Effect of b value on contrast during diffusion-weighted magnetic resonance imaging assessment of acute ischemic stroke

Purpose: To examine the effect of varying the diffusion encoding strength (b value) on the contrast (signal difference, DeltaS) between damaged and normal tissue during diffusion-weighted magnetic resonance imaging (DWI) assessment of acute ischemic stroke, Materials and 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 (5002500 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 DeltaS (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. Conclusion: 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. (C) 2002 Wiley-Liss, Inc.