"Generating Coefficients for Regularization Terms in Nonrigid Registration of Contrast-Enhanced MRI"
Xi Liang, R. Kotagiri, Q. Yang, Marius Staring and A. Pitman
Nonrigid registration is a technique to recover spatial deformations between images. It can be formulated as an optimization problem to minimize the image dissimilarity. A regularization term is used to reduce undesirable deformations which are usually employed in a homogeneous or spatial-variant fashion. When spatial-variant regularization is used in nonrigid registration of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI), the local coefficients have been determined by manual segmentation of tissues of interest. We propose a framework to generate regularization coefficients for nonrigid registration in DCE-MRI, where tumor locations are to be transformed in a rigid fashion. The coefficients are obtained by applying a sigmoid function on subtraction images from a pre-registration. All parameters in the function are automatically determined using k-means clustering. The validation study compares three regularization weighting schemes in nonrigid registrations: a constant coefficient for a volume-preserving term, binary coefficients obtained by manual segmentation and a real-value coefficients using the proposed method on a rigidity term. Evaluation is performed using displacements, intensity changes and volume changes of tumors on synthetic and clinical DCE-MR breast images. As a result, the registration using spatial-variant rigidity terms performs better than using homogeneous volume-preserving terms. For the coefficient generation methods of a rigidity term, the proposed method can replace the binary coefficients requiring manual tumor segmentation.