Differentiation of linear and reticular opacities in high resolution computed tomography (HRCT) in interstitial lung diseases.

High resolution computed tomography revolutionized imaging diagnosis of interstitial lung disease. HRCT reveals changes invisible on chest radiograms and in conventional CT, enabling their assessment on the level of the smallest structural unit of the lung interstitium, lung lobule. A large group of morphological changes seen on HRCT in interstitial lung disease are linear and reticular opacities. HRCT enables evaluation of linear and reticular densities invisible on chest radiograms and even on conventional CT. The linear and reticular opacities occur in different interstitial lung disease. They are typical of both active inflammatory changes, potentially treatable and irreversible pulmonary fibrosis. The linear opacities without evident lung architecture distortion suggest active process, whilst cystic changes, honeycombing and evident lung distortion suggest presence of irreversible fibrosis. The tree-in-bud pattern is a very specific finding in pathology of airways, and suggests the presence of bronchiolitis. Apart from that, the linear and reticular densities separately do not allow reliable differential diagnosis, and must be assessed in association with other HRCT findings.