CSF cleft sign

The CSF cleft sign in neuroimaging can be used to distinguish an extra-axial lesion from an intra-axial lesion, and is typically used in the description of a meningioma.

Classically, the cleft was regarded as representing a thin rim of CSF between tumour and brain parenchyma. However, it often is of high signal on FLAIR imaging (i.e. does not suppress on FLAIR) and most likely represents a non-CSF tumour-brain interface or CSF admixed with solutes. 

Nevertheless, it remains a good sign that a mass is extra-axial and typically consists of:

  • high T2 signal (representing CSF +/- solutes)
  • vessels: pial or larger vessels.
  • hypointense dura (on both T1 and T2 on the surface of the mass (if extradural)

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