Hi Simon
Calculating RD and AD values *per fibre direction* is an ill-posed problem given standard diffusion (HARDI) data. In particular, there is a strong correlation between partial volume fractions and diffusivities.
In bedpostx, we assume that RD=0, and so we can calculate partial volume fractions unambiguously.
If you want to calculate RD values per fibre, you need a more complex model and appropriate data (e.g. multiple b-values).
Cheers,
Saad.
Thanks Saad,
This helps, but one of my assumptions was flawed--that i could reconstruct RD from bedpostx output, and then correct it with tbss_x. I'm aware of what precludes the former, but is there any means of improving the RD calculated from a dtifit output using tbss_x? Is it meaningful to use tbss_x on anything but bedpostx output?
--
Saad Jbabdi
University of Oxford, FMRIB Centre
JR Hospital, Headington, OX3 9DU, UK
(+44)1865-222466 (fax 717)
