Objective:To investigate the predictive value of fluid-attenuated inversion recovery (FLAIR) signal strength ratio (SIR) in onset time≤4.5 h in patients with acute ischemic stroke.Methods:A retrospective analysis was performed; 180 acute ischemic stroke patients admitted to Department of Neurology, Nanjing Hospital Affiliated to Nanjing Medical University from January 2020 to June 2023 were chosen. Hypoperfusion intensity ratio (HIR) was used to evaluate the collateral circulation (poor collateral circulation: HIR≤0.4; good collateral circulation: HIR>0.4); clinical data and imaging indexes between poor collateral circulation and good collateral circulation groups were compared. Univariate and multivariate Logistic regressions were used to analyze the influencing factors for onset time≤4.5 h in patients with acute ischemic stroke. Correlation between SIR and onset time was analyzed in patients with acute ischemic stroke. Role of HIR as agency between SIR and onset time was explored. Receiver operating characteristic (ROC) curve was used to analyze the predictive efficacy of SIR and diffusion weighted imaging (DWI)-FLAIR mismatch in onset time≤4.5 h in acute ischemic stroke patients.Results:Of the 180 patients, 100 were into the good collateral circulation group and 80 were into the poor collateral circulation group; compared with the good collateral circulation group, the poor collateral circulation group had significantly higher percentage of patients with hyperlipidemia, larger DWI infarction volume before treatment, larger perfusion weighted imaging (PWI)-DWI mismatch volume and higher SIR ( P<0.05). In these 180 patients, 76 had onset time≤4.5 h and 104 had onset time>4.5 h. Univariate Logistic regression analysis showed that hyperlipidemia, DWI infarct volume before treatment, DWI-FLAIR mismatch, HIR and SIR were influencing factors for onset time≤4.5 h in acute ischemic stroke patients ( P<0.05). Multivariate Logistic regression analysis showed that hyperlipidemia ( OR=6.654, 95% CI: 5.751-8.824, P<0.001), HIR ( OR=0.724, 95% CI: 0.521-1.321, P=0.041) and SIR ( OR=739.881, 95% CI: 383.296-14 258.065, P<0.001) were independent influencing factors for onset time≤4.5 h in acute ischemic stroke patients. Pearson correlation analysis showed that SIR was positively correlated to onset time in patients with acute ischemic stroke ( r=0.420, P<0.05), and SIR was positively correlated to onset time in patients from poor collateral circulation group ( r=0.781, P<0.05). ROC curve showed that AUC of SIR in predicting onset time≤4.5 h was 0.917 (95% CI: 0.814-1.000, P<0.001) and that of DWI-FLAIR mismatch in predicting onset time≤4.5 h was 0.530 (95% CI: 0.509-0.757, P=0.075) in poor collateral circulation group, enjoying significant difference in predictive efficacy. Conclusion:Acute ischemic stroke patients with low HIR and SIR have higher odds of onset time≤4.5 h; SIR can more accurately predict the onset time in these patients with poor collateral circulation.