OBJECTIVE: Cerebral perfusion imaging with computed tomography (CT) or magnetic resonance (MR) is widely available. The optimum perfusion values to identify tissue at risk of infarction in acute stroke are unclear. We systematically reviewed CT and MR perfusion imaging in acute ischemic stroke. METHODS: We searched for papers on MR or CT perfusion performed <24 hours after stroke that assessed perfusion thresholds, mean perfusion lesion values, or lesion volumes. We extracted definitions and perfusion values. We compared definitions and evaluated perfusion thresholds for "nonviable"/"at risk" and "at risk"/"not at risk tissue" thresholds. RESULTS: Among 7,152 papers, 69 met inclusion criteria for analysis of definitions (49 MR and 20 CT), 21 MR (n = 551), and 10 CT (n = 266) papers, median sample size 22, provided thresholds. We found multiple definitions for tissue states, eg, tissue at risk, 18; nonviable tissue, 12; 16, no definition. Perfusion parameters varied widely; eg, 9 different MR, 6 different CT parameters for the "at risk"/"not at risk threshold." Median threshold values varied up to 4-fold, eg, for the "at risk"/"not at risk threshold," median cerebral blood flow ranged from 18 to 37ml/100g/min; mean transit time from 1.8 to 8.3 seconds relative to the contralateral side. The influence of reperfusion and duration of ischemia could not be assessed. INTERPRETATION: CT and MR perfusion imaging viability thresholds in stroke are derived from small numbers of patients, variable perfusion analysis methods and definitions of tissue states. Greater consistency of methods would help determine reliable perfusion viability values for wider clinical use of perfusion imaging.