Background: Distinctive aspects of acute ischemic stroke (AIS) due to internal carotid artery occlusion (ICAO) include severe presenting deficits, extensive infarct volumes, frequent poor functional outcome, and mortality. Decision-making for thrombectomy, hemicraniectomy, and palliative care depends crucially upon infarct extent but the most widely used current image rating scale, ASPECTS, assesses only 10 regions confined to the MCA field. Methods: We developed the hemispheric-ASPECTS (H-ASPECTS) rating scale that assesses the entire extent of the ICA-supplied brain. The 16-point H-ASPECTS score adds to original MC-ASPECTS (10 regions), assessments of 3 ACA regions, 2 supratentorial PCA regions (thalamus, occipital - potentially fetal ICA-supplied), and 1 anterior choroidal artery region (Figure). We rated initial and 24h CT/MR in consecutive AIS-ICAO patients. Results: Among the AIS-ICAO patients (n=17), age was 68.5 (±15.8), 52.9% female, NIHSS median 20 (IQR 14-22). Occlusion locations were: carotid T 47%; cervical 47%; isolated intracranial ICA 6%. Treatment included EVT ± IV lysis in 59% and hemicraniectomy in 6%. Time from last known well to 1st imaging was 9.6h and 2nd imaging 35h. On 1st imaging H-ASPECTS scores were median 12 (IQR 11-14); on 2nd imaging 10 (IQR 6.5-12.5). Involvement of regions outside MCA, detected by the H-ASPECTS, occurred in 47% of patients, including ACA in 35%, PCA in 12%, and AChA in 6%. Second image H-ASPECTS correlated strongly with discharge mRS, r=0.66 (p=0.007). Conclusions: H-ASPECTS expands the scope of AIS imaging scores, captured abnormalities outside the MCA field in 47% of patients with ICAO, and correlated well with discharge disability level outcome. The additional information H-ASPECTS provides may enhance decision-making in ICAO patients with large ischemic cores, including for endovascular thrombectomy and hemicraniectomy.