Hypersensitivity pneumonitis, also known as extrinsic allergic alveolitis,
is caused by inhalation of specific environmental organic antigens. This di
sease may have typical high-resolution CT findings that, in the appropriate
clinical setting, can be sufficiently characteristic to allow a confident
diagnosis without the need for a lung biopsy. In this pictorial essay, the
high-resolution CT patterns of hypersensitivity pneumonitis are illustrated
. The authors emphasize the correlation among the radiologic presentation,
functional abnormalities, and pathologic findings.