C-arm orbits for metal artifact avoidance (MAA) in cone-beam CT

Abstract

Purpose: Metal artifacts present a challenge to cone-beam CT (CBCT) image-guided surgery, obscuring visualization of metal instruments and adjacent anatomy. We present a method to reduce the influence of metal artifacts by prospectively defining an C-arm source-detector orbit that mitigates metal-induced biases in the projection data. Methods: The metal artifact avoidance (MAA) method is compatible with simple mobile C-arms, does not require exact prior information on the patient or metal implants, and is consistent with more advanced polyenergetic model-based image reconstruction, and metal artifact reduction (MAR) post-processing methods. The MAA method consists of: (i) coarse localization of metal objects in the field-of-view (FOV) via two or more low-dose scout projection views and segmentation (e.g., a simple U-Net) in coarse backprojection; (ii) model-based prediction of metal-induced x-ray spectral shift for all possible views (gantry rotation and tilt angles); and (iii) identification of a circular or non-circular orbit that reduces the variation in spectral shift. The method was evaluated in a series of experiments, including simulation, phantom, and cadaver studies in the context of image-guided spine surgery. Results: The MAA method accurately predicted tilted circular and non-circular orbits that reduced the magnitude of metal artifacts. Realistic distributions of metal instrumentation were successfully localized (0.71 median Dice coefficient) from 2-6 low-dose scout views even in complex anatomical scenes. The MAA-predicted tilted circular orbits reduced root-mean-square error (RMSE) in 3D image reconstructions by 46-70% and "blooming" artifacts (apparent width of the screw shaft) by 20-45%. Non-circular orbits defined by MAA achieved a further ~46% reduction in RMSE compared to the best (tilted) circular orbit. Conclusion: The MAA method presents a practical means to predict C-arm orbits that minimize spectral bias from metal instrumentation. Resulting orbits exhibited substantial reduction of metal artifacts in raw CBCT reconstructions by virtue of higher fidelity projection data.