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Statistical atlases built by point distribution models (PDMs) using a novel hybrid 3D shape model were used for surface reconstruction. The hybrid shape model removes the need for global scaling in aligning training examples and instance generation, thereby allowing the PDM to capture a wider range of variations. The atlases can be used to reconstruct, or deformably register, the surface model of an object from just two to four 2D x-ray projections of the object. The methods was tested using proximal and distal femurs. Results of simulated projections and fluoroscopic images of cadaver knees show that the new instances can be registered with an accuracy of about 2 mm.

Fiducial registration is useful both in applications where other registration techniques have poor performance and for validation of new registration techniques. Registration of 3D CT or MR images to 2D X-ray images is particularly difficult, in part because automated contour extraction from 2D images is not yet well solved and in part because of the considerable computational expense in matching the contours to the 3D images.