Hi,
This is a tricky subject, as it is for every format. Here is a nice summary here:
and here:
also here:
Within SPM, it looks like its version of the MNI template (IXI549space) is very similar to MNI152NLin6 :
- SPM: Note that in later iterations, SPM defined its own “space” based on scans acquired within the IXI dataset, leveraging on the concept of segmentation-based normalizations (as applied in the Unified Segmentation, DARTEL and SHOOT approaches). This space, referred to as the IXI549Space is largely similar to MNI152 and of similar resolution (where it’s likely safe to say that differences do not matter as much for most applications). Along the same lines, both spaces are unsuitable for use in DBS imaging, in our opinion. More info here.
If I refer to this discussion, SPM IXI549 space matches the MNI152NLin6Asym template:
Concerning spaces, the TPM.nii in SPM12 matches an asymmetric variant of the MNI ICBM 152 non-linear 6th Gen space (this is not identical to the symmetric one available here http://nist.mni.mcgill.ca/?p=858 ), although there are slight differences, as the tissue priors were generated from different subjects (the IXI data set) with different registration techniques. The CAT12 templates were generated from the IXI data set via Geodesic Shooting aka Shoot, and “finally transformed to MNI space” as stated in a CAT12 help text. Although not explicitly mentioned it seems to be the MNI ICBM 152 non-linear 6th Gen asymmetric space, and not the (somewhat larger) MNI ICBM 2009 space. Again, due to different subjects there would be slight differences to the original atlas.
The MNI space is visible through the TPM template in the spm12 folder: spm12/tpm/TPM.nii
You can get the MNI152Nlin6Asym template images here: https://www.templateflow.org
Let us know if the statements above are correct for you?