title: Acute stroke: automatic perfusion lesion outlining using level sets.
abstract: OBJECTIVE
  To develop a user-independent algorithm for the delineation of hypoperfused tissue on perfusion-weighted images and evaluate its performance relative to a standard threshold method in simulated data, as well as in acute stroke patients.
  
  
  METHODS
  The study was approved by the local ethics committee, and patients gave written informed consent prior to their inclusion in the study. The algorithm identifies hypoperfused tissue in mean transit time maps by simultaneously minimizing the mean square error between individual and mean perfusion values inside and outside a smooth boundary. In 14 acute stroke patients, volumetric agreement between automated outlines and manual outlines determined in consensus among four neuroradiologists was assessed with Bland-Altman analysis, while spatial agreement was quantified by using lesion overlap relative to mean lesion volume (Dice coefficient). Performance improvement relative to a standard threshold approach was tested with the Wilcoxon signed rank test.
  
  
  RESULTS
  The mean difference in lesion volume between automated outlines and manual outlines was -9.0 mL ± 44.5 (standard deviation). The lowest mean volume difference for the threshold approach was -25.8 mL ± 88.2. A significantly higher Dice coefficient was observed with the algorithm (0.71; interquartile range [IQR], 0.42-0.75) compared with the threshold approach (0.50; IQR, 0.27- 0.57; P , .001). The corresponding agreement among experts was 0.79 (IQR, 0.69-0.83).
  
  
  CONCLUSIONS
  The perfusion lesions outlined by the automated algorithm agreed well with those defined manually in consensus by four experts and were superior to those obtained by using the standard threshold approach. This user-independent algorithm may improve the assessment of perfusion images as part of acute stroke treatment.
  
  
  BACKGROUND
  http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.13121622/-/DC1.
authors.affiliation: Center of Functionally Integrative Neuroscience, Department of Neuroradiology, Århus University Hospital, Nørrebrogade 44, Bldg 10G, 5th Floor, DK-8000 Århus C, Denmark.
authors.forename: Kim
authors.initials: K
authors.lastname: Mouridsen
authors.forename: Kartheeban
authors.initials: K
authors.lastname: Nagenthiraja
authors.forename: Kristjana Ýr
authors.initials: KÝ
authors.lastname: Jónsdóttir
authors.forename: Lars R
authors.initials: LR
authors.lastname: Ribe
authors.forename: Anders B
authors.initials: AB
authors.lastname: Neumann
authors.forename: Niels
authors.initials: N
authors.lastname: Hjort
authors.forename: Leif
authors.initials: L
authors.lastname: Østergaard
mesh_terms: D000368:Aged; D000465:Algorithms; D001783:Blood Flow Velocity; D003287:Contrast Media; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D009942:Organometallic Compounds; D010363:Pattern Recognition, Automated; D019047:Phantoms, Imaging; D020521:Stroke / Q000175:diagnosis*
chemical_list: D003287:Contrast Media; D009942:Organometallic Compounds; C090600:gadobutrol