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Radiological Features Of Subarachnoid Cisterns

In this article, we are going to describe briefly the radiological features of subarachnoid cisterns. This article describes which modalities can be used to look for subarachnoid cisterns and how they are seen on different modalities.


  • Cisterna magna usually measures 2-10 mm in the 2nd and 3rd trimesters in an antenatal ultrasound.
  • Cisterna magna is the only subarachnoid cistern that can be seen in an infant by the cranial ultrasound.
Antenatal Ultrasound Cisterna Magna
Antenatal Ultrasound Cisterna Magna

Subarachnoid Cisterns on CT and MRI:

CT and MRI are very helpful in visualizing all the subarachnoid cisterns.

  • The subarachnoid cisterns are best visualized on axial & sagittal planes on CT & MRI scans as CSF-filled spaces between parts of the brain and the skull.
  • On CT the CSF appears hypodense so are the subarachnoid cisterns.
  • On T1 weighted sequence of MRI the cisterns appear hypointense, and on T2 weighted sequence they appear hyperintense.
  • Cisterna magna is located behind the medulla and below the cerebellar hemispheres. It measures 3-8mm in the midsagittal plane measuring from the posterior margin of the foramen magnum to the inferior margin of the vermis.
CT Brain (Axial View) - Cisterna magna
CT Brain (Axial View) – Cisterna magna
  • At the level of the midbrain, the interpeduncular, ambient and the quadrigeminal cisterns are seen around the midbrain.
  • Anterior to the interpeduncular cistern there is the suprasellar cistern. Posterior to the quadrigeminal cistern is the superior vermian cistern.
subarachnoid cisterns
CT Brain (Axial View) Subarachnoid Cisterns
  • At the level of superior part of the lateral ventricles, the pericallosal cistern can be seen between the falx and the genu of the corpus callosum anteriorly and between the splenium and the falx posteriorly.
MRI T2WI (Sagital View) Pericallosal Cistern
MRI T2WI (Sagital View) Pericallosal Cistern
  • The vessels can be seen in all cisterns.
  • The acoustic nerve in the pontine cistern is visualized.
  • The optic nerve and the optic chiasm are seen well on the T2-wieghted MRI.
  • The small structures in the cisterns, surrounded by the CSF are well demonstrated due to the marked difference in relaxation constant in MR imaging between the water and the brain tissue.
  • The cistern of velum interpositum when present is seen as fluid-filled space inferior to the splenium of the corpus callosum and between the thalamus and the fornix.
  • It is triangular in axial plane with its apex anteriorly.
MRI (T2WI) Cistern of Velum Interpositum.
MRI (T2WI) Cistern of Velum Interpositum.

Cervical myelography:

  • A cervical myelogram is the radiographic procedure of the cervical spinal cord in which the contrast media is injected into the cerebrospinal fluid within the subarachnoid space.
  • A needle is introduced above the C2 level and is directed upwards.
  • At a depth of 3cm, the needle is felt to penetrate the atlantooccipital membrane and to enter cisterna magna.
  • There is the risk of injury to the spinal cord, the medulla oblongata anterior to the cistern and the posterior inferior cerebellar artery with its branches within the cistern.


  • This was the old technique in which brain was imaged by replacing air in the subarachnoid cisterns. CT and MRI have replaced this technique.

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