[Freesurfer] problem bbregister onto longitudinal pipeline

Dear Freesurfer Users,

I'm having a problem to register my PET images into the longitudinal pipeline of freesurfer (version 5.1).

I have use bbregister without any problem to put the PET image into the crossectional images but it fails to register to the long version of it. Someone can tell me if I'm doing something wrong?

bbregister -s P09_TA --mov pet_09.nii --init-fsl --reg matrix.mat --o pet2free.mgz

This last command works perfectly, but

bbregister -s P09_TA.long.P09 --mov pet_09.nii --init-fsl --reg matrix.mat --o pet2free.mgz

Fails completely.

My objective is to project my pet cortex into the surface in longitudinal that is why I'm interested in this processing. Maybe you can tell my another way of doing this.

Thank you very much.

Daniel