Swirl sign

The swirl sign refers to the non-contrast CT appearance of acute extravasation of blood into a hematoma, for example an extradural hematoma or subdural hematoma. It represents unclotted fresh blood which is of lower attenuation than the clotted blood which surrounds it 1).

Black curved arrow

It is the corollary of the spot sign on CTA which represents the same phenomenon, but in that case caused by contrast extravasation 2).

see Gupta VK, Seth A. “Swirl Sign” in Extradural Hematoma. World Neurosurg. 2018 Oct 10. pii: S1878-8750(18)32302-7. doi: 10.1016/j.wneu.2018.10.010. [Epub ahead of print] PubMed PMID: 30315986.

1)
Al-Nakshabandi NA. The swirl sign. Radiology. 2001 Feb;218(2):433. PubMed PMID: 11161158.
2)
Wada R, Aviv RI, Fox AJ, Sahlas DJ, Gladstone DJ, Tomlinson G, Symons SP. CT angiography “spot sign” predicts hematoma expansion in acute intracerebral hemorrhage. Stroke. 2007 Apr;38(4):1257-62. Epub 2007 Feb 22. PubMed PMID: 17322083.