SPM_1:   Spatial registration and normalisation of images
Friston KJ, Ashburner J, Frith CD, Poline J-B, Heather JD
& Frackowiak RSJ (1995)
Human Brain Mapping 2:165-189

Abstract
   This paper concerns the spatial and intensity transformations that
   map one image onto another.  We present a general technique that
   facilitates non-linear spatial (stereotactic) normalisation and
   image realignment.  This technique minimises the sum of squares
   between two images following non-linear spatial deformations and
   transformations of the voxel (intensity) values.  The spatial and
   intensity transformations are obtained simultaneously, and
   explicitly, using a least squares solution and a series of
   linearising devices.  The approach is completely noninteractive
   (automatic), non-linear and non iterative.  It can be applied in
   any number of dimensions.  Various applications are considered,
   including the realignment of functional MRI time-series, linear
   (affine) and non-linear spatial normalisation of PET and
   structural MRI images, the coregistration of PET to structural MRI
   and, implicitly, the conjoining of PET and MRI to obtain high
   resolution functional images.

This paper describes the general approach to estimating movement
parameters and is a useful background read that deals with the many
instantiations of this theory in SPM95