In 1933, Irish radiologist Peter Kerley mapped anomalous linear opacities on chest radiographs of patients experiencing chronic mitral stenosis. His observations, historically chronicled in The New England Journal of Medicine, categorized three distinct linear configurations: A, B, and C lines. Among these, the B variant became an indelible cornerstone of pulmonary imaging.
Kerley B lines are short, straight, horizontal linear densities measuring 1 to 2 centimeters in length and roughly 1 millimeter in width. They appear most prominently at the lung bases, running perpendicular to the lateral chest wall near the costophrenic angles. Anatomic reality drives this appearance: the secondary pulmonary lobules in the peripheral lung zones are bordered by interlobular septa that contain connective tissue, pulmonary venules, and lymphatic vessels. In healthy lungs, these delicate membranes remain far below the spatial resolution limits of a projection radiograph.
When left ventricular end-diastolic pressure spikes, pulmonary venous hypertension ripples backward into the capillary beds. Capillary hydrostatic pressures exceeding 18 to 20 mmHg overwhelm Starling forces, pushing transudate into the interstitium. The resulting interstitial fluid expansion causes lymphatic engorgement and swollen interlobular septa. What the radiologist sees on a standard posteroanterior projection is not an optical illusion. It is the direct radiopaque attenuation of X-ray beams passing edge-on through dense, fluid-laden tissue.
In contrast, Kerley A lines represent deeper interstitial septa radiating toward the hila, measuring up to 6 centimeters, while Kerley C lines represent reticular superimpositions across the lung bases. Kerley B lines remain the most clinically reliable plain-film herald of impending alveolar flooding.
