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Superior opening of cerebral aqueduct

The superior opening of the cerebral aqueduct, also known as the proximal or rostral opening of the aqueduct of Sylvius, is the narrow channel connecting the third ventricle to the midbrain portion of the cerebral aqueduct. It is located posteriorly in the midbrain tegmentum at the level of the mesencephalon, beneath the posterior part of the third ventricle. This structure allows cerebrospinal fluid (CSF) to flow from the third ventricle into the cerebral aqueduct, which then continues to the fourth ventricle, playing a crucial role in ventricular CSF circulation and intracranial pressure regulation.

Anatomically, it is surrounded by periaqueductal gray matter, which has functional significance in pain modulation and autonomic regulation. Its small caliber makes it prone to obstruction, leading to aqueductal stenosis and hydrocephalus, making imaging evaluation critical.

Function

  • Acts as the proximal entry point for CSF into the cerebral aqueduct

  • Maintains CSF circulation between the third and fourth ventricles

  • Plays a role in intracranial pressure regulation

  • Surrounding periaqueductal gray contributes to pain modulation and autonomic functions

Synonyms

  • Proximal opening of cerebral aqueduct

  • Rostral aqueduct opening

  • Aqueduct of Sylvius superior aperture

MRI Appearance

T1-weighted images:

  • Appears as a narrow, hypointense channel connecting the posterior third ventricle to the midbrain aqueduct

  • Surrounded by intermediate signal gray matter (periaqueductal gray)

  • Normal CSF within the aqueduct appears hypointense on T1

T2-weighted images:

  • CSF within the aqueduct appears hyperintense, contrasting sharply with the hypointense surrounding midbrain tissue

  • The superior opening is visible as the transition zone between third ventricle CSF and aqueduct CSF

  • Aqueductal stenosis or obstruction can be identified as narrowing or absent high-signal CSF flow

STIR (Short Tau Inversion Recovery):

  • Surrounding midbrain tissue remains intermediate signal, CSF remains hyperintense

  • Useful for detecting adjacent edema, demyelination, or gliosis

T1 Post-Contrast (Gadolinium-enhanced):

  • Normally, there is no enhancement of the aqueduct or its superior opening

  • Abnormal enhancement may indicate tumor, infection, or ependymitis

CT Appearance:

  • The superior opening itself is not directly visualized on conventional CT due to its small size

  • Surrounding midbrain structures and third ventricle are clearly seen

  • Ventricular enlargement (hydrocephalus) secondary to obstruction is often the indirect indicator

  • High-resolution CT or CT ventriculography may demonstrate narrowing of the aqueduct

MRI images

Superior opening of cerebral aqueduct mri sag image -img-00000-00000

MRI images

Superior opening of cerebral aqueduct AXIAL