Ivy Sign

In normal subjects, intracranial structures including venous sinuses, cortical veins, and choroids plexus are commonly but not always enhanced on gadolinium-enhanced MR images, but there is no apparent leptomeningeal enhancement. Leptomeningeal enhancement on postcontrast MR images has been described in association with meningoencephalitis, meningeal carcinomatosis, and vasculitis.     

Demaerel et al observed marked meningeal enhancement in a girl with moyamoya disease. The ivy sign was described by Ohta et al, referring to the diffuse leptomeningeal enhancement that resembles creeping ivy on stones in patients with moyamoya disease.

Creeping Ivy on Stones.

The ivy sign can be found on FLAIR images, as well as on contrast-enhanced MR images. Characteristic enhancement is caused by an engorged pial network via leptomeningeal anastomosis. Surgical findings of engorged pial vessels with thickened arachnoid membranes and edema are described in some cases, which may explain the leptomeningeal enhancement on contrast-enhanced MR images. Dilated pial vessels were pre-existing rather than newly formed. The source of leptomeningeal high signal intensity on FLAIR images may be complex and is not clear. As for the leptomeningeal enhancement on post-contrast T1-weighted images, it seems largely due to slow-flowing vessels of the engorged pial network.

Transverse postcontrast T1-weighted MR image shows diffuse enhancement along leptomeningeal surfaces (arrowheads), predominantly in right hemisphere.

Transverse unenhanced FLAIR MR image shows bilateral areas of high signal intensity (arrowheads) along leptomeninges.

Thickening with congestion of the leptomeninges can take part in the mechanism causing high signal intensity. Thus, the mechanism of the leptomeningeal ivy sign can be explained mainly by an engorged pial network and possibly by congested leptomeninges.