Table 6.

Chest HRCT Scan Features of the Fibrotic HP Pattern

HRCT Pattern	Typical HP	Compatible with HP	Indeterminate for HP
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 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	Variant patterns of lung fibrosis:	Lone patterns (i.e., not accompanied by other findings suggestive of HP) of:
		• UIP pattern: basal and subpleural distribution of honeycombing with/without traction bronchiectasis (per 2018 diagnosis of IPF guidelines [20])	• UIP pattern (as per 2018 IPF diagnosis guidelines [20])
	The distribution of fibrosis may be:
	• Random both axially and craniocaudally or	• Extensive GGOs with superimposed subtle features of lung fibrosis	• Probable UIP pattern (as per 2018 IPF diagnosis guidelines [20])
	• Mid lung zone–predominant or		• Indeterminate pattern for UIP (as per 2018 IPF diagnosis guidelines [20])
	• Relatively spared in the lower lung zones	Variant (predominant) distributions of lung fibrosis:	• Fibrotic NSIP pattern
		• Axial: peribronchovascular, subpleural areas	• Organizing pneumonia–like pattern
	HRCT abnormalities indicative of small airway disease:	• Craniocaudal: upper lung zones	• Truly indeterminate HRCT pattern
	• Ill-defined, centrilobular nodules and/or GGOs
	• Mosaic attenuation, three-density pattern,* and/or air trapping (often in a lobular distribution)	HRCT abnormalities indicative of small airway disease:
		• Ill-defined centrilobular nodules, or
		• Three-density pattern* and/or air trapping

Definition of abbreviations: GGO = ground-glass opacity; HP = hypersensitivity pneumonitis; HRCT = high-resolution computed tomography; IPF = idiopathic pulmonary fibrosis; NSIP = nonspecific interstitial pneumonia; UIP = usual interstitial pneumonia.

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.

^*The three-density pattern was formerly called the “headcheese sign.” It is described in detail in Table 4.