Objective: Mechanical thrombectomy (MT) is now an established treatment foracute ischemic stroke (AIS) with large vessel occlusion (LVO) within 6 hours. Since2018, MT is also recommended from 6-24 hours after selecting with additional multimodalimaging including perfusion imaging. We sought to investigate patients withsignificant discrepancy in core infarct between computed tomography (CT) and CTperfusion (CTP). Methods: In this retrospective study, patients with AIS who were evaluated for MTusing the RAPID software (IschemaView, Redwood City, CA, USA) from February2018 to March 2019 were included. Cases with discrepancy between infarct volumeon non-contrast CT and core volume (cerebral blood flow <30%) as analyzed by RAPIDon CTP were analyzed. Results: In the study period, 635 patients were evaluated for acute stroke symptoms.Non-contrast head CT was performed in 635 patients, and CTP with RAPID softwarepost processing was performed in 134 patients. Among the 134 patients, 8 (5.9%)patients had gross discrepancy in core infarct between CT and CTP, with underestimationof infarct by CTP. Evaluation of these cases shows that the likely reason forthis discrepancy is recanalization of a LVO, which then leads to erroneously normal orgross underestimate of the core infarct volume determined from CTP post processinganalysis. Conclusions Recanalization of a LVO can lead to erroneously normal or gross underestimationof the core infarct as determined by post processing software analysisof CTP data. The whole composite of hyperacute CT imaging should be examinedwhile making decisions. This caveat of perfusion imaging interpretation has not beenreported previously.