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Median aperture of fourth ventricle (foramen of Magendie)

The median aperture, also known as the foramen of Magendie, is a midline opening in the posterior wall of the fourth ventricle, located in the roof of the ventricle near the cerebellar vermis. It serves as a critical communication pathway between the fourth ventricle and the cisterna magna, allowing cerebrospinal fluid (CSF) to flow from the ventricular system into the subarachnoid space.

The foramen is triangular in shape and is bounded by the inferior medullary velum and the obex. Its patency is essential for normal CSF circulation, and obstruction can lead to non-communicating hydrocephalus. The median aperture, together with the lateral apertures (foramina of Luschka), regulates the outflow of CSF from the fourth ventricle, maintaining intracranial pressure and CSF homeostasis.

Function

  • Provides a ventricular-subarachnoid communication

  • Allows CSF drainage into the cisterna magna

  • Maintains normal CSF circulation and intracranial pressure

  • Plays a role in ventricular system homeostasis

  • Obstruction can result in hydrocephalus

Synonyms

  • Foramen of Magendie

  • Median foramen of the fourth ventricle

  • Posterior ventricular aperture

MRI Appearance

T1-weighted images:

  • The median aperture is typically not directly visualized due to its small size

  • CSF appears hypointense (dark) relative to brain parenchyma

  • Surrounding structures such as the cerebellar vermis and medulla are clearly seen

T2-weighted images:

  • CSF within the fourth ventricle and foramen is hyperintense (bright)

  • The median aperture may be inferred by the continuity of high signal CSF into the cisterna magna

  • Obstruction may be suggested by dilated fourth ventricle and absent CSF flow

STIR (Short Tau Inversion Recovery):

  • CSF remains hyperintense, while surrounding parenchyma is suppressed

  • Useful for detecting associated edema or posterior fossa anomalies

T1 Post-Contrast (Gadolinium-enhanced):

  • The foramen itself does not enhance

  • Surrounding meninges or abnormal lesions (arachnoid cysts, tumors) may show enhancement, highlighting obstruction or pathology

CT Appearance:

  • Direct visualization is limited due to small size

  • CSF within the fourth ventricle and cisterna magna appears hypodense (dark)

  • Obstruction or abnormality is inferred from ventricular enlargement, hydrocephalus, or mass effect

MRI images

Median aperture of fourth ventricle (foramen of Magendie) mri axial image -img-00000-00000

MRI images

Median aperture of fourth ventricle (foramen of Magendie)