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Fetal fourth ventricle

The fetal fourth ventricle is a diamond-shaped cavity of the ventricular system, located in the hindbrain. It develops early from the neural tube and connects the cerebral aqueduct superiorly to the central canal of the spinal cord inferiorly. It plays an essential role in cerebrospinal fluid (CSF) circulation and is a crucial landmark in prenatal brain imaging.

The size and shape of the fourth ventricle are important markers of fetal brain development. Enlargement may indicate congenital malformations such as Dandy–Walker malformation, Chiari malformations, or aqueductal stenosis.

Synonyms

  • IV ventricle (fetal)

  • Hindbrain ventricle

  • Rhombencephalic ventricle

Structure and Development

  • Shape: Diamond-shaped cavity in axial plane, tent-shaped in sagittal plane

  • Roof: Superior medullary velum and cerebellum

  • Floor (rhomboid fossa): Formed by pons and medulla

  • Openings:

    • Foramen of Magendie (median aperture)

    • Foramina of Luschka (lateral apertures)

  • Development: Forms from the rhombencephalon (hindbrain vesicle) around the 5th week of gestation

Relations

  • Anteriorly: Pons and medulla oblongata (rhomboid fossa)

  • Posteriorly: Cerebellum and cerebellar vermis

  • Superiorly: Cerebral aqueduct

  • Inferiorly: Central canal of spinal cord

Function

  • Acts as a conduit for CSF flow from third ventricle (via aqueduct) to central canal and subarachnoid space

  • Maintains CSF circulation and pressure equilibrium in the fetal brain

  • Serves as an imaging marker of posterior fossa development and cerebellar formation

Clinical Significance

  • Dandy–Walker malformation: Enlarged fourth ventricle with absent or hypoplastic cerebellar vermis

  • Arnold–Chiari malformations: Altered shape and compression of fourth ventricle

  • Aqueductal stenosis: Dilatation of all ventricles including the fourth

  • Posterior fossa cysts: Can mimic or communicate with fourth ventricle

  • Hydrocephalus: Ventricular enlargement is a key diagnostic marker on prenatal MRI and ultrasound

MRI Appearance

T2 HASTE (T2 GRE):

  • CSF within the fourth ventricle: very bright hyperintense signal

  • Ventricular margins: sharply defined against adjacent brainstem and cerebellum

  • Cerebellar vermis and posterior fossa structures: darker (intermediate) T2 signal compared to fluid

  • Enlargement or malformation: easily seen as expansion of bright CSF space

T1 GRE:

  • CSF within the fourth ventricle: low signal (dark)

  • Surrounding cerebellum, pons, and medulla: intermediate signal

  • Hemorrhage or proteinaceous CSF within ventricle: bright hyperintense areas

MRI image

Fetal fourth ventricle MRI image