[Fleischner Society] Anatomy.—The perilobular region comprises the structures bordering the periphery of the secondary pulmonary lobule. CT scans.—This pattern is characterized by distribution along the structures that border the pulmonary lobules (ie, interlobular septa, visceral pleura, and vessels) (102). The term is most frequently used in the context of diseases (eg, perilobular organizing pneumonia) that are distributed mainly around the inner surface of the secondary pulmonary lobule (103) (Fig 49). This may resemble indistinct thickening of the interlobular septa.