Background: Changes of signal intensities (SIs) across intracranial atherosclerosis (ICAS) on magnetic resonance angiography (MRA) may reflect hemodynamic impact of the lesion. We evaluated the interobserver reproducibility of an index termed signal intensity ratio (SIR), developed in a previous study to represent the changes of SIs across ICAS on MRA. Methods: Symptomatic ICAS on MRAwere retrospectively recruited. Two observers respectively evaluated the images and calculated the SIR as follows, blinded to each other's readings: SIR 5 (mean poststenotic SI 2mean background SI)/(mean prestenotic SI - mean background SI). Statistical analyses were performed to evaluate the interobserver reproducibility of this index. Results: Atotal of 102 symptomatic ICASs were enrolled, with 36 (35.3%) lesions of 50%-69% MRA stenoses and others being 70%-99% stenoses or flow void on MRA. Overall, mean SIRs were not significantly different between the 2 observers (.92 +/- .17 versus .93 +/- .17; mean difference 2.006 +/- .09; P = .496 for paired t test). Pearson correlation coefficients were.. 80 for all analyses, indicating strong linear correlations between SIRs by the 2 observers. Bland-Altman analysis for SIRs of all cases showed no systematic bias between the 2 observers. For different cut-points ranging from .75 to 1.00, the kappa statistics were mostly greater than .6 and interobserver agreements were all greater than 80%, implying substantial agreement between observers. Conclusions: SIR was demonstrated to be highly reproducible between observers in the present study. Future studies are warranted to further explore the role of this index in comprehensive evaluation and risk stratification of symptomatic ICAS. (C) 2013 by National Stroke Association