Selection of the optimum b factor for diffusion-weighted magnetic resonance imaging assessment of ischemic stroke

The purpose of this study was to determine the diffusion sensitivity factor b that optimizes the contrast‐to‐noise ratio (CNR) for both diffusion‐weighted signal intensity and the apparent diffusion coefficient (ADCNR) when evaluating ischemic stroke by diffusion‐weighted MRI. The relative contrast, noise levels, CNR, and ADCNR were calculated for typical ADC values in human brain, 780 μm2/s in adults and 1200 μm2/s in neonates in normal tissue, 20–40% less in acute and subacute stroke, and 50% more in chronic stroke. The optimum b factor depends strongly on the ADC, whether TE is fixed or varies with the b factor, whether CNR or ADCNR is measured, and anisotropy. The optimum b factor in adults is 1000 s/mm2 in acute and chronic stroke, and 1200 s/mm2 in subacute stroke. The optimum values are about 200 s/mm2 lower in neonates than in adults. The CNR and ADCNR are within 10% of the optimum over at least a 2‐fold range of b factors, from 68–136% of the optimum b factor. If a single b factor is to be used for all situations, a diffusion b factor of 1000 s/mm2 is recommended. Magn Reson Med 51:996–1001, 2004. © 2004 Wiley‐Liss, Inc.