-dirs parameter for dwi2response

Hello everyone,

I am looking to use the manual parameterization of dwi2response function.
dwi2response manual input in_voxels output -dirs …
Is there any MRTRIX Function that provides the (X Y Z 3) directions of the
needed voxels by the tensor for the -dirs input?
More specifically, in my understanding it is right to use in this case:
tensor2metric tensor.mif -vector dirs.mif for calculation of the principal eigen-
vector, hence, the main directionality. Or this is not what is required as input?

Thanks a lot!

Yes, you can certainly use tensor2metric -vector for that. I’d also add the -modulate none option to make sure they’re all unit length – I can’t remember whether the approach performs normalisation internally…

Perfect, thanks a lot for the answer!

Just a follow-up question on the -modulation selection. Are the calculated vectors by tensor2metric normalized by default (thus, no modulation will result to unit length vectors)?
The same happens with FSL calculated vectors ? Could not find that info in FSLWiki…
Thank you!

By default, they’re scaled by FA – means the -vector option produces a DEC map by default. You need to specify none to get unit-normalised vectors.

Sorry, I’ve no idea! Hopefully someone else will have the answer to that one…

Thanks for the answer.
May I ask, do the normalized vectors make a difference in the case of tensor2metric directions parameter or make the calculations more robust in some sense?

**Update: Vectors from FSL seem to be normalized as their magnitude is 1 in all cases.

I’m assuming you’re taking about the directions passed to dwi2response? There’s no such parameter for tensor2metric. If so, then no, it won’t make any difference: I’ve just checked, the direction vector will be normalised before use.

Yes, I meant dwi2response, sorry about that. Great, thanks a lot.