Why is the noise greater with greater bvalue? In any case, it just uses ordinary LS, so assume homoscedastic.
On 05/08/2015 01:54 PM, Anders Eklund wrote: > Yes, but the signal can have rather different variance due to the different > b-values. So it would make sense to use a heteroscedastic GLM, where the > variance is allowed to change over the samples. In the more conventional > homoscedastic GLM, the variance is assumed to be constant over time / > samples. Is this ignored in freesurfer, or is it solved by using weighted > least squares? > > - Anders > > ________________________________________ > Från: freesurfer-boun...@nmr.mgh.harvard.edu > [freesurfer-boun...@nmr.mgh.harvard.edu] för Douglas N Greve > [gr...@nmr.mgh.harvard.edu] > Skickat: den 8 maj 2015 19:40 > Till: freesurfer@nmr.mgh.harvard.edu > Ämne: Re: [Freesurfer] DTI tensor estimation > > It is standard dti analysis, so it computes the log(y), then sets up a > seven column matrix with > 1 b*gx*gx b*gx*gy b*gx*gz b*gy*gy b*gy*gz b*gz*gz > > The b, gx, gy, and gz change with each row according to the bval and > bvec files > > doug > > > On 05/08/2015 05:29 AM, Anders Eklund wrote: >> Dear freesurfer experts, >> >> I looked at the help text for the dt_recon function, and wonder how >> the GLM is setup for the DTI data, is there a paper that describes how >> freesurfer does it? Specifically, I'm interested in how a dataset with >> 3-4 different b-values is analyzed. Is there a dummy variable in the >> GLM that models the different b-values, since the data from different >> b-values have very different variance? >> >> http://freesurfer.net/fswiki/dt_recon >> >> Regards, >> Anders >> >> >> _______________________________________________ >> Freesurfer mailing list >> Freesurfer@nmr.mgh.harvard.edu >> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > -- > Douglas N. Greve, Ph.D. > MGH-NMR Center > gr...@nmr.mgh.harvard.edu > Phone Number: 617-724-2358 > Fax: 617-726-7422 > > Bugs: surfer.nmr.mgh.harvard.edu/fswiki/BugReporting > FileDrop: https://gate.nmr.mgh.harvard.edu/filedrop2 > www.nmr.mgh.harvard.edu/facility/filedrop/index.html > Outgoing: ftp://surfer.nmr.mgh.harvard.edu/transfer/outgoing/flat/greve/ > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > > > The information in this e-mail is intended only for the person to whom it is > addressed. If you believe this e-mail was sent to you in error and the e-mail > contains patient information, please contact the Partners Compliance HelpLine > at > http://www.partners.org/complianceline . If the e-mail was sent to you in > error > but does not contain patient information, please contact the sender and > properly > dispose of the e-mail. > > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > > -- Douglas N. Greve, Ph.D. MGH-NMR Center gr...@nmr.mgh.harvard.edu Phone Number: 617-724-2358 Fax: 617-726-7422 Bugs: surfer.nmr.mgh.harvard.edu/fswiki/BugReporting FileDrop: https://gate.nmr.mgh.harvard.edu/filedrop2 www.nmr.mgh.harvard.edu/facility/filedrop/index.html Outgoing: ftp://surfer.nmr.mgh.harvard.edu/transfer/outgoing/flat/greve/ _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
