On a CT scan, the ‘air crescent’ is strongly suggestive of a fungal infection.
Lungs normally look black on a CT scan because they are full of air, with the ribs around them appearing bright white because bone is much denser.
In invasive aspergillosis, the growing nodule may at first show up with a fuzzy white ‘halo’ made up of microscopic bleeds. A couple of weeks later, the dying lung tissue shrinks down into a firm ball, leaving an air-filled gap that looks like a crescent moon on a CT scan.
Non-invasive fungal balls that grow in pre-existing cavities (e.g. following tuberculosis) also produce an air crescent, but in this case the entire ball is made up of a dense mass of fungal strands, mucus and cell debris.
Read more about air crescent (also known as Monad sign).