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 microm(2)/s in adults and 1200 microm(2)/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/mm(2) in acute and chronic stroke, and 1200 s/mm(2) in subacute stroke. The optimum values are about 200 s/mm(2) 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/mm(2) is recommended.