Ariel, We recommend people to use these calculated b-values (including cross-terms) instead of the input b-values. This was explained explicitly at page 137, 2nd paragraph in
http://www.ncbi.nlm.nih.gov/pubmed/23702418 "Small b value variations due to the cross-terms between diffusion and imaging gradients are calculated (vendor supplied algorithm) in units of 5 s/mm2 before the gradient nonlinearity correction" Regards Gordon On Aug 5, 2013, at 4:46 PM, Ariel Rokem <[email protected]> wrote: > Hello everyone, > > First of all, thanks for all the hard work on putting this data-set together. > I have been looking at some of the diffusion data and it's been a good > experience so far. > > I have a question concerning the encoding of b values in this data set. > Nominally, data was collected in 3 different shells (1000, 2000 and 3000 > s/mm^2), but the values encoded in the files are not exactly these three > values. That is, values are +/- 5-10 units from these three shells (and from > 0). As in:995, 990, 1005, 3005, 5, etc. > > Should we take these values literally? Was there a difference in the > diffusion encoding between these different directions? If so, why? > > Another related question: I recall a talk (at ISMRM?) in which one of the > researchers affiliated with HCP mentioned fluctuations of b value across the > volume (I apologize not remembering who this was...). Does anyone have a good > reference about these fluctuations, for this data, or other data? > > Thanks again for your help with this question and for all the work on this, > > Ariel Rokem > Department of Psychology > Stanford University > _______________________________________________ > HCP-Users mailing list > [email protected] > http://lists.humanconnectome.org/mailman/listinfo/hcp-users > _______________________________________________ HCP-Users mailing list [email protected] http://lists.humanconnectome.org/mailman/listinfo/hcp-users
