What is the difference between the ACPC, MNI, SPM and TAL coordinate systems?
There are a number of coordinate systems that have the origin, i.e., the [0,0,0] point, at the Anterior Commissure and that have a RAS orientation (i.e. the x-axis pointing to the right, the y-axis pointing to anterior, the z-axis pointing to superior).
- ACPC is used if the geometry is according to the individual subject head/brain size.
- TAL is used if the geometry is piecewise scaled to match the Talairach-Tournoux (1988) atlas.
- MNI, MNI152 or SPM is used if the geometry is spatially warped to the MNI152 template brain.
- MNI305 or FSAVERAGE is used if the geometry is spatially warped to the MNI305 template brain.
Coordinates expressed in the TAL, MNI/MNI152/SPM, and MNI305/FSAVERAGE coordinate system all imply that some spatial deformation was done; they can be looked up in the corresponding template or atlas. For ACPC coordinates no deformation was done; they can be used to express location in an individual subject, or measure distance, volume or surface area.
See also this frequently asked question that explains how different EEG, MEG and MRI coordinate systems are defined.