DTI preprocessing in FSL: B-vector correction

Using FSL to pre-process DTI data is easy to do, and the use of a standard FMRIB pipeline is ubiquitous in the literature. An ongoing concern with this common pre-processing pipeline is how to appropriately deal with eddy currents. FSL uses a 12 degree of freedom affine registration as a method of eddy current correction. By definition, therefore, it corrects for both eddy currents AND gross subject motion. So far this seems great... however...

It is really important to alter your b-vector file based on subject motion (Leemans and Jones 2009), but one should NEVER correct b-vectors based on eddy currents, since these vary across the brain (randomly) depending on a variety of factors. This can have profound effects on not only tractography, but also general diffusion modelling (DTIfit). Current wisdom on the FSL forum suggests that using the downloadable script “rotbvecs” will solve your b-vector correction issue. The problem is that this will correct for both subject motion AND eddy currents.

Here is a simple solution that will allow you to keep using FSL for preprocessing, and keep you up to date on appropriate b-vector corrections:

1) Create a new eddy_correct script that only uses 6-degrees of freedom, and call it eddy_correct_6dof. To do this, just add “-dof 6” to the following line of code:

${FSLDIR}/bin/flirt –in $i –ref ${output}_ref –nosearch –o $i –paddingsize 1

Then run the rotbvecs script as normal (this changes the bvecs file to reflect subject motion).

If you still want to correct for eddy_currents, simply run the regular eddy_correct script, and don’t touch the b-vector file. This might seem like a time consuming extra step, but probabilistic tractography is incredibly sensitive to the initial quality of the data. If you are looking for more detailed instructions, just send me a quick email (look to http://csl.psychol.cam.ac.uk/people/) for my contact info.

Of course ... one can also simply choose not to use FSL to pre-process the data... some great alternatives are emerging.