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 decreased perfusion of the poorly ventilated regions, e.g. bronchiectasis, cystic fibrosis, constrictive bronchiolitis
- occlusive vascular disease (can be termed a mosaic perfusion pattern in this setting 7): low attenuation regions are abnormal and reflect relative oligaemia, e.g. chronic pulmonary embolism, chronic thromboembolic pulmonary hypertension
- parenchymal disease: most common cause of mosaic attenuation. high attenuation regions are abnormal and represent ground-glass opacity
The term is best used where the differentiation between mosaic perfusion, mosaic oliagemia, 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 2,5:
- peripheral vessels: if vessels in hypoattenuated regions of the lung are smaller than in the other regions, the pattern is due to mosaic perfusion (i.e. airways or vascular disease rather than ground-glass)
- central vessels: pulmonary hypertension, reflected as dilatation of the central pulmonary arteries, suggests a vascular cause
- small airways: the presence of abnormally dilated or thick walled airways in the relatively lucent lung confirms underlying airway disease, see small airways disease
- parenchymal changes: ground glass opacity is the likely cause for mosaic attenuation if other features of infiltrative disease are present, such as reticular opacities (i.e. crazy paving pattern) or nodules
- air trapping: refers to regions of lung which following expiration do not show the normal increase in attenuation, or show little change in cross-sectional area 5 (i.e. this is an expiratory HRCT finding); the presence of air trapping suggests airway disease
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