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Topic

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Marginal sinus

The marginal sinus is a small venous channel located at the foramen magnum, encircling the margin of the foramen and connecting the inferior sagittal sinus, basilar venous plexus, and occipital sinus. It plays a crucial role in cranial venous drainage, particularly in connecting intracranial and extracranial venous systems. The marginal sinus is an important structure in neuroimaging and neurosurgery due to its involvement in venous outflow at the craniocervical junction.

Synonyms

  • Peri-foraminal sinus

  • Circumferential sinus of foramen magnum

  • Foramen magnum marginal sinus

Function

  • Serves as a venous ring around the foramen magnum.

  • Connects the inferior sagittal sinus, basilar venous plexus, occipital sinus, and sometimes the internal vertebral venous plexus.

  • Facilitates venous drainage between intracranial and cervical venous systems.

  • Plays a role in collateral venous circulation, especially in cases of sinus thrombosis or occlusion elsewhere.

MRI Appearance

T2-weighted Imaging:

  • Appears as a flow void or low signal intensity rim around the foramen magnum due to fast venous blood flow.

  • May be difficult to differentiate if small or partially thrombosed.

T1-weighted Imaging:

  • Typically shows a low to intermediate signal intensity ring at the foramen magnum.

  • Flow-related signal loss may make the sinus appear as a void.

Post-Contrast 3D (PC 3D) MRI / Post-Contrast T1:

  • Shows enhancement of the marginal sinus as a thin, enhancing rim encircling the foramen magnum.

  • Enhancement is best appreciated in high-resolution, post-contrast T1-weighted images.

  • The venous ring can be visualized clearly if the sinus is patent.

CT Appearance

  • Seen as a thin, low-attenuation rim at the foramen magnum on non-contrast CT (if visible).

  • May be difficult to identify unless enlarged, thrombosed, or opacified by contrast.

CTV (CT Venography) Appearance

  • Marginal sinus enhances with intravenous contrast, appearing as a thin enhancing venous ring around the foramen magnum.

  • CTV can demonstrate its continuity with the occipital sinus, basilar plexus, and sometimes the vertebral venous plexus.

  • Thrombosis, if present, appears as a filling defect within the expected location of the marginal sinus.

MRI images

Marginal sinus MRI 3T axial image

MRI images

Marginal sinus MRI 3T coronal image

MRI images

Marginal sinus MRI 3T sagittal image