Cobweb sign | Radiology Signs

Answer:

The cobweb sign is the appearance of thin, linear, low-attenuation strands crossing the lumen of a dissected aorta on CT angiography. The strands are residual ribbons of the tunica media that were incompletely sheared away as the aortic wall split, and they remain tethered across the newly created channel. Because such remnants can only persist in the space produced by that shearing, the sign is highly specific for the false lumen and is one of the most reliable discriminators when the usual criteria are equivocal. It is infrequent, so its absence means nothing; its presence, however, settles the question. Correct identification of the false lumen matters for planning endovascular repair, for choosing a stent landing zone, and for recognising branch vessels arising from the false lumen. Complementary discriminators include the beak sign, the larger cross-sectional area of the false lumen, and outer wall calcification favouring the true lumen.

Why is it called so?

The fine linear strands stretched across the false lumen resemble the threads of a cobweb.

Pathophysiology

Dissection propagates through the outer third of the tunica media. The separation is not perfectly clean: slender bands of medial tissue remain attached to both the intimal flap and the outer wall, bridging the newly formed channel. On contrast-enhanced CT these bands are surrounded by opacified blood within the false lumen and are therefore rendered as thin low-attenuation lines. The true lumen, being the original aortic channel, contains no such remnants, which is why the finding is specific to the false lumen.

Alternative names: None

Other associated named signs: Beak sign, Mercedes-Benz sign of a triple-channel dissection, and the floating viscera sign

References

1. Chen D, Matsunaga F. Aortic cobweb sign. Abdom Radiol (NY). 2022;47(4):1505-1506. PMID: 35178613.