Validation of volume-preserving non-rigid registration: Application to contrast-enhanced Mr-mammography

Abstract

In this paper, we present a validation study for volume preserving nonrigid registration of 3D contrast-enhanced magnetic resonance mammograms. This study allows for the first time to assess the effectiveness of a volume preserving constraint to improve registration accuracy in this context. The validation is based on the simulation of physically plausible breast deformations with biomechanical breast models (BBMs) employing finite element methods. We constructed BBMs for four patients with four different deformation scenarios each. These deformations were applied to the post-contrast image to simulate patient motion occurring between pre- and post-contrast image acquisition. The original pre-contrast images were registered to the corresponding BBM-deformed post-contrast images. We assessed the accuracy of two optimisation schemes of a non-rigid registration algorithm. The first solely aims to improve the similarity of the images while the second includes the minimisation of volume changes as another objective. We observed reductions in residual registration error at every resolution when constraining the registration to preserve volume.Within the contrast enhancing lesion, the best results were obtained with a control point spacing of 20mm, resulting in target registration errors below 0.5mm on average. This study forms an important milestone in making the non-rigid registration framework applicable for clinical routine use.