Purpose: Metal artifacts pose a challenging problem to cone-beam CT (CBCT) in image-guided orthopedic interventions. Highly attenuating metal implants effectively create a missing-data problem and this incomplete data yields reconstruction artifacts. Previous work has demonstrated that non-circular orbits can improve data completeness and potentially avoid missing data. In this work, we present the first implementation of continuous non-circular orbit acquisitions on a clinical robotic C-arm for reduction of metal artifacts.
Methods: A Siemens ARTIS pheno robotic C-arm with a Siemens Test Automation Control System (TACS) was used to image a 3D-printed (copper/PLA blend) cervical spine phantom implanted with four stainless steel pedicle screws using two non-circular orbits: a sawtooth orbit and a double-circle orbit. The sawtooth orbit rotated 360° continuously in the LAO/RAO direction tilted between +25° and -25° in two cycles in the CRAN/CAUD direction. For the double-circle orbit, two 360° LAO/RAO rotations were conducted with fixed 30° and -30° CRAN/CAUD tilt. To accommodate the irregular projection data, model-based iterative reconstruction was applied using a penalized-likelihood approach. Comparisons between orbits including a conventional circular scan are presented. Additionally, we analyze the effect of precision of system geometry on image quality.
Results: Both non-circular orbits significantly reduce the artifacts surrounding the implants, permitting better visualization of pedicle screws and the surrounding anatomy. In extreme cases where two screws coincide in the same axial plane, the anatomical structures are highly obscured in the conventional scan, while the non-circular orbits preserve anatomical details of the vertebrae, surrounding soft tissue, and threads on the screws. Moreover, by further refining the machine recorded geometry, we reduce the noise level by 40.67%.
Conclusion: This is the first continuous acquisition of non-circular orbits on a clinical imaging system. In an emulated cervical spine scan, we demonstrate the ability of non-circular orbits to dramatically reduce metal artifacts.
Funding Support, Disclosures, and Conflict of Interest: This research was supported in part by NIH grant R01EB027127. This research was supported in part by a Siemens grant. The concept and information presented in this paper is based on research and is not commercially available. Due to regulatory reasons, its future availability cannot be guaranteed.