<aside>

The fourth ventricle is a diamond-shaped cavity of the ventricular system located in the posterior fossa. It is filled with CSF and lies between the pons and medulla anteriorly and the cerebellum posteriorly.

</aside>

Boundaries:

Roof	Superior and inferior medullary vela + cerebellum.
Floor (rhomboid fossa)	Formed by dorsal surfaces of pons and medulla.
Apex	Extends into the obex inferiorly, continuous with central canal of medulla/spinal cord.

Communications:

Superiorly	With cerebral aqueduct (of Sylvius).C
Inferiorly	Central canal of medulla/spinal cord.
Laterally	Foramina of Luschka.
Midline	Foramen of Magendie.

These permit CSF to exit into the subarachnoid space (cisterna magna).

Function

<aside>

CSF formed in choroid plexus of lateral ventricles → passes through foramen of Monro → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space via Magendie & Luschka.

</aside>

Function: Acts as a central reservoir for CSF circulation around brainstem and cerebellum.

Radiology

Normal MRI: Diamond-shaped cavity, CSF signal intensity, visible foramina.
Dilated 4th ventricle: Seen in communicating hydrocephalus.
Compressed/obliterated 4th ventricle: Seen in posterior fossa masses (e.g., medulloblastoma, cerebellar infarct, tonsillar herniation).
Expansile intraventricular lesion: Suggests ependymoma, subependymoma, or choroid plexus tumor.

![(a) Midline sagittal T1-weighted MRI image of the brainstem shows normal triangular or diamond shaped appearance of the fourth ventricle. (b) Coronal T1-weighted MRI image demonstrates the anatomy of the floor of the fourth ventricle.

****Yap CW, Yong C, Soon BKH. The different shapes of the fourth ventricle. Clinical Radiology. 2023;78(12):875-884. doi:10.1016/j.crad.2023.07.012](attachment:e8ffa43a-8894-43ef-856e-3773ec3eafa0:image.png)

(a) Midline sagittal T1-weighted MRI image of the brainstem shows normal triangular or diamond shaped appearance of the fourth ventricle. (b) Coronal T1-weighted MRI image demonstrates the anatomy of the floor of the fourth ventricle.

****Yap CW, Yong C, Soon BKH. The different shapes of the fourth ventricle. Clinical Radiology. 2023;78(12):875-884. doi:10.1016/j.crad.2023.07.012

![Axial T2-weighted MRI image showing a normal fourth ventricle that has a “Kaiser Wilhelm Helmet” shape. A picture of the Kaiser Wilhelm Helmet is included for reference.

****Yap CW, Yong C, Soon BKH. The different shapes of the fourth ventricle. Clinical Radiology. 2023;78(12):875-884. doi:10.1016/j.crad.2023.07.012](attachment:ab1fe35a-1c69-4778-b859-83aa415af415:image.png)

Axial T2-weighted MRI image showing a normal fourth ventricle that has a “Kaiser Wilhelm Helmet” shape. A picture of the Kaiser Wilhelm Helmet is included for reference.

****Yap CW, Yong C, Soon BKH. The different shapes of the fourth ventricle. Clinical Radiology. 2023;78(12):875-884. doi:10.1016/j.crad.2023.07.012

Etiology	Pathology	Key feature
Developmental / Malformative	‣	Enlarged 4th ventricle with vermian agenesis and enlarged posterior fossa.
	‣	Downward herniation of cerebellar tonsils compressing the 4th ventricle.
	‣	Secondary dilation of 4th ventricle.
Neoplasms (children)	‣	Midline, roof of 4th ventricle; hyperdense on CT; can fill ventricle and cause obstructive hydrocephalus.
	‣	Arises from ependymal lining; may extend through foramina of Magendie/Luschka (“toothpaste tumor”).
	‣	Cyst with mural nodule, may project into 4th ventricle.
Neoplasms (adults)	‣	Benign, non-enhancing 4th ventricular lesion, often incidental.
	‣	Posterior fossa tumor, cystic with mural nodule, may compress ventricle.
	‣	Less common, but can lodge in 4th ventricle.
Other Lesions	‣	Rare, arise from choroid plexus in 4th ventricle, cause hydrocephalus by overproduction or obstruction.
	Arachnoid cysts / epidermoids	May compress 4th ventricle.

Function

Radiology

Clinical Relevance