A reticular pattern on CT almost always represents significant ILD. Morphologically, a reticular pattern may be caused by thickened interlobular or intralobular septa or honeycomb (fibrotic) destruction. Numerous thickened interlobular septa indicate an extensive interstitial abnormality and causes include infiltration by fibrosis (interstitial fibrosis), abnormal cells (lymphangitis carcinomatosa), or fluid (pulmonary oedema). Although thickened interlobular septa can be a consequence of infiltration by fibrosis, this feature is not a frequent finding in idiopathic pulmonary fibrosis (IPF). Interlobular septal thickening is usually described as smooth (seen in pulmonary oedema and alveolar proteinosis) or irregular (e.g. lymphangitic spread of tumour), but the distinction is not always easily made. 

Sarcoidosis causes nodular septal thickening, although this pattern is not usually the dominant feature of parenchymal involvement. In some diseases, a perilobular distribution may give the spurious impression of thickening of the interlobular septa. However, such a pattern reflects a pathologic process that is 'smeared' around the internal lobular surface and is most frequently associated with organising pneumonia.

Intralobular septal thickening manifests as a fine reticular pattern on HRCT and is seen in all ILDs but most commonly in IPF. Often, the intralobular septal thickening may be so fine that HRCT does not demonstrate discrete intralobular opacities but a generalised increase in lung density (ground-glass opacification). Severe pulmonary fibrosis usually results in a coarse reticular pattern made up of interlacing irregular linear opacities. The reticular pattern of end-stage fibrotic (honeycomb) lung is characterised by cystic air spaces surrounded by irregular walls. The certain identification of honeycombing, as opposed to other forms of reticulation, is not always straightforward but is of particular relevance as it may directly impact patient care. The distortion of normal lung morphology by extensive fibrosis results in irregular dilatation of segmental and subsegmental airways (traction bronchiectasis/bronchiolectasis); in the periphery of the lung, it can be difficult to distinguish dilated airways from true honeycomb destruction.

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