A Photogrammetric System for 3d Reconstruction of a Scoliotic Torso

Scoliosis is a deformity of the human spine most commonly encountered with children. Its early detection is vital for the patient’s treatment, so periodic examinations via X-rays are traditionally used to measure its progression. Since this can cause a significant dose of radiation leading to an increased risk of cancer, it is imperative to minimize the use of radiographs and introduce a non-invasive and radiation-free scoliosis detection methodology. Quantifying the scoliotic deformity of the torso surface is a valid alternative, because the internal spine curvature and the torso surface deformation are highly correlated. This work proposes a low-cost multi-camera photogrammetric system for the reconstruction of a torso surface with a sub-millimetre level accuracy, which will be suitable for the routine clinical monitoring of scoliosis with no harmful side effects. The paper first describes the system design for optimal accuracy. Secondly, it explains the system calibration in terms of the camera calibration, stability analysis, and establishing the exterior orientation parameters of the implemented cameras. Then, it covers the entire reconstruction procedure giving insights on the pattern projection, epipolar resampling, feature extraction, matching and tracking, intersection, and the final surface registration. The final accuracy is tested by evaluating the goodness of fit between the reconstructed surface and a more accurate surface that has been measured by a coordinate measuring machine.