It is often performed in research-based functional neuroimaging where one wants to find common brain activation across multiple human subjects.
The brain scan can be obtained from magnetic resonance imaging (MRI) or positron emission tomography (PET) scanners.
There are two steps in the spatial normalization process: The estimation of the warp-field can be performed in one modality, e.g., MRI, and be applied in another modality, e.g., PET, if MRI and PET scans exist for the same subject and they are coregistered.
Spatial normalization typically employs a 3-dimensional nonrigid transformation model (a "warp-field") for warping a brain scan to a template.
For this reason, flows which generalize the ideas of additive groups allow for generating large deformations that preserve topology, providing 1-1 and onto transformations.