Re: [HCP-Users] Diffusion data b values

[Glasser, Matthew]

Yes you should take the bvalues in the bvals files literately.  The actual average bvalue applied varies because of interactions between the imaging and diffusion gradients (cross-terms).   If you use FSL, you can deal with the bvalue/bvector gradient deviations with existing code.  If not, new code will need to be written.  This is discussed in the HCP diffusion paper: http://www.sciencedirect.com/science/article/pii/S105381191300551X Peace, Matt. From: Ariel Rokem <aro...@gmail.com> Date: Monday, August 5, 2013 3:46 PM To: "hcp-users@humanconnectome.org" <hcp-users@humanconnectome.org> Subject: [HCP-Users] Diffusion data b values 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 HCP-Users@humanconnectome.org http://lists.humanconnectome.org/mailman/listinfo/hcp-users   The materials in this message are private and may contain Protected Healthcare Information or other information of a sensitive nature. If you are not the intended recipient, be advised that any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited. If you have received this email in error, please immediately notify the sender via telephone or return mail. _______________________________________________ HCP-Users mailing list HCP-Users@humanconnectome.org http://lists.humanconnectome.org/mailman/listinfo/hcp-users