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Hi - I guess this is not an easy question - here is my take. If you are fitting a single diffusion tensor, the diffusivity (and FA) values that you measure will depend on b-value, as apparent diffusion is not mono-exponential in the brain, as the diffusion tensor model assumes. Therefore, if you collect at more than a single b-value, your model fit to a diffusion tensor model will be worse. You will measure something between what you would have measured with each one of the b- values. My guess is that the error in FA/MD will be higher than for a single b-value shell acquisition, but I have not spent a long time thinking about it. However, remember that for single shell experiments, your measurements are necessarily conditioned on you choice of b-value. If all you are interested in is diffusion orientation (for tractography), I again think you are better off spreading your orientations equally on a single shell. The model in bedpost also assumes mono-exponential decay of each compartment, so I expect that the multi b-value experiment will look like more noise to the bayesian estimation (as the model fit will be worse) and therefore will inflate uncertainty. Having said this, we do not have practical experience. T On 26 Mar 2007, at 20:43, Stephen Towler wrote: > For TBSS and probtrack analysis, what benefit (if any) does one > derive from acquiring three b-values (b0, bmax/2, bmax) instead of > two b-values (b0 & bmax)? When collecting 2-3 mm isovoxels at 32 > or sixty directions, omitting the intermediate b-value creates > substantial time savings which is a huge help with compromised > subjects. > > Thanks! > Stephen > > > -- > (please note email change from mbi.ufl.edu to ufl.edu) > > Stephen Towler > [log in to unmask] > 352-294-0048 office > 352-258-6409 mobile > 352-392-8347 fax > > Leonard Lab > Department of Neuroscience > PO Box 100244 > University of Florida HSC > Gainesville, FL 32610