Estimation of out-of-plane vertebra rotations on radiographic projections using CT data: a simulation study

This study extends previous research concerning in vivo intervertebral motion by means of single-plane fluoroscopy in an attempt to overcome 2D analysis limitations. Knowledge of out-of-plane vertebra rotations will extend the results provided by planar kinematic studies, which is particularly important for lateral bending investigation where axial rotation accompanies side bending, but is also valuable in sagittal analysis (e.g. indicating an absence of coupled axial rotation). Combining a fluoroscopic projection of a vertebra with volumetric information provided by CT data, vertebra 3D position can be estimated. Out-of-plane vertebral rotations are estimated by comparing Digitally Reconstructed Radiographs (DRRs) in different orientations with a reference fluoroscopic projection, maximising the image cross-correlation index. DRRs have been computed from CT-data using a ray-casting algorithm. In this work a feasibility study of the method was performed by means of a computer simulation. To this end the CT volume (vertebra L4, segmented) provided by the Visible Human Project was utilised and reference fluoroscopic projections were simulated in different orientations adding various levels of noise. Accuracy and precision of the proposed method was determined. Error analysis reveals that an accuracy of less than 1 degree can be achieved in computation of out-of-plane vertebral angles.