Dear FS experts

I want to use Freesurfer registration tools. I check

https://surfer.nmr.mgh.harvard.edu/fswiki/FreeSurferCommandsRegistration?hig...

1). Here it is the term " modality" I want to get clarification. It is easy to know Modality is different between CT, MRI and PET. But if MRI use different sequence, T1, T2, DWI, ASL to get different images, it is the same modality or different modality?

2). mri_coreg is strongly recommended for use. But I can not find the instruction in Wiki.

3.) If I register ASL source image and subtraction images to T1w images, I guess that preciseness of registration will be higher if use source images to register. Is it right? If it is correct, If I use ASL source images to register to get .lta file, how can I apply .lta file to apply for subtraction images? Do you have instruction?

Many thanks

Best Regards

Frank Yu