A two-stage algorithm for model-based registration of medical images

Digital subtraction has shown to be the best noninvasive tool for diagnosing small changes of internal structures in medical images. However, subtraction techniques require perfect geometric match. Geometric projection of most medical imaging techniques could be modeled by perspective projection. Eight parameters must be determined for the perspective back-projection of the subsequent image into the geometry of the reference. Perspective invariants which allow the determination of the total eight parameters are still unknown. Therefore, rotations, translations, and scalings (RST-movements) are detected in a first stage using the Fourier-Mellin invariant. This pre-registration allows the linkage of corresponding landmarks in both images by local correlation. In the second stage, the coordinates of automatically detected landmarks are used to determine the perspective parameters for fine-adjustment. The two stages form a robust registration algorithm which is applied to the alignment of intraoral radiographs acquired without individual adjustment aids. The excellent results allow the clinical establishment of the method.