Figure 4

Projective projection of CT bone data to obtain a Digitally Reconstructed Radiograph (DRR). The projective projection of the CT bone data was performed simulating the standard X-ray imaging of the ankle on a digital radiography system (CXDI-40EG, CANON, USA) with the X-ray focus 1 meter away from the image plane. The ankle model was positioned in the standard pose with the anatomical axes parallel to those of the X-ray system depending on the direction of the imaging. The DRR of the bone was generated by casting rays from the X-ray source through the volume of the bone model. Each of these rays went through a number of voxels of the volume, the attenuation coefficients of which were then integrated along the ray and projected onto the imaging plane to obtain a DRR image resembling a radiograph. During the generation of the DRR for 2D measurements, the landmarks (red) associated with the morphological parameters determined were also generated automatically using the geometrical features of the 3D volumetric bone models (Table 2), and projected onto the 2D image plane. The projected 3D landmarks were used to label the landmarks automatically using a 2D-to-3D registration procedure [24].