Characteristics of irreversibility in high-resolution computed tomography in chronic hypersensitivity pneumonitis

From a radiological point of view, the statement of irreversibility can only be made with certainty for the signs of irregular reticulation in conjunction with traction bronchiectasis and for honeycomb cysts in the HR-CT morphologically fibrotic phenotype. The HR-CT signs of the inflammatory phenotype can behave differently. Ground-glass nodules always have a reversible potential. In contrast, the underlying pathology of ground-glass opacity can only be assessed over time, as it does not necessarily correlate with inflammation but can also correspond to fine fibrosis. Similarly, air trapping on HR-CT is an important sign of hypersensitivity pneumonitis in both the radiologically inflammatory and fibrotic form. The persistence of air trapping in the inflammatory type over years suggests the possibility of irreversibility of the bronchiolitis in these cases. Whether a higher resolution in HR-CT, such as through photon-counting CT, can lead to clearer differentiation and prediction of reversibility and irreversibility in the future remains to be seen.