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Age: Female

Sex: 80

Indication: Headache

Arachnoid Granulation

Mimicking Thrombus

Findings Relating to the Mimic

  • Small rounded filling defect within the right transverse sinus, compatible with an arachnoid granulation.


  • Arachnoid granulations are common, with a prevalence of approximately 60%.
  • They are protrusions of arachnoid through the dura mater, which allow CSF to extend from the subarachnoid space into the venous sinus or into the calvarium.
  • They most commonly occur in the superior sagittal and transverse sinuses.
  • On non-contrast head CT they are CSF density and on post-contrast imaging they appear as round, focal filling defect in the dural sinus.  Usually they are small (<10mm), but occassionally can be “giant” (>10mm).  If they involve the calvarium, they appear as a circumscribed lucent lesion.
  • Usually these granulations are easy to differentiate from thrombosis given their round, well-circumscribed appearance and typical location.


Typical appearance of an arachnoid granulation (red arrow).

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