Simplified calibration system for stereoradiography in scoliosis

Stereoradiography is a well known method to obtain 3D images of the spine and the thorax. The main algorithm used is the DLT, which is a very general one yielding to 11 implicit parameters per view. In order to calibrate the geometrical configuration of the stereoradiographic setup with this algorithm, there is a need of a wide calibrating object, leading to systems hard to use in clinical practice. The aim of this work was to modify the algorithm in order to simplify the calibration object. We used assumptions related to the specific case of stereoradiography, which reduced the problem of calibration to only 6 independent explicit parameters. A geometrical calibration performed plane by plane enabled to design a singular calibration object composed of steel balls along two vertical lines and three horizontal ones. A simulation of real configuration both for the previous method and the current algorithm associated to the singular calibrating object give for thirty 3D points a 2 RMS (95% confidence interval) error of reconstruction of respectively 0.6 and 0.3 mm. This study yielded to the transfer in clinics of two simplified systems of calibration which will be easier to use in clinical practice.