Slide 1: What are the faint linear opacities (3 are arrowed) on this AP chest X-ray called? Chest x-ray shown with arrows pointing to peripheral area of the left lung.
Slide 2: These are called Kerley B lines or septal lines. What are they and why do they occur? To understand septal lines, we have to learn about the secondary pulmonary lobule, a polygonal structure that makes up the smallest unit of lung completely surrounded by connective tissue.
Slide 3: Anatomy of the secondary pulmonary lobule and etiology of septal lines. Graphic showing pulmonary lobule. Lymphatics (green) and pulmonary venules (red) are embedded in peripheral connective tissue (black). The pulmonary arteriole (blue) and bronchiole (grey) are the centrilobular structures. Alveolar sacs arranged around centrilobular structures. The most common etiology for septal lines is pulmonary edema, but anything that causes (1) lymphatic distension (e.g. lymph-angitic carcinomatosis) or (2) proliferation of connective tissue (e.g. pulmonary fibrosis) can create septal lines.
Slide 4: As you can see here (CT scan shown), normal septae are hard to see. Check out these images, both from a patient in heart failure. If septal lymphatics get distended, the polygonal secondary lobule (red arrows) becomes evident. CT scan showing central dots (red circle), which are arterioles.
Tags: chest xray, CT scan, CXR, Kerley B lines, lymphatic distension, pulmonology, radiology, reading room, septal lines