title: Which time-to-peak threshold best identifies penumbral flow? A comparison of perfusion-weighted magnetic resonance imaging and positron emission tomography in acute ischemic stroke.
abstract: OBJECTIVE
  In acute ischemic stroke, the hypoperfused but viable tissue is the main therapeutic target. In clinical routine, time-to-peak (TTP) maps are frequently used to estimate the hemodynamic compromise and to calculate the mismatch volume. We evaluated the accuracy of TTP maps to identify penumbral flow by comparison with positron emission tomography (PET).
  
  
  METHODS
  Magnetic resonance imaging (MRI) and PET were performed in 11 patients with acute ischemic stroke (median 8 hours after stroke onset, 60 minutes between MRI and PET imaging). The volumes defined by increasing TTP thresholds (relative TTP delay of >2, >4, >6, >8, and >10 seconds) were compared with the volume of hypoperfusion (<20 mL/100 g per min) assessed by 15O-water PET. In a volumetric analysis, each threshold's sensitivity, specificity, and predictive values were calculated.
  
  
  RESULTS
  The median hypoperfusion volume was 34.5 cm3. Low TTP thresholds included large parts of the hypoperfused but also large parts of normoperfused tissue (median sensitivity/specificity: 93%/60% for TTP >2) and vice versa (50%/91% for TTP >10). TTP >4 seconds best identifies hypoperfusion (84%/77%). The positive predictive values increased with the size of hypoperfusion.
  
  
  CONCLUSIONS
  This first comparison of quantitative PET-CBF with TTP maps in acute ischemic human stroke indicates that the TTP threshold is crucial to reliably identify the tissue at risk; TTP >4 seconds best identifies penumbral flow; and TTP maps overestimate the extent of true hemodynamic compromise depending on the size of ischemia. Only if methodological restrictions are kept in mind, relative TTP maps are suitable to estimate the mismatch volume.
authors.affiliation: Max Planck Institute for Neurological Research, Cologne, Germany. j.sobesky@pet.mpin-koeln.mpg.de
authors.forename: J
authors.initials: J
authors.lastname: Sobesky
authors.forename: O
authors.initials: O
authors.lastname: Zaro Weber
authors.forename: F-G
authors.initials: FG
authors.lastname: Lehnhardt
authors.forename: V
authors.initials: V
authors.lastname: Hesselmann
authors.forename: A
authors.initials: A
authors.lastname: Thiel
authors.forename: C
authors.initials: C
authors.lastname: Dohmen
authors.forename: A
authors.initials: A
authors.lastname: Jacobs
authors.forename: M
authors.initials: M
authors.lastname: Neveling
authors.forename: W-D
authors.initials: WD
authors.lastname: Heiss
mesh_terms: D000328:Adult; D000368:Aged; D001921:Brain / Q000098:blood supply / Q000473:pathology; D002560:Cerebrovascular Circulation; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D049268:Positron-Emission Tomography; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D020521:Stroke / Q000175:diagnosis*