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Cisterna magna

The cisterna magna, also known as the cerebellomedullary cistern, is the largest subarachnoid cistern in the posterior fossa. It is located between the cerebellum and the dorsal surface of the medulla oblongata, inferior to the cerebellar hemispheres, and continuous with the fourth ventricle via the foramen of Magendie. The cisterna magna contains cerebrospinal fluid (CSF), arteries (vertebral and posterior inferior cerebellar arteries), veins, and cranial nerve roots, providing a protective and cushioning space for the brainstem and cerebellum.

It is a critical landmark for neurosurgeons, radiologists, and neurologists, particularly in procedures like posterior fossa surgery, CSF sampling, and evaluation of hydrocephalus, Chiari malformations, or posterior fossa masses.

Function

  • Contains and circulates cerebrospinal fluid (CSF)

  • Provides a buffer and protective space for the cerebellum and brainstem

  • Acts as a landmark for posterior fossa imaging and surgical planning

  • Supports arterial and venous structures in the posterior fossa

Synonyms

  • Cerebellomedullary cistern

  • Posterior fossa cistern

  • Cisterna subarachnoidea magna

MRI Appearance

T1-weighted images:

  • The cisterna magna appears as a well-defined hypointense (dark) CSF-filled space posterior to the medulla and inferior to the cerebellum

  • Brain tissue surrounding the cistern (cerebellum and medulla) appears intermediate signal

  • Pathological masses, hemorrhage, or CSF obstruction appear as altered signal intensities

T2-weighted images:

  • CSF within the cisterna magna appears bright hyperintense, providing excellent contrast with surrounding cerebellar and brainstem tissue

  • Useful for detecting arachnoid cysts, dilatation, or posterior fossa edema

STIR (Short Tau Inversion Recovery):

  • CSF is hyperintense, while surrounding brain tissue is intermediate to low signal

  • STIR is particularly sensitive to edema or inflammatory changes in the posterior fossa

T1 Post-Contrast (Gadolinium-enhanced):

  • Normal cisterna magna shows no intrinsic enhancement, as CSF does not enhance

  • Pathological lesions such as arachnoiditis, tumors, abscesses, or vascular malformations may show rim or internal enhancement

CT Appearance:

  • The cisterna magna appears as a well-defined low-density (hypodense) CSF space in the posterior fossa

  • Bony boundaries of the foramen magnum and occipital bone are hyperdense, providing clear anatomical reference

  • CT is useful for detecting hemorrhage, mass effect, hydrocephalus, or congenital malformations

MRI images

Cisterna magna mri axial image

MRI images

Cisterna magna mri sagittal image