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 a tumor 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 tumor-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)
