Reticular, linear, and irregular linear opacities in the lungs are manifestations of interstitial lung disease, and the pathological changes can be exudation or transudation, infiltration of inflammatory cells or tumor cells, and hyperplasia of fibrous connective tissue or granulation tissue. Common interstitial lung diseases include chronic bronchitis, idiopathic pulmonary fibrosis, lymphangitic carcinomatosis, pneumoconiosis, and connective tissue disease. Due to different pathological properties, lesion ranges, and occurrence times, their imaging manifestations are also diverse.
X-ray
Interstitial pulmonary disease at different sites and caused by different causes has different manifestations. Interstitial lesions around large bronchi and blood vessels are manifested by thickened, hazy, and disordered lung markings. Lesions around bronchioles, blood vessels, and interlobular septa are manifested by reticular, linear, or honeycomb opacities. Localized linear opacity, such as linear opacity between the mass in lung cancer and the hilum or pleura, can be seen in intrapulmonary lesions along the interstitium extending to the hilum or periphery. After pulmonary tuberculosis heals, the peripheral interstitium may develop fibrosis, manifested by irregular linear opacities. Fluid or tissue hyperplasia in the interlobular septa can be manifested by septal lines in different parts; the most common are septal B lines, also known as Kerley B lines, manifested by several, 2cm long, 1- 2mm wide, linear opacities perpendicular to the costal pleura in the outer zones near the costophrenic angle of the two lower lung fields, mostly in pulmonary venous hypertension and pulmonary interstitial edema.
Figure 1 Interstitial lung disease
a. Chest plain film shows diffuse reticular opacities in the lungs; b. CT lung window shows linear and reticular opacities in both lungs, predominantly in the subpleural area, forming a honeycomb pattern, this is idiopathic pulmonary fibrosis.
CT
HRCT can detect early mild pulmonary fibrosis, manifested by subtle changes such as thickening of the interlobular septum, and has great value in the diagnosis of interstitial lung lesions.
CT manifestations of interlobular septal thickening are related to the involved area, severity, and etiology. In the early stage, the manifestations are 1- 2 cm long, thin linear opacities connected to the pleura, and polygonal reticular opacities when the lesion is apparent; in the progressive stage, due to extensive interlobular septal thickening, adjacent thickened interlobular septa are connected, and within 1 cm below the pleura, 2 - 5 cm long curvilinear opacities parallel to the chest wall can be seen, which are termed subpleural lines; in the late stage, honeycomb opacities can be seen under the pleura of the middle and lower lung fields, and can involve the middle and inner zones inward and the upper lung fields upward.
MRI
Under normal circumstances, the signal in lung fields is very low, so the visibility of reticular and thin linear lesions is not satisfactory. Large irregular linear lesions can sometimes be seen on the dark background (low signal) of lung fields, and are isointense on T1WI and T2WI.