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Mastoid emissary vein

The mastoid emissary vein (MEV) is a small but clinically significant venous channel that traverses the mastoid foramen, connecting the sigmoid sinus (a dural venous sinus) with the extracranial venous system, typically draining into the posterior auricular or occipital vein. The MEV serves as an important collateral pathway for venous outflow, especially in cases of jugular vein obstruction. Due to its variable size and course, the MEV is relevant in skull base surgery and radiology, as it may be a source of bleeding or route for infection spread. Awareness of its imaging characteristics is vital for differentiating it from pathological lesions.

SYNONYMS

  • Mastoid emissary vein (MEV)

  • Emissary vein of the mastoid foramen

  • Vena emissaria mastoidea

  • Posterior mastoid emissary vein (less commonly used)

FUNCTION

  • Connects the sigmoid sinus (intracranial venous sinus) with extracranial veins (usually posterior auricular or occipital vein)

  • Acts as a collateral pathway for cerebral venous drainage, especially if internal jugular vein flow is compromised

  • May serve as a conduit for the spread of infection or tumor between intracranial and extracranial compartments

MRI APPEARANCE

T2-Weighted Images

  • Appears as a flow void (signal loss) within the mastoid bone, reflecting fast-flowing blood

  • May appear as a small tubular or round structure, sometimes mimicking a small cyst or vessel

T1-Weighted Images

  • Shows as a signal void (dark structure) in the mastoid region due to flowing blood

  • Surrounding bone may appear with high or intermediate signal, vein itself remains hypointense

3D Phase-Contrast (PC 3D) MRI

  • The MEV is visualized as a high-flow vascular channel

  • 3D reconstructions highlight its connection between the sigmoid sinus and extracranial veins

  • Useful for surgical planning and for differentiating from other vascular anomalies

Post-Contrast T1-Weighted MRI

  • The MEV typically enhances due to its vascular nature

  • Appears as a small enhancing tubular or round structure in the mastoid region

  • Useful for confirming vascularity and distinguishing from non-vascular lesions

CT APPEARANCE

  • Appears as a well-defined bony canal (mastoid foramen) in the posterior aspect of the temporal bone

  • May see a small, round or oval lucency (foramen) traversing the mastoid bone

  • The vein itself may not be directly visualized without contrast, but the foramen can be identified

CTV (CT Venography) Appearance

  • MEV is opacified with contrast

  • Appears as a small, enhancing vascular structure extending from the sigmoid sinus through the mastoid foramen to extracranial veins

MRI images

Mastoid emissary vein MRI 3T axial image