T2-FLAIR mismatch sign
The T2-FLAIR mismatch sign describes the MRI appearance considered highly specific for diffuse astrocytoma (IDH-mutant, 1p/19q-non-codeleted molecular status), as opposed to other lower-grade gliomas. It is particularly helpful in distinguishing a diffuse astrocytoma from an oligodendroglioma that will not demonstrate T2-FLAIR mismatch.
On T2 weighted images, these tumors have extensive areas of fairly homogeneous and strikingly high signal. On T2-FLAIR, instead, the majority of these areas become relatively hypointense in signal due to incomplete suppression. At the margins of the tumor, a rim of hyperintensity is usually seen. This appearance is typical of the entity previously known as protoplasmic astrocytoma which is, however, no longer recognized as a distinct entity in the current WHO classification of CNS tumors.