A visual 3D-tracking and positioning technique for stereotaxy with CT scanners

In this paper, we present a 3-D tracking technique inspired by visual servoing, specifically designed for Computed Tomography (CT). This work has been developed within the framework of robotand computer-assisted interventional radiology, using stereotactic external fiducials made of radiopaque rods. These fiducials produce a set of image feature points that are used in a pose estimation algorithm, with only one slice. Intra-operative movements, such as those due to the patient, can then be tracked with the proposed algorithm by means of a motion field approach, so as to update the 2-D/3-D registration. Therefore, the proposed method solves a fundamental safety issue associated to the robotic assistance of CT-guided interventions. The contributions of the paper are threefolds. Firstly, the stereotactic visual feedback is modelled using the Plücker representation of 3-D straight lines, while the CT plane slice provides corresponding image points. It is shown that, with this representation, the number of features needed to compute the pose is minimal compared to the known previous techniques. Secondly, the Jacobian matrix which relates the image points displacements to the velocity screw of the stereotactic frame is computed, providing the CT motion field. Thirdly, the update of the Jacobian matrix is investigated. It requires an on-line stereotactic registration. As with the CT imaging modality, the 2-D/3-D registration is highly inaccurate when the fiducials are moving, this paper provides a CT visual 3-D tracking method inspired by the image-based visual servoing, which may alleviate the Jacobian matrix updates. To validate this technique, we first present simulations of the CT visual tracking. Finally, the proposed method is applied to real images, obtained with a moving operation table. Preprint submitted to Robotics and Autonomous Systems 22 October 2007