A Thesis submitted to The University of Arizona College of Medicine - Phoenix in partial fulfillment of the requirements for the Degree of Doctor of Medicine. / Current stroke imaging protocol at Barrow Neurological Institute calls for a noncontrast computed tomography (NCCT), a computed tomography angiography (CTA), and a computed tomography perfusion (CTP) at the time of presentation to the emergency department (ED), and follow up imaging includes magnetic resonance diffusion weighted imaging (MR‐DWI). This information is used to determine the appropriateness and safety of tissue plasminogen activator (tPA) administration. Previous studies have shown the risk for post‐tPA hemorrhagic conversion rises significantly as the size of the infarct core increases. Thus, it is of great importance to have an accurate method of measuring core infarct size in patients presenting with acute ischemic stroke. The purpose of our study is to determine if CTP correctly identifies the infarct core and if post‐tPA hemorrhagic conversion is related to the size of the infarct core and/or the accuracy of CTP in identifying the infarct core. The ultimate goal is to improve patient outcomes by decreasing the morbidity and mortality associated with tPA administration. This study is a retrospective chart review of all patients who presented to the ED during a one year period with signs and symptoms of acute ischemic stroke who then subsequently received tPA. Imaging was also reviewed, including the NCCT, CTA, CTP, and MRDWI for each patient. In this study, MR‐DWI is used as the gold standard for determining the presence or absence of an infarct core. CTP and MR‐DWI are in agreement of the presence of an infarct core in 7 patients, or 10 percent of the time. Similarly, CTP and MR‐DWI are in agreement of the absence of an infarct core in 31 patients, or 44 percent of the time. In the other 32 patients, CTP and MR‐DWI are in disagreement. The percent correlation between CTP and MR‐DWI was found to be 24 percent with a p‐value < 0.05. As for post‐tPA hemorrhagic conversion, 12 percent of patients had hemorrhagic conversion, and when the hemorrhage rate was compared to the size of the infarct core, the odds of post‐tPA hemorrhagic conversion were 56 times higher in the group of patients with infarct cores larger than one‐third of a vascular territory than in patients with smaller infarct cores with a p‐value < 0.001. Although no significant correlation was found between the accuracy of CTP data and the rate of post‐tPA hemorrhagic conversion, patients with concordant CTP and MR data had a 46% lower likelihood of post‐tPA hemorrhagic conversion than did patients with contradictory CTP and MR‐DWI data. Conclusion: Because patients with infarct cores larger than one‐third of a vascular territory are 56 times more likely to hemorrhage than patients with smaller infarct cores and CTP is less accurate than MR‐DWI in identifying the infarct core in patients presenting with acute ischemic stroke, CTP studies should not be part of the acute stroke imaging protocol. Another imaging modality, such as MR‐DWI, may be preferential in the setting of acute ischemic stroke to identify the infarct core.