Cleft sign - one of the first things to determine when you see an intracranial mass is its location: is it within the brain (intra-axial eg. astrocytoma) or outside the brain (extra-axial eg. meningioma)? Extra-axial masses are typically separated from the brain by a thin cleft of signal or density difference. Classically, this cleft was regarded as a rim of CSF however it is now thought that much of the appearance, particularly on MRI, is due to a non-CSF tumour-brain interface phenomenon. Either way, it is an excellent sign to help differentiate an extra-axial mass. Both the above examples are meningiomas.