[Freesurfer] PET/MRI registration failed with bbregister or mri_coreg

Dear Freesurfer's experts,

I would like to register PET image onto T1 anatomical. I used bbregister with the following command and it failed : *bbregister --s SUBJID --init-fsl --t2 --mov PET.nii.gz --reg Pet2T1.register.dof6.dat --lta Pet2T1.register.dof6.lta --init-reg-out Pet2T1.init.register.dof6.dat --o rPET.nii.gz *

Having heard about mri_coreg solving some bbregister fails, I tested it with the following command : *mri_coreg --s SUBJID --mov PET.nii.gz --reg Pet2T1.register.dof6.mri_coreg.lta --regdat Pet2T1.register.dof6.mri_coreg.dat*

But it failed in the same way than bbregister. Do you have any advice on this problem ?

Best regards, Matthieu