Three density sign

![Imaging patterns in HP. (a) Axial section of chest HRCT showing ill-defined centrilobular nodules (white asterisk) and areas of air-trapping (arrows) suggesting small airway involvement. (b) Ground-glass opacities and mosaic attenuation (ground glass alternating with air-trapping (arrows)) suggestive of predominant interstitial infiltration. (c) “Three density sign” comprised of lung lobules with normal density (white asterisk), lobules with ground-glass attenuation (arrowheads),and lobules with decreased density due to air-trapping (arrows). (d) Fibrotic HP presented by areas of ground glass accompanied by tractionbronchiectasis (black arrow) and honeycombing (white arrow)

Calaras D, David A, Eirini Vasarmidi, Antoniou K, Alexandru Corlateanu. Hypersensitivity Pneumonitis: Challenges of a Complex Disease. Canadian Respiratory Journal. 2024;2024:1-14. doi:https://doi.org/10.1155/2024/4919951](attachment:84ae2ed6-38e2-4d04-ac4d-d23252f9e9f3:image.png)

Imaging patterns in HP. (a) Axial section of chest HRCT showing ill-defined centrilobular nodules (white asterisk) and areas of air-trapping (arrows) suggesting small airway involvement. (b) Ground-glass opacities and mosaic attenuation (ground glass alternating with air-trapping (arrows)) suggestive of predominant interstitial infiltration. (c) “Three density sign” comprised of lung lobules with normal density (white asterisk), lobules with ground-glass attenuation (arrowheads),and lobules with decreased density due to air-trapping (arrows). (d) Fibrotic HP presented by areas of ground glass accompanied by tractionbronchiectasis (black arrow) and honeycombing (white arrow)