The “Outline Sign”: Thin Hyperenhancing Perimeter as an MR Imaging Feature of Meningioma. A Useful Tool in the Temporal Bone Region for Differentiating Meningiomas from Schwannomas and Paragangliomas

A, Contrast-enhanced axial T1-weighted image in a 46-year-old man demonstrates a large mass in the anterior cranial fossa continuing to the olfactory groove, which was subsequently resected and confirmed to be a meningioma. The mass demonstrates a thin rim of hyperenhancement (“outline sign”) that is especially prominent along its right posterolateral margin (arrow). B, A magnified view of this meningioma shows areas of rim enhancement (arrowheads) with a jagged appearance (“serrated edges”). Axial (C) and coronal (D) contrast-enhanced T1-weighted images in a 47-year-old with pathologically proven meningioma show an outline sign around the periphery of the lesion (arrows). Note the serrated pattern of this enhancing edge.

Potential pitfalls of the outline sign. A, Axial contrast-enhanced T1-weighted image in a 34-year-old with pathologically proven vestibular schwannoma shows small focal short/discrete (left arrow) or punctate/rounded/nodular (right arrow) areas of enhancement including some near the periphery (arrows). The short/discrete or punctate/nodular rather than thin curvilinear appearance is not consistent with the outline sign. B, Axial contrast-enhanced T1-weighted image in a 46-year-old with pathologically proven vestibular schwannoma. There is linear enhancement along the margin of the internal auditory canal (arrows), and it is difficult to determine whether this is intrinsic to the dural lining or if it is along the periphery of the tumor that may have cystic components at its lateral aspect. However, as this linear enhancement reaches the cerebellopontine angle cistern (arrowheads), it appears to continue along the posterior petrous ridge away from the tumor, favoring the former theory of being related to the dura. As such, this is equivocal for an outline sign. C, Axial contrast-enhanced T1-weighted image shows a pathologically proven left vestibular schwannoma in a 49-year-old patient. The central portion of the tumor shows diminished enhancement within areas of focal cystic change, falsely exaggerating the degree of mural enhancement (arrows). This enhancing periphery is thicker than 1 mm and does not have the thin appearance of the outline sign.