reticular and linear pulmonary opacification

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In chest radiology, reticular and linear opacification refers to a broad subgroup of pulmonary opacification caused by a decrease in the gas to soft tissue ratio due to a pathological process centered in or around the pulmonary interstitium. This includes thickening of any of the interstitial compartments by blood, water, tumor, cells, fibrous disease or any combination thereof. The thickening of the interstitium can be reticular, reticulonodular, or linear where the predominant pattern is a result of the underlying pathological process.

Reticular

The reticular interstitial pattern refers to a complex network of curvilinear opacities that usually involved the lung diffusely. They can be subdivided by their size (fine, medium or coarse). The subdivision refers to the size of the lucent spaces created by the intersection of lines:

fine "ground-glass" (1-2 mm): seen in processes that thicken the pulmonary interstitium to produce a fine network of lines, e.g. interstitial pulmonary edema
medium "honeycombing" (3-10 mm): commonly seen in pulmonary fibrosis with involvement of the parenchymal and peripheal interstitium
coarse (>10 mm): cystic spaces caused by parenchymal destruction, e.g. usual interstitial pneumonia, pulmonary sarcoidosis, pulmonary Langerhans cell histiocytosis

Reticulonodular

A reticulonodular interstitial pattern is produced by either overlap of reticular shadows or by the presence of reticular shadowing and pulmonary nodules. While this is a relatively common appearance on a chest radiograph, very few diseases are confirmed to show this pattern pathologically. Examples include:

Linear

Linear interstitial patterns are seen in processes that thicken the axial (bronchovascular) interstitium or the peripheral pulmonary interstitium

axial: diffuse thickening along the bronchovascular tree seen as parallel opacities radiating from the hila (seen transversely) or peribronchial cuffing (seen en face)
peripheral: thickening of the peripheral interstitium (either medially or laterally) produces Kerley lines

Axial interstitial thickening is difficult to distinguish from airways disease that result in bronchial wall thickening, (e.g. bronchiectasis, asthma) and most often seen in interstitial pulmonary edema. Peripheral interstitial involvement is seen in interstitial pulmonary edema, lymphangitis carcinomatosis and acute viral or atypical bacterial pneumonia.