Lines (linear opacities) on HRCT

A predominantly-linear pattern CT scan can be divided into one of two categories, based upon the types of lines seen -- Septal Lines or Non-Septal Lines.

Septal Lines

Septal lines are caused by thickening of the intra and/or interlobular septa. Normally, only a few septa should be visible on a HRCT.

• Pulmonary Edema -- Generally will be smooth septal thickening
• Hemorrhage -- Can look just like edema
• Lymphoma -- May be smooth or nodular
• Lymphantgitis Carcinomatosis -- Often has a nodular appearance

Non-Septal Lines

Linear opacities that are not septal lines usually indicate fibrosis.

• UIP - Usual Interstitial Pneumonitis -- UIP has a classic appearance with the fibrosis showing a predominantly peripheral and basilar distribution. The differential for the etiology of UIP should be well know to you (it's the "IPF and friends" differential).
• Non-UIP Pattern -- If the pattern is not classic for UIP, you should consider other possible etiologies of pulmonary fibrosis, such as Hypersensitivity Pneumonitis, Sarcoidosis, and Chronic Aspiration, among others.