Dear FSL experts

I have a basic question below.

I'm trying to extract FA and MD value for each subcortical GM structure (e.g., thalamus, hippocampus) in each individual's native space.

To this end, I have first done FIRST using T1 volumetric images, and then split *_all_fast_origsegs.nii.gz into 15 subcortical GM structures.

Then I made binary masks for the 14 subcortical GM structures (excluding brainstem).

Here I encountered the problem.

How can I coregister T1 image with FA and MD images in order to extract FA/MD values from subcortical GM structures that are made from FIRST ?

My T1 images are 1 mm^3 and DTIs are 2.5 mm^3.

Sorry for the basic question.

Please advice me to find a way.

Thank you in advance.

Lee K.