|E||Eosinophilic granuloma / Langerhans cell histiocytosis||Young male smokers|
|P||Pneumocystis Pneumonia (PCP)||Immunocompromised|
|R||Radiation fibrosis||Head and neck cancers|
In general, keep in mind that the upper lung zones have increased aeration and decreased perfusion relative to the lower lung zones due to the effects of gravity.
Source: Radiology Notes