Chronic HP Mimicking NSIP

As previously discussed, it is often difficult to distinguish between chronic HP, UIP, and NSIP.27 Here, we have inspiratory and expiratory axial images and a coronal image from a patient with a parenchymal abnormality with a central bronchovascular distribution of ground glass opacities and areas of peribronchial thickening. There is subpleural sparing, so at first glance one might consider NSIP. However, there are areas of lucency that suggest lobular air trapping and, on end-expiratory scan, the areas of air trapping are exaggerated. It is the presence of air trapping that confirms a diagnosis of hypersensitivity pneumonitis rather than NSIP, which the inspiratory images suggested at first review.