Fig. 21

Interface sign. Irregular appearance of pleural surface due to fibrosis (white arrows) in a case of UIP. In cases of interstitial disease with fibrous component, interstitial thickening with focal retractions of the pulmonary parenchyma may develop—creating an alteration of the regularity of the profiles between lung parenchyma, vessels, bronchi, or visceral pleura. In these cases, it is possible to observe a “jagged appearance”