When interpreting a chest X-ray, the detection of fine, subtle lines in the lung periphery can provide a critical diagnostic clue. Among the most important of these are Kerley B lines —a classic radiographic sign of interlobular septal thickening, most commonly associated with pulmonary edema. What Are Kerley B Lines? Kerley B lines are thin, linear opacities seen predominantly in the lung peripheries. They represent thickening of the interlobular septa —the connective tissue partitions that separate the secondary pulmonary lobules.