Efficient implementation of KL entropy estimator for image registration Ivan Vnucko Fundamental problem of mutual information image registration is the estimation of entropy and mutual information. We reimplemented the Kozatchenko-Leonenko estimator based on nearest neighbor distances. All-nearest neighbor search uses a kD tree with Best Bin First node traversal. Our main goal was the speed. In the presentation we will shortly introduce the KL estimator and the BBF kD tree, describe the implementation and results of experiments.