Mosaic attenuation pattern in lung

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 ^5,10, 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 oligemia, e.g. chronic pulmonary embolism, chronic thromboembolic pulmonary hypertension 

• other vascular causes giving patchy ground glass changes

  • acute

    • pulmonary edema ¹⁰

    • pulmonary hemorrhage¹⁰

• parenchymal disease: considered most common cause of mosaic attenuation (50%) ⁵; high attenuation regions are abnormal and represent ground-glass opacity

  • acute infection

  • subacute infection

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 ^2,5:

1. 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)

2. central vessels: pulmonary hypertension, reflected as dilatation of the central pulmonary arteries, suggests a vascular cause

3. small airways: the presence of abnormally dilated or thick walled airways in the relatively lucent lung confirms underlying airway disease, see small airways disease

4. parenchymal changes: ground glass opacity is the likely cause for mosaic attenuation if other features of the infiltrative disease are present, such as reticular opacities (i.e. crazy paving pattern) or nodules

5. air trapping: refers to regions of the 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

A practical approach to the etiological diagnosis of a mosaic attenuation pattern is to ask the following question: 'Which is the pathological lung area?' ^5,10

• Is the abnormal lung hyperattenuating?

  • parenchymal lung disease (most common cause, 50%)

  • pulmonary edema

  • pulmonary hemorrhage

• Is the abnormal lung area hypoattenuating? 

  • occlusive vascular disease (oligemia)

  • obstructive small airways disease (air-trapping plus secondary vasoconstriction)