Ischemic territorial mismatch due to posterior cerebral artery occlusion illustrated by the FLAIR ivy sign in moyamoya disease.

Cerebral infarctions in moyamoya disease frequently do not conform to anatomical arterial territories. This phenomenon is closely associated with delayed steno-occlusive involvement of the posterior cerebral artery (PCA) following progression of internal carotid artery disease, which compromises PCA-derived leptomeningeal collateral circulation that had previously supported the anterior circulation. Although PCA involvement is common in moyamoya disease, the hierarchical vulnerability of PCA-derived collateral pathways and their relationship to the fluid-attenuated inversion recovery (FLAIR) ivy sign remain incompletely understood. This pictorial essay illustrates a hypothesis-generating conceptual framework of ischemic territorial mismatch caused by PCA steno-occlusion and clarifies the hemodynamic significance and clinical utility of the FLAIR ivy sign. PCA-derived leptomeningeal collateral circulation consists of two functionally distinct pathways: superficial collaterals supplying the middle cerebral artery (MCA) territory and deep collaterals supplying the anterior cerebral artery (ACA) territory via the posterior pericallosal artery. Because superficial collaterals traverse long cortical distances, they are more vulnerable to progressive PCA steno-occlusion than deep collateral pathways. Functional insufficiency of superficial collaterals may result in selective hemodynamic compromise of the MCA territory, while the PCA and ACA territories remain relatively preserved, producing ischemic territorial mismatch. Representative imaging findings from a pediatric cross-sectional case and an adult longitudinal case are presented to demonstrate this hemodynamic framework. Across the presented cases, the FLAIR ivy sign tends to preferentially appear in the MCA territory during superficial collateral insufficiency. In the longitudinal case, progression of PCA steno-occlusion was accompanied by emergence of the ivy sign in the MCA territory, which resolved after successful revascularization, highlighting its dynamic and reversible nature as a marker of hemodynamic compromise. In conclusion, ischemic territorial mismatch reflects hierarchical vulnerability of PCA-derived leptomeningeal collaterals. The FLAIR ivy sign provides a practical, noninvasive imaging marker of superficial collateral decompensation and hemodynamic stabilization after revascularization, offering guidance for longitudinal follow-up.