intrasubject, intramodality registration (e.g., PET to PET or MRI to MRI)
intrasubject, intermodality registration (e.g., PET to MRI)
intersubject registration (e.g. subject to subject or subject to atlas template)

Intrasubject registration of brain images uses a rigid-body model. Intermodality registration has been validated for some (MRI-PET), but not all modalities. Intersubject registration can be performed using any of a variety of linear or nonlinear models to register different subjects to one another or to an atlas template (for example, an averaged brain in "Talairach space"). These models may also be useful for intrasubject registration of organs that are more deformable than brain or for tracking intrasubject developmental changes over time.

In addition to 3D models, AIR 3.0 includes homologous 2D deformation models that may be useful in selected circumstances (e.g., single slice fMRI data, pure 2D images, etc.).

Once the registration parameters are known, the AIR package allows the data to be resampled to generate a final image using any of the following interpolation models:

• nearest neighbor
• trilinear
• windowed sinc
• mixed linear/windowed sinc
• unwindowed sinc
• chirp-z
• mixed linear/chirp-z

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