We evaluated the correlation between cerebrovascular reserve (CVR) on acetazolamide (ACZ) -stress SPECT brain scans and collaterals on ASL MRI in ICA stenosis. 86 patients with ICA stenosis (>70%) were enrolled in this study. On ASL, late-arriving flow appears as serpiginous high ASL signal within cortical vessels, which has been termed arterial transit artifact (ATA). 82/86 ICA stenosis patients underwent SPECT imagings with Tc-99m-ECD in the resting and after ACZ challenge. Significant positive relationship was observed between normal CVR group and ATA showing group in ICA stenosis patients on ASL brain perfusion (p=0.035, chi-square test).