Consistent groupwise non-rigid registration for atlas construction

This paper describes a groupwise, non-rigid registration algorithm to simultaneously register all subjects in a population to a common reference (or natural) coordinate system, which is defined to be the average of the population. This natural coordinate system is calculated implicitly by constraining the sum of all deformations from itself to each subject to be zero. To do this, the gradient projection method for constrained optimization is applied to maximize the similarity between the images, subject to the constraint being satisfied. The algorithm has been tested on a group of 19 brain MR images acquired from a population of subjects with schizophrenia.