Registration of one atlas template to another atlas space!

Hello Everyone,
I need urgent help!!!
I am trying to register one pig brain atlas space to another atlas space so that I can transform white matter (WM) and CSF masks from one atlas to the other. I attempted this using antsRegistrationSyN, and while the outer brain contour appears to align reasonably well, the internal brain structures do not align properly at all. I am unsure how to proceed and would appreciate guidance on the best approach to handle this situation.
Thanks
Sanjida