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The cleft sign 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 parencyma. However, it often is of high signal on FLAIR imaging and most likely represents a non-CSF tumour-brain interface.
Nevertheless, it remains a good sign that a mass is extraaxial and is typically made up of:
hypointense dura (on both T1 and T2 on the surface of the mass. (if extra dural)
1. Takeguchi T, Miki H, Shimizu T et-al. Evaluation of the tumor-brain interface of intracranial meningiomas on MR imaging including FLAIR images. Magn Reson Med Sci. 2003;2 (4): 165-9. Magn Reson Med Sci (link) - Pubmed citation