title: Computed tomography and magnetic resonance perfusion imaging in ischemic stroke: definitions and thresholds.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Neurology, Institute of Neurological Sciences, University of Glasgow Southern General Hospital, Glasgow, Scotland.
authors.forename: Krishna A
authors.initials: KA
authors.lastname: Dani
authors.forename: Ralph G R
authors.initials: RG
authors.lastname: Thomas
authors.forename: Francesca M
authors.initials: FM
authors.lastname: Chappell
authors.forename: Kirsten
authors.initials: K
authors.lastname: Shuler
authors.forename: Mary J
authors.initials: MJ
authors.lastname: MacLeod
authors.forename: Keith W
authors.initials: KW
authors.lastname: Muir
authors.forename: Joanna M
authors.initials: JM
authors.lastname: Wardlaw
mesh_terms: D001921:Brain / Q000473:pathology; D002545:Brain Ischemia / Q000150:complications / Q000000981:diagnostic imaging / Q000473:pathology*; D002544:Cerebral Infarction / Q000000981:diagnostic imaging / Q000473:pathology; D002560:Cerebrovascular Circulation / Q000502:physiology; D003627:Data Interpretation, Statistical; D057225:Data Mining; D006801:Humans; D008279:Magnetic Resonance Imaging; D010477:Perfusion; D012107:Research Design; D018570:Risk Assessment; D020521:Stroke / Q000000981:diagnostic imaging / Q000209:etiology / Q000473:pathology*; D014057:Tomography, X-Ray Computed