Mosaic attenuation (MA) is the term used to describe lung containing patchy areas with slightly higher and/or lower attenuation than normal lung. The term is not applied to frank consolidation or lung destruction. A crucial step in determining the etiology of mosaic attenuation is to establish whether it is the darker or whiter lung that is abnormal: the best way of doing this is to evaluate the size of the vessels. If the vessels are the same size in both the white and black lung, then the white lung is abnormal and the patient has "ground-glass opacity" or GGO. If the vessels are smaller in the black lung than in the white lung, then it is the black lung that is abnormal. Both pulmonary emboli and reflex vasoconstriction (decreased vessel size in poorly ventilated areas of air trapping) can result in diminished vessel caliber. In patients with pulmonary emboli, expiration results in decreased volume of both black and white lung, preserving the relative density difference between white and dark lung. With air trapping, the normal white lung is compressed and gets even whiter, but because air is "trapped" in the black lung, the volume and density of the black lung does not change, resulting in an increase in the density difference between normal white lung and obstructed black lung. This case is a good example of this phenomenon. Apply this approach with caution: some diseases (e.g. hypersensitivity pneumonitis) may have patchy areas of normal lung, air trapping and ground-glass opacification.