CT Findings in Hypersensitivity Pneumonitis
The key CT findings in hypersensitivity pneumonitis (HP) are ground-glass opacities, mosaic attenuation, air trapping on expiratory images, and ill-defined centrilobular nodules, with different patterns distinguishing nonfibrotic from fibrotic HP. 1, 2
Nonfibrotic HP CT Findings
- Diffusely distributed ground-glass opacities (GGO) and mosaic attenuation are characteristic features of lung infiltration in nonfibrotic HP 1, 2
- Ill-defined, small (<5 mm) centrilobular nodules on inspiratory images are highly suggestive of nonfibrotic HP 1
- Air trapping on expiratory CT images is a key diagnostic feature and should be specifically evaluated with expiratory imaging 1, 3
- Craniocaudal distribution is typically diffuse without zonal predominance 2
- Axial distribution is typically diffuse without central or peripheral predominance 2, 4
- The "three-density pattern" may be present, showing normal lung attenuation, ground-glass opacities, and decreased attenuation (air trapping) 1, 2
Fibrotic HP CT Findings
- Coexisting lung fibrosis and signs of bronchiolar obstruction (mosaic attenuation, air trapping) are highly suggestive of fibrotic HP 1, 2
- Irregular fine or coarse reticulation with architectural distortion is a typical manifestation of fibrosis in HP 1, 5
- Septal thickening with or without traction bronchiectasis in areas of ground-glass opacities is commonly seen 1, 2
- Fibrosis is typically most severe in the mid or mid and lower lung zones 1, 6
- Relative basal sparing helps differentiate fibrotic HP from idiopathic pulmonary fibrosis 1, 2
- On axial images, there is often no central or peripheral predominance of lung fibrosis, though peripheral distribution may be seen in some cases 1, 6
- Honeycombing may be present in severe forms of fibrotic HP 1, 5
Prognostic CT Findings
- The presence of air trapping and mosaic attenuation on CT is associated with better survival in chronic HP 7
- Traction bronchiectasis and increased pulmonary artery/aorta ratio are associated with worse survival 6
- CT pattern of usual interstitial pneumonia (UIP) or probable UIP is an independent risk factor for fibrotic progression and acute exacerbation 5
- Ground-glass opacity predominance (without extensive fibrosis) is associated with improved survival 6
Recommended CT Acquisition Parameters
- Noncontrast examination is standard 1, 2
- Volumetric acquisition with submillimetric collimation is recommended 1
- Thin-section CT images (<1.5 mm) using a high-spatial-frequency algorithm 1
- Both inspiratory and expiratory acquisitions are essential to evaluate for air trapping 1, 2
- Recommended radiation dose is 1-3 mSv ("reduced" dose) 1
- Prone imaging is optional but may help differentiate dependent atelectasis from true pathology 1
Diagnostic Pitfalls and Caveats
- CT findings alone are insufficient for definitive diagnosis of HP, especially in fibrotic HP where appearances can overlap with other interstitial lung diseases 2, 6
- Smoking may decrease the typical HP appearance on CT and increase the likelihood of a UIP-like pattern 6
- The CT appearance of chronic HP can mimic nonspecific interstitial pneumonia (NSIP) or usual interstitial pneumonia (UIP) patterns 3, 6
- Fibrotic progression in chronic HP often shows persistent areas of mosaic attenuation that are eventually replaced by fibrosis 5
- Integration with exposure history, clinical information, and multidisciplinary discussion is essential for accurate diagnosis 2