The swirl sign refers to the non-contrast CT appearance of acute extravasation of blood into a hematoma, for example an intracerebral hemorrhage, extradural hematoma or subdural hematoma. It represents unclotted fresh blood which is of lower attenuation than the clotted blood which surrounds it 1,5,6. It is in some ways the corollary of the spot sign on CTA which represents the same phenomenon, but in that case, caused by extraluminal focal accumulation/pooling/extravasation of contrast 2,5.
In an intracerebral hemorrhage, the swirl sign is one of many predictors of hematoma expansion 5 (see intracerebral hemorrhage for further discussion).
Terminology
The swirl sign is only one or many imaging features described in an attempt to predict hematoma growth, and in many instances, these signs overlap.
For example, the black hole sign is in many respects a swirl sign that is A) encapsulated by the hematoma and B) >28 HU lower in density than the surrounding hematoma 6.
Radiographic features
CT
The swirl sign denotes one of more areas within, or in continuity with, the hematoma that are of lower attenuation than the surrounding/adjacent clot. This can vary from isoattenuating to hypoattenuating to the adjacent brain 6.
Their morphology is variable; truly swirly (curvilinear, streaklike), rounded or branching 6.
