ImportNiiDiff[]

will prompt for the *.nii,*.bvec and *.bval file to import.

ImportNiiDiff[*.nii]

will import the *.nii file and automatically also imports the *.bvec and *.bval is they have the same name.

ImportNiiDiff[*.nii,*.bvec,*.bval]

will import the given files. The output will be {data,grad,bvec,vox}.