A coarse 8mm search for the motion parameters is then carried out using the cost function specified followed by two subsequent searches at 4mm using increasingly tighter tolerances.
All optimizations use trilinear interpolation.
As part of the initial 8mm search, an identity transformation is assumed between the middle volume and the adjacent volume. The transformation found in this first search is then used as the estimate for the transformation between the middle volume and the volume beyond the adjacent one. This scheme should lead to much faster optimization and greater accuracy for the majority of studies where subject motion is minimal. In the pathalogical cases, this scheme does not penalise the quality of the final correction. Finally, if a 4-stage correction has been specified, a further optimization pass is carried out using sinc interpolation for greater accuracy.