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

Abstract

In this paper, we present a 3D-tracking technique inspired by visual servoing, specifically designed for Computed Tomography (CT). This work has been developed within the framework of robot- and 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. The patient’s movements can then be tracked with the proposed algorithm by means of a motion field approach, so as to update the 2D/3D 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 threefold. First, the stereotactic visual feedback is modelled using the Plücker representation for 3D straight lines, while the CT plane slice provides corresponding image points. It is shown that the number of features needed to compute the pose is minimal compared to the known previous techniques. Second, the Jacobian matrix which relates the image points displacements to the velocity screw of the stereotactic frame is computed, providing the CT motion field. Third, the update of the Jacobian matrix is investigated. It requires the on-line stereotactic registration. As with the CT imaging modality, the 2D/3D registration is highly inaccurate when the fiducials are being moving, this paper provides a CT visual 3D 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 stereotactic markers mounted on a robotic platform and placed in a CT scanner.