Hypersensitivity Pneumonitis: Imaging Findings and Patterns

Recently published guidelines on the diagnosis of hypersensitivity pneumonitis from the American Thoracic Society/Japanese Respiratory Society/Asociación Latinoamericana de Tórax and the American College of Chest Physicians place increased importance on CT analysis as a component of the multidisciplinary diagnosis. Accurate identification and characterization of CT findings is critical as they have downstream effects on the rest of the diagnostic workup, which places imaging early in their diagnostic algorithms. Though the imaging features of hypersensitivity pneumonitis are well established, it remains a challenging diagnosis for radiologists due to its variable appearance and lack of specific CT findings. This review summarizes the imaging features and patterns of nonfibrotic and fibrotic hypersensitivity pneumonitis using the newest guidelines and classifications as a framework. Additional information relevant to the interpreting radiologist is also briefly discussed, including progressive pulmonary fibrosis, acute exacerbation, and differentiating fibrotic hypersensitivity pneumonitis from a usual interstitial pneumonia pattern of fibrosis.