Dear Sevim,

Regarding your first question: There is still a problem with your path, it is trying to write a file with the extension "." , while most likely it has to be ".nii". fn_fa needs to be something like "fa.nii". 

With respect to your second question, you need to turn off interpolation. Interpolation will result in an FA map that has different dimensions than your tensors.

Best, 
Emilie