Blend sign | Radiology Signs

Answer:

The blend sign is a haematoma containing two adjacent, well-demarcated regions of different attenuation on non-contrast CT: a relatively hypoattenuating area abutting the hyperdense clot, with a visible boundary between them. Three criteria define it: the two regions must have a clearly discernible border, the density difference must be at least 18 Hounsfield units, and the hypoattenuating region must not be completely encapsulated by the hyperdense component. It is an independent predictor of haematoma expansion in spontaneous intracerebral haemorrhage, with low sensitivity of roughly 13-43% but high specificity of about 89-95%, so it identifies high-risk patients but cannot be used to exclude expansion. It sits within a family of non-contrast CT expansion markers that must be kept distinct: the black hole sign, in which the hypoattenuating region is completely encapsulated and the density difference is at least 28 HU; the island sign, three or more separate small haematomas adjacent to the main clot; and the swirl sign.

Why is it called so?

The lower-density region appears to blend into the adjacent hyperdense clot along a definable margin.

Pathophysiology

A haematoma that has stopped bleeding retracts and becomes uniformly hyperdense as the clot organises. Continued or recurrent bleeding introduces fresh, unclotted blood into or alongside the established clot. Because unretracted fresh blood has a lower haematocrit and lower attenuation than retracted clot, it appears as a relatively hypodense region abutting the older component, and the interface between the two remains sharp because they are of different ages rather than being mixed. The finding therefore images ongoing bleeding directly, which is why it predicts expansion and poor outcome.

Alternative names: None

Other associated named signs: Black hole sign, island sign, swirl sign, satellite sign, and the CT angiography spot sign

References

1. Zhu C, Sun J, Li H, Yang L, Wei H. Predictive performance of non-contrast CT signs for hematoma expansion: A comparative analysis across traditional, revised, and conservative definitions. Neuroradiology. 2026;68(4):887-899. PMID: 41848861.