The term 'mosaic attenuation pattern', or more simply mosaic pattern, refers to regional attenuation differences demonstrated on HRCT. The attenuation of a given area of lung depends on the amount of blood, parenchymal tissue and air in that area, and thus the sign of a mosaic attenuation pattern is non-specific. It is the dominant abnormality in three completely different types of diffuse pulmonary disease: small airways disease, chronic occlusive vascular disease and infiltrative lung disease. In the first two processes, the decreased attenuation ('black') lung is abnormal; in infiltrative lung disease it is the 'grey' lung that is abnormal. In a study of 70 patients in whom a mosaic attenuation pattern was the dominant abnormality, Worthy et al. showed that small airways disease and infiltrative lung disease were correctly identified but the mosaic attenuation pattern caused by occlusive vascular disease was frequently misinterpreted. Bronchial abnormalities and, to a lesser extent, the presence of air trapping on expiratory CT are the most useful discriminatory features in identifying small airways disease as the cause of mosaic attenuation. However, the phenomenon of hypoxic bronchodilatation in chronic occlusive vascular disease, and the fact that air trapping may be seen on expiratory CT in chronic thromboembolic disease, complicates interpretation. Nevertheless, the differentiation between the three basic causes of a mosaic attenuation pattern is usually easily made when clinical and physiological information is taken into account.

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