We present an automatic and accurate technique for 3D co-registration of SPECT and CT. The method will allow for attenuation correction of SPECT images and fusion of the anatomic details from CT and the functional information from SPECT. Registration was achieved by optimizing the mutual information metric over the parameter space defined by the translation and rotation parameters. To improve the robustness and accuracy of the algorithm, registration was performed in a coarse-to-fine manner. We

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... applied the algorithm on three clinical data sets originating from 1 pelvic and 2 thoracic studies. Validation was done by inspecting the 2D and 3D fusion of the registered images and by observing the convergence in the metric and the transformation parameters. We also evaluated quantitatively the effects of the choice of the parameters, the number of multiresolution levels, and initial misalignment of the paired volumes. Registration of both studies converged close to a final alignment with a maximum translational error of 1.41mm ± 0.78mm and rotational error of 1.21 • ± 0.46 • for the thoracic study and a maximum translational error of 1.96mm ± 1.27mm and rotational error of 0.57 • ± 0.34 • for the pelvic studies. The average computation time on a 3.0GHz PC was < 4 minutes for the entire registration procedure. We conclude that the algorithm had successfully co-registered the CT and SPECT images.