BACKGROUND AND PURPOSE: Perfusion imaging is commonly utilized to assess collateral status in patients with acute ischemic stroke due to anterior large-vessel occlusion. However, its potential role in identifying the etiology of extracranial ICA lesions before endovascular therapy (EVT) remains underexplored. We aimed to evaluate the utility of perfusion imaging parameters-particularly the hypoperfusion intensity ratio (HIR)-in detecting extracranial ICA steno-occlusion caused by atherosclerotic disease before EVT. MATERIALS AND METHODS: We conducted a retrospective cohort study of patients with consecutive EVT stroke in our institutional registry from January 2019 to December 2022 who underwent CT perfusion imaging and had ≥70% extracranial ICA stenosis. Patients were categorized into those with atherosclerotic and nonatherosclerotic ICA based on clinical and radiologic characteristics. Perfusion parameters, including CBF, CBV, time-to-maximum (Tmax), and Tmax mismatch ratios, were compared between groups. Multivariable logistic regression was performed to identify independent predictors of atherosclerotic disease. RESULTS: Of the 622 patients who underwent EVT, 78 met the inclusion criteria (median age 68 [60-76], 39.7% women, median NIHSS 15 [10-18]), and 53 (67.9%) had atherosclerotic ICA lesions. Compared with nonatherosclerotic cases, these patients had significantly lower median volumes of CBF <20% (0 versus 4 mL), lower HIR (0.27 versus 0.47, P < .01), and reduced Tmax mismatch ratios across several thresholds. In multivariable analysis, lower HIR (adjusted OR 0.01; 95% CI: 0.00-0,16) and several Tmax mismatch ratios, including >10 seconds/>8 seconds (adjusted OR 0.05; 95% CI: 0.00-0.53), >10 seconds/>4 seconds (adjusted OR 0.01; 95% CI: 0.00-0.26), >8 seconds/>6 seconds (adjusted OR 0.04; 95% CI: 0.00-0.58), and >8 seconds/>4 seconds (adjusted OR 0.04; 95% CI: 0.00-0.86), were independently associated with atherosclerotic ICA lesions. CONCLUSIONS: HIR and Tmax mismatch ratios (>10 seconds/>8 seconds, >10 seconds/4 seconds, >8 seconds/6 seconds, >8 seconds/4 seconds) were associated with acute steno-occlusive ICA due to atherosclerotic disease before EVT. Early identification of the underlying etiology may facilitate a more tailored cervical approach during endovascular treatment.