CTICasesHypersensitivity Pneumonitis Demonstrated by the "Gray Lung" Window/Level Sign
Imaging findings
A symptomatic patient with an abnormal DLCO and a history of significant bird exposure had a CT that appeared largely unremarkable at conventional window settings, but when displayed at a wider, more inflated window/level (approximately window width 1152, level -373) revealed diffusely increased, homogeneous "gray" lung attenuation compared to the black air of the trachea and bronchi, with subtle mosaic attenuation and conspicuous visible bronchi ("black bronchus" sign) reflecting diffuse interstitial cellular infiltration; a follow-up study after removal of the birds, displayed at the same window settings, showed resolution of the diffuse gray-lung appearance, supporting a diagnosis of hypersensitivity pneumonitis.
Key takeaways
Diffuse, non-nodular cellular infiltration of hypersensitivity pneumonitis can be extremely subtle or entirely missed at conventional lung window settings and is best appreciated as an overall "gray lung" using a wider window and different level, with conspicuously visible small airways (black bronchus sign) as an associated clue; deliberately varying window/level settings, and comparing to a follow-up study at identical settings after removal of the inciting antigen, can reveal and confirm this diffuse pattern that would otherwise be overlooked.
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