A simple algorithm for geometric registration of tomographic images was developed to be applied to images of the heart obtained by Positron Emission Tomography (PET). Commonly, image misregistrations are not only caused by simple positioning differences like translation and rotation, but also by deformations that cannot be expressed by simple geometric transformations. The algorithm that we developed is a combination of two methods: a �rigid� registration followed by an �elastic� deformation. A variable number of reference points is used to register the images. The first part of this method consists of calculating the best match between the reference points of the two images based on the evaluation of the center of gravity, the axes of inertia and the surface extension. A set of geometric rotation, translation and scaling operations are then applied to the image to match. In the second phase an elastic model is further applied to perfectly match the reference points. An algorithm based on a weighted average of the displacement of every reference point is used. This second phase produces local deformations of the image to match every reference point. A set of validation experiments was performed using digitized images of isolated slices of a dog's heart where several deformations were applied. Further experiments on PET images showed that this method provides adequate automatic 2D registration of the tomographic images. The same approach could potentially be used to match images from different imaging modalities like MRI or CT scans. © 1988, SPIE.