title: Effect of b value on contrast during diffusion-weighted magnetic resonance imaging assessment of acute ischemic stroke.
abstract: OBJECTIVE
  To examine the effect of varying the diffusion encoding strength (b value) on the contrast (signal difference, Delta S) between damaged and normal tissue during diffusion-weighted magnetic resonance imaging (DWI) assessment of acute ischemic stroke.
  
  
  METHODS
  Phantoms with diffusion values approximating those expected in acutely infarcted and normal tissue were constructed from a mixture of agar and formaldehyde and imaged at varying b values (0-3000 mm(-2) second). Ten patients were imaged with multiple b values (500-2500 mm(-2) second) within 12 hours of stroke onset.
  
  
  RESULTS
  Theoretical calculations showed that for any combination of diffusion coefficients there existed an optimal b value that was higher than the standard setting of 1000 mm(-2) second, and this was confirmed by the phantom studies. In the patients, increasing b from 1000 to 1500 mm(-2) second increased Delta S (average, 22.4%; P = 0.001), but no consistent benefit was seen at b = 2000 mm(-2) second (P = 0.408). This compared favorably with the average optimal b value of 1662 mm(- 2) second calculated from the patients.
  
  
  CONCLUSIONS
  These results suggest that increasing the b value from 1000 to 1500 mm(-2) second would increase contrast between infarcted and normal tissue in the setting of acute ischemic stroke.
authors: Department of Radiology, University of Calgary, Calgary, Alberta, Canada.
authors: Raoul S
authors: RS
authors: Pereira
authors: Ashley D
authors: AD
authors: Harris
authors: Robert J
authors: RJ
authors: Sevick
authors: Richard
authors: R
authors: Frayne
mesh_terms: D000208:Acute Disease; D000328:Adult; D000368:Aged; D000704:Analysis of Variance; D002545:Brain Ischemia / Q000473:pathology*; D004058:Diffusion; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D019047:Phantoms, Imaging