorder, such as respiratory bronchiolitis or constritive bronchiolitis, or be part of parenchymal lung disease (hypersensivity pneumonitis, bronchiectasis and asthma). In these patients, air cannot escape the obstructed small airways, so, at expiratory CT the lungs show a relatively diffuse decrease in attenuation (air trapping). Direct visualization of the small airways is not possible on CT in normal individuals. Therefore, the presence of centrilobular nodules in conjunction with mosaic attenuation is a good clue that injury to the small airways is the cause of the mosaicism. Primary pulmonary vascular disease results in mosaic attenuation because of the regional differences in lung perfusion. Helpful clues to differentiate vascular causes of mosaic attenuation from the others are the morphology and the size of the peripheral and central pulmonary arteries. The presence of enlarged pulmonary arteries should prompt evaluation for pulmonary hypertension as a cause of mosaic attenuation. At CT, ground-glass opacity occurs when there is increased lung attenuation and the underlying airway and vessels remain visible. Diffuse ground-glass opacity is a nonspecific finding with numerous causes. Pathologically, include thickening of the alveolar wall due to inflammation or fibrosis; alveolar collapse; increased capillary blood volume; the presence of fluid, cells, or amorphous material partially filling the alveolar spaces; or a combination of these.

Conclusion
Mosaic attenuation is a common finding among a wide range of causes that can involve the airways, pulmonary vasculature, alveoli, and interstitium. The major difficulty with this pattern lies in the fact that it can be difficult for a radiologist to determine which areas of lung attenuation are normal and which are abnormal. The differential diagnosis can be narrowed by using a step-by-step approach and recognizing various imaging manifestations.