 |
 |
Hi Steve, Thank you for your reply. I examined the values by overlaying (in afni) "all_MD_skeletonized" on "all_MD", and comparing the overlay and underlay MD values of the same subject at each pixel on the skeleton. My understanding is that the pixel values on the skeleton in "all_MD_skeletonized" are derived from searching perpenticularly from the original skeleton for the maximum value, representing the center of the track. These should therefore be >= the same pixels on "all_MD", which represent the values before the projection. Our FA data are consistent with this, although MD are not. Does this sound like I have an incorrect understanding of the process somehow? Thank you in advance for your valueble time. Win On Fri, 27 Oct 2006 09:03:23 +0100, Steve Smith <[log in to unmask]> wrote: >Hi Win, > >It sounds like something's very wrong - in particular any null >validation of the permutation testing should definitely give the >right results. How does the all_MD (the 4D nonlinear aligned data) >look - view as a movie loop in FSLView ? > >How are you judging whether values go up or down after projection? >The projected values stay exactly the same (there's no interpolation >involved) so I'm not sure what measure you're using. > >Cheers, Steve. > > >On 26 Oct 2006, at 21:16, Win Gongvatana wrote: > >> Hi all, >> >> I was wondering what has been your experiences with tbss and mean >> diffusivity (or other non-FA) data. We've successfully used tbss on >> the F >> A >> data in our lab, but still have some trouble with MD. The >> recommended ste >> ps >> were followed through to tbss_non_FA to get the projected MD >> skeletons fo >> r >> individual subjects (based on the original FA skeleton, as >> recommended). >> Performing t-tests on our various data sets processed with this >> technique >> , >> "randomise" yielded wildly significant group differences (1-alpha >> > .99 f >> or >> the majority of voxels) even with our "control vs. control" analyses. >> >> One thing that I find unusual is that the projected MD values on the >> skeleton are sometimes lower than the values before projection, >> which is >> counter-intuitive to my understanding of the "perpendicular search" >> strat >> egy >> for the projection. Inspecting the projected FA values shows them >> to alwa >> ys >> be >= the values before projection. >> >> If this matters, my scaling factor for MD at step 1 was 2000 (our >> values >> range from about 0 to 5, with no negatives). >> >> Any advice/suggestions would be greatly appreciated. >> >> I'd also like to thank the FSL/TBSS team for the incredibly useful >> tools >> you >> made available. >> >> Win > > >------------------------------------------------------------------------ >--- >Stephen M. Smith, Professor of Biomedical Engineering >Associate Director, Oxford University FMRIB Centre > >FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK >+44 (0) 1865 222726 (fax 222717) >[log in to unmask] http://www.fmrib.ox.ac.uk/~steve >------------------------------------------------------------------------ >--- >=========================================================================