Hindawi Publishing Corporation EURASIP Journal on Advances in Signal Processing Volume 2010, Article ID 806094, 8 pages doi:10.1155/2010/806094 Research Article 2D-3D Registration of CT Vertebra Volume to Fluoroscopy Projection: A Calibration Model Assessment P. Bifulco, 1 M. Cesarelli, 1 R. Allen, 2 M. Romano, 1 A. Fratini, 1 and G. Pasquariello 1 1 Department of Biomedical, Electronic and Telecommunication Engineering, University of Naples Federico II, 80125 Napoli, Italy 2 Institute of Sound and Vibration Research, University of Southampton, Southampton S017 1BJ, UK Correspondence should be addressed to P. Bifulco, pabifulc@unina.it Received 28 April 2009; Accepted 22 May 2009 Academic Editor: Jo˜ ao Manuel R. S. Tavares Copyright © 2010 P. Bifulco et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This study extends a previous research concerning intervertebral motion registration by means of 2D dynamic fluoroscopy to obtain a more comprehensive 3D description of vertebral kinematics. The problem of estimating the 3D rigid pose of a CT volume of a vertebra from its 2D X-ray fluoroscopy projection is addressed. 2D-3D registration is obtained maximising a measure of similarity between Digitally Reconstructed Radiographs (obtained from the CT volume) and real fluoroscopic projection. X-ray energy correction was performed. To assess the method a calibration model was realised a sheep dry vertebra was rigidly fixed to a frame of reference including metallic markers. Accurate measurement of 3D orientation was obtained via single-camera calibration of the markers and held as true 3D vertebra position; then, vertebra 3D pose was estimated and results compared. Error analysis revealed accuracy of the order of 0.1 degree for the rotation angles of about 1 mm for displacements parallel to the fluoroscopic plane, and of order of 10 mm for the orthogonal displacement. 1. Introduction Intervertebral kinematics closely relates to the function- ality of spinal segments and can provide useful diagnos- tic information. Direct measurement of the intervertebral kinematics in vivo is very problematic due to its intrinsic inaccessibility. The use of a fluoroscopic device can provide a continuous 2D screening of a specific spinal tract (e.g., cervical, lumbar) during spontaneous motion of the patient, with an acceptable, low X-ray dose. 2D kinematics can be extrapolated from fluoroscopic sequences. Most of the previous works [1–8] were confined to the estimation of planar motion (most on sagittal plane) and are based on the assumption of absence of out-of-plane coupled motion (e.g., axial rotation). Coupled motion can be neglected in sagittal (flexion-extension) motion (mainly due to anatomic symmetry), but in lateral bending, where a coupled axial rotation is certainly present [9], this approximation is no longer valid. The knowledge of 3D positioning (pose) of vertebrae with time can lead to full 3D kinematics analysis, or at least to evaluate the presence of out-of-plane motion (rotation). External skin markers do not provide accurate intervertebral motion description [10, 11], and invasive positioning of markers inserted in the vertebrae is not generally viable. In order to allow clinical application, 3D kinematics analysis should be performed by means of readily available, and minimally invasive, instrumentation, combined with an appropriate image processing technique. In this study, a method for 3D pose estimation of vertebrae based on single plane-projection (e.g., Digital Video Fluoroscopy, DVF) combined with available CT-data [12–17] is proposed. By processing common CT slices it is possible to extract a 3D model of a vertebra, and by subsequent processing using ray-casting techniques it is possible to produce Digitally Reconstructed Radiographs (DRRs), simulating the 3D radiograph formation process. Comparing the DRRs with the real fluoroscopic image it is possible to estimate the real 3D orientation of the vertebra when screened by fluoroscopy (Figure 1). To assess the accuracy and the repeatability of the method invitro, a calibration model consisting of a sheep dry vertebra rigidly fixed to an X-raytransparent frame of reference was designed in order to independently evaluate its 3D pose by