Registration of high resolution images of the retina

Abstract

A method of image registration is presented for the case when the deformation between two images can be well approximated with a combination of translation, rotation and global scaling. The method achieves very high accuracy by combining a global optimization in the 4-dimensional discrete parameter space with a local optimization in the 4-dimensional continuous parameter space. The 4-dimensional global optimization is accomplished with two 2-dimensional optimizations. The Fourier magnitude is used to decouple translation from rotation and scaling, and a log-polar mapping of the Fourier magnitude is used to convert rotation and scaling into shifts. Optimal rotation and scaling parameters are determined with a cross-correlation in the log-polar domain. After compensation for rotation and scaling differences, cross-correlation in the spatial domain yields the translation parameters. The four registration parameters are further refined with a local optimization using the correlation coefficient as a similarity measure in the 4-dimensional continuous parameter space. Results are shown from simulations and from registration of retinal images. For simulated images with a signal-to-noise ratio of -5 dB, the accuracy of the registration method is estimated to be better than 0.07 degrees, 0.1 %, and 0.3 pixels for rotation, scaling, and translation, respectively. In the case of 512x512 pixel images the computation resource requirements are compatible with high end PCs, i.e., approximately 25 minutes on an Intel 80486/33MHz based IBM/PC compatible.