Hi Naveed,

As Peter suggested, those files are not volume files and cannot be written directly into BRIK format. You can map cortical thickness values into the volume, but that would be a separate step with various options that you must consider.

If you have not done so already, you should run @SUMA_Make_Spec_FS -GIFTI -sid YOUR_SUBJECT
on the output of FreeSurfer. This will bring the thickness results into a format that SUMA and AFNI can handle. FreeSurfer surfaces, surface and volume parcellations end up in a directory called SUMA/ .

You can then take those surface thickness files and map them into the volume with the program 3dSurf2Vol.

cheers,
Ziad