Septal Lines |top| -

On CT scans, septal lines appear as thickened interlobular septa, which can be seen in a variety of patterns, including:

| Type | Location | Key Feature | |------|----------|--------------| | | Upper/mid zones, central | Long (2–6 cm), radiating from hila into lung, unbranched. Less common now due to better CT correlation—often represent thickened deep lymphatics. | | Kerley B lines | Peripheral, especially costophrenic angles | Short (1–2 cm), horizontal, reaching pleura at right angles. Most common and specific type. | | Kerley C lines | Lower zones, reticular pattern | Finer, reticular (net-like) opacities—actually represent overlapping B lines in a limited area. Now considered non-specific. | septal lines

On high-resolution CT (HRCT), septal lines are seen as: On CT scans, septal lines appear as thickened

In the complex, spongy anatomy of the human lung, structure dictates function. While the primary purpose of the lung is the gas exchange that sustains life, this process relies on a meticulous architectural framework. Buried deep within the secondary lobules—the basic functional units of the lung—lie the septal lines. These are the thin, connective tissue divisions that separate one lobule from another. Most common and specific type

It is the lymphatics and the veins within these septa that are of primary interest to the pathologist. Because the interlobular septa house the pulmonary lymphatic channels, they are uniquely positioned to react to changes in fluid dynamics and cellular infiltration. In a healthy lung, these septa are so thin—often less than 0.1 millimeters—that they are barely perceptible on imaging. However, when pathology strikes, they transform into visible, distinct lines known radiologically as "septal lines."