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Topic

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Frenulum veli

The frenulum veli is a slender, midline band of neural tissue located in the posterior aspect of the brainstem, specifically at the junction between the midbrain and the cerebellum. It extends from the inferior aspect of the inferior colliculi and continues caudally to merge with the superior medullary velum, a thin sheet forming part of the roof of the fourth ventricle. The frenulum veli acts as an important anatomical landmark and is closely associated with the exit of the trochlear nerve (cranial nerve IV), which emerges dorsally and curves around the brainstem at this location.

Anatomical Features

  • Location: Posterior midbrain, dorsal to the pons, forming part of the roof of the fourth ventricle.

  • Borders: Bounded superiorly by the inferior colliculi and inferiorly by the superior medullary velum.

  • Associations: Trochlear nerve emerges at the lateral margins.

Function

  • Serves as an anatomical landmark for the identification of cranial nerve IV.

  • Contributes to the structural support of the superior medullary velum and indirectly to the formation of the fourth ventricle’s roof.

  • Has no direct functional role in sensory or motor activity.

Arterial Supply

  • Supplied primarily by branches of the superior cerebellar artery.

  • Additional minor supply may arise from the posterior cerebral artery branches.

Venous Drainage

  • Drains into the veins of the superior medullary velum.

  • Venous outflow continues to the great cerebral vein of Galen and nearby midline venous channels.

MRI Appearance

  • T1-weighted Imaging:

    • The frenulum veli is isointense to the brainstem (gray matter).

    • Appears as a thin, midline linear structure; distinction from surrounding CSF is limited due to similar signal intensity to brain parenchyma.

  • T2-weighted Imaging:

    • The frenulum veli is also isointense to the brainstem parenchyma.

    • Appears as a subtle, thin line surrounded by the hyperintense CSF of the fourth ventricle, providing a natural contrast.

  • FLAIR Imaging:

    • The frenulum veli remains isointense to the brainstem; more conspicuous due to the suppression of CSF signal on FLAIR, allowing the structure to be better appreciated against the dark CSF background.

CT Appearance

  • The frenulum veli is not directly visualized on standard CT scans due to its very thin and soft-tissue nature.

  • Its location can be inferred indirectly in relation to the fourth ventricle and brainstem, but direct visualization is rarely possible even on thin-section CT.

MRI images

Frenulum veli mri 3t axial image