Procrustes analysis is the theoretical framework of the ILM technique. Given two configurations of points which describe homologous points in the MR and SPECT volumes of the same brain, a rigid linear transformation is constructed for an optimal registration using only 9 degrees of freedom: 3 translations, 3 rotations/reflections and an optional scale change in each dimension (no shears). A review of Procrustes analysis, its use in the ILM technique, and other validations of the ILM technique are presented here as a basis for the development of the work presented in this thesis.