Radiology can be beautiful.

Recently I came across an example of 'mosaic attenuation' on a high resolution CT (HRCT) scan of the chest. There are many rather evocative descriptive terms used in reference to changes seen on this modality of CT scan, and I think that mosaic attenuation is perhaps my favourite.

The attached photo demonstrates this abnormality. Areas of black lung are nestled against areas of 'ground glass' change (think about the smoothed, opaque appearance of a glass fragment found on the beach). These represent areas of differential ventilation and perfusion. On HRCT of the chest, blood is seen as white, air as black and areas where there is a significant amount of both blood flow (perfusion) and air ventilation come up grey. When we have a 'mosaic' there are areas of lung alongside each other which have very different ratios of blood flow to ventilation. Black (more air less blood flow) against grey (more blood flow relatively).

Understanding this, it's apparent that disease process which have an impact on blood circulation in the lungs, or on air flow in tiny airways, in a variable way throughout the lungs may lead to mosaic attenuation. This includes such entities as hypersensitivity pneumonitis (which effects tiny airways, the terminal bronchioles) and pulmonary hypertension. In the attached film, there is evidence of widespread bronchiectasis - which may be the cause of the mosaic attenuation....

But bronchiectasis on CT is another story.