Chest HRCT scan features of the nonfibrotic and fibrotic HP patterns

HRCT pattern	Typical HP	Compatible with HP	Indeterminate for HP
Nonfibrotic HP pattern
Description	The "typical HP" pattern is suggestive of a diagnosis of HP. It requires (A) at least one HRCT abnormality indicative of parenchymal infiltration and (B) at least one HRCT abnormality indicative of small airway disease, both in a diffuse distribution.	"Compatible-with-HP" patterns are nonspecific patterns that have been described in HP.	N/A
Relevant radiological findings	HRCT abnormalities indicative of parenchymal infiltration: GGOs Mosaic attenuation^* HRCT abnormalities indicative of small airway disease: Ill-defined, centrilobular nodules Air trapping Distribution of parenchymal abnormalities: Craniocaudal: diffuse (with or without some basal sparing) Axial: diffuse	Parenchymal abnormalities: Uniform and subtle GGOs Airspace consolidation Lung cysts Distribution of parenchymal abnormalities: Craniocaudal: diffuse (variant: lower lobe predominance) Axial: diffuse (variant: peribronchovascular)	N/A
Fibrotic HP pattern
Description	The "typical HP" pattern is suggestive of a diagnosis of HP. It requires (A) an HRCT pattern of lung fibrosis (as listed below) in one of the distributions and (B) at least one abnormality that is indicative of small airway disease.	"Compatible-with-HP" patterns exist when the HRCT pattern and/or distribution of lung fibrosis varies from that of the typical HP pattern; the variant fibrosis should be accompanied by signs of small airway disease.	The "indeterminate-for-HP" pattern exists when the HRCT is neither suggestive of nor compatible with a typical and probable HP pattern.
Relevant radiological findings	HRCT abnormalities indicative of lung fibrosis are most commonly composed of irregular linear opacities/coarse reticulation with lung distortion; traction bronchiectasis and honeycombing may be present but do not predominate. The distribution of fibrosis may be: Random both axially and craniocaudally, or Mid lung zone-predominant, or Relatively spared in the lower lung zones HRCT abnormalities indicative of small airway disease: Ill-defined, centrilobular nodules and/or GGOs Mosaic attenuation^*, three-density pattern^¶, and/or air trapping (often in a lobular distribution)	Variant patterns of lung fibrosis: UIP pattern: basal and subpleural distribution of honeycombing with/without traction bronchiectasis (per 2018 diagnosis of IPF guidelines^[1]) Extensive GGOs with superimposed subtle features of lung fibrosis Variant (predominant) distributions of lung fibrosis: Axial: peribronchovascular, subpleural areas Craniocaudal: upper lung zones HRCT abnormalities indicative of small airway disease: Ill-defined centrilobular nodules, or Three-density pattern^¶ and/or air trapping	Lone patterns (ie, not accompanied by other findings suggestive of HP) of: UIP pattern (as per 2018 IPF diagnosis guidelines^[1]) Probable UIP pattern (as per 2018 IPF diagnosis guidelines^[1]) Indeterminate pattern for UIP (as per 2018 IPF diagnosis guidelines^[1]) Fibrotic NSIP pattern Organizing pneumonia-like pattern Truly indeterminate HRCT pattern

Rarely, fibrotic HP may be seen (1) as a component of combined pulmonary fibrosis and emphysema or pleuroparenchymal fibroelastosis with emphysema, (2) as a pure emphysematous form of HP, or (3) in acute exacerbation.

GGO: ground-glass opacity; HP: hypersensitivity pneumonitis; HRCT: high resolution computed tomography; N/A: not applicable; IPF: idiopathic pulmonary fibrosis; NSIP: nonspecific interstitial pneumonia; UIP: usual interstitial pneumonia.
* Mosaic attenuation corresponding to parenchymal infiltration is created by GGOs adjacent to normal-appearing lung.
¶ Combination of three attenuations on inspiratory CT images: normal-appearing lung, high attenuation (GGO), lucent lung (regions of decreased attenuation and decreased vascularity). This pattern indicates a mixed obstructive and infiltrative process.

Reference:

Raghu G, Remy-Jardin M, Myers JL, et al. Diagnosis of idiopathic pulmonary fibrosis: an official ATS/ERS/JRS/ALAT clinical practice guideline. Am J Respir Crit Care Med 2018; 198:e44.

Reprinted with permission of the American Thoracic Society. Copyright © 2021 American Thoracic Society. All rights reserved. Ganesh Raghu, Martine Remy-Jardin, Christopher J Ryerson, et al. Diagnosis of Hypersensitivity Pneumonitis in Adults. An Official ATS/JRS/ALAT Clinical Practice Guideline. Am J Respir Crit Care Med 2020; 202:e36. The American Journal of Respiratory and Critical Care Medicine is an official journal of the American Thoracic Society.

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Chest HRCT scan features of the nonfibrotic and fibrotic HP patterns