Enlarged emissary veins that normally involute during development can be a dominant draining source to the internal jugular vein when associated with a hypoplastic sigmoid venous sinus or jugular foramen1.
This non-contrasted CT suggests the presence of a trans-mastoid emissary vein collateral. Further imaging is required to confirm this diagnosis.
The patient's complaint of tinnitus may be secondary to the prominent trans-mastoid emissary vein, or the vein may be of no consequence.