Mosaic attenuation is a descriptive term used in describing a patchwork of regions of differing pulmonary attenuation on CT imaging. It is a non-specific finding, although is associated with the following:
- obstructive small airways disease: low attenuation regions are abnormal and reflect two phenomena occurring at the same time: 1. air-trapping in lung areas with obstructive small airways disease, and 2. reduced perfusion in these pathological areas secondary to vasoconstriction (in order to maintain constant ventilation/perfusion ratio in these hypoventilated areas). This can also lead to relative hyperemia in adjacent healthy lung areas, further contributing to the mosaic attenuation pattern , e.g. bronchiectasis, cystic fibrosis, constrictive bronchiolitis
- occlusive vascular disease (can be termed a mosaic perfusion pattern in this setting): low attenuation regions are abnormal and reflect relative oligemia, e.g. chronic pulmonary embolism, chronic thromboembolic pulmonary hypertension
- other vascular causes giving patchy ground glass changes
- parenchymal disease: considered most common cause of mosaic attenuation (50%) ; high attenuation regions are abnormal and represent ground-glass opacity
- acute infections
- subacute infections
The term is best used where the differentiation between mosaic perfusion, mosaic oligemia, and ground-glass opacity cannot be confidently made.
Differentiating the cause
Although "mosaic attenuation" is non-specific, a radiologist should search for ancillary imaging findings which may help identify the underlying process in order to help guide clinical management.
Ascertaining the underlying cause for mosaic attenuation is often possible on the basis of clinical information, combined with the assessment of other lung features on HRCT :
A practical approach to the etiological diagnosis of a mosaic attenuation pattern is to ask the following question: 'which is the pathological lung area?'
- Is the lung area with hyperattenuation?
- parenchymal lung disease (most common cause, 50%)
- pulmonary edema
- pulmonary haemorrage
- Is the lung area with hypoatenuation?
- occlusive vascular disease (oligoaemia)
- obstructive small airways disease (air-trapping plus secondary vasoconstriction)