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 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.

In all cases the origin is at the Anterior Commissure and the negative y-axis is passing through the Posterior Commissure.

Coordinates in the tal, mni/mni152/spm, and mni305/fsaverage all imply that some spatial deformation was done; they can be looked up in a template or atlas. ACPC coordinates can be used to measure distance, volume or surface area in an individual subject.

See also this page which describes the TT and MNI space in more detail and this page that explain the differences between MNI152 and MNI305.

See also this frequently asked question that explains how the different EEG, MEG and MRI coordinate systems are defined.