Empty delta sign | Radiology Signs

Answer:

The empty delta sign is a triangular filling defect in the posterior superior sagittal sinus on contrast-enhanced CT or post-contrast T1-weighted MRI: the dural walls of the sinus enhance while the intraluminal thrombus does not, so the enhancing rim encloses a non-enhancing centre. It is most conspicuous on images through the torcular region, where the sinus is triangular in cross-section. The sign is specific but insensitive, appearing in only about a quarter to a third of cases of dural venous sinus thrombosis and typically not before the first week, so a negative study never excludes the diagnosis. Its unenhanced counterpart is the dense sinus or cord sign, and the definitive tests are CT venography and MR venography with T2*-weighted or susceptibility-weighted sequences, on which thrombus blooms. Important pitfalls are a high haematocrit or dehydration making a normal sinus look dense, a hypoplastic or atretic transverse sinus, prominent arachnoid granulations, and the normal non-opacified sinus on an image acquired too early after injection. Recognition matters because the clinical presentation, headache, seizure, papilloedema or a lobar haemorrhage that crosses arterial territories, is non-specific, and treatment is anticoagulation even in the presence of venous haemorrhagic infarction.

Why is it called so?

The enhancing dural walls form the outline of a Greek letter delta whose interior is empty because the thrombus does not enhance.

Pathophysiology

Thrombus forms in the dural sinus in the setting of a prothrombotic state, dehydration, pregnancy and the puerperium, oral contraceptive use, sepsis, adjacent mastoiditis or head injury. The sinus wall itself is composed of dura with a rich vascular plexus that continues to enhance normally with contrast, whereas the clot is avascular and remains hypoattenuating. Collateral dural vessels around the thrombosed segment enlarge and add to the peripheral enhancement over time, which is why the sign becomes more obvious in the subacute phase and then disappears once the thrombus organises and recanalises. Obstruction of venous outflow raises cerebral venous pressure, producing vasogenic oedema, impaired cerebrospinal fluid absorption with raised intracranial pressure, and eventually venous infarction with a characteristic tendency to haemorrhage.

Alternative names: Delta sign; empty triangle sign

Other associated named signs: Cord sign of a dense thrombosed vein on unenhanced CT and the dense triangle sign of acute sinus thrombus

References

1. Sadik JC, Jianu DC, Sadik R, Purcell Y, Novaes N, Saragoussi E, et al. Imaging of Cerebral Venous Thrombosis. Life (Basel). 2022;12(8). PMID: 36013394.