Interpolation artefacts in non-rigid registration.

Voxel based non-rigid registration of images involves finding a similarity maximising transformation that deforms a source image to the coordinate system of a target image. In order to do this, interpolation is required to estimate the source intensity values corresponding to transformed target voxels. These interpolated source intensities are used when calculating the similarity measure being optimised. In this work, we compare the extent and nature of artefactual displacements produced by voxel based non-rigid registration techniques for different interpolators and investigate their relationship to image noise and global transformation error. A per-voxel similarity gradient is calculated and the resulting vector field is used to characterise registration artefacts for each interpolator. Finally, we show that the resulting registration artefacts can generate spurious volume changes for image pairs with no expected volume change.