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Posterior cerebellomedullary cistern (cisterna magna)

The posterior cerebellomedullary cistern, more commonly known as the cisterna magna, is the largest of the subarachnoid cisterns of the brain. Located in the posterior fossa, it lies between the inferior surface of the cerebellum and the dorsal surface of the medulla oblongata. This CSF-filled space serves as a critical communication point between the fourth ventricle and the subarachnoid space, as CSF exits the fourth ventricle via the foramen of Magendie and enters the cisterna magna. Its size and shape can vary significantly between individuals, and it is bordered anteriorly by the medulla, posteriorly and superiorly by the cerebellar vermis, and inferiorly by the occipital bone.

Synonyms

  • Cisterna magna

  • Posterior cerebellomedullary cistern

  • Magna cistern

Function

  • Acts as a major reservoir for cerebrospinal fluid (CSF) in the posterior fossa.

  • Facilitates the circulation of CSF from the fourth ventricle into the larger subarachnoid space surrounding the brain and spinal cord.

  • Cushions and protects adjacent neural structures, such as the cerebellum and medulla.

  • Allows the passage of vessels and cranial nerves.

MRI Appearance

  • T1-weighted imaging:

    • The cisterna magna appears as a well-defined hypointense (dark) CSF-filled space posterior to the medulla and inferior to the cerebellum. It is clearly demarcated from brain tissue, which appears more isointense or hyperintense relative to CSF.

  • T2-weighted imaging:

    • On T2, the cisterna magna is hyperintense (bright), reflecting the high water content of CSF. The boundaries remain well defined against the surrounding cerebellum and medulla, which are less bright.

  • FLAIR (Fluid-Attenuated Inversion Recovery):

    • The CSF within the cisterna magna is hypointense (dark) or nearly null, as FLAIR suppresses the signal from free fluid. Any abnormal signal within this space (e.g., increased intensity) may indicate the presence of blood, protein, infection, or neoplastic processes.

CT Appearance

  • The cisterna magna is seen as a low-density (hypodense, dark) CSF-filled space in the posterior fossa, between the cerebellum and medulla. It is outlined by the higher density (gray) of adjacent neural structures and the occipital bone.

  • On non-contrast CT, it may not be as sharply demarcated as on MRI but remains visible as a region of lower attenuation than brain parenchyma.

  • Any hyperdensity (bright area) in this space on CT may indicate pathology such as subarachnoid hemorrhage.

MRI images

posterior cerebellomedullary cistern; Cisterna magna mri 3t sagittal image

CT image

Posterior cerebellomedullary cistern (cisterna magna) CT sag anatomy image