Reticular opacities seen in HRCT images are a result of the thickening of the interlobular septa or fibrosis (honeycombing).2
septal thickening is defined as the thickening of the interstitium that
surrounds the secondary pulmonary lobule by fluid, fibrous tissue, or
infiltration by cells. It can have a smooth, nodular or irregular appearance.
Visualisation of numerous septa, which are normally about 0.1 mm thick, indicates an abnormal condition.2,6,9