What causes the interface sign on high-resolution CT of the chest?
Let me know in the comments.
Click to reveal the answer
Answer:
The interface sign is irregularity and shagginess of the boundaries between aerated lung and adjacent structures, that is, the pleural surfaces, the interlobar fissures and the walls of vessels and bronchi, on high-resolution CT. Normally these interfaces are smooth and sharply drawn; when fine fibrosis or infiltration thickens the interlobular septa and the peribronchovascular interstitium abutting them, the outlines become serrated. It is one of the earliest and most sensitive indicators of interstitial disease and is often present before honeycombing or overt reticulation, so it is worth looking for specifically at the fissures and along the pleural surface. Its weakness is that it is entirely non-specific as to cause, appearing in usual interstitial pneumonia, non-specific interstitial pneumonia, hypersensitivity pneumonitis, sarcoidosis, lymphangitic carcinomatosis, asbestosis and pulmonary oedema alike. The diagnosis therefore comes from the accompanying pattern and distribution: basal and subpleural reticulation with traction bronchiectasis and honeycombing indicates a usual interstitial pneumonia pattern, upper-zone perilymphatic nodularity indicates sarcoidosis, nodular septal thickening with preserved lobular architecture indicates lymphangitic spread, and mosaic attenuation with centrilobular nodules indicates hypersensitivity pneumonitis. Assessment requires thin sections and a high-spatial-frequency algorithm, and prone imaging to exclude dependent atelectasis.
Why is it called so?
The sign describes the interfaces between lung and adjacent structures, which lose their normal smooth outline.
Pathophysiology
The pulmonary interstitium is a continuous connective tissue skeleton extending from the peribronchovascular sheath at the hilum through the interlobular septa to the subpleural interstitium. Fibrosis, oedema or tumour infiltration expands this skeleton, and because the septa and subpleural interstitium meet the pleura and fissures at right angles, thickening of their ends creates small projections along an otherwise smooth line. The eye perceives this as irregularity of the interface. The same process along vessels and bronchi thickens their walls and produces the shaggy vascular and bronchial contours, and as fibrosis matures, the distortion progresses to traction bronchiectasis, architectural distortion and honeycombing.
Alternative names: Irregular interface sign; shaggy interface
Other associated named signs: Honeycombing, traction bronchiectasis and the subpleural line of early fibrosis
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
Improve Content And Help Your Colleagues!
Found an error in the post? Please let us know using our contact page and we will update it with due credits!
If you wish to contribute radiological images for this case (or any other article on the website), you can submit them here and you will be duly credited: Submit radiology case