Mosaic attenuation describes a CT pattern that comprises a patchwork of regions of differing pulmonary attenuation. The term can only be applied to CT acquired during full inspiration ¹⁷. Causes fall into the following categories:

• obliterative small airways disease: regional air-trapping causes abnormally low attenuation, exacerbated by reflex vasoconstriction. Low attenuation is maintained on expiratory CT in contrast to the increased attenuation of normal lung. Relative hyperaemia can increase attenuation in unaffected areas ^5,10. Causes include bronchiectasis, constrictive bronchiolitis and hypersensitivity pneumonitis

• occlusive vascular disease causes mosaic perfusion ⁷: low attenuation regions are oligaemic and normal areas may be hyperaemic, with increased attenuation. Attenuation of all regions increases on expiration. Chronic thromboembolic pulmonary hypertension is a common cause.

• multifocal disease causing ground glass opacity in a mosaic pattern is commonly (about 50% ⁵) due to

  • acute pulmonary oedema ¹⁰

  • pulmonary haemorrhage¹⁰

  • pneumonia

• a combination of normal lung, air trapping and ground glass opacity:

  • hypersensitivity pneumonitis, a three density pattern is characteristic

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 aetiological 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 oedema

  • pulmonary haemorrhage

• Is the abnormal lung area hypoattenuating? 

  • occlusive vascular disease (oligaemia)

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